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FTHOM A ECENT PHOTO AAP
FROM A RECENT PHOTOGRAPH
FOR THE USE OF
SCHOOLS, ACADEMIES, AND COLLEGES.
EDWIN J. HOUSTON, A.M., PH.D.,
EMERITUS PROFESSOR OF PHYSICAL GEOGRAPHY AND NATURAL PHILOSOPHY IN THE CENTRAL
HIGH SCHOOL OF PHILADELPHIA; PROFESSOR OF PHYSICS IN THE FRANKLIN
INSTITUTE OF THE STATE OF PENNSYLVANIA.
PUBLISHED BY ELDREDGE & BROTHER,
No. 17 North Seventh Street.
Entered, according to Act of Congress, in the year 1891, by
ELDREDGE & BROTHER,
in the Office of the Librarian of Congress, at Washington.
WESTCOTT & THOMSON,
THE GEORGE S. FERGUSON CO.,
TO THE ORIGINAL EDITION.
N the preparation of this work, an endeavor has been made to supply a concise yet comprehensive
text-book, suited to the wants of a majority of our schools.
The Author, in the course of his teaching, has experienced the need of a work in which unneces-
sary details should be suppressed, and certain subjects added, which, though usually omitted in works
on Physical Geography, seem, in his judgment, to belong properly to the science. The variety of
topics necessarily included under the head of Physical Geography renders it almost impossible to
cover the entire ground of the ordinary text-books during the time which most schools are able to
devote to the study, and the feeling of incompleted work thus impressed on the mind of both teacher
and scholar is of the most discouraging nature.
To remove these difficulties, the Author, during the past few years, has arranged for his own
students a course of study, which, with a few modifications, he has at last put into book form, thinking
that it may prove beneficial to others.
The division of the text into large and small print has been made with a view of meeting the
wants of different grades of schools, the large type containing only the more important statements, and
the small type being especially designed for the use of the teacher and the advanced student. The
maps have been carefully drawn by the Author according to the standard works and the latest
authorities. Neither time nor expense has been spared to insure accuracy of detail and clearness
Throughout the work no pains have been spared to insure strict accuracy of statement. Clearness
and conciseness have been particularly aimed at; for which reason the names of authorities for state-
ments which are now generally credited have been purposely omitted.
The Author has not hesitated to draw information from all the standard works on Geography,
Physics, Geology, Astronomy, and other allied sciences; and in the compilation of the Pronouncing
Vocabulary he acknowledges his indebtedness to Lippincott's Gazetteer of the World.
Acknowledgments are due to Mr. William M. Spackman, of Philadelphia, and Prof. Elihu
Thomson, of the Central High School, for critical review of the manuscript. Also to Mr. M. Benja-
min Snyder, of the Central High School, for revision of the proof-sheets of the chapter on Mathe-
matical Geography. E. J. H.
CENTRAL HIGH SCHOOL, Philadelphia, Pa.
TO THE REVISED EDITION.
THE marked progress which has been made in most of the departments of science embraced
in the study of Physical Geography since the issue of the original edition of "The
Elements of Physical Geography" has rendered the preparation of a revised edition a matter
The study of Physical Geography, including as it does not only the crust of the earth and
s heated interior, but also the distribution of its land, water, air, plants, and animals, includes,
in its range, a great variety of topics, and necessitates for its proper elucidation many branches
of science. Some knowledge of the elementary principles of these sciences is necessary to the
proper study of Physical Geography. The number of such principles is great, and the temptation
naturally exists to encumber even an elementary text-book with such an abundance of leading
principles as to render it either incomprehensible, or too extended for actual use in the school-
The author has endeavored in the revised edition to avoid undue multiplicity either of ele-
mentary principles or unimportant details. His object has been to develop forcibly the close inter-
dependence of the inanimate features of the earth's surface, the land, water, and air, with its
animate features, its flora, and fauna, and to show the marked influence which all of these exert
on the development of the human race, and, therefore, on history itself.
Recognizing, from his standpoint of a teacher, the inadvisability of crowding a book with
new matter simply because it is new, the author has carefully avoided the introduction of new
theories unless they have been generally accepted by the best authorities. Old theories are in all
cases given the preference of new ones, unless the latter bear the stamp of general approval.
At the same time the results of recent investigations have been freely given in all cases where
they have been considered sufficiently authoritative.
In order to avoid confusing the mind of the student, controversial matters have been carefully
avoided. When, however, opinion on any subject is fairly divided, a brief statement is made of the
The favorable reception accorded by the teaching profession to the earlier editions of the book,
and the flattering increase in the number of schools using it, have satisfied the author of the
inadvisability of changing, to any considerable extent, the order of sequence of topics discussed, or
the general manner of explanation therein adopted.
In the preparation of the revised edition the author has freely consulted the latest standard
authorities in the many sciences represented.
The maps have all been re-drawn according to the best authorities, and are printed and colored
by processes that in point of clearness and beauty leave little room for improvement.
EDWIN J. HOUSTON.
CENTRAL HIGH SCHOOL,
The first chapter of this book is intended mainly for reference, containing as it does, an abstract.
of the elementary principles of Mathematical Geography, with which most pupils beginning the study
of Physical Geography are familiar. In many schools in which the book is used, it is customary
to begin the formal study of the book with the Syllabus, page 31, which presents a comprehensive
review of the chapter, and in practice and results this plan has proved satisfactory.
THE EARTH AS A PLANET.
L MATHEMATICAL GEOGRAPHY .
REVIEW QUESTIONS . . .
THE INSIDE OF THE EARTH.
I. THE HEATED INTERIOR . .
II. VOLCANOES .............
III. EARTHQUAKES . . .
REVIEW AND MAP QUESTIONS . .
THE OUTSIDE OF THE EARTH.
I. THE CRUST OF THE EARTH . .
II. DISTRIBUTION OF THE LAND AREAS .
III. ISLANDS ..............
IV. RELIEF FORMS OF THE LAND .
V. RELIEF FORMS OF THE CONTINENTS .
REVIEW QUESTIONS ......... .
MP QUESTIONS . .
L PHYSICAL PROPERTIES OF WATER . 57
I. DRAINAGE ............... 59
III. RIVERS .............
IV. TRANSPORTING POWER OF RIVERS
V. DRAINAGE SYSTEMS . .
VI. LAKES .............
SYLLABUS . . . .
REVIEW AND MAP QUESTIONS .
I. THE OCEAN ..............
II. OCEANIC MOVEMENTS . . .
III. OCEAN CURRENTS . . .
SYLLABUS .......... .......
REVIEW AND MAP QUESTIONS . .
I. GENERAL PROPERTIES OF THE ATMOSPHERE
II. CLIMATE . . . .
III. THE WINDS . . . .
IV. STORMS ................
SYLLABUS . . . . .
REVIEW QUESTIONS . . .. .
MAP QUESTIONS .......... .....
MOISTURE OF THE ATMOSPHERE.
I. PRECIPITATION OF MOISTURE . .
II. HAIL, SNOW, AND GLACIERS . .
III. ELECTRICAL AND OPTICAL PHENOMENA .
SYLLABUS . . . . .
REVIEW QUESTIONS . . ... .
MAP QUESTIONS .......... .....
S1. Geography is a description of the earth.
\ The earth may be considered in three different
(1.) In its relations to the solar system;
(2.) In its relations to government and society;
(3.) In its relations to nature.
Hence arise three distinct branches of geog-
raphy-Mathematical, Political, and Physical.
2. Mathematical Geography treats of the earth
in its relations to the solar system.
Mathematical Geography forms the true basis for
accurate geographical study, since by the view we thus
obtain of the earth in its relations to the other members
of the solar system, we are enabled to form clearer concep-
tions of the laws which govern terrestrial phenomena.
Here we learn the location of the earth in space, its size,
form, and movements, its division by imaginary lines, and
the methods of representing portions of its surface on maps.
3. Political Geography treats of the earth in
its relations to the governments and societies of
men, of the manner of life of a people, and of
their civilization and government.
4. Physical Geography treats of the earth in
its relations to nature and to the natural laws by
which it is governed. It treats especially of the
systematic distribution of all animate and inani-
mate objects found on the earth's surface. It not
only tells of their presence in a given locality,
but it also endeavors to discover the causes and
results of their existence.
Physical Geography, therefore, treats of, the
distribution of six classes of objects-Land, Water,
Air, Plants, Animals, and Minerals.
Geography deals with the inside as well as with the out-
side of the earth. It encroaches here on the province of
geology. Both treat of the earth: geography mainly with
the earth's present condition; geology with its condition
both in the past and present, though mainly during the past.
Some authors make physical geography a branch of geol-
ogy, and call it physiographic geology, but we prefer the
word "physical," or as the etymology would make it,
THE EARTH AS A PLANET.
Fig, 1. The Earth in Space.
5. The Earth moves through empty space
around the sun. The old idea of the earth
resting on, or being supported by something, is
erroneous. The earth rests on nothing.
A book or other inanimate object placed on a
support will remain at rest until something or
somebody moves it, because it has no power of
self-motion. This property, is called inertia.
Inertia is not confined to bodies at rest. If
the bpok be thrown up through the air, it ought
to keep on moving upward for ever, because it
has no more power to stop moving than to begin
to move. We know, however, that in reality it
stops very soon, and falls to the earth; because-
(1.) The earth draws or attracts it;
(2.) The falling body gives some of its motion
to the air through which it moves.
Were the book thrown in any direction through
the empty space in which the stars move, it would
continue moving in that direction for ever, unless
it came near enough to some other body which
would attract it and cause it to change its motion.
Our earth moves through empty space on ac-
count of its inertia, and must continue so moving
for eternities. There are ample reasons for believ-
ing that all heavenly bodies continue their mo-
tion solely on account of their inertia. The curved
paths in which the earth and the other planets
move are resultant paths produced in a manner
that will be explained hereafter.
Space is not absolutely empty, but is everywhere filled
with a very tenuous substance called ether, which trans-
mits to us the light and heat of the heavenly bodies.
Wherever the telescope reveals the presence of stars we
must believe the ether also extends.
6. The Stars.-The innumerable points of light
that dot the skies are immense balls of matter
which, like our earth, are moving through empty
space. Most of them are heated so intensely that
they give off heat and light in all directions.
They are so far from the earth that they would
not be visible but for theirjimmense size. Beyond
them are other balls, also self-luminous, but too
far off to be visible except through a telescope.
Beyond these, again, we have reason to believe
that there are still others. These balls of matter
are called stars.
All the heavenly bodies, however, do not shine
by their own light. A few-those nearest the
earth-shine by reflecting the light of the sun.
These are called planets, and move with the earth
around the sun.
- 7. The Solar System comprises the sun, eight
large bodies called planets, and, as far as now
known, three hundred and eighty-four smaller
bodies called planetoids or asteroids, besides nu-
merous comets and meteors. Some of the planets
have bodies called moons or satellites moving
around them. These also belong to the solar
Fig. 2 represents the solar system. In the
centre is the sun. The circles drawn around
the sun show the paths or orbits of the planets.
These orbits are represented as circular, but in
reality they are slightly flattened or elliptical.
The elongated orbits mark the paths of the comets.
j F TC
7. L' AI'II
Fig. 2. The Solar System,
The drawing shows the order of the planets from
the sun their common centre, together with the
satellites or moons of some of the planets, and
the rings of Saturn.
8. Names of the Planets.-The planets, named
in their regular order from the sun, beginning
with the nearest, are as follows-viz.: Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune. The first four-Mercury, Venus, Earth,
and Mars-are comparatively small; the second
four-Jupiter, Saturn, Uranus, and Neptune-are
very large, Jupiter being nearly fourteen hun-
dred times larger than the earth. The. initial
letters of the last three planets, Saturn, Uranus,
and Neptune, taken in their order from the sun:
s, u, and n-spell the name of their common
Mercury has a mean or average distance of 36,000,000
of miles from the sun; Venus, 67,200,000; Earth, 92,900,000;
and Mars, 141,500,000.
Jupiter is 483,000,000; Saturn, 886,000,000; Uranus,
1,781,900,000; Neptune, 2,791,600,000. The asteroids move
around the sun in the space between the orbits of Mars and
* Calculated in round numbers for
the mean solar distance of
C P T- U I I E~
12 PHYSICAL GEOGRAPHY.
It is difficult to obtain clear conceptions of distances that
are represented by millions of miles. We may learn the
numbers, but in general they convey no definite ideas.
Should a man travel forty times around the earth at the
equator, he would only have gone over about 1,000,000
miles. Now, Mercury, the nearest of the planets, is thirty-
six times farther from the sun than the entire distance the
man would have travelled, while Neptune is nearly three
thousand times the distance he would have travelled.
S9. The Satellites.-A satellite is a body that
revolves around another body: the planets are
satellites of the sun; the moon is a satellite of
the earth. Mars has two moons. So far as is
known, neither Mercury nor Venus has a satel-
lite. All the planets whose orbits are beyond the
orbit of the earth have moons: Jupiter has five,
Uranus six, Saturn eight, and Neptune one. Be-
sides its moons, Saturn has a number of curious
ring-like accumulations of separate solid or liquid
particles revolving around it. The earth's moon
is about 240,000 miles from the earth. Its vol-
ume is about one-forty-ninth that of the earth's.
10. The Sun is the great central body of the
solar system. Around it move the planets with
their satellites, receiving their light and heat
from it. The sun is a huge heated mass about
1,300,000 times the size of the earth. Its diam-
eter is about 866,500 miles. It appears the
largest self-luminous body in the heavens because
it is comparatively near the earth. Many stars
which appear as mere dots of light are much
larger than the sun.
The sun is a body heated to luminosity, and gives out or
emits light and heat like any other highly-heated body.
If no causes exist to maintain its heat, it will eventu-
ally cool and fail to emit light. The sun's heat is partly
kept up by a variety of causes, the principal of which is
the heat developed by meteoric showers that fall on its
surface. If a meteor fall toward the sun from inter-
planetary space, it will reach the surface with enormous
velocity, and its motion will there be converted into
heat. Since, however, the increase of the sun's mass so
necessitated' is not confirmed by astronomical observa-
tions, it is believed that the sun's heat is not being main-
tained in this way, and that the sun must eventually cool
-an event, however, so remote in time that the life of the
solar system may be regarded as practically infinite.
Size of the Sun.-Were the sun hollow and the earth
placed at its centre, there would not only be sufficient room
to enable the moon to revolve at its present actual distance
around the earth, but it would still, in all parts of its orbit,
be nearly 200,000 miles below the surface of the sun.
All the fixed stars are distant suns, and probably have
worlds like our own moving around them.
From the enormous distances of the fixed stars, we are
obliged, in estimating their distances, to use for our unit
of measurement the velocity of light. Any other common
unit would be too small. Light moves through space at
the rate of about 186,000 miles a second, which is over
11,000,000 miles a minute. Notwithstanding this prodig-
ious velocity, it would take over three thousand years for
light to reach the earth from some of the stars that are
visible to the naked eye. But beyond these stars the tele-
scope reveals myriads of others, whose number is limited
only by the power of the instrument. We may conclude
that the universe is as boundless as space; that is, light
can never reach its extreme limits.
11. Cause of the Earth's Revolution.-The earth
continues its motion through space solely on account of
its inertia. Its curved path around the sun is a resultant
caused by the constant action of two forces: one, a pro-
jectile force probably imparted to it when it began its
separate existence; the other, the sun's attraction, which
causes the earth to fall toward the sun. Under the influ-
ence of the projectile force alone the earth would, in a
given time, move from a to b (Fig. 3); but during this time
Fig. 3. Cause of the Curved Shape of the Earth's Orbit.
it has been continually changing its direction by an
amount equivalent to a direct fall from b to c along b d;
hence its real orbit, during this time, is along the curved
line a c.
12. Position of the Solar System in Space.-
The sun, with all the bodies which move around
it, is in that portion of the heavens called the
Milky Way. The sun is an insignificant star
among the millions of other stars the telescope
has revealed to us.
It was formerly believed that the sun was stationary, for
it was not then known that the positions of the fixed stars
were undergoing slight variations as regards the earth.
It is now generally conceded that the sun, with all the
planets, is moving through space with tremendous veloc-
ity, the direction at present being toward the constella-
tion Hercules. The astronomer Maedler, however, believes
that the grand centre around which the solar system is
moving is Alcyone, the brightest star in the constellation
of the Pleiades. The estimated velocity of the sun in its
immense orbit is 1,382,000,000 miles per year. As the earth
is carried along with the sun in its orbit, it is continually
entering new realms of space.
13. The Earth. -The shape of the earth is that
of a round ball or sphere slightly flattened at two
opposite sides. Such a body is termed a spheroid.
There are two kinds of spheroids-oblate and pro-
late; the former has the shape of an orange, the
latter that of a lemon.
MATHEMATICAL GEOGRAPHY. 13
The straight line that runs through the centre
of a sphere or spheroid and terminates at the cir-
cumference is called the diameter. If the sphere
rotates-that is, moves around like a top-the
Fig, 4, Oblate Spheroid.
Fig. 5. Prolate Spheroid.
diameter on which it turns is called its axis. In
the oblate spheroid the axis is the shorter diam-
eter; in the prolate spheroid the axis is the longer
~;- ~~- - ~
--- W- -- -=~-- .
l^_ sLS=-_ -..- _--- _- -- __ -i| .
-- -- -
--- -- -
-_,, :: z-. ... h- =-
Fig. 6, Curvature of the'Earth's Surface.
The shape of our earth is that of an oblate
spheroid. The polar diameter is 26.47 miles
shorter than the equatorial diameter.
14. Proofs of the Rotundity of the Earth.-
The earth is so large a sphere that its surface
everywhere appears flat. The following simple
considerations will prove, however, that its form
is nearly spherical:
(1.) Appearance of Approaching Objects.-If
the earth were flat, as soon as an object appeared
on the horizon we would see the upper and lower
parts at the same time; but if it were curved, the
top parts would first be seen. Now, when a ship
is coming into port we see first the topmasts, then
the sails, and finally the hull; hence the earth
must be curved; and, since the appearance is the
same no matter from what direction the ship is
approaching, we infer that the earth is evenly
curved, or spherical.
(2.) Circular Shape of the Horizon.-The hori-
zon-or, as the word means, the boundary-is the
line which limits our view when nothing inter-
venes. The fact that this is always a circle fur-
nishes another proof that the earth is spherical.
The horizon would still be a circle if the earth were
perfectly flat, for we would still see equally far in all di-
rections; but it would not everywhere be so, since to an
observer near the edges some other shape would appear.
It is on account of the spherical form of the earth that our
field of view on a plain is so soon limited by the apparent
meeting of the earth and sky. As we can see only in
straight lines, objects continue visible until they reach
such a distance as to sink below the horizon, so that a
straight line from the eye will pass above them, meeting
the sky far beyond, on which, as a background, the objects
on the horizon are projected.
(3.) Shape of the Earth's Shadow.-We can
obtain correct ideas of the shape of a body by
the shape of the shadow it casts. Now, the
shadow which the earth casts on the moon dur-
ing an eclipse of the moon is always circular,
and as only spherical bodies cast circular shad-
ows in all positions, we infer that the earth is
(4.) Measurement.-The shape of the earth has
been accurately ascertained by calculations based
on the measurement of an arc of a meridian. We
therefore not only know that the earth is oblately
spheroidal, but also approximately the amount of
(5.) The Shape of the Great Circle of Illumi-
nation, or the line separating the portions of the
earth's surface lighted by the sun's rays from
those in the shadow, is another evidence of the
rotundity of the earth.
\ 15. The Dimensions of the Earth.-The equa-
torial diameter of the earth, or the distance
through at the equator, is, approximately, 7926
miles; its polar diameter, or the length of its
axis, is 7899 miles. The circumference is 24,899
miles. The entire surface is equal to nearly
197,000,000 square miles.
The specific gravity of the earth is about 5j; that is, the
average weight of all its matter is five and two-third
times heavier than an equal volume of water.
16. Imaginary Circles.-In order to locate
places on the earth, as well as to represent por-
tions of its surface on maps, we imagine the earth
to be encircled by a number of curved lines
called great and small circles.
A great circle is one which would be formed
on the earth's surface by a plane passing through
the earth's centre, hence dividing it into two
equal parts. All great circles, therefore, divide
the earth into hemispheres.
The formation of a great circle on a sphere by cutting
it into two equal parts is shown in Fig. 7.
Fig. 7. Great Circle,
The shortest distance between any two places on the
earth is along the are of a great circle.
All planes passing through the earth's centre form ap-
proximately great circles on its surface.
A small circle is one formed by a plane which
does not cut the earth into two equal parts.
The formation of a small circle by cutting a sphere into
unequal parts is shown in Fig. 8.
Fig, 8, Small Cirole,
The great circles employed most frequently in
geography are the equator and the meridian
circles. The small circles are the parallels.
If we divide the circumference of any circle, whether
great or small, into three hundred and sixty equal parts,
each part is called a degree. The one-sixtieth part'of a
degree is a minute; the one-sixtieth part of a minute is a
second. These divisions are represented as follows: 340,
12', 38'"; which reads, thirty-four degrees twelve minutes
and thirty-eight seconds.
The Equator is that great circle of the earth
which is equidistant from the poles.
Meridian Circles are great circles of the earth
which pass through both poles.
The Meridian of any given place is that half
of the meridian circle which passes through thit
place and both poles. A meridian of any place
reaches from that place to both poles, and there-
fore is equal to one-half of a great circle, and,
with the meridian directly opposite to it, forms
a great circle called a meridian circle. There
are as many meridian circles as there are places
on the equator or on any parallel.
In large cities the meridian is generally assumed to pass
through the principal observatory.
* o---- __--
Fig. 9, Meridians and Parallels,
Parallels are small circles which pass around
the earth parallel to the equator.
The meridians extend due north and south, and are
everywhere of the same length; the parallels extend due
east and west, and decrease in length as they approach the
The Tropics are parallels which lie 230 27'
north and south of the equator: the northern
tropic is called the Tropic of Cancer, the south-
ern tropic is called the Tropic of Capricorn.
The Polar Circles are parallels which lie 230
27' from each pole. The circle in the Northern
Hemisphere is called the Arctic Circle; that in
the Southern. Hemisphere, the Antarctic Circle.
17. Latitude is distance north or south from
the equator toward the poles, measured along
the meridians. It is reckoned in degrees.
The meridian circles are divided into nearly
equal parts by the parallels, and it is the number
of these parts that occur on the meridian of any
place between it and the equator which deter-
mines the value of its latitude. If we conceive
eighty-nine equidistant parallels drawn between
the equator and either pole, they will divide all
the meridians into ninety nearly equal parts; the
value of each of these parts will be one degree
of latitude. Therefore, if the parallel running
through a place is distant from the equator forty-
five of these parts, its latitude is 45. If more
than eighty-nine parallels be drawn, the value
of each part will be less than one degree.
Places north of the equator are in north lati-
tude; those south of it are in south latitude.
Since the distance from the equator to the poles
is one-fourth of an entire circle, and there are
only 3606 in any circle, 900 is the greatest value
of latitude a place can have. Latitude 90' N.
therefore corresponds to the north pole.
To recapitulate: Latitude is measured on the
meridians by the parallels.
18. Longitude is distance east or west of any
Places on the equator have their longitude measured
along it; everywhere else longitude is measured along the
The meridian from which longitude is reckoned
is called the Prime Meridian. Most nations take
the meridians of their own capitals for their prime
meridian. The English reckon from the me-
ridian which runs through the observatory at
Greenwich; the French from Paris. In the
United States we reckon from Washington.
Any prime meridian circle divides all the par-
allels into two equal parts. A place situated east
of the prime meridian is in east longitude; west
of it is in west longitude.
Since there are only 1800 in half a circle, the greatest
value the longitude can have is 1800; for a place 1810 east
of any meridian would not fall within the eastern half of
the parallel on which it is situated, but in the western
half; and its distance, computed from the prime meridian,
would be 1790 west. .
It is the meridians that divide the parallels
into degrees; therefore longitude is measured on
the parallels by the meridians.
19. Value of Degrees of Latitude and Longi-
tude.-As latitude is distance measured on the
arc of a meridian, the value of one degree must
be the Iy-th part of the circumference along that
meridian, since there are only 3600 in all. This
makes the value of a single degree approximately
equal to 69Q miles. Near the poles the flattening
of the earth causes the value of a degree slightly
to exceed that of one near the equator.
The value of a degree of longitude is subject
to great variation. It is equal to the ya th part
of the earth's circumference, provided the place
be situated on the equator; otherwise, it is the
j--th part of the parallel passing through the
place that is taken; and as the parallels decrease
in size as we approach the poles, the value of a
degree of longitude must likewise decrease as the
latitude increases, until at either pole the longi-
tude becomes equal to zero.
The value of a single degree of longitude on the equator,
or at lat. 0, is equal to about 69J miles.
At latitude 450 it is equal to about 49 miles.
90 0 "
Geographical Mile.-The y-0th of the equatorial
circumference, or the one-sixtieth of a degree of longitude
at the equator, is called a nautical or geographical mile.
The statute mile contains 1760 yards; the geographical or
nautical mile, 2028 yards. The nautical mile is sometimes
called a knot.
20. Map Projections.-The term projection as
applied to map-drawing means the various methods
adopted for representing portions of the earth's
surface on the plane of a sheet of paper.
The projections in most common use are Merca-
tor's, the orthographic, the stereographic, and the
conical projections. Of these the stereographic is
best adapted to ordinary geographical maps, and
Mercator's to physical maps. All -projections
must be regarded as but approximations.
1. The Orthographic Projection is that by which the
earth's surface is represented as it would appear to an
observer viewing it from a great distance.
2. The Stereographic Projection is that by which the
earth's surface is represented as it would appear to an
observer whose eye is directly on the surface, if he looked
through the earth as through a globe of clear glass, and
drew the details of the surface as they appeared projected
on a transparent sheet of paper stretched in front of his
eye across the middle of the earth. There may be an
almost infinite number of such projections, according to
the position of the observer. The two stereographic pro-
jections in most common use are the Equatorial and the
Mercator's Projection represents the earth on
a map in which all the parallels and meridians
are straight lines.
Mercator's charts are drawn by conceiving the
earth to have the shape of a cylinder instead of
that of a sphere, and to be unrolled from this
cylinder so as to form a flat surface. The me-
ridians, instead of meeting in points at the north
and south poles, are drawn parallel to each other.
This makes them as far apart in the polar regions
16 PHYSICAL GEOGRAPHY.
as at the equator, and consequently any portion
of the earth's surface represented on such a chart,
if situated toward the poles, will be dispropor-
Pig. 10, The Earth on Mercator's Projection.
tionally large. In order to avoid the distortion
in the shape of the land and water areas, the dis-
tance between successive parallels is increased as
they approach the poles. The dimensions of the
land or water, however, are greatly exaggerated
in these regions. The immediate polar regions
are never represented on such charts, the poles
being supposed to be at an infinite distance.
Mercator's charts are generally employed for physical
maps, on account of the facility they afford for showing
direction. The distortion they produce in the relative
size of land or water areas must be carefully borne in
mind, or wrong ideas of the relative size of various parts
of the world will be obtained.
Mercator's charts make bodies of land and
water situated near the poles appear much larger
than they really are.
In an Equatorial Projection of the entire earth
the equator passes through the middle of each
hemisphere, and a meridian circle forms the
In a Polar Projection of the entire earth the
Fig, 11. The Earth on an Equatorial Projection.
poles occupy the centres of each hemisphere, and represented as if drawn on the frustum of a cone
the equator forms the borders, and afterward unrolled. This projection is. suit-
In a Conical Projection the earth's surface is able where only portions of the earth's surface,
Pig. 12, The Earth on a Polar Projection,
and not hemispheres, are to be represented. The In maps as ordinarily constructed it is not true that the
cone is supposed to be placed so as to touch the upper part is north, the lower part south, the right hand
earth at the central parallel of the country to be east, and the left hand west, except in those on Merca-
r tor's projection. In all maps due north and south lie along
represented. the meridians, and due east and west along the parallels, since
Fig. 13, The Conical Projection.
in most maps both parallels and meridians are curved lines.
Therefore, in most maps due north and south and due east
and west will lie along the meridians and parallels, and
not directly toward the top and bottom, or the right- and
7 21. The Hemispheres.-The equator divides the
earth into a Northern and a Southern Hemisphere.
The meridian of long. 200 W. from Greenwich
is generally taken as the dividing-line between
the Eastern and Western Hemispheres.
22. The Movements of the Earth; Rotation.-
The earth turns around from west to east on its
diameter or axis. This motion is called its ro-
That the earth rotates from west to east the following
consideration will show: To a person in a steam-car mov-
ing rapidly in any direction, the fences and other objects
along the road will appear to be moving in the opposite
direction: their motion is of course apparent, and is caused
by the real motion of the car. Now, the motion of the
sun and the other heavenly bodies, by which they appear
to rise in the east and set in the west, is apparent, and is
caused by the real motion of the earth on its axis; this
motion must therefore be from west to east. The sun, the
planets, and their satellites, so far as is known, also turn
on their axes from west to east.
The earth makes one complete rotation in about
every twenty-four hours-accurately, 23 hours 56
minutes 4.09 seconds. The velocity of its rota-
tion is such that any point on the equator will
travel about 1042 miles every hour. The veloci-
ty of course diminishes at points distant from the
equator, until at the poles it becomes nothing.
23. Change of Day and Night.-The earth re-
ceives its light and heat from the sun, and, being
an opaque sphere, only one-half of its surface can
be lighted at one time. The other half is in dark-
ness, since it is turned from the sun toward por-
tions of space where it only receives the dim light
of the fixed stars. The boundary-line between the
light and dark parts forms approximately a great
circle called the Great Circle of Illumination. Had
the earth no motion either on its axis or in its
orbit, that part of its surface turned toward the
sun would have perpetual day, and the other part
perpetual night; but by rotation different portions
of the surface are turned successively toward and
away from the sun, and thus is occasioned the
change of day and night.
24. The Revolution of the Earth.-The earth has
also a motion around the sun, called its revolution.
The revolution of the earth is from west to east;
this is also true of all the planets and asteroids,
and of all their satellites, except those of Uranus,
and probably of Neptune.
The phrases "rotation of the earth on its axis" and
"revolution in its orbit" are often used in reference to
the earth's motion; but the simple words rotation" and
"revolution" are sufficient, since the first refers only to
the motion on its axis, and the second only to the motion
in its orbit.
The earth makes a complete revolution in 365
days 6 hours 9 minutes 9.6 seconds. This time
forms what is called a sidereal year. The tropical
year, or the time from one March equinox to the
next, is somewhat shorter, or 365 days 5 hours 48
minutes 49.7 seconds. The latter value is the one
generally given for the length of the year. It is
nearly 3651 days.
It will be found that the sum of the days in all the
months of an ordinary year is only equal to 365, while the
true length is approximately one-quarter of a day greater.
This deficiency, which in every four years amounts to an
entire day, is met by adding one day to February in every
fourth or leap year. The exact time of one revolution,
however, is some 11 minutes less than 6 hours. These
eleven extra minutes are taken from the future, and are
paid by omitting leap year every hundredth year, except
that every 400 years leap year is counted. In other words,
1900 will not be a leap year, since it is not divisible by 400,
but the year 2000 will be a leap year.
The length of the orbit of the earth is about
577,000,000 miles. Its shape is that of an el-
lipse which differs but little from a circle. The
sun is placed at one focus of the ellipse, and, as
this is not in the centre of the orbit, the earth
must be nearer the sun at some parts of its revo-
lution than at others.
When the earth is in that part of its orbit which is near-
est to the sun, it is said to be at its perihelion; when in
that part farthest from the sun, at its aphelion. The peri-
helion distance is about 90,259,000 miles; the aphelion dis-
tance, 93,750,000 miles. The earth reaches its perihelion
about January 1st.
The earth does not move with the same rapidity through
all parts of its orbit, but travels more rapidly in perihelion
than in aphelion. Its mean velocity is about 19 miles a
second, which is nearly sixty times faster than the speed
of a cannon-ball.
18 PHYSICAL GEOGRAPHY.
25. Laplace's Nebular Hypothesis.-The uniformity
in the direction of rotation and revolution of the planets
has led to a very plausible supposition as to the origin of
the solar system, by the celebrated French astronomer La-
place. This supposition, known as Laplace's nebular hy-
pothesis, assumes that, originally, all the materials of which
the solar system is composed were scattered throughout
space in the form of very tenuous or nebulous matter. It
being granted that this matter began to accumulate around
a centre, and that a motion of rotation was thereby ac-
quired, it can be shown, on strict mechanical principles,
that a system resembling the solar system might be evolved-
As the mass contracted on cooling, the rapidity of its
rotation increased. The equatorial portions bulged out
through the centrifugal force, until ring-like portions
separated, and, collecting in spherical masses, formed the
planets. The planets in a similar manner detached their
satellites. At the time of the separation of Neptune the
nebulous sun must have extended beyond the orbit of this
planet. The temperature requisite for so great an expan-
sion must have been enormous.
Although a mere hypothesis, there are many facts which
tend to sustain it, and it is now generally accepted.
26. The Plane of the Earth's Orbit is a per-
fectly flat surface so placed as to touch the earth's
orbit at every point. It may be regarded as an
imaginary plane of enormous extent on which the
earth moves in its journey around the sun.
\ 27. Causes of the Change of Seasons.-The
change of the earth's seasons is caused by the
revolution of the earth, together with the fol-
Fig, 14. Inclination bf-Ais-to'rbit and Ecliptic.
(1.) The inclination of the earth's axis to the
plane of its orbit. The inclination is equal to
The ecliptic is the name given to a great circle whose
plane coincides with the plane of the earth's orbit. Since
the earth's axis is 900 distant from the equator, the plane
of the ecliptic must be inclined to the plane of the equator
900 minus 660 33', or 230 27'.
The mere revolution of the earth would be unable to
produce a change of seasons, unless the earth's axis were
inclined to the plane of its orbit. If, for example, the
axis of the earth stood perpendicularly on the plane of its
orbit, the sun's rays would so illumine the earth that the
great circle of illumination would always be bounded by
some meridian circle. The days and nights would then
be of equal length, and the distribution of heat the same
throughout the year. Under these circumstances therf'
could be no change of seasons, since the sun's rays would
always fall perpendicularly on the same part of the earth:
on the equator.
(2.) The Constant Parallelism of the Earth's
Axis.-During the earth's revolution its axis
always points nearly to the same place in the
heavens, viz. to the north star. It is therefore
always approximately parallel to any former
Unless the axis were constantly parallel to any former
position, the present change of seasons would not occur.
On account of the spherical form of the earth,
only a small part of its surface can receive the
vertical rays of the sun at the same time. This
part can be regarded as nearly a point; and since
only one-half of the earth is lighted at any one
time, the great circle of illumination must extend
900 in all directions from the point which receives
the vertical rays. By rotation all portions of
the surface situated anywhere within the tropics
in the same latitude, at some time or another
during the day, are turned so as to receive the
vertical rays of the sun, and consequently, the
portion so illumined has the form of a ring or
zone. Other things being equal, this zone con-
tains the hottest portions of the surface, the heat
gradually diminishing as we pass toward either
On account of the inclination of its axis, the
earth receives the vertical rays of the sun on new
portions of its surface every day during its revo-
lution; and it is because different portions of the
surface are constantly being turned toward the sun
that the change of seasons is to be attributed.
As the earth changes its position in its orbit, the
sun's rays fall vertically on different parts of the
surface, so that during the year one part or an-
other of the surface within 230 27' on either side
of the equator receives the vertical rays.
The astronomical year begins on the 20th
of March, and we shall therefore first consider
the position of the earth in its orbit at that
An inspection of Fig. 15 will show that at this
time the earth is so turned toward the sun that
the vertical rays fall exactly on the equator. The
great circle of illumination, therefore, reaches to
the poles, and the days and nights are of an equal
length all over the earth. This time is called the
March equinox. Spring then begins in the North-
ern Hemisvhere, and autumn in the Southern.
This is shown more clearly in Fig. 16, which
represents the relative positions of the illumined
and non-illumined portions at that time.
MATHEMATICAL GEOGRAPHY. 19
x1g. .Lo. -~e uroin ox me Eartn, showing One uhange 05 easohns,
As the earth proceeds in its orbit, the inclina-
tion of the axis causes it to turn the Northern
Hemisphere more and more toward the sun. The
vertical rays, therefore, fall on portions farther
and farther north until, on the 21st of. June, the
Fig. 16. The Earth at an Equinox.
vertical rays reach their farthest northern limit,
and fall directly on the Tropic of Cancer, 23 27'
N., when the sun is said to be at its summer sol-
Since the portions receiving the vertical rays
of the sun are now or 'the Tropic of. Cancer,
the light and heat must extend in the Northern
Hemisphere to 23 27' beyond the north pole, or
to the Arctic Circle; while in the Southern Hemi-
sphere they must fall short of the south pole by
the same number of degrees, or reach to the Ant-
Fig. 17, The Earth at the Summer Solstice.
arctic Circle. The Northern Hemisphere then be-
gins its summer, and the Southern its winter.
The relative positions of the illumined and
non-illumined portions of the earth at the sum-
mer solstice are more clearly shown in Fig. 17.
Here, as is shown, the great circle of illumination
\P2 PHSIA GEORAHY
extends in the Northern Hemisphere as far over
the pole as the Arctic Circle.
After the 21st of June the Northern Hemi-
sphere is turned less toward the sun, and the
vertical rays continually approach the equator,
all the movements of the preceding season being
reversed, until on the 22d of September, the time
of the September equinox, the equator again receives
the vertical rays, the great circle of illumination
again coinciding with the meridian circles. The
earth has now moved from one equinox to an-
other, and has traversed one-half of its orbit.
The Southern Hemisphere then begins its spring,
the Northern its autumn.
From the 22d of September until the 20th of
March, while the earth moves through the other
half of its orbit, the same phenomena occur in
the Southern Hemisphere that have already been
noticed in the Northern. Immediately after the
22d of September the inclination of the axis
causes the earth to be so turned toward the sun
that its rays begin to fall south of the equator;
and, as the earth proceeds in its orbit, the South-
ern Hemisphere is turned more and more toward
the sun, and the vertical rays fall farther and
farther toward the pole. This continues until
the 21st of December, when the rays fall vertically
on the Tropic of Capricorn, and the December sol-
stice is reached. The great circle of illumination
now extends beyond the south pole as far as the
Antarctic Circle, but falls short of the north pole
23' 27', reaching only the Arctic Circle. Sum-
mer then commences in the Southern Hemisphere,
and winter in the Northern.
After the 21st of December the Southern
Hemisphere is turned less and less toward the
N ORTH TEMAPELRA 1'-
ToI6 TORRI NED
SOUTH TEMPERA' r:
CAf-_ TRCTlC C, PCLEC
Iig. 18. Mathematical Climatic Zones.
sun, and the part receiving the vertical rays
approaches the equator, until on the 20th of
March the equator again receives the vertical
rays, and, with the March equinox, spring com-
mences in the Northern Hemisphere, and with
it a new astronomical year.
The equinoxes and solstices as a rule occur on the dates
named. Occasionally they occur immediately before or
after said dates.
28. Mathematical Zones.-The Torrid Zone.-
That belt of the earth's surface which lies be-
tween the tropics is called the Torrid Zone.
During one time or another throughout the
year every part of its surface receives the ver-
tical rays .of the sun.
The Temperate Zones are included between the
tropics and the polar circles. The northern zone
is called the North Temperate Zone, and thb south-
ern zone, the South Temperate Zone.
The Polar Zones are included between the
polar circles and the poles. The northern zone
is called the North Frigid Zone, and the southern
zone, the South Frigid Zone.
These zones, which are separated by the parallels of lati-
tude, are generally termed the astronomical or mathematical
zones to distinguish tbn m from others called physical zones,
which are bounded by the lines of mean annual temper-
It will be noticed that the distance of the tropics from
the equator and of the polar circles from the poles is 230
27', or the value of the'inclination of the plane of the
ecliptic to the plane of the equator.
29. Length of Day and Night.--Whenever
more than half of either the Northern or South-
ern Hemisphere is illumined, the great circle of
illumination will divide the parallels unequally,
and the length of the daylight in that hemisphere
will exceed that of the night in proportion as the
length of the illumined part, measured along any
of the parallels, exceeds that of the dark part.
The length of daylight or darkness may exceed
that of one complete rotation of the earth. The
great circle of illumination may at times pass
over the poles as far beyond them as 230 27';
and places situated within this limit may remain
during many rotations exposed to the rays of the
A little consideration will show that the longest day
must occur at the poles, since the poles must continue
to receive the sun's rays from the time they are first illu-
mined at one equinox until the sun passes through a sol-
stice and returns to the other equinox. Nowhere, outside
the polar circles, will the length of daylight exceed one
entire rotation of the earth.
The length of the longest day at the equator, latitude
0, is 12 hours.
Of the longest day a; the poles, latitude 900, is six
There are three kinds of geography-Mathematical, Po-
litical, and Physical.
Physical Geography treats of Land, Water, Air, Plants,
Animals, and Minerals.
Geography deals mainly with the earth as it is; geology
mainly with the earth as it was.
The earth continues its motion around the sun in conse-
quence of its inertia.
The distant stars are balls of fire like our sun, and prob-
ably'have worlds resembling ours revolving around them.
The sun and the bodies that revolve around it consti-
tute the solar system.
The sun is about 1,300,000 times larger than the earth.
The sun is a body heated to luminosity, and gives out or
emits light and heat like any other highly-heated body.
The shape of the earth is that of an oblate spheroid
whose equatorial diameter is about 26 miles longer than
its polar. That the earth is round and not flat is proved
-1st, by the appearance of approaching or receding ob-
jects; 2d, by the circular shape of the horizon; 3d, by the
circular shape of the earth's shadow; 4th, by actual meas-
urement; and 5th, by the shape of the great circle of
The earth's diameter is nearly 8000 miles, its circumfer-
ence not quite 25,000 miles, and its area about 197,000,000
The imaginary circles used in geography are the Equa-
tor, the Meridian Circles, and the Parallels.
Latitude is measured on the meridians by the parallels.
The greatest number of degrees of latitude a place can
have is 900; the greatest of longitude, 1800. The latitude
at the equator is 0 N. or S. The longitude at the poles or
on the prime meridian is 00 E. or W.
Longitude is measured on the equator, or on the parallels,
by the meridians.
Maps are drawn on different projections: the Equatorial,
the Polar, and Mercator's projections are in most general
use. A Mercator's projection causes places near the poles
to appear larger than they really are.
On all maps due north and south lies along the merid-
ians; due east and west, along the parallels: when these
are curved lines, the top and bottom of the map will not
always represent north and south, nor the right and left
hand east and west.
The inclination of the earth's axis to the plane of its
orbit, and the constant parallelism of the axis with any
former position, together with the revolution around the
sun, cause the change of seasons.
The astronomical year begins March 20th.
On the 20th of March and on the 22d of September the
days and nights are of equal length all over the earth.
From the 20th of March the days increase in length in the
Northern Hemisphere until the 21st of June, when they
attain their greatest length; they then decrease until the
22d of September, when they again become equal.
The Torrid Zone is the hottest part of the earth, because,
during one time or another throughout the year, every part
of its surface receives the vertical rays of the sun.
The Solar System.
How does the principle of inertia apply to the earth's
motion around the sun?
What do you understand by the solar system?
Describe the earth's position in the solar system. Which
of the planets are between the earth and the sun? Which
are beyond the orbit of the earth?
How does the size of the sun compare with that of the
Are any of the distant stars larger than our sun?
What is a satellite ? Which of the planets have satellites ?
Explain the cause of the circular shape of the earth's
In what part of space is the solar system?
Has our sun any motion through space?
Enumerate the proofs of the rotundity of the earth.
State accurately the length of the equatorial diameter
of the earth; of its polar diameter; of its circumference.
What is its area ?
How many times heavier is the earth than an equally
large globe of water? V/>
Define great and small circles. Name the circles most
commonly used in geography.
What do you understand by latitude? How is latitude
reckoned? Of what use is latitude in geography? Why
can the value of the latitude never exceed 90? Of what
use are meridians and parallels in measuring latitude?
What do you understand by longitude? How is longi-
tude reckoned? Of what use is longitude in geography?
Why can its value never exceed 1800? Of what use are
meridians and parallels in measuring longitude?
Where is the value of a degree of latitude the greatest ?
Of a degree of longitude? Why?
What effect has a Mercator's chart on the appearance of
bodies of land or water in high northern or southern lati-
What is an equatorial projection? A polar projection?
A conical projection? What is the position of the poles in
an equatorial projection? In a polar projection ?
Movements of the Earth.
Prove that the earth turns on its axis from west to east.
Explain the cause of the change of day and night.
Define a sidereal year; a tropical year. Which value is
generally taken for the length of the civil year?
Describe Laplace's nebular hypothesis.
Enumerate the causes which produce the change of
On what days of the year will the sun's rays fall verti-
cally on the equator ? On what days will its rays fall ver-
tically on the Tropic of Cancer? On the Tropic of Capri-
ALTHOUGH water occupies much the larger portion of the earth's surface, yet, when compared with
the entire volume of the globe, its quantity is comparatively insignificant; for the mean depth of the
ocean probably does not exceed two and one-third miles, and underneath this lies the solid crust,
with its heated interior.
The crust and heated interior are composed of a variety of simple and compound substances. Simple
or elementary substances are those which have never been separated into components. Compound
substances are those which are composed of two or more simple or elementary substances combined
under the influence of the chemical force.
THE INSIDE OF THE EARTH.
The Heated Interior.
30. The Proofs of the Earth's Original Fluidity
or fused condition through heat are-
(1.) Its Spherical Shape, which is the shape
the earth would have taken had it been placed
in space when in a melted condition. This is
the shape of nearly all the heavenly bodies.
(2.) The fact that the rocks which were first
formed give evidence by their appearance of
having been greatly heated. These rocks are
generally highly crystalline.
(3.) The general climate of the earth during
the geological past was much warmer than at
Very little of the internal heat now reaches the surface.
According to Poisson, all that escapes would raise the mean
annual temperature only -hth of a degree Fahr.
31. Laplace's Nebular Hypothesis agrees very well
with the idea of a former igneous fluidity, since, at the
time of its separation from the nebulous sun, the earth
must have had a temperature sufficient not only to fuse,
but even to volatilize, most of its constituents.
32. Proofs of a Present Heated Interior.-The
following considerations show that the inside of
the earth is still highly heated:
(1.) The deeper we penetrate the crust, the
higher the temperature becomes. Moreover, the
rate of increase, though varying in different lo-
calities with the character of the materials of the
crust, is nearly uniform over all parts of the sur-
face, the average value of the increase being 10
Fahr. for every 55 feet of descent.
This would seem to indicate that the entire
inside of the earth.is heated, and that the heat
increases as we go toward the centre.
We cannot, however, estimate the thickness of the crust
from this fact-
1. Because we have never penetrated the crust more
than a few thousand feet below the level of the sea, and
therefore we do not know that this rate of increase of
temperature continues the same;
2. Even if it did continue uniform, since the melting-
point of solids increases with the pressure, we do not
know what allowance should be made for this increase.
(2.) In all latitudes prodigious quantities of
melted rock escape from the interior through
the craters of volcanoes. The interior, there-
fore, must be hot enough to melt rock.
33. Condition of the Interior.-We do not
know the condition of the material which fills
the interior of the earth. It might be supposed,
since rock escapes from the craters of volca-
noes in a fluid or molten condition, that the in-
terior is filled with molten matter; but this is
not necessarily so, since the enormous pressure
to which the interior is subjected would prob-
ably be sufficient to compress it into a viscous
or pasty mass, or, possibly, even to render it solid.
The lava which issues from the crater of a vol-
cano is necessarily more mobile than the interior
of the earth; for, coming, as it does, from great
depths, it must grow more and more liquid as it
approaches the surface and is thus relieved of its
pressure. Indeed, the most viscous rock conceiv-
able, if highly heated when ejected from pro-
found depths, would become comparatively fluid
on reaching the surface.
34. Views Concerning the Condition of the
Interior.-Considerable difference of opinion ex-
ists as to the exact condition of the interior of
the earth. The following opinions may be men-
(1.) That the earth has a solid centre and
crust, with a heated or pasty layer between.
(2.) That the crust is solid, but the interior
highly heated, so as to be in a fused or pasty
(3.) That the earth is solid throughout, but
highly heated in the interior.
Of the above views, the second is perhaps the
most tenable, and will be adopted as serving in
the simplest manner to explain the phenomena
of the earth arising from the presence of a highly
heated interior. Admitting the crust to be suf-
ficiently thin, and in such a condition as to per-
mit of but a small degree of warping, then all
the phenomena can be satisfactorily explained.
35. Thickness of the Crust.-We cannot as-
sign a definite limit to the thickness of the crust,
since the portions that are solid from having
cooled, most probably pass insensibly into those
that are nearly solid from the combined influence
of loss of heat and increasing pressure. It seems
probable that the portion solidified by cooling is
thin, when compared with the whole bulk of the
earth; in other words, the heated interior lies
comparatively near the surface.
36. Effects of the Heated Interior.-As the
crust loses its heat it shrinks or contracts, and,
growing smaller, the materials of the interior are
crowded into a smaller space, and an enormous
force is thus exerted, both on the interior and on
the crust itself; tending either to change the shape
of the crust, to break it, or to force out some of
the interior. The following phenomena are there-
fore caused by the contraction of the crust:
(3.) Non-volcanic igneous eruptions;
(4.) Gradual elevations or subsidences of the
37. Volcanoes.-One of the most striking proofs
of the existence of a heated interior is the ejection
of enormous quantities of melted rock through
openings in the crust.
A volcano is a mountain, or other elevation,
through which the materials of the interior escape
to the surface. The opening is called the crater,
and may be either on the top or on the sides of
-- :_ -'
Fig 19. An Eruption of Mount Vesuvius,
38. Peculiarities of Craters.-The crater, as its name
indicates, is cup-shaped. The rim, though generally entire,
is sometimes broken by the force of the eruption, as in
Mount Vesuvius, where the eruption in 79 A. D.-the first=
on record-blew off the northern half of the crater. The
material thus detached, together with the showers of ashes
and streams of lava, completely buried the cities of Her-
culaneum and Pompeii, situated near its base.
The crater is often of immense size. Mauna Loa, on the
island of Hawaii, has two craters-one on the summit, and
the other on the mountain-side, about 4000 feet above the
sea. The latter-Kilauea-is elliptical in shape, and about
71 miles in circumference; its area is nearly 4 square miles,
and its depth, from 600 to 1000 feet.
Volcanic mountains are of somewhat different
shapes, but near the crater the conical form pre-
dominates, and serves to distinguish these moun-
tains as a class. The shape of the volcanic eone
is caused by the ejected materials accumulating
around the mouth of the crater in more or less
39. The ejected materials are mainly as fol-
(1.) Melted Rock, or Lava.-Lava varies, not
only with the nature of the materials from which
it was formed, but also with the conditions under
which it has cooled, and the quantity of air or
vapor entangled in it. Though generally of a
dark gray, it occurs of all colors; and its texture
varies from hard, compact rock to porous, spongy
material that will float on water.
When just emitted from the crater, ordinary lava flows
about as fast as molten iron would on the same slope. On
steep mountains, near the crater, the lava, when very
hot, may flow faster than a horse can gallop; but it soon
cools, and becomes covered with a crust that greatly re-
tards the rapidity of its flow, until its motion can only be
determined by repeated observations.
At Kilauea, jets of very liquid lava are sometimes
thrown out, which, falling back into the crater, are drawn
out by the wind into fine threads, thus producing what
the natives call P616's hair, after their mythical goddess.
The volume of the ejected lava is often very great. Vol-
canic islands are generally formed entirely by lava streams.
Hawaii and Iceland were probably formed entirely of lava
emitted from numerous volcanic cones.
(2.) Ashes or Cinders.-These consist of minute
fragments of lava that are ejected violently from
the crater; at night they appear as showers of
brilliant sparks. When they fall directly back
on the mountain, they aid in rearing the cone.
More frequently, they are carried by the wind to
points far distant. The destructive effects of
volcanic eruptions are caused mainly by heavy
showers of ashes. The ashes, when exceedingly
fine, form what is called volcanic dust.
At the beginning of an eruption large frag-
ments of rock are sometimes violently thrown
out of the crater.
(3.) Vapors, or Gases.-The vapor of water
often escapes in great quantities from the crater,
especially at the beginning of the eruption. On
cooling, it condenses and forms dense clouds, from
which torrents of rain fall. These clouds, lighted
by the glowing fires beneath, appear to be actually
burning, and thus give rise to the erroneous belief
that a volcano is a burning mountain. To the
condensation of this vapor is probably to be as-
cribed the lightning which often plays around the
summit of the volcano during an eruption. Be-
sides the vapor of water, various gases escape, of
which sulphurous acid is the most common.
When a large quantity of rain mingles with the ashes,
torrents of mud are formed, which move with frightful
velocity down the slopes of the mountain, occasioning con-
siderable damage. During the eruption of Galungung, in
Java, more than one hundred villages were thus destroyed.
The rock that is formed by the hardening of volcanic mud
is called tufa.
40. The Inclination of the Slopes of the vol-
canic cones depends on the nature of the material
of which they are formed. Where lava is the
main ingredient, the cone is broad and flat. The
inclination of a lava cone ranges from 30 to 100,
Fig, 20, Lava Cone, Inclination from 3 to 10,
according to the liquidity of the lava. A very
stiff lava will form a much steeper cone.
45. The number of volcanoes is not accurately
known. The best authorities estimate it at about
672, of which 270 are active. Of the latter, 175
are on islands, and 95 are on the coasts of the con-
46. Regions of Volcanoes.-The principal vol-
canic regions of the earth are-*
(1.) Along the Shores of the Pacific, where an
immense chain of volcanoes, with but few breaks,
encircles it in a huge "Sea of Fire."
On the Eastern Borders, in the Andean range,
are the volcanic series of Chili, Bolivia, and Ecua-
dor; those of Central America and Mexico; in
the United States are the series of the Sierra
Nevada and Cascade ranges and of Alaska; and
finally, connecting the system with Asia, the vol-
canic group of the Aleutian Islands.
On the Western Borders volcanoes occur in the
following districts: the Kamtchatkan Peninsula,
with its submerged ranges of the Kurile Islands;
the Japan, the Loo Choo, and the Philippine
Islands; the Moluccas; the Australasian Island
Chain, terminating in New Zealand; and finally,
nearly in a line with these, the volcanoes of Ere-
bus and Terror on the Antarctic continent.
(2.) In the Islands of the Pacific.-Volcanic
activity is not wanting over the bed of the Pa-
cific. The Sandwich Islands, the Society Group,
the Marquesas, Friendly Islands, New Hebrides,
Ladrones, and many others, are volcanic.
(3.) Scattered over the Seas that divide the
Northern and Southern Continents, or in their
vicinity, viz.: in the neighborhood of the Carib-
bean Sea, in the Mediterranean and Red Seas,
and in the Pacific and Indian Oceans between
Asia and Australia.
In the neighborhood of the Caribbean Sea.-This
region includes the two groups of the Antilles in
the Caribbean Sea, and the Gallapagos Islands in
the Pacific Ocean.
In the neighborhood of the Mediterranean and
Red Seas.-This region includes the volcanoes of
the Mediterranean and its borders, those of Italy,
Sicily, the Grecian Archipelago, of Spain, Central
France, and Germany, together with those near
the Caspian and Red Seas.
Between Asia and Australia.-This region in-
cludes the Sunda Islands, Sumatra, Java, Sum-
bawa, Flores, and Timor, which contain numerous
craters. In Java there are nearly 50 volcanoes,
28 of which are active, and there are nearly as
* We follow mainly the classification of Dana.
many in Sumatra. There are 109 volcanoes in
the small islands near Borneo.
(4.) In the Northern and Central Parts of the
All the islands in the deep ocean which do not
form a part of the continent are volcanic; as,
for example, the island of St. Helena, Ascension
Island, the Cape Verdes, the Canaries, the Azores,
and Iceland. The Cameroons Mountains, on the
African coast near the Gulf of Guinea, together
with some of the islands in the gulf, are volcanic.
(5.) In the Western and Central Parts of the
Volcanoes are found in Madagascar and in the
adjacent islands. They also occur farther south,
in the island of St. Paul and in Kerguelen Land,
and in Kilimandjaro, near the eastern coast of
47. Submarine Volcanoes.-From the difficulty in ob-
serving them, submarine volcanoes are not so well known
as the others. The following regions are well marked:
In the Mediterranean Sea, near Sicily and Greece.
Near the island of Santorin the submarine volcanic en-
ergy is intense. It has been aptly described as a region
"Where isles seem to spring up like fungi in a wood."
In the Atlantic Ocean; off the coast of Iceland; near
St. Michael, in the Azores; and over a region in the nar-
rowest part of the ocean between Guinea and Brazil.
In the Pacific Ocean; near the Aleutian Islands,
where two large mountain-masses have risen from the
water within recent time. Near the Japan Islands, where,
about twenty-one centuries ago, according to native his-
torians, Fusi Yama, the highest mountain in Japan, rose
from the sea in a single night.
In the Indian Ocean, the island of St. Paul, in the
deep ocean between Africa and Australia, exhibits signs
of submarine activity.
48. Peculiarities of Distribution.-Nearly all
volcanoes are found near the shores of continents
or on islands.
The only exceptions are found in the region
south of the Caspian Sea, and in that of the
Thian Shan Mountains. As volcanoes are but
openings in the earth's crust which permit an es-
cape of materials from the pasty interior, they
will occur only where the crust is weakest. This
will be on the borders of sinking oceans, in the
lines of fracture formed by the gradual separa-
tion of the ocean's bed from the coasts of the
continent. The floor of the ocean in all latitudes
is covered with a layer of quite cold water, so
that the difference in the amount of the contrac-
tion will in general be most marked on the bor-
ders of the oceans or on the edges of the conti-
In most regions the volcanoes lie along lines
28 PHYSICAL GEOGRAPHY.
more or less straight. Lines joining such a series
may be considered as huge cracks in the crust,
the volcanic phenomena occurring in their weak-
The frequent occurrence of volcanoes in moun-
tainous districts is caused by the crust being
broken and flexed, so as to admit of an easy
passage for the molten rock.
Where one system of fissures crosses another the
crust becomes weak, the openings numerous, and the
volcanic activity great. The two antipodal points
of the Antilles and the Sunda Islands are excel-
lent examples, and are the most active volcanic
regions on the earth.
Efforts have been made to show some connection be-
tween certain states of the weather and periods of vol-
canic activity; but, so far, these have amounted to mere
predictions of coming changes, based on observations of
the direction of upper currents of air from the clouds
of ashes or smoke ejected by the volcano. No law of
periodicity of eruption has, as yet, been discovered.
49. Other Volcanic Phenomena:
Mud Volcanoes are small hillocks that emit
streams of hot mud and water from their craters,
but never molten rock. They are found in vol-
Solfataras are places where sulphur vapors es-
cape and form incrustations. They occur in vol-
Geysers are sometimes ranked with volcanic phe-
nomena. They are described under Hot Springs.
------- o:= oo----
50. Earthquakes are shakings of the earth's
crust, of degrees varying in intensity from
scarcely perceptible tremors to violent agita-
tions that overthrow buildings and open huge
fissures in the ground. They may therefore be
divided into two classes:
(1.) A shaking movement without any perma-
nent change in the surface;
(2.) A shaking movement accompanying an
uplift or subsidence.
An earthquake is sometimes called a seismic
51. Facts concerning Earthquakes.-A careful
study of earthquakes appears to establish the fol-
(1.) The place or origin of the shock is not
deep-seated or far below the earth's surface, but
Fig. 23. Fissures produced by the Charleston Earthquake of 1886.
is near the surface, probably never deeper than
thirty miles, and often much less.
(2.) .The area of disturbance depends not only
on the energy of the shock, but also on the depth
of its origin below the surface: the deeper the
origin, the greater the area.
(3.) The shape of the origin is generally that
of a line, often many miles in length.
(4.) The direction of the motion at the surface
is nearly upward over the origin, and more in-
clined as the distance from the origin increases.
(5.) The shape of the area of disturbance de-
pends on the nature of the materials through
which the wave is moving. If these are of
nearly uniform elasticity in all directions, the
area is nearly circular; if more elastic in one
direction than in another, the area is irregular
52. The Varieties of Earthquake Motion at the
Earth's Surface are-
(1.) A wave-like motion, in which the ground
rises and falls like waves in water.
(2.) An upward motion, somewhat similar to
that which follows an explosion of powder below
the surface. This has been known to occur with
sufficient force to throw heavy bodies considerable
distances up.into the air.
(3.) A rotary motion, which, from its destruc-
tive effects, is fortunately of rare occurrence.
Humboldt mentions an earthquake that happened in
Chili where the ground was so shifted that three great
palm trees were twisted around one another like willow
There are two kinds of movement transmitted through
the crust during earthquakes: these are the earthquake
motion proper, and the motion that produces the accompanying
53. The Velocity of Earthquake Motion varies
according to the intensity of the shock and the na-
ture of the material through which it is trans-
mitted. No average result can therefore be
given. Various observers have estimated it at
from 8 to 30 miles per minute.
54. The Sounds Accompanying Earthquakes
vary both in kind and intensity. Sometimes
they resemble the hissing noises heard when red-
hot coals are thrown into water; sometimes they
are rumbling, but more frequently they are of
greater intensity, and are then comparable to
discharges of artillery or peals of thunder.
The confused roaring and rattling are probably caused
by the different rates of transmission of the sound through
the air and rocks.
55. Duration of the Shocks.-When the area
of disturbance is large, shocks of varying intensity
generally follow each other at irregular intervals.
Though, in general, the violence of the shock is
soon. passed, disturbances may occur at intervals
of days, weeks, or even years.
During the earthquake in Calabria in 1783, when nearly
100,000 persons perished, the destructive vibrations lasted
scarcely two minutes, but the tremblings of the crust con-
tinued long afterward. During the earthquake at Lisbon
in 1755, when about the same number perished, the shock
which caused the greatest damage continued but five or
six seconds, while a series of terrible movements followed
one another at intervals during the space of five minutes.
56. Cause of Earthquakes.-It is generally be-
lieved that the principal cause of earthquakes is the
force produced by the contraction of a cooling crust.
During the cooling of the earth the crust ,cn-
tinually contracts, and the pressure so produced,
slowly accumulating for years, at last rends it
in vast fissures, thus producing those violent
movements of its crust called earthquakes. If
this theory be admitted-and it is a probable one
-the earth's crust must every now and then be
in such a strained condition that the slightest
increase of force from within, or of diminished
resistance from without, would disturb the con-
ditions of equilibrium, and thus result in an
57. Strain Caused by Contraction consequent on
cooling is well exhibited in the so-called "Prince Ru-
pert's Drops," which are made by allowing melted glass
to fall in drops through cold water. The sudden cooling
of the outside produces powerful forces, which tend to
compress the drop; but, since these forces balance one
another, no movement occurs until, by breaking off the
long end of the drop, one set of forces is removed, when
the others, no longer neutralized, tear the drop into almost
Similar effects are produced by unequal contraction and
expansion. Hot water poured into a tumbler will often
crack it. The crackling sound of a stovepipe when sud-
denly heated or cooled is a similar effect.
58. Other Causes of Earthquakes. -Earth-
quakes may also be occasioned by-
(1.) The sudden evolution of gases or vapors
from the pasty interior.
This is probably the cause of many of the
slight shocks that occur in the neighborhood of
active volcanic regions.
(2.) Shocks caused by falling masses.
Those who deny the existence of a pasty interior, en-
deavor to explain the production of earthquakes by the
shock caused by the occasional caving in of huge masses
of rocks, in caverns hollowed out by the action of subter-
ranean waters; or by the'gradual settling of the upturned
strata in mountainous districts. There can be no doubt
that even moderately severe shocks are caused by falling
masses; but such a force is utterly inadequate to produce
a shock like that which destroyed Lisbon, when an area
of nearly 7,500,000 square miles was shaken.
59. Periodicity of Earthqnakes.-It was for-
merly believed that earthquakes occurred with-
out any regularity, but by a comparison of the
times of occurrence of a great number it has been
discovered that they occur more frequently-
(1.) In winter than in summer;
(2.) At night than during the day;
(3.) During the new and full moon, when the
attractive force of the sun and moon acts simul-
taneously on the same parts of the earth.
Earthquake shocks are more frequent in winter,
and during the night, because the cooling, and
consequent contraction, occur more rapidly at
these times, and therefore the gradually accumu-
lating force is more apt to acquire sufficient inten-
sity to rend the solid crust.
Earthquakes are more frequent during new
and full moon, because the increased force on
the earth's crust caused by the position of the
sun and moon at these times, is then added to
the accumulated force produced by cooling.
It has been asserted that in the equatorial regions earth-
quakes are especially frequent during the setting in of
periodical winds called the monsoons, at the change of
the rainy season or during the prevalence of hurricanes.
These facts, however, are not well established.
60. Distribution of Earthquakes. Earth-
quakes may occur in any part of the world, but
are most frequent in volcanic districts. They are
more frequent in mountainous than in flat coun-
tries. They are especially frequent in the high-
est mountains. According to Huxley, fairly pro-
nounced earthquake shocks occur in some part of
the earth at least three times a week.
There is, in many instances, an undoubted connection
between volcanic eruptions and earthquakes. Humboldt
relates that during the earthquake at Riobamba, when
some 40,000 persons perished, the volcano of Paste ceased
to emit its vapor at the exact time the earthquake began.
The same is related of Vesuvius at the time of the earth-
\quake at Lisbon.
S61. Phenomena of Earthquakes.-In order to give
some idea of the phenomena by which severe earthquake
shocks are attended, we append a brief description of the
earthquake which destroyed the city of Lisbon, on the 1st
of November, 1755. The loss of life on this occasion was
the more severe, since the shock occurred on a holy day,
when nearly the whole population was assembled in the
churches. A sound like thunder was heard, and, almost
immediately afterward, a series of violent shocks threw
down nearly every building in the city. Many who es-
caped the falling buildings perished in the fires that soon
kindled, or were murdered by lawless bands that after-
ward' pillaged the city.
The ground rose and fell like the waves of the sea; huge
chasms were opened, into which many of the buildings
were precipitated. In the ocean a huge wave, over 50 feet
high, was formed, which, retreating for a moment, left the
bar dry, and then rushed toward the land with frightful
force. This was repeated several times, and thousands
perished from this cause alone. The neighboring moun-
tains, though quite large, were shaken like reeds, and
were rent and split in a wonderful manner.
This earthquake was especially remarkable for the im-
mense area over which the shock extended. It reached
as far north as Sweden. Solid mountain-ranges-as, for
example, the Pyrenees and the Alps-were severely shaken.
A deep fissure was opened in France. On the south, the
earthquake waves crossed the Mediterranean and destroyed
a number of villages in the Barbary States. On the west,
the waves traversed the bed of the Atlantic, and caused
unusually high tides in the West Indies. In North Amer-
ica the movements were felt as far west as the Great Lakes.
Feebler oscillations of the ground occurred at intervals for
several weeks after the main shock.
62. Non-volcanic Igneous Eruptions.-In re-
gions remote from volcanoes, melted rock has
been forced up from the interior through fissures
in the rocks of nearly all geological formations.
On cooling, the mass forms what is called a dyke.
Dykes vary in width from a few inches to several
yards. They are generally much harder than the
rocks through which they were forced, and, being
less subject to erosion, often project considerably
above the general surface.
From their mode of formation, dykes are gen-
erally without traces of stratification, but by cool-
ing a series of transverse fractures are sometimes
produced. The dykes thus obtain the appearance
of a series of columns, called basaltic columns.
Igneous rocks of this description are found in
all parts of the continents, but are especially com-
mon near the borders of mountainous districts.
Fingal's Cave, in Scotland, is a noted example
of basaltic columns.
Pig. 24, Basaltic Columns, Fingal's Cave, Scotland,
63. Gradual Elevations and Subsidences.-Be-
sides the sudden changes of level produced by
earthquakes, there are others that take place
slowly, but continuously, by which large portions
of the surface are raised or lowered from their
former positions. The rate of movement is very
slow-probably never exceeding a few feet in a
century. The following examples are the most
The Scandinavian peninsula (Norway and Swe-
den) is slowly rising in the north and sinking in
The southern part of the coast of Greenland is
The North American coast, from Labrador to
New Jersey, is rising.
The Andes Mountains, especially near Chili,
are gradually rising.
The Pacific Ocean, near the centre, is sinking
over an area of more than 6000 miles.
The cause of these movements is to be traced
to the warping action caused by gradual contrac-
tion of a cooling crust.
The earth was originally melted throughout. It after-
ward cooled on the surface and formed a crust. The earth's
original fluidity is rendered probable-
(1.) By the spherical shape of the earth;
(2.) By the crystalline rocks underlying all others;
(3.) By the greater heat of the earth during geological
The interior is still in a highly-heated condition. This
is proved-ist. By the increased heat of the crust as we go
below the surface; 2d. By the escape of lava from volca-
noes in all latitudes.
The following opinions are held concerning the condi-
tion of the interior of the earth:
(1.) That the earth has a solid centre and crust, with a
heated layer between.
(2.) That the earth has a solid crust only, and an inte-
rior sufficiently heated to be in a fused or in a pasty con-
(3.) That the earth is solid throughout, but highly
heated in the interior.
The thickness of the crust is not known. It is probable
that the portions solidified by cooling pass insensibly into
those that are nearly solid from the combined influence
of loss of heat and increasing pressure. The heated
interior, however, must lie comparatively near the sur-
The effects produced by the heated interior on the crust
are-ist. Volcanoes; 2d. Earthquakes; 3d. Non-volcanic
igneous eruptions; and 4th. Gradual elevations or subsi-
Volcanic mountains are of a variety of shapes. Near
their craters the cone shape predominates, and serves to
distinguish these mountains as a class.
The ejected materials of volcanoes are-ist. Melted rock
or lava; 2d. Ashes or cinders; 3d. Vapors or gases.
These materials are brought up from great depths into
the volcanic mountain by the force produced by a contract-
ing globe. They may escape from the crater-1st. By the
pressure of highly-heated vapors; or, 2d. By the pressure
of a column of melted lava.
The inclination of the slopes of the volcanic cone de-
pends on the materials of which it is composed. Ash-
cones are steeper than those formed of lava.
Eruptions are of two kinds, explosive and non-explo-
High volcanic mountains are, as a rule, characterized by
Volcanoes occur both on the surface of the land and on
the bed of the ocean.
Those on the land occur mainly near the borders of
sinking oceans, where the crust is weakest.
The principal volcanic districts of the world are-1.
Along the shores of the Pacific; 2. On the islands which
are scattered over the Pacific; 3. Scattered over the seas
which divide the northern and southern continents; 4. In
the northern and central parts of the Atlantic Ocean; 5.
In the western and central parts of the Indian Ocean.
The centres of volcanic activity' are found in the An-
tilles and in the Sunda Islands, where several lines of
fracture cross each other.
Subordinate volcanic phenomena are seen in-1. Mud
volcanoes; 2. Solfataras; 3. Geysers.
Earthquakes are shakings of the earth's crust; they may
occur with or without a permanent displacement.
The following facts have been discovered as to earth-
(1.) Their place of origin is not very deep-seated.
(2.) The area of disturbance increases with the energy
of the shock and the depth of the origin.
(3.) The shape of the origin is that of a line, and not
that of a point.
(4.) The shape of the area of disturbance depends on
the elasticity of the materials through which the shock
(5.) The earthquake motion travels through the earth
as spherical waves which move outward in all directions
from the origin of the disturbance.
The movement at the earth's surface may be--st. In
the form of a gentle wave; 2d. An upward motion; 3d. A
The velocity with which the earthquake motion is trans-
mitted varies with the intensity of the shock and the
nature of the materials through which it is propagated.
There are two distinct kinds of motion accompanying
earthquake waves: the earthquake motion proper, and
the motion producing the accompanying sounds.
As a rule, the earthquake shocks which produce the
greatest damage are of but short duration, generally but
a few seconds or minutes. Slighter disturbances may fol-
low the main shock at intervals of days, weeks, or even
Earthquake shocks are more frequent-1st. In winter
than in summer; 2d. At night than during the day; 3d.
During the time of new and full moon than at any other
Earthquakes are mainly caused by the gradually in-
creasing force produced by the contraction of the crust.
Earthquakes are also to be attributed to the forces which
eject the molten matter from the craters of volcanoes.
Slight earthquake shocks may be occasioned by the fall-
ing in of masses of rock from the roofs of subterranean
caverns, or by the settling of upturned strata.
Earthquakes may occur in any part of the earth, but are
most frequent in volcanic and in mountainous regions.
Dykes are masses of rock formed by the gradual cooling
of melted matter which has been forced up through fis-
sures from the interior.
Basaltic columns are formed by dykes. They owe their
columnar structure to fractures produced on cooling.
The crust of the earth is subject to gradual as well as to
sudden changes of level.
The Scandinavian peninsula is rising on the north and
sinking on the south.
The southern coast of Geeenland is sinking.
The North American coast, from Labrador to New Jer-
sey, is rising.
The range of the Andes near Chili is rising.
The bed of the Pacific in the neighborhood of the Poly-
nesian island chain is sinking.
These movements are caused by the contraction of a
32 PHYSICAL GEOGRAPHY.
The Heated Interior.
Enumerate the proofs that the interior of the earth is
still in a highly-heated condition.
Name some circumstances which render it probable that
the earth was originally melted throughout.
What is the average rate of increase of temperature
with descent below the surface?
How can it be shown that the whole interior of the
earth is filled with highly-heated matter?
Why is it so difficult to assign a definite limit to the
thickness of the earth's crust?
Is the interior of the earth supposed to be in as fluid a
condition as that of the lava which escapes from a volcano ?
What four classes of effects are produced in the crust by
the heated interior?
What are volcanoes? What connection have they with
the interior of the earth? How do active volcanoes differ
from those which are extinct?
Explain the origin of the conical form of volcanic
Which generally produces the more destructive effects,
ashes or lava? Why?
Enumerate the materials which are ejected from the in-
terior of the earth through the craters of volcanoes.
What is tufa? How is it formed ?
Which has the greater inclination, a lava-cone or an
Explain in full the manner in which the shrinkage, or
contraction of the earth on cooling, produces a pressure
both in the interior and in the crust.
By what forces are volcanic eruptions produced ?
Into what two classes may all volcanic eruptions be di-
vided? How are those of each class caused?
Give an example of each of these classes.
What is the highest volcano in the world?
Under what five regions may all the volcanoes in the
world be arranged?
In what parts of the world are volcanoes most numer-
Why are volcanoes more numerous here than elsewhere?
Name some of the regions of submarine volcanoes.
Why are all volcanoes found near the coasts of the con-
tinents or on islands?
What are mud volcanoes? Solfataras?
What are earthquakes? Into what two classes may they
Name some facts that have been discovered about earth-
Name three kinds of earthquake motion. Which is the
Describe the sounds which accompany earthquakes.
What is the main cause of earthquakes? To what other
causes may they be attributed?
What facts have been discovered respecting the period-
icity of earthquakes?
Give a short description of the earthquake which de-
stroyed the city of Lisbon.
Are any portions of the earth free from earthquake
In what parts of the earth are earthquake shocks most
What are dykes? How were they formed?
Enumerate some of the gradual changes of level which
are now occurring in the crust of the earth. By what are
these changes caused?
Trace on the map the five principal volcanic districts of
Which contains the greater number of volcanoes, the
Atlantic or the Pacific shores of the continents?
Does the eastern or the western border of the Indian
Ocean contain the greater number of volcanoes?
Name the principal volcanic islands of the Atlantic.
Of the Indian. Of the Pacific.
Locate the following volcanoes: Hecla, Pico, Kilauea,
Sarmiento, Llullayacu, Egmont, Cosiguina, Teneriffe,
Autisana, Kilimandjaro, Demavend, Peshan, Osorno, Ere-
bus, and Terror.
-NName the principal volcanic mountains of North America.
In what part of the Atlantic Ocean are submarine erup-
tions especially frequent ?
Name three noted volcanoes of the Mediterranean
Name the portions of the earth which were shaken by
the earthquake of Lisbon. When did this earthquake
What noted volcanoes are found in the region visited by
the earthquake of Lisbon?
In what portions of the Eastern Hemisphere are earth-
quake shocks especially frequent? In what portions of
the Western Hemisphere?
THE CRUST OF THE EARTH. 33
THE OUTSIDE OF THE EARTH.
The Crust of the Earth.
64. Composition of the Crust.-The elementary
substances are not equally distributed throughout
the earth's crust. Many of these substances occur
only in extremely small quantities, while others
are found nearly everywhere.
Although the deepest cutting through the earth's crust
does not extend vertically more than about two miles be-
low the level of the sea, yet the upturning of the strata, or
the outcropping of the different formations, enables us to
study a depth of about sixteen miles of the earth's crust.
A careful study of the composition of this part of the
crust shows that oxygen constitutes nearly one-half of it,
by weight. Silicon, an element which, when combined
with oxygen, forms silica or quartz, constitutes, either as
sand, or combined with various bases as silicates, one-
fourth; so that these two elements form at least three-
fourths, by weight, of the entire crust. The following are
also prominent ingredients of rocks-aluminium, which,
when combined with oxygen, forms alumina, the basis of
clay; magnesium, calcium, potassium, sodium, iron, and car-
bon. These nine substances, according to Dana, form
1%- ths, by weight, of the entire crust.
Sulphur, hydrogen, chlorine, and nitrogen also occur fre-
quently. The remaining elements are of comparatively
65. The Origin of Rocks.-When the earth was
yet a melted globe, the water which now covers
the larger portion of its surface hung over it,
uncondensed, either as huge clouds or as masses
of vapor. After a comparatively thin crust had
formed, the vapor was condensed as rain, and cov-
ered the earth with a deep layer of boiling water.
Occasionally the cooling crust was broken by the
increasing tension, and portions of the molten in-
terior were forced out- and spread over the sur-
face. The muddy waters then cleared by depos-
iting layers of sediment over the ocean's bed.
When, by long-continued cooling, the crust be-
came thicker, the breaking out of the interior oc-
curred less frequently, and contraction, wrinkling
the surface in huge folds, caused portions to
emerge from the ocean and form dry land. Dur-
ing all this time the waters were arranging the
looser materials in layers or strata which were
originally more or less horizontal; but wher-
ever the contraction forced the melted interior
through the crust or upturned it in huge folds,
the horizontal position of the deposits was de-
stroyed; and even when not so disturbed, the
heat of the interior, escaping through fissures,
often produced such alterations as to confuse or
completely to obliterate all traces of their regu-
The almost inconceivable extent of geological time may
be inferred from the calculations of Helmholtz, based on
the rapidity of the cooling of lava. These calculations
show that in passing from a temperature of 20000 C. to
2000 C. a time equal to three hundred and fifty million years
must have elapsed. Before this a still greater time must
have elapsed, and after it came the exceedingly great ex-
tent of geological time proper.
66. According to their Origin, rocks may be
divided into three distinct classes:
(1.) Igneous Rocks, or those ejected in a melted
condition from the interior, and afterward cooled.
(2.) Aqueous Rocks, or those deposited as sedi-
ment by water. When mineral matter settles in
water, the coarser, heavier particles reach the bot-
tom first, so that a sorting action occurs, which
makes the different layers or strata vary in the
size and density of their particles, and, to a great
extent, in their composition.
Aqueous rocks are sometimes called sediment-
(3.) Metamorphic Rocks, or those originally
deposited in layers, but afterward so changed by
the action of heat as to lose all traces of stratifi-
This change, which is called metamorphism, is caused by
heat acting under pressure in the presence of moisture. Under
these conditions a far less intense heat is required to re-
move all traces of stratification. Metamorphism appears
to consist mainly in a rearrangement of the chemical con-
stituents of the rocks.
67. According to their Condition, rocks may
be divided into two classes:
(1.) Stratified Rocks, or those arranged in
regular layers. Aqueous rocks are always strati-
fied, and sometimes, though rarely, metamorphic
rocks are stratified.
34 PHYSICAL GEOGRAPHY.
Fig, 25. Stratified Bock,
In Fig. 25 the different layers or strata are shown by
the shadings. Stratified rocks are the most common form
of rocks found near the earth's surface.
Stratified rocks are largely composed of fragments of
older rocks; for this reason they are sometimes called
(2.) Unstratified Rocks, or those destitute of
any arrangement in layers. They are of two kinds:
(1.) Igneous, or those which were never stratified.
(2.) Metamorphic, or those which were once
stratified, but have lost their stratification by
the action of heat.
Unstratified rocks are sometimes called crystal-
line rocks, because they consist of crystalline
68. Fossils are the remains of animals or plants
which have been buried in the earth by natural
causes. Generally, the soft parts of the organism
have disappeared, leaving only the harder parts.
Sometimes the soft parts have been gradually re-
moved, and replaced by mineral matter, generally
lime or silica; thus producing what are called
petrifactions. At times the mere impression of
the animal or plant is all that remains to tell
of its former existence.
Fig, 26, Fossil Enorinite,
When the remains of an animal or plant are exposed to
the air or buried in dry earth, they generally decompose
and pass off almost entirely as gases; but when buried
under water or in damp earth, their preservation is more
probable. Therefore, the species most likely to become
fossilized are those living in water or marshes, or in the
neighborhood of water or marshes.
69. According to the Presence or Absence of
Fossil Remains, rocks may be divided into two
(1.) Fossiliferous Rocks, or those which con-
tain fossils. They are stratified and are of
aqueous origin. Metamorphic rocks, in very
rare instances, are found to contain fragments
(2.) Non-fossiliferous Rocks, or those destitute
of fossils. They include all igneous rocks and
most of those that are metamorphic.
70. Paleontology is the science which treats of fossils..
Paleontology enables us to ascertain the earth's condi-
tion in pre-historic times, since by a careful examination
of the fossils found in any rocks we discover what animals
and plants lived on the earth while such rocks were being
deposited. The earth's strata thus become the pages of a
huge book; and the fossils found in them, the writings
concerning the old life of the world. By their careful
study geologists have been enabled to find out much of
the earth's past history.
71. Division of Geological Time.-A compari-
son of the various species of fossils found in the
earth's crust discloses the following facts:
(1.) The fossils found in the lowest rocks bear
but a slight resemblance to the animals and
plants now living on the earth.
(2.) The fossils found in the intermediate strata
bear a resemblance to existing species, though
this resemblance is not so strongly marked as in
the upper strata.
(3.) The fossils found in the upper strata bear
a decided resemblance to existing species.
It is on such a basis that the immense extent
of geological time is divided into the following
shorter periods or times:
(1.) Archmean Time, or the time which wit-
nessed the dawn of life. This time included an
extremely long era, during most of which the con-
ditions of temperature were such that no life could
possibly have existed. Toward its close, however,
the simplest forms of life were created.
The lower Archman rocks resulted from the
original cooling of the molten earth, and cover
its entire surface, including the floor of the ocean.
On these rest less ancient Archman rocks, formed
as sedimentary deposits of the older rocks.
The rocks of the Archean Time in North America in-
clude the Laureitian, the lowest, named from the river
St. Lawrence, near which they occur, and the Huroniau,
named from their occurrence near Lake Huron.
(2.) Paleozoic Time, or ancient life, included
the time during which the animals and plants
bore but little resemblance to those now living.
(3.) Mesozoic Time, or middle life, included
the time during which the animals and plants
began to resemble those now living.
THE CRUST OF THE EARTH.
(4.) Cenozoic Time, or recent life, included the
time during which the animals and plants bore
decided resemblance to those now living.
These times are divided into ages.
Archsean Time includes-
(1.) The Azoic Age;
(2.) The Eozoic Age.
Paleozoic Time, or, as it is sometimes called,
the Primary, includes-
(1.) The Age of Invertebrates, or the Silurian;
(2.) The Age of Fishes, or the Devonian;
(3.) The Age of Coal-plants, or the Carbon-
Mesozoic Time, or, as it is sometimes called,
the Secondary, includes the Age of Reptiles.
Cenozoic Time includes-
(1.) The Tertiary, or the Age of Mammals;
(2.) The Quaternary, or the Age of Man.
Where no disturbing causes existed, and the
land remained under the seas, the rocks deposited
during these periods were thrown down in regu-
lar strata, one over the other. The Archman
were the lowest; above them were the Paleozoic,
then the Mesozoic, and finally those of the Ceno-
zoic. Generally, however, frequent dislocations
of the strata have disturbed the regular order
72. The Azoic Age included all the time from
the first formation of the crust to the appearance
of animal and vegetable life.
The Eozoic Age is that which witnessed the
dawn of life. The sedimentary rocks of this age
are so highly metamorphosed that nearly all traces
of life have been obliterated. Among plants, the
marine algca, or sea-weeds, and among animals,
the lowest forms of the protozoa, were probably
the chief species.
73. The Age of Invertebrates, or the Silurian,
is sometimes called the Age of Mollusks. Among
plants, algce, or sea-weeds, are found; among ani-
.mals, protozoa, radiates, articulates, and mollusks,
but no vertebrates. Hence the name, Age of In-
vertebrates. Mollusks were especially numerous.
The name Silurian is derived from the ancient Silures,
a tribe formerly inhabiting those parts of England and
Wales where the rocks abound.
74. The Age of Fishes, or the Devonian.-
During this age all the sub-kingdoms of animals
are found, but the vertebrates first appear, being
represented by fishes, and from this fact the name
has been given to the age. Land-plants are also
found. Immense beds of limestone and red sand-
stone were deposited.
The name Devonian is derived from the district of Dev-
onshire, England, where the rocks abound.
75. The Age of Coal-Plants, or the Carbonif-
erous.-The continents during this age consisted
mainly of large, flat, marshy areas, covered with
luxuriant vegetation, subject, at long intervals, to
extensive inundations. The decaying vegetation,
decomposing under water, retained most of its
solid constituent, carbon, and formed beds of coal.
All the sub-kingdoms of animals were represented
and reptiles also existed. The comparatively few
land-plants of the preceding age now increased
and formed a dense vegetation.
To favor such a luxuriant vegetation the air
must have been warm and moist. Since all the
coal then deposited previously existed in the air
as carbonic acid, the Carboniferous Age was nec-
essarily characterized by a purification of the
Fig. 27, Carboniferous Landscape. (A restoration.)
Formation of Coal.-In every 100 parts of dry vege-
table matter there are about 49 parts of carbon, 6 of hydro-
gen, and 45 of oxygen. The carbon is a solid; the hydro-
gen and oxygen are gases. It is from the carbon that coal
is mainly formed. When the decomposition of the vege-
table matter takes place in air, the carbon passes off with
the hydrogen and oxygen as various gaseous compounds;
but when covered by water, most of the carbon is retained,
together with part of the oxygen and hydrogen. Although
every year our forests drop tons of leaves, no coal results,
the deposit of one year being almost entirely removed
before that of the next occurs.
It has been computed that it would require a depth of
eight feet of compact vegetable matter to form one foot.of
bituminous coal, and twelve feet of vegetable matter to
form one foot of anthracite coal. Anthracite coal differs
THE CRUST OF
36 PHYSICAL GEOGRAPHY.
from bituminous mainly in the greater metamorphism to
which it has been subjected; it contains a greater propor-
tion of carbon and less hydrogen and oxygen.
76. The Age of Reptiles.-In this age the ani-
mals and plants begin to resemble existing species.
The age is characterized mainly by the prepon-
derance of reptiles, many of which were very
large, as, for example, the plesiosaurus, an animal
with a long, snake-like neck and a huge body, or
the ichthyosaurus, with a head like a crocodile and
short neck and large body. Both of these ani-
mals were furnished with fin-like paddles, and
lived in the water. Huge pterodactyls, or bat-
like saurians, flew in the air or paddled in the
water. Mammals and birds also occur.
Pig. 28, 'The Age of Reptiles. (A restoration.)
77. The Age of Mammals, or the Tertiary Age.
-Mammals, or animals that suckle their young,
occurred in great numbers, and, being the highest
Fig, 29. Mastodon giganteus. An Animal of the Mammalian Age
type of life, gave the name to the age. The ani-
mals and plants of the Mammalian Age closely
resembled existing species, though most of them
were much larger; as, for example, the dinothe-
rium, a huge animal, with a trunk like an ele-
phant, but with downward-turned tusks; the
palceotherium, and many others.
78. The Era of Man, or the Quaternary Age,
witnessed the introduction of the present animals
and plants and the creation of man.
79. Changes Now Occurring in the Earth's
Crust.- Geological time was characterized by ex-
tensive changes, both in the kind and luxuriance
of life, and in the nature of its distribution.
The earth is still undergoing extensive changes,
which are caused by the following agencies:
(1.) By the Winds, which often carry sand
from a desert and distribute it over fertile plains:
in this manner the narrow tract of fertile land on
the borders of the Nile, in Egypt, receives much
sand from the Sahara. The winds are also piling
up huge mounds of sand along the sea-coasts,
forming what are called dunes, or sandhills.
(2.) By the Moisture of the Atmosphere, soak-
ing into porous rocks or running into the crevices
between solid ones. This water in freezing ex-
pands with force sufficient to rend the rock into
fragments, which are carried away by the rivers
or, when sufficiently small, by the winds.
(3.) By the Action of Running Water.-Rivers
wash away portions of their banks or cut their
Fig, 30, Onrions Effect of Erosion.
way through their channels. This action is
called ero 1n. It occurs even in the hardest
DISTRIBUTION OF THE LAND-AREAS.
rocks. The materials thus carried away are
spread over the lowlands near the mouth of the
river or thrown into the sea, where they often
form large deposits. By the constant action of
these causes the mean heights of the continents are
decreasing and their breadths increasing.
The most remarkable instance of erosion is
found in the caftons of the Colorado River, where.
the waters have eaten a channel through the hard
limestones and granites that form the bed of the
stream, until they now run through gorges whose
walls ascend almost perpendicularly to the height
of from 3000 to 6000 feet.
A good idea of this great depth may be obtained by
walking along a straight street for about a mile (5280
feet), ana then imagining the street set upright in the air.
On looking down toward the starting-place, we would see
it as it would appear at the bottom of a hole about 6000
The forms produced by erosion are often extremely fan-
tastic. Tall, slender, needle-like columns, capped by a
layer of harder rock, sometimes occur, thus showing in a
marked manner an effect of erosion.
(4.) By the Action of Ocean Waves, changing
the outlines of coasts; as may be seen in portiofis
of the coasts of England and Scotland.
(5.) By the Agency of Man, witnessed mainly
in the destruction of the forests over extended
(6.) By the Contraction of a Cooling Crust,
resulting in-1. Earthquakes; 2. Volcanoes; 3.
Gradual uplifts and subsidences.
Distribution of the Land-Areas.
80. Geographic Effects of Light, Heat, and
Moisture.-The peculiarities observed in the dis-
tribution of animal and vegetable life are caused
by differences in the distribution of light, heat,
and moisture. Since light, heat, and moisture
are influenced by the interaction of land, water,
and air, we must first study the distribution and
grouping of these inorganic or dead forms before
we can understand those that are living.
81. The Distribution of the Land.-Of the
197,000,000 square miles that make up the
4arth's surface, about 144,000,000 are water and
53,000,000 land. The proportion is about as the
square of 5 is to the square of 3. If, therefore,
we erect a square on a side of five, its entire area
will represent the relative water-area of the globe;
while a square whose side is three will represent
the relative land-area.
_- =:WATER-AREA, -
_-_.144,000,000 square miles. .
L i N.Ai. l. \, ~-
Fig, 31, Relative Land- and Water-Areas.
82. The Distribution of the Land can be best
studied when arranged under two heads:
(1.) The Horizontal Forms of the Land, or the
different shapes produced in the land-areas by the
coast lines, or by the contact of land and water;
(2.) The Vertical Forms of the Land, produced
by the irregularity of the surface of the high
lands and low lands.
83. The Horizontal Forms.-The land-areas
are divided into continents and islands.
The Eastern Hemisphere contains four conti-
nents: Europe, Asia, Africa, and Australia. The
first three form one single mass, which is called
the Eastern Continent.
Though the word "continent" strictly refers to an ex-
tended area of land entirely surrounded by water, usage
has sanctioned the application of the term to the grand
divisions of the land. It is quite correct, therefore, to
speak of the North American Continent, the Asiatic Con-
The Western Hemisphere contains two conti-
nents: North and South America; these consti-
tute what is called the Western Continent.
The following are the extremities of the conti-
In the Eastern Continent-
Most northern point, Cape Chelyuskin, lat. 780 16' N.
Most southern point, Cape Agulhas, lat. 340 51' S.
Most eastern point, East Cape, long. 170 W.
Most western point, Cape Verd, long. 170 34' W.
In the Western Continent--
Most northern point, Point Barrow, lat. 720 N.
Most southern point, Cape Firoward, lat. 530 53' S.
Most western point, Cape Prince of Wales, long. 1680 W.
Most eastern point, Cape St. Roque, long. 350 W.
38 PHYSICAL GEOGRAPHY.
84. Peculiarities in the Distribution of the
(1.) The continents extend farther to the north
than to the south.
(2.) The land masses are crowded together near
the north pole, which they surround in the shape
of an irregular ring.
(3.) The three main southern projections of
the land-South America, Africa, and Australia
-are separated from each other by extensive
85. Land and Water Hemispheres.-The ac-
cumulation of the land in the north and its sepa-
ration in the south lead to a curious result-nearly
all the land is collected in one hemisphere.
If one point of a pair of compasses be placed at
the north pole of a globe, and the other stretched
out to reach to any point on the equator, they
will describe on the surface of the globe a great
circle, and consequently will divide the globe into
hemispheres. If, while they are stretched this dis-
tance apart, one of the points be placed at about
the city of London, a circle swept with the other
point will divide the earth into land and water
hemispheres. Such a great circle would pass
through the Malay Peninsula and the coast of
The Land Hemisphere contains all of North
America, Europe, and Africa, and the greater part
of South America and Asia. The Water Hemi-
sphere contains the southern portions of South
America, the Malay Peninsula, and Australia.
Fig. 32, Land and Water Hemispheres.
86. Double Continents.-The six grand divis-
ions or continents may be divided into three pairs,
called Double or Twin Continents.
Each Double Continent consists of a northern
and southern continent, almost separated from
each other, but connected by a narrow isthmus
or island chain.
The three double continents are North and
South America, Europe and Africa, and Asia
and Australia. There are, therefore, three north-
ern and three southern continents.
The northern continents lie almost entirely in
temperate latitudes, while the southern lie mainly
in the tropics.
87. Lines of Trend.-The study of any map
of the world on a Mercator's projection will dis-
close the following peculiarities in the earth's
There are two great systems of courses, trends, or
lines of direction, along which the shores of the con-
tinents, the mountain-ranges, the oceanic basins, and
the island chains extend.
These trends extend in a general north-easterly
and north-westerly direction, and intersect each
other nearly at right angles.
North-east Trends.-A straight ruler can be so placed
along the south-eastern coasts of Greenland and the south-
eastern coasts of North America that its edge will touch
most of their shore lines. Its general direction will be
It can be similarly placed along the south-eastern coast
of South America, the north-western coast of Africa, and
most of the western coast of Europe; along the south-
eastern coasts of Africa; the south-eastern coast of Hin-
dostan; and along the eastern coast of Asia, without its
general direction differing much from north-east.
North-west Trends.-A straight ruler can be so placed
as to touch most of the western shores of North America
and part of the western coast of South America; most
of the western coasts of Greenland, or the north-eastern
coasts of North America, and part of the western coasts
of Africa. All these courses are sensibly north-west.
If placed with one end at the mouth of the Mackenzie
River, and the other on the south-western extremity of
Lake Michigan, it will cut nearly all the great lakes in
Central British America. The direction of the island
chains of the Pacific Ocean in particular is characterized
by these two trends, many of the separate islands being
elongated in the direction of the trend of their chain.
88. Continental Contrasts. The main pro-
longation of the western continent extends in the
line of the north-western trend, while that of the
eastern continent extends in the line of the north-
eastern trend. The axes of the continents, or
their lines of general direction, therefore, inter-
sect each other nearly at right angles.
The western continent extends far north and
south of the equator, while the eastern lies mainly
north of the equator. The Western Continent,
therefore, is characterized by a diversity of cli-
mates; the Eastern Continent, by a similarity.
The distribution of vegetable and animal life
in each continent is necessarily affected by the
peculiarities of its climate.
It is from the prevalence of the lines of trend that the
general shape of the continents is mainly triangular. An
excellent system of map-drawing has been devised on this
The following peculiarities exist in the coast
lines of the continents:
The coast lines of the northern continents are
very irregular, the shores being deeply indented
with gulfs and bays, while those of the southern con-
tinents are comparatively simple and unbroken.
The continents are most deeply indented near
the regions where the pairs of northern and south-
ern continents are nearly separated from each
'other. These regions correspond with the lines of
great volcanic activity, and appear to be areas over
which considerable subsidence has occurred.
The continents differ greatly from one another
in their indentations. Europe is the most indented
of all the continents. The area of her peninsulas,
compared with that of her entire area, is as 1 to 4.
Asia comes next in this respect, the proportion
being 1 to 51, while in North America it is but
1 to 14.
The following Table gives in the first column the area
of each of the continents, in the second the length of coast
line, and in the third the number of square miles of area
to one mile of coast line:
CONTINENTS. AREA. COAST LINE. for 1.
Asia ................. 17,500,000 sq. miles. 35,000miles. 500
Africa .............. 12,000,000 16,000 750
North America.. 8,400,000 22,800 368
South America... 6,500,000 14,500 449
Europe............. 3,700,000 19,500 190
Australia.......... 3,000,000 10,000 300
Europe has, in proportion to its area,
About three times as much coast line as Asia.
About four'times as much as Africa.
About twice as much as North America.
More than twice as much as South America.
Europe is the most, and Africa the least, deeply
indented of the continents.
89. Relative Continental and Insular Areas.-
Of the 53,000,000 square miles of land, nearly
3,000,000, or about one-seventeenth, is composed
90. Varieties of Islands.-Islands are either
continental or oceanic.
Continental Islands are those that lie near the
shores of the continents. They are continuations
of the neighboring continental mountain-ranges
or elevations, which they generally resemble in
geological structure. They may, therefore, be re-
garded as projections of submerged portions of the
neighboring continents. Continental islands have,
in general, the same lines of trend as the shores of
the neighboring mainland.
Continental islands, as a rule, are larger than oceanic
islands. This is caused by the shallower water in which
continental islands are generally situated. Papua and
Borneo have each an area of about 250,000 square miles;
either of these islands is more than twice as large as the
combined areas of Great Britain and Ireland.
91. American Continental Island Chains.
(1.) The Arctic Archipelago comprises the
large group of islands north of the Dominion
of Canada. It consists of detached portions of
the neighboring continent.
(2.) The Islands in the Gulf of St. Lawrence
and its neighborhood are apparently the northern
prolongations of the Appalachian mountain-sys-
(3.) The Bahamas lie off the south-eastern coast
of Florida, to which they belong by position and
structure. Their general trend is north-west.
(4.) The West Indies form a curved range,
which connects the peninsula o Yucatan with
the coast-mountains of Venezuela. Here both
trends appear, though the north-western pre-
Fig, 33. West India Island Chain,
1, Cuba; 2, Hayti; 3, Jamaica; 4, Porto Rico; 5, Caribbee Islands,
(5.) The Aleutian Islands form another curved
range, which connects the Alaskan Peninsula with
Kamtchatka; their general trend is north-east.
They are connected with the elevations of the
North American continent.
40 PHYSICAL GEOGRAPHY.
(6.) The Islands west of the Dominion of Can-
ada and Alaska. These are clearly the summits
of submerged northern prolongations of the Pa-
cific coast ranges.
(7.) The Islands of the Patagonian Archi-
pelago are the summits of submerged prolonga-
tions of the Andes of Chili.
92. Asiatic Continental Island Chains consist
of a series of curved ranges extending along the
entire coast, and intersecting each other nearly at
(1.) The Kurile Islands are a prolongation of,
the Kamtchatkan range.
(2.) The Islands of Japan extend in a curve
from Saghalien to Corea.
(3.) The Loo Choo Islands extend in a curve
from the islands of Japan to the island of For-
(4.) The Philippines form two diverging chains,
which merge on the south into the Australasian
Island chain. The eastern chain extends to the
southern extremity of Celebes, and the western
to that of Borneo.
The Asiatic chains belong to a submerged mountain-
range extending from Kamtchatka to the Sunda Islands.
Their general direction is parallel to the elevations of the
93. The Australasian Island Chain.
The Australasian Island chain is composed of
a number of islands extending along curved
trends over a length of nearly 6000 miles, from
Sumatra to New Zealand. The islands extend
along three curved lines, whose general direction
Fig. 34. Australasian Island Chain.
1, Sumatra; 2, Java; 3, Sumbawa; 4, Flores; 5, Timor; 6, Borneo;
7, Celebes; 8, Gilolo; 9, Ceram; 10, Papua; 11, Louisiade Archipel-
ago; 12, New Caledonia; 13, New Zealand; 14, Admiralty Islands;
15, Solomon's Archipelago; 16, Santa Cruz; 17, New Hebrides.
The Australasian chain was probably connected with the
Asiatic continent during recent geological time, and sepa-
rated from it by subsidence. Its numerous volcanoes and
coral formations prove that subsidence is still taking
94. Peculiarity of Distribution.--The follow-
ing peculiarity is noticed in the distribution of
Each of the continents has an island, or a group
of islands, near its south-eastern extremity. For
example, North America has the Bahamas and
the West Indies; Greenland has Iceland; South
America has the Falkland Islands; Africa has
Madagascar; Asia has the East Indies; and
Australia has Tasmania.
95. Oceanic Islands are those situated far away
from the continents. They occur either in vast
chains, which generally extend along one or the
other of the two lines of trend, or as isolated
Oceanic Island Chains.
The following are the most important:
(1.) The Polynesian Chain;
(2.) The Chain of the Sandwich Islands;
(3.) The Tongan or New Zealand Chain.
Fig, 35. Polynesian Island Chain.
1, Marquesas; 2, Paumotu; 3, Tahitian; 4, Rurutu group; 5, Her-
vey group; 6, Samoan, or Navigator's; 7, Vakaafo group; 8, Vaitupu;
9, Kingemill; 10, Ralick; 11, Radack; 12, Carolines; 13, Sandwich.
The Polynesian Chain consists of a series of
parallel chains, extending from the Paumotu and
the Tahitian Islands to the Carolines, the Ralick,
and the Radack groups. Their general direction
is north-west; the total length of the chain is
about 5500 miles.
The Chain of the Sandwich Islands extends in
a north-westerly direction. Its length is about
The New Zealand Chain extends north-east as
far as the Tonga Islands, cutting the Australasian
chain at right angles.
96. Isolated Oceanic Islands are mainly of two
kinds: the Volcanic and the Coral. As a rule, the
Volcanic islands are high, while Coral islands sel-
'dom rise more than twelve feet above the water.
Volcanic Islands are not confined to isolated
groups, but occur also in long chains. The Poly-
nesian, Sandwich, and New Zealand Chains con-
tain numerous volcanic peaks. But the high, iso-
lated oceanic islands are almost always of volcanic
origin, and, consisting of the summits of subma-
rine volcanoes, are generally small. Some of the
Canary and Sandwich Islands, which are of this
class, rise nearly 14,000 feet above the sea.
97. Coral Islands, or Atolls, though of a great
variety of shapes, agree in one particular:
They consist of a low, narrow rim of coral rock,
enclosing a body of water called a lagoon.
Fig, 36. A Coral Island.
98. Mode of Formation of Coral Islands.-The
reef forming the island is of limestone, derived
from countless skeletons of minute polyps that
once lived beneath the surface of the waters.
The skeletons, however, are not separate. The
polyp propagates its species by a kind of bud-
ding; that is, a new polyp grows out of the body
of the old. In this way the skeletons of count-
less millions of polyps are united in one mass and
assume a great variety of shapes.
One of the most common species of reef-forming corals,
the madrepora, is shown in Fig. 37. Many other forms
The delicate coral structures, together with
shells from various shellfish, are ground into frag-
ments by the action of the waves, and by the in-
Fig. 37, Coral.
filtration of water containing lime in solution,
they become compacted into hard limestone, on
which new coral formations grow.
The growth of the coral mass is directed up-
ward, and ceases when low-water mark is reached,
because exposure to a tropical sun kills the polyps.
But the action of the waves continues, and the
broken fragments are gradually thrown up above
the general level of the water. In this way a reef
is formed, whose height is limited by the force of
the waves, and seldom exceeds twelve feet.
On the bare rock, which has thus emerged, a
soil is soon formed and a scanty vegetation ap-
pears, planted by the hardy seeds scattered over
it by the winds and waves.
The coral island never affords a very comfortable resi-
dence for man. The palm tree is almost the only valuable
vegetable species; the animals are few and small, and the
arable soil is limited. Moreover, the island is subject to
occasional inundations by huge waves from the ocean.
99. Distribution of Coral Islands. -According
to Dana, the reef-forming coral polyp is found
only in regions where the winter temperature
of the waters is never lower than 680 Fahr.
Some varieties, however, will grow in colder
water. Coral islands are confined to those parts
of tropical waters where the depth does not greatly
exceed 100 feet, and which are protected from cold
ocean-currents, from the influence of fresh river-
waters, muddy bottoms, and remote from active vol-
canoes, whose occasional submarine action causes
the death of the coral polyp. Though some coral
polyps grow in quiet water, the greater part thrive
best when exposed to the breakers. The growth is
therefore more rapid on the side toward the ocean
than on the side toward the island.
Coral islands are most abundant in the Pacific Ocean.
The following groups contain numerous coral islands:
the Paumotus, the Carolines, the Radack, the Ralick,
and the Kingsmill groups, and the Tahitian, Samoan,
and Feejee Islands, and New Caledonia.
In the Indian Ocean the Laccadives and the Maldives are
In the Atlantic Ocean the West Indies and the Bermudas
100. Varieties of Coral Formations.- There
are four varieties of coral formations:
(1.) Fringing Reefs, or narrow ribbons of coral
rock, lying near the shore of an ordinary island.
(2.) Barrier Reefs, which are broader than
Fringing Reefs, and lie at a greater distance
from the shore, but do not extend entirely around
A barrier reef off the coast of New Calqdonia has a
length of 400 miles. One extends along the north-eastern
shore of Australia for over 1000 miles. Barrier reefs are
not continuous, but often have breaks in them through
which vessels can readily pass.
(3.) Encircling Reefs are barrier reefs extend-
ing entirely around the island. As a rule, en-
circling reefs are farther from the shores of the
island than barrier reefs. Tahiti, of the Society
Islands, is an example of an encircling reef.
(4.) Atolls.-This name is given to reefs that
encircle lagoons or bodies of water entirely free
The varieties of reefs just enumerated mark
successive steps or stages in the progress of for-
mation of the coral island.
When a more careful study of the habits of the reef-
forming coral polyp disclosed the fact of its inability to
live in the ocean at greater depths than 100 or 120 feet,
the opinion, which formerly prevailed, of coral islands
rising from profound depths, had to be abandoned. The
idea had its foundation in the fact that a sounding-line,
thrown into the water near the shore of a coral island,
almost invariably showed depths of thousands of feet, and
yet brought up coral rock. In no case, however, did the
rock contain living polyps. An ingenious hypothesis of
Darwin, which appears well sustained by the extensive
observations of Dana and others, explains the great.depth
if coral formations.
101. Darwin's Theory of Coral Islands.-Ac-
cording to this distinguished naturalist, the coral
formation begins near the shore of an island that
is slowly sinking. If the growth of the reef up-
ward equals the sinking of the island, the thick-
ness of the reef is limited only by the time during
which the operation continues.
In Fig. 38 is shown, in plan and section, an island with
elevations A, and B, and river a. The coral island begins
as a fringing reef somewhere off the coast of an ordinary
island at c, c, c, when the conditions are favorable. The
Fig, 38. Growth of a Coral Island.
coral reef must gradually extend around the island, since its
growth toward the ocean is soon limited by the increasing
depth, and toward the shore of the island by the muddy
waters near the surf and the absence of the breakers.
Meanwhile, as the island is sinking, the channel sepa-
rating the reef from the coast increases in breadth. A
barrier reef is thus formed, which at last completely sur-
rounds the island, and becomes an encircling reef. The
higher portions of land, which are still above the waters,
form islands in the central lagoon. Opposite the mouth
of the river a, the growth is prevented by the fresh water,
and a break in the reef is thus produced. These breaks
are sometimes sufficient to permit a ship to enter the
lagoon. At last all traces of the old island disappear, and
its situation is marked by a clear lake, surrounded by a
narrow rim of coral which follows nearly the old coast
A coral island, therefore, is always of an ap-
proximately circular or oval form, and encloses a
clear space in the ocean. Extended systems of coral
formations occurring in any region are a proof of
Relief Forms of the Land.
102. By the Forms of Relief of the Land is
meant the elevation of the land above the mean
level of the sea.
The highest land in the world is. Mount Everi
est, of the Himalayas; it is 29,000 feet high.
The greatest depression is the Dead Sea, in Pales-
tine, which is about 1312 feet below the level of
the ocean. The sum of these is somewhat less
than six miles.
An elevation of six miles is insignificant when
RELIEF FORMS OF THE LAND. 43
compared with the size of the earth. If repre-
sented on an ordinary terrestrial globe, it would
be scarcely discernible, since it would project
above the surface only about the Ty-rth of the
diameter. The highest elevations of the earth are
proportionally much smaller than the wrinkles on
the skin of an orange.
Fig. 39, Relative Height of Mountains.
If, as in Fig. 39, a sphere be drawn to represent the size
of the earth, its radius will be equal to about 4000 miles.
If, now, the line A B be drawn equal to the radius, it
will represent a height of 4000 miles. One-half this
height would be 2000 miles; one-half of this 1000, and
successive halves 500 and 250 miles. An elevation of 250
miles would not therefore be very marked.
Although the irregularities of the surface are
comparatively insignificant, they powerfully affect
the distribution of heat and moisture, and conse-
quently that of animal and vegetable life. An
elevation of about 350 feet reduces the tempera-
ture of the air 10 Fahr.-an effect equal to a
difference of about 70 miles of latitude. High
mountains, therefore, though under the tropics,
may support on their higher slopes a life similar
to that of the temperate and the polar regions.
103. The Relief Forms of the Land are divided
into two classes:
Low Lands and High Lands.
The boundary-line between them is taken at
1000 feet, which is the mean or average elevation
of the land.
Low Lands are divided into plains and hills.
High Lands are divided into plateaus and
If the surface is*comparatively flat or level, it
is called a plain when its elevation above the sea
is less than 1000 feet, and a plateau when its ele-
vation is 1000 feet or over.
If the surface is diversified, the elevations are
called hills when less than 1000 feet high; and
mountains when 1000 feet or over.
I04. Plains and Hills cover about one-half of
the land surface of the earth. In the Eastern
Continent they lie mainly in the north; in the
Western, they occupy the central portions.
Plains generally owe their comparatively level
surface to the absence of wrinkles or folds in the
crust, in which case the general level is preserved,
but the surface rises and falls in long undulations:
these may therefore be called undulating plains,
The flat surface may also be due to the gradual
settling of sedimentary matter. In this case the
plains are exceedingly level. They are called
marine when deposited at the bottom of a sea or
ocean, and alluvial when deposited by the fresh
water of a river or lake. Alluvial plains occur
along the lower course of the river or near its
Marine and alluvial plains, from their mode of forma-
tion, are generally less elevated than undulating plains.
105. Plateaus are generally found associated
with the mountain-ranges of the continents. Their
connection with the adjacent plains is either ab-
rupt, as where the plateau of Anahuac joins the
low plains on the Mexican Gulf; or gradual, as
where the plains of the Mississippi Valley join
the plateaus east of the Rocky Mountains.
106. Mountains. -In a mountain-chain the
crest or sumnm.it of the range separates into a num-
ber of detached portions called peaks; below the
peaks the entire range is united in a solid mass.
The breaks in the ridge, when extensive, form
The influence of inaccessible mountains, like the Pyr-
enees and Himalayas, in preventing the intermingling of
nations living on their opposite sides, is well exemplified
by history. In the past, mountains formed the boundaries
of different races. Some mountains, like the Alps and the
Appalachians, have numerous passes,
A Mountain-System is a name given to several
connected chains or ranges. Mountain-systems
are often thousands of miles in length and hun-
dreds of miles in breadth.
The Axis of a Mountain-system is a line extend-
ing in the general trend of its chains.
Where several mountain-axes intersect one an-
other, a complicated form occurs, called a Moun-
The Pamir Knot, formed by the intersection of the
Karakorum, Belor, and Hindoo-Koosh Mountains, is an
example. It lies on the southern border of the elevated
plateau of Pamir.
Fig, 40, A Mountain-Pass.
107. Orology treats of mountains and their
The force which upheaved the crust into moun-
tain-masses and plateaus had its origin in the
contraction of a cooling globe. There are good
reasons for believing that no extensive mountains
existed during the earlier geological ages, since
the crust was then very thin, and would have
been fractured before sufficient force could accu-
mulate to upheave it into mountain-masses.
The great mountain-systems of the world are
formed from sedimentary deposits that slowly ac-
cumulated over extended areas until they acquired
very great thickness. The deposits forming the
Appalachians, according to Dana, were, in places,
40,000 feet in depth, and covered the eastern bor-
der of the continent from New York to Alabama,
varying from 100 to.200 miles in breadth.
After the accumulation of these strata they
were, through the contraction of the crust, sub-
jected to the gradual effects of lateral pressure,
by which they were sometimes merely flexed or
folded, but more frequently crushed, fractured, or
mashed together, and thus thickened and thrust
upward. That side of the deposit from which
the thrust came would have a steeper slope than
the opposite side, which received a thrust arising
from the resistance.
This theory of mountain-formation, which is
generally accepted, explains the following facts:
(1.) All mountains have two slopes-a short
steep slope, facing the ocean, and a long gentle
slope, facing the interior of the continent.
(2.) The strata on the short steep slope are
generally highly metamorphosed; those on the
long slope are in general only partially metamor
phosed, or wholly unchanged.
(3.) The mountain-systems are situated on the
borders of the continents where the sedimentary
(4.) Slaty cleavage, or the readiness with which
so many of the rocks of mountains cleave or split
in one direction, is a proof of these rocks having
been subjected to intense, long-acting, lateral pres-
sure, since such pressure can be made to develop
slaty cleavage in plastic material.
Isolated Mountains.-Nearly all high isolated moun-
tains were formed, by the ejection of igneous rocks from
the interior; that is, they are of volcanic origin and have
been upheaved by a vertical strain or true projectile force,
as in the volcanic range of Jorullo in Mexico.
108. Valleys in mountainous regions are either
longitudinal or transverse.
Longitudinal Valleys are those that extend in
the dire;ion of the length of the mountains.
Tralsverse Valleys extend across the moun-
tain. 'It is in transverse valleys that most passes
Although valleys, like mountains, owe their origin to
the contraction of a cooling crust, yet their present shapes
are modified by the operation of other forces. By the
action of their water-courses, valleys are deepened in one
place and filled up in another. Extensive land-slides often
alter their configuration. During the Glacial Period many
valleys were greatly changed by the action of huge mov-
ing masses of ice. Fiord-valleys were formed in this
In level countries valleys generally owe their
origin to the eroding power of water.
109. Peculiarities of Continental Reliefs.-
The following peculiarities are noticeable in the
relief forms of the continents:
(1.) The continents have, in general, high bor-
ders and a low interior.
(2.) The highest border lies nearest the deep-
est ocean; hence, the culminating point, or the
highest point of land, lies out of the centre of the
(3.) The greatest prolongation of a continent
is always that of its predominant mountain-sys-
(4.) The prevailing trends of the mountain-
RELIEF FORMS OF THE CONTINENTS.
masses are the same as those of the coast lines, and
are, in general, either north-east or north-west. *
In describing the relief forms of the continents
we shall observe the following order:
(1.) The Predominant System, or a system of
elevations exceeding all others in height, and con-
taining the culminating point of the continent.
(2.) The Secondary System or Systems, inferior
to the preceding in height.
(3.) The Great Low Plains.
Fig. 41. Orographic Chart of North America, (Light portions, mountains; shaded portions, plains.)
1, Rocky Mountain System; 2, System of the Sieira Nevada and Cascade Ranges; 3, Sierra Madre; 4, Great Interior Plateau; 5, Wahsatch
Mountains; 6, Appalachians; 7, Plateau of Labrador; 8, Height of Land; 9, Arctic Plateau; 10, Mackenzie River; 11, Nelson River; 12, St.
Lawrence River; 13, Mississippi River.
Relief Forms of the Continents.
I. NORTH AMERICA
110. Surface Structure.-The Predominant
Mountain-System lies in the west.
The Secondary Systems lie in the east and north.
The Great Low Plains lie in the centre.
111. The Pacific Mountain-System, the pre-
dominant system, extends, in the direction of the
greatest prolongation of the continent, from the
Isthmus of Panama to the Arctic Ocean. It con-
sists of an immense plateau, from 300 to 600
miles in breadth, crossed by two nearly parallel
mountain-systems: the Rocky Mountains on the
east and the system of the Sierra Nevada and
Cascade ranges on the west. The eastern moun-
tain-system is highest near the south; the west-
ern range is highest near the north. Between
these lie numerous parallel ranges enclosing lon-
gitudinal valleys, connected in places by trans-
verse ranges forming basin-shaped valleys.
The Rocky Mountain System.-The Rocky
Mountains rise from the summits of a plateau
whose elevation, in the widest part of the system,
varies from 6000 to 7000 feet above the sea;
therefore, although the highest peaks range from
11,000 to nearly 15,000 feet, their elevation above
the general level of the plateau is comparatively
inconsiderable. The plateau on the east rises by
almost imperceptible slopes from the Mississippi
River. The upper parts of the slopes, near the
base of the mountains, form an elevated plateau
called the "Plains," over which, at one time,
roamed vast herds of buffalo or bison. This ani-
mal is rapidly becoming extinct.
Though the name "Rocky Mountains" is generally con-
fined to those parts of the chain which extend through
British America and the United States, yet, in connection
with the Sierra Nevada Mountains, it is continued through
Mexico by the Sierra Madre Mountains, and by smaller
ranges to the Isthmus of Panama.
46 PHYSICAL GEOGRAPHY.
Fig. 42. On the Plains.
The Rocky Mountain System forms the great
watershed of the continent, the eastern slopes
draining mainly through the Mississippi into the
Atlantic, and the western slopes draining through
the Columbia and the Colorado into the Pacific.
It slopes gradually upward from the Arctic Ocean
toward the Mexican plateau, where it attains its
greatest elevation in the volcanic peak of Popo-
catepetl, 17,720 feet above the sea.
- The System of the Sierra Nevada and Cascade.'
Mountains extends, in general, parallel to the
Rocky Mountain System. It takes the name of
Sierra Nevada in California and Nevada, and of
the Cascade Mountains in the remaining portions
of the continent. It reaches its greatest eleva-
tion in Mount St. Elias, in Alaska, 19,500 feet
above the sea. This is the culminating point of
the North American continent.
In the broadest part of the plateau of the Pacific system,
between the Wahsatch Mountains on the east, and the
Sierra Nevada and Cascade ranges on the west, lies the
plateau of the Great Basin. Its high mountain borders
rob the winds of their moisture, and the rainfall, except
on the mountain-slopes, is inconsiderable. The Great
Basin has a true inland drainage.
The heights of all mountains, except those much fre-
quented, must generally be regarded as but good approxi-
mations, since the methods employed for estimating heights
require great precautions to secure trustworthy results.
Even the culminating points of all the continents have
not, as yet, been accurately ascertained.
112. The Secondary Mountain-Systems of North
America comprise the Appalachian system, the
Plateau of Labrador, the Height of Land, and
the Arctic Plateau. The last three have but an
The Appalachian Mountain System consists of
a number of nearly parallel chains extending
from the St. Lawrence to Alabama and Georgia.
It is high at the northern and southern ends, and
slopes gradually toward the middle. The highest
peaks at either end have an elevation of about
The Appalachian system is broken by two deep depres-
sions, traversed by the Hudson and Mohawk Rivers. Be-
tween the foot of the system and the ocean lies a low coast
plain, whose width varies from 50 to 250 miles.
113. The Great Low Plain of North America
lies between the Atlantic system on the east and
the Pacific system on the west. It stretches from
the Arctic Ocean to the Gulf of Mexico.
Near the middle of the plain the inconsider-
able elevation of the Height of Land divides it
into two gentle slopes, which descend toward the
Arctic Ocean and the Gulf of Mexico. A gen-
tle swell extending from north-west to south-east
divides the northern portion of the plain into
two parts. The eastern and western basins, so
formed, are connected by a break in the water-
shed, through which the Nelson River empties
into Hudson Bay.
SThe southern part of the plain is traversed, in
-its lowest parts, by the Mississippi River.
The tributaries of this river descend the long, gentle
slopes of the Atlantic and Pacific systems.
114. The Relief Forms of a Continent are best
understood by ideal sections, in which the base-
line represents the sea-level, and the scale of
heights on the margin represents the elevation
of the various parts.
In all such sections the vertical dimensions of the land
are necessarily greatly exaggerated.
Fig. 43, Section of North America from East to West.
1, St. Elias; 2, Sierra Nevada; 3, Rocky Mountains; 4, Mississippi
Valley; 5, Appalachian System.
115. Approximate Dimensions of North America,
Area of continent, 8,400,000 square miles.
Greatest breadth from east to west, about 3100 miles.
Greatest length from north to south, about 4500 miles.
Coast line, 22,800 miles.
Culminating point, Mount St. Elias, 19,500 feet.
RELIEF FORMS OF THE CONTINENTS. 47
Fig, 44, Orographic Chart of South America,
(Light portions, mountains; shaded portions, plains.)
1, Systernof the Andes; 2, Plateau of Quito; 3, Plateau of Bolivia;
4, Aconcagua; 5, Plateau of Guiana; 6, Plateau of Brazil; 7, The
Orinoco; 8, The Amazon; 9, The La Platte.
II. SOUTH AMERICA.
116. Surface Structure. The Predominant
Mountain-System of South America is in the west.
The Secondary Systems are in the east.
The Great Low Plain lies between them.
117. The System of the Andes, which extends
along the western border of the continent, is the
.predominant mountain-system. It is composed
mainly of two approximately parallel chains
separated by wide and comparatively level val-
leys. On the north there are three chains, and
on the south but one; in the centre, mainly two.
The chains are connected by transverse ridges,
forming numerous mountain-knots.
The Andes System forms a continuation of
the Pacific Mountain-System. A wide depression
at the Isthmus of Panama marks their separation.
From this point the Andes increase in height
toward the south, probably reaching their high-
est point in Chili, where the volcanic peak of
Aconcagua, 23,910 feet, is believed to be the cul-
minating point of South America, and of the West-
Nevada de Sorata was formerly believed to be the cul-
minating point of South America, but recent recalculations
of the observations have resulted in a loss of nearly 4000
feet of the supposed height of Sorata. Some authorities
still claim that several peaks in Bolivia reach an ele-
vation of nearly 25,000 feet.
The Andes Mountain-System terminates ab-
ruptly in the precipitous elevations of Cape Horn.
Numerous table-lands are included between the parallel
ranges: the most important are-the plateau of Quito, 9543
feet; the plateau of Pasco, in North Peru, 11,000 feet; the
plateau of Bolivia, from 12,000 to 14,000 feet. From most
of these higher plateaus volcanic peaks arise.
118. The Secondary Mountain-Systems of South
America are the plateaus of Brazil and Guiana.
They both lie on the eastern border.
The Plateau of Brazil is a table-land whose
average height is about 2500 feet. Narrow
chains or ridges separate the river-valleys.
The plateau of Brazil forms the watershed between the
tributaries of the Amazon and the La Plata. Along the
Atlantic a moderately continuous range descends in steep
terraces to the ocean. The average altitude is more than
double that of the western portion of the plateau. The
highest peaks are somewhat over 8000 feet.
The Plateau of Guiana, smaller than the Plateau
of Brazil, but about equally elevated, forms the
watershed between the Orinoco and the Amazon.
Pig. 45. Amazon River Scenery,
119. The Great Low Plain of South America
lies between the predominant and the secondary
mountain-systems. It is mainly of alluvial origin,
but slightly elevated, and is much more level than
the great plain of North America.
This plain is drained by the three principal river-sys-
48 PHYSICAL GEOGRAPHY.
teams of the continent, by which it is divided into three
parts: the Llanos of the Orinoco, the Selvas of the Amazon,
and the Pampas of the La Platte.
The Llanos are grassy plains which, during the rainy
season, resemble our prairies, but during the dry weather
The Selvas, or forest plains, are covered by an uninter-
rupted luxuriant forest. The vegetation here is so dense
that in some places the broad rivers form the only ready
means of crossing the country. Near the river-banks are
vast stretches of swampy ground.
The Pampas are grassy plains which in some respects
resemble the Llanos.
A coast plain lies between the Andes and the
Pacific. It is widest near the Andes of Chili,
Fig. 46. Section of South America from East to West,
1, Volcano Arequipa; 2, Lake Titicaca; 3, Nevada de Sorata; 4,
Central Plain; 5, Mountains of Brazil.
where in some places it is 100 miles in breadth.
Between the parallels of 270 and 230 the plain
is an absolute desert, called the Desert of Ata-
cama. Here rain never falls and vegetation is
120. Approximate Dimensions of South America.
Area of continent, about 6,500,000 square miles.
Greatest breadth from east to west, 3230 miles.
Greatest length from north to south, 4800 miles.
Coast line, 14,500 miles.
Culminating point, Aconcagua, 23,910 feet.
121. Contrasts of the Americas.-In both North
and South America the predominant system lies in
the west, the secondary systems in the east, and the
low plains in the centre.
They differ in the following respects:
In North America the predominant system is a
broad plateau, having high mountain-ranges; the
principal secondary system is narrow, and formed
of parallel ranges; the low plains are character-
ized by undulations, and contain several deep de-
pressions occupied by extensive lake-systems.
In South America the predominant system is nar-
row; the secondary systems are broad; the low plain
is alluvial, extremely flat, contains no depressions,
and consequently no extensive lake-systems.
Fig. 47, Orographia Chart of Europe, (Light portions, mountains; shaded portions, plains.)
1, The Alps; 2, Mont Blanc; 3, Pyrenees; 4, Cantabrian; 5, Sierra Estrella; 6, Sierra Nevada; 7, Mountains of Castile; 8, Apennines,
9, Dinaric Alps; 10, Balkan; 11, Pindus; 12, Tautrs;. 13, Caucasus; 14, Cevennes; 15, Plateau of Auvergne; 16, Vosges; 17, Black Forest; 18,
Jura; 19, Hartz; 20, Bohemian Plateau; 21, Carpathians; 22, Hungarian Forest; 23, Transylvanian Mountains; 24, Kiolen Mounfains; 25, Urals.
III. EUROPE. The Great Low Plain lies between the Pre-
122. Surface Structure.- The Predominant dominant and Secondary Systems.
Mountain-System is in the south.
Mountain-System is in the south. A line drawn from the Sea of Azov to the mouth of the
The Secondary Systems are in theinorth and east. Rhine River divides Europe into two distinct physical
, -" *TI"
RELIEF FORMS OF THE CONTINENTS. 49
regions. The great low plain lies on the north, and the
predominant mountain-system on the south. The coun-
try north of this line is sometimes called Low Europe, and
that south of it, High Europe.
123. The Predominant Mountain-System of Eu-
rope is composed of a highly complex series of
mountain-systems extending along the northern
shores of the Mediterranean in a curve, from the
Straits of Gibraltar to the shores of Asia Minor.
The system is highest in the centre, where the
Alps form the culminating point of the continent.
SThe average elevation of the Alps ranges from
10,000 to 12,000 feet. The highest peak, Mont
Blanc, 15,787 feet, is the culminating point of the
European continent. Matterhorn and Monte Rosa
are but little inferior in height. On the south-
west the system is continued to the Atlantic by
the Cevennes and adjoining ranges in France, and
the Pyrenees and Cantabrian in the northern part
of the Spanish peninsula. The Pyrenees are an
elevated range, with peaks over 11,000 feet high.
On the east the system extends in two curves to
the Black Sea by the Carpathian and Transylva-
nian Mountains on the north, and the Dinaric
Alps and the Balkan Mountains on the south.
124. Divisions of Predominant System.-The
predominant mountain-system of Europe may be
conveniently regarded as consisting of a central
body or axis, the Alps, with six projections or
limbs-three on the north, and three on the south.
The three divisions on the north include-
The Western Division, or the mountains of
France, including the mountains lying west of
the valleys of the Rhine and the Rhone;
The Central Division, or the mountains of Ger-
many, situated between the Western Division and
the upper valleys of the Oder and the Danube;
The Eastern Division, or the mountains of
Austria-Hungary, situated between the Central
Division and the Black Sea.
These divisions contain a highly complicated system of
minor elevations. Their complexity is due to the fre-
quent intersection of the north-eastern and north-western
trends. Basin-shaipd plateaus, like the Bohemian and
Transylvanian, are thus formed.
The Western Division includes most of the mountains
of France, as the Cevennes, the mountains of Auvergne,
and the Vosges Mountains.
The Central Division includes the Jura Mountains in
Switzerland, the Swiss and the Bavarian plateaus, the
Black Forest Mountains, the Hartz Mountains, 'and the
The Eastern Division includes most of the mountains
of Austria, as the Carpathians, the Hungarian Forest, and
the Transylvanian Mountains.
125. The three projections on the south are the
three mountainous peninsulas of Southern Eu-
The Iberian Peninsula, including Spain and
The Italian Peninsula;
The Turco-Grecian Peninsula.
The Iberian Peninsulg--The principal mountains are
the Sierra Estrella, the mountains of Castile, and the
Sierra Nevada. The Pyrenees separate the Peninsula from
France. The Cantabrian Mountains extend along the
The Italian Peninsula contains the Apennines, ex-
tending mainly in the direction of the north-western
The Turco-Grecian Peninsula.-The Dinaric Alps
extend along the coast of the Adriatic; the Balkan Moun-
tains extend from east to west, through Turkey; and the
Pindus from north to south, through Turkey and Greece.
126. The Secondary Mountain-Systems of Eu-
rope comprise the system of the Scandinavian
peninsula, the Ural Mountains, and the Cauca-
The System of the Scandinavian Peninsula
includes the elevations of Norway and Sweden.
With the exception of the Kiolen Mountains in
the north, the system does not embrace distinct
mountain-ridges, but consists mainly of a series
Fig, 48. Fiord on Norway Coast,
of broad plateaus that descend abruptly on the
west in numerous deeply-cut valleys called fiords,
through which the sea penetrates nearly to the
heart of the plateaus. Fiords are valleys that
were deeply eroded by slowly moving masses of
50 PHYSICAL GEOGRAPHY.
ice, called glaciers, and subsequently partially sub-
merged. On the east the slopes are more gradual,
and are occupied by numerous small lakes.
The System of the Urals is composed of a
moderately elevated range extending from the
Arctic Ocean on the north to the plains of the
Caspian on the south. The elevated island of
Nova Zembla may be considered as forming a
part of its northern prolongation.
The Caucasus Mountains bear peaks exceeding
in elevation those of the Alps. They belong,
however, more properly to the elevations of
127. The Great Low Plain of Europe lies be-
tween the predominant and secondary mountain-
systems, and stretches north-eastwardly from the
Atlantic to the Arctic. It is remarkably level,
and is highest in the middle, where the Valdai
Hills form the principal watershed of Europe.
Westward the plain is continued under the North
Sea to the British Isles, where a few inconsider-
able elevations occur.
South of the Alps the large plain of the Po
River stretches across the northern part of Italy.
128. Approximate Dimensions of Europe.
Area of continent, 3,700,000 square miles.
Coast line, 19,500 miles.
Greatest breadth from north to south, 2400 miles.
Greatest length from north-east to south-west, 3370
Culminating point, Mont Blanc, 15,787 feet.
Fig. 49. Orographio Chart of Asia, (Light portions, mountains; shaded portions, plains.)
1, Himalaya Mountains; 2, Karakorum; 3, Kuen-lun; 4, Belor; 5, Thian Shan; 6, Altai; 7, Great Kinghan; 8, Yablonol; 9, Nanhng;
10, Peling; 11, Vindhya; 12, Ghauts; 13, Hindoo-Koosh; 14, Elburz; 15, Suliman; 16, Zagros; 17, Taurus; 18, Caucasus; 19, Asiatic Island Chain.
129. Surface Structure.- The Predominant
Mountain-System is in the south.
The Secondary Systems surround the Predomi-
The Great Low Plain is on the north and west,
and lies between the mountain-systems of Asia
and the secondary system of the Urals.
Europe and Asia are sometimes considered as geographic-
ally united in one grand division called Eurasia.
130. The mountain-systems of Asia are nearly
all connected in one huge mass which extends in
RELIEF FORMS OF THE CONTINENTS. 51
the line of the north-east trend, from the Arctic to
the Indian Ocean. Though in reality one vast
system, yet they are most conveniently arranged
in one predominant and several secondary systems.
The Predominant System is the plateau of
Thibet, the loftiest table-land in the world. It
is between 15,000 and 16,000 feet high, and is
crossed by three huge, nearly parallel, mountain-
ranges: the Himalayas on the south, the Kuen-
lun on the north, and the Karakorum between
them. The Himalayas, the loftiest mountains
Fig. 60. Himalaya Mountains.
in the world, rise abruptly from the plains of
Northern Hindostaa. Like the Alps, their axis
is curved, but in the opposite direction. The
breadth of the system varies from 100 to 200
miles; the length is about 1500 miles. The high-
est point is Mount Everest, 29,000 feet above the
sea; it is the culminating point of the Asiatic con-
tinent and of the world. Kunchinjunga and Dha-
walaghiri are scarcely inferior in height.
131. The Secondary Systems lie on all sides of
the predominant system, though mainly on the
north and east of the predominant system. Like
Europe, the Asiatic continent projects on the
south in the three mountainous peninsulas of
Arabia, Hindostan, and Indo-China.
On the north and east of the plateau of Thibet
is an extended region called the plateau of Gobi,
considerably lower than the surrounding country.
The Kuen-lun and Great Kinghan Mountains
bound- it on the south and east, and the Altai
Mountains on the north. On the west lie the
Thian Shan and Altai, which by their open val-
leys afford ready communication with the low
plains on the west.
The plateau of Gobi varies in average height from 2000
to 4000 feet. The greatest depression is in the west, and
is occupied by Lake Lop and the Tarim River. A small
part of the region near the mountain-slopes is moderately
fertile, the remainder is mainly desert.
The Altai Mountains are but little known, but some of
their peaks exceed 12,000 feet. They are continued east-
ward by the Yablonoi Mountains. East of the plateau of
Gobi a range extends north-easterly through Mantchooria.
On the south and west of Thibet lie the pla-
teaus of Iran, Armenia, and Asia Minor.
The Plateau of Iran includes Persia, Afghan-
istan, and Beloochistan. It is a basin-shaped
region from 3000 to 5000 feet high. The Elburz
and Hindoo-Koosh Mountains form its borders on
the north, the Suliman on the east, and the Za-
gros on the south and west.
The Suliman Mountains rise abruptly from the plains
of the Indus. Across these mountains occurs the only
practicable inland route between Western Asia and the
The Plateaus of Armenia and of Asia Minor
lie west of the Plateau of Iran. Armenia is 8000
feet high, and bears elevated mountains: Mount
Ararat, 16,900 feet, is an example. On the west,
the peninsula of Asia Minor, or Anatolia, extends
between the Black and Mediterranean Seas, and
is traversed by the Taurus Mountains.
The CaucaSus Mountains lie north of the pla-
teau of Armenia. They are an elevated range
extending between the Black and Caspian Seas,
and form part of the boundary-line between Eu-
rope and Asia. Mount Elburz, the Watch-
Tower," the culminating peak, is 18,493 feet
The Arabian Plateau occupies the entire penin-
sula of Arabia. It is separated from the plateau
of Iran by the Persian Gulf and the valleys of
the Tigris and the Euphrates.
The Plateau of Deccan occupies the lower part
of the peninsula of Hindostan. It is crossed on
the north by the Vindhya Mountains, and along
the coasts by the Eastern and Western Ghauts.
The Peninsula of Indo-China is traversed by
a number of mountain-ranges which diverge from
the eastern extremity of the Himalayas. The
Nanling and Peling extend from east to west
- 132. The Great Low Plain is, in reality, but a
continuation of the European plain. It extends
from the Arctic Ocean south-westerly to the Cas-
pian and Black Seas. It is hilly on the east, but
level on the west. South of the 60th parallel it
is comparatively fertile. Around the shores of
the Arctic are the gloomy Tundras.
The Tundras are vast regions which in summer are
covered with occasional moss-beds, huge shallow lakes,
and almost interminable swamps, and in winter with thick
ice. The tundras are caused as follows: The rivers that
flow over the immense plain of Asia rise in the warmer
regions on the south. Their upper courses thawing while
the lower courses are still ice-bound, permits large quan-
tities of drift ice to accumulate at their mouths, which,
damming up the water, causes it to overflow the adjoining
Depressions of the Caspian and Sea of Aral.-
Two remarkable depressions occur in the basins
of the Caspian and Sea of Aral, and that of the
Dead Sea. These are all considerably below the
level of the ocean. The waters of the Caspian
and Sea of Aral were probably once connected
in a great inland sea.
The Smaller Asiatic Plains are drained by
several river-systems. These are the Plain of
Mantchooria, drained by the Amoor; the Plain
of China, drained by the Hoang-Ho and the
Yang-tse-Kiang; the Plain of India, drained by
the Indus, the Ganges, the Brahmapootra, and
the Irrawaddy; and the Plain of Persia, drained
by the Tigris and the Euphrates.
133. Approximate Dimensions of Asia.
Area of continent, 17,500,000 miles.
Coast line, 35,000 miles.
Greatest length from north-east to south-west, 7500 miles.
Greatest breadth from north to south, 5166 miles.
Culminating point, Mount Everest, 29,000 feet.
134. Comparison of the Relief Forms of Eu-
rope and Asia.-In both Europe and Asia the
chief elevations are in the south and the great low
plains in the north. Asia, like Europe, extends
toward the south in three great peninsulas: Ara-
bia, Hindostan, and Indo-China.
Fig. 51, Section of Asia from North to South,
1, Cape Comorin; 2, Deccan; 3, Plain of India; 4, Himalayas; 5, Everest; 6, Kuen-lun; 7, Karakorum; 8, Thibet; 0, Upper Tartary; 10,
Ararat; 11, Elburz; 12, Thian Shan; 13, Altai; 14, Mountains of Kamtchatka; 15, Arctic Ocean, mouth of Yenesei
S1357 Surface Structure.-Nearly the entire con-
tinent of Africa is a moderately elevated plateau.
? It therefore has no great low plains; but the in-
-terior is lower than the marginal mountain-sys-
tems, and in this respect the true continental type,
S high borders and a low interior, is preserved.
136. The Predominant Mountain-System is in
The Secondary Systems are in the south, west,
a The great interior depression is in the middle,
4' and is surrounded by the predominant and sec-
S ondary systems.
\ A narrow, low plain extends along most of the
coast. It is broadest on the north-west, between
Sthe plateau of the Sahara and the Atlas Moun-
Fig, 52, Orographic Ohart of Africa, 137. The Predominant Mountain-System ex-
(Light portions represent mountains; shaded portions, plains.) tends along the entire eastern shore, from the
1, Abyssinian Plateau; 2, 3, Kenia and Kilimandjaro; 4, Lupata; Mediterranean Sea to the southern extremity of
5.Dragon; 6, Nieuveldt; 7, Mocambe; 8, Crystal; 9, Cameroons; 10,n t cetre i
Kong; 11, Atlas; 12, Lake Tchad; 13, Madagascar. the continent. It is highest near the centre, in
RELIEF FORMS OF THE CONTINENTS.
the plateaus of Abyssinia and Kaffa. The culmi-
nating point is probably to be found in the vol-
canic peaks of Kenia and Kilimandjaro, whose
estimated heights are taken at about 19,000 feet.
In the Abyssinian plateau, on the north, an aver-
age elevation of from 6000 to 8000 feet occurs.
Upon this, rising in detached groups, are peaks
the highest of which are over 15,000 feet.
From the Abyssinian plateau the system is con-
tinued northward to the Mediterranean by a suc-
cession of mountains which stretch along the
western shores of the Red Sea. Some of the
peaks are from 6000 to 9000 feet. South of the
plateau of Kaffa the system is continued by the
Lupata and Dragon Mountains to the southern
extremity of the continent. The Zambesi and
Limpopo Rivers discharge their waters into the
Indian Ocean through deep breaks in the system.
138, Secondary Systems.--On the south the
Nieuveldt and Snow Mountains stretch from east
to west, with peaks of over 10,000 feet. Table
Mountain is on the south.
Fig. 53, Table Mountain,
On the west the Mocambe and Crystal Mountains
extend from the extreme south to the Gulf of
Guinea. Near the northern end of this range,
but separate from it, are the volcanic peaks of
the Cameroons Mountains, 13,000 feet high.
The Kong Mountains extend along the north-
ern shores of the Gulf of Guinea in a general
east-and-west direction. Some of the peaks are
snow-capped. In the extreme north of Africa
are the Atlas Mountains, which rise from the
summit of a moderately elevated plateau. Some
of the peaks are 13,000 feet high.
139. The Great Interior Depression north of
the equator is divided into two distinct regions.
A straight line extending from Cape Guardafui
to the northern shores of the Gulf of Guinea
marks the boundary. The mountain-systems
north of this line have a general east-and-west
direction; those south of it have a general north-
The Plateau of the Sahara occupies the north-
ern part of the interior depression. Its general
elevation is about 1500 feet, though here and
there plateaus of from 4000 to 5000 feet occur,
and even short mountain-ranges with peaks of
6000 feet. The main portion of the region is cov-
ered with vast sand-fields, with occasional rocky
masses, and is one of the most absolute deserts
in the world.
-'+ -. ." .'',
Fig. 54, Desert of Sahara,
Near long. 140 E. from Greenwich, in the district of
Fezzan, the plateau is divided from north to south by a
broad valley. In this occur many remarkable depressions,
some of which are several hundred feet below the level of
the Mediterranean. Here fertile spots, called oases, are
South of the Sahara is the Soudan, a remark-
ably well-watered and fertile region. Lake Tchad
occupies the greatest depression. The interior,
which lies south of this, is but little known. It
is probably a moderately elevated plateau. Ex-
tensive lake-basins-Albert and Victoria Nyan-
zas and Tanganyika-lie near the predominant
140. Approximate Dimensions of Africa.
Area of continent, 12,000,000 square miles.
Coast line, 16,000 miles.
Greatest breadth from east to west, 4800 miles.
Greatest length from north to south, 5000 miles.
Culminating point, Mount Kenia, or Kilimandjaro,
about 19,000 feet.
54 PHYSICAL GEOGRAPHY.
Fig. 55. Orographic Chart of Australia,
(White portions, mountains; shaded portions, plains.)
1, Australian Alps; 2, Kosciusko; 3, 4, 5, Secondary Systems; 6,
141. Surface Structure, -The Predominant
Mountain-System is in the east.
The Secondary Systems are in the west and
The Great Low Plain lies between the pre-
dominant and secondary systems, and slopes
gently to the southern coast.
The Predominant System extends along the
entire eastern shore, from Torres Straits to the
southern extremity of Tasmania. It is for the
most part composed of broad plateaus. The
system is highest in the south-east, where the
name Australian Alps is given to the range.
Mount Kosciusko, 7000 feet, probably forms the
culminating point of the Australian continent.
The system descends abruptly on the east, but
on the west it descends by gentle slopes to the
low plains of the interior.
142. The Secondary Systems, on the west and
north-west, are of but moderate elevation.
143. The Great Low Plain lies in the interior. Ac-
curate information as to its peculiarities is yet wanting.
A moderate elevation on the north connects the eastern
and western systems. The south-eastern portion, which
is the best known, is well watered and remarkably fertile.
Basin-shaped valleys are found in the west. The lower
parts are occupied by Lake Eyre, Torrens, and Gairdner.
144. Approximate Dimensions of Australia.
Area of continent, 3,900,000 square miles.
Coast line, 10,000 miles.
Greatest length from east to west, 2400 miles.
Greatest breadth from north to south, 2000 miles.
Culminating point, Mount Kosciusko, 7000 feet.
145. Contrasts of Africa and Australia.-In
the north, the African continent resembles .Europe
and Asia in the arrangement of its forms of
relief. In the south, it resembles the Americas.
As a whole, the African continent resembles
Australia more closely than any other. In both
Fig. 56. Australian Scenery.
Africa and Australia the predominant system is
in the east, and extends along the entire coast.
In each the secondary systems are in the west
and north. But Africa terminates in a plateau
which descends abruptly to the sea, while Australia
is terminated by a great low plain which descends
by long, gentle slopes from the interior.
Rock-masses are divided, according to their origin, into
igneous, aqueous, and metamorphic. According to their con-
dition, into stratified and unstratified. According to the
presence or absence of organic remains, into fossiliferous
and non-fossiliferous. Stratified rocks are sometimes called
fragmental. Unstratified rocks are sometimes called crys-
talline. Aqueous rocks are sometimes called sedimentary.
Aqueous rocks are stratified. Igneous rocks are un-
stratified. Metamorphic rocks were originally stratified,
but lost their stratification through metamorphism.
REVIEW QUESTIONS. 55
Aqueous rocks may contain fossils. Igneous rocks never
contain fossils. Metamorphic rocks, in rare instances, may
contain fragments of fossils.
Geological time is divided into Archlan, Palmozoic, Meso-
zoic, and Cenozoic.
Archman Time includes the Azoic and the Eozoic Ages.
Paleozoic Time, or, as it .is sometimes called, the Pri-
mary, includes the Silurian, Devonian, and Carboniferous
Mesozoic Time, or the Secondary, includes the Age of
Cenozoic Time includes the Age of Mammals, or the Ter-
tiary, and the Era of Man, or the Quaternary Age.
The changes to which the earth's crust is now subject
are produced by the following agencies:
1. By the winds; 2. By the moisture of the atmosphere;
3. By the action of running water; 4. By the action of
ocean waves; 5. By the agency of man; 6. By the con-
traction of a cooling crust.
'- There is more water than land surface on the earth, in
proportion of 25 : 9, or as 5': 3'.
The land-masses surround the north pole in the shape
of an irregular ring.
Nearly all the land-areas are collected in one hemi-
sphere, and the water-areas in another.
The Land Hemisphere comprises the whole of North
America, Europe, and Africa, all of Asia except a small
part of the Malay Peninsula, and the greater part of South
The Water Hemisphere comprises_ ole of Australia
and the southern portions of South America and the Ma-
The northern continents are almost entirely in the tem-
perate latitudes; the southern are mainly in the tropics.
The land-masses may be divided into three doublets,
consisting of pairs of northern and southern continents,
almost or entirely separated from each other.
There are two great systems of trends or lines of direc-
tion, along which the continents, the coast lines, the
mountain-ranges, the oceanic basins, and the island chains
are arranged. These trends are north-east and north-west.
The northern continents are characterized by deeply in-
dented coast lines; the southern are comparatively simple
and unbroken. Europe is the most, and Africa the least,
deeply indented of the continents.
In proportion to her area, Europe has three times as
much coast line as Asia, and four times as much as Africa.
One-seventeenth of the land-area is composed of islands.
Islands are either continental or oceanic.
There are four successive stages in the formation of a
coral island or atoll: 1. The fringing reef; 2. The barrier
reef; 3. The encircling reef; 4. The coral island or atoll.
The greatest elevations and depressions in the earth's
surface are small when compared with its size.
Low lands are either plains or hills.
High lands are either plateaus or mountains.
Plains are-1. Undulating; 2. Marine; 3. Alluvial.
Mountains were produced by the contraction of the
crust, producing a lateral pressure on thick, extended de-
posits of sedimentary rocks. Slaty cleavage was caused
by this lateral pressure.
Valleys are either longitudinal or transverse.
All continents have high borders and a low interior.
The highest border faces the deepest ocean.
The greatest prolongation of a continent is that of its
predominant mountain-system. The culminating point is
always out of the centre.
North and South America resemble each other in the
arrangement of their relief forms. Their predominant
systems are in -the west; their secondary systems are in
the east; their great low plains are between the predomi-
nant and secondary systems.
The predominant system of North America is the Pa-
cific mountain-system. The secondary systems are-the
Appalachian system, the plateau of Labrador, the Height
of Land, and the Arctic plateau.
The predominant system of South America is the sys-
tem of the Andes. The secondary systems are-the pla-
teaus of Guiana and Brazil. The great low plains are-
the Llanos of the Orinoco, the Selvas of the Amazon, and
the Pampas of the La Plata.
Europe and Asia resemble each other. .Their predomi-
nant systems are in the south; their great low plains are
north of their predominant systems. The predominant
system of Europe is in the south.
The secondary systems are-the mountains of the Scan-'
dinavian Peninsula, the Ural Mountains, and the Caucasus
The predominant mountain-system of Asia is the pla.
teau of Thibet.
The secondary systems are-the plateau of Gobi, the
Thian-Shan and Altai Mountains, the plateau of Indo-
China, the plateau of Deccan, the plateau of Iran, the pla-
teau of Asia Minor, and the plateau of Arabia.
Africa and Australia resemble each other. Their pre-
dominant systems are in the east; their secondary systems
are in the west and north; their depressed areas are be-
tween the two.
The predominant mountain-system of Africa includes
the mountains of the eastern coast.
The secondary systems include the Nieuveldt and Snow
Mountains in the south, the Mocambe, Crystal, Cameroons,
and Kong Mountains in the west, and the Atlas Mountains
in the north.
The predominant mountain-system of Australia includes
the mountains of the eastern coast.
The secondary systems include those found in the south,
west, and north.
What two elementary substances form the greater part
by weight of the earth's crust?
Into what classes may rocks be divided according to
their condition? According to their origin? According
to the presence or absence of fossils?
What is paleontology?
Define Archaean Time, Palmozoic Time, Mesozoic Time,
and Cenozoic Time.
Explain the nature of the changes, which the atmo-
sphere is now effecting in the earths surface. Which the
water is effecting. Which man is effecting.
What must be the areas of two squares whose areas
56 PHYSICAL GEOGRAPHY.
represent the relative land- and water-areas of the earth?
What are the actual areas in square miles?
How would you draw a circle around the earth which
will divide it into land and water hemispheres?
Do the continents extend farther to the north pole or to
the south pole ?
What do you understand by lines of trend?
Which have the more diversified coast lines, the north-
ern or the southern continents?
Define continental and oceanic islands, and give exam-
ples of each. Why are continental islands to be regarded
as detached portions of the neighboring mainland?
Name the American island chains. The Asiatic chains.
'D)escribe the Australasian island chain. The Polynesian
Which are the higher, volcanic islands or coral islands?
Name the four principal steps or stages in the progress
of formation of a coral island.
Is the coral island built by the coral animalcule or by
the waves? Explain your answer.
What is Darwin's theory for the presence of a lagoon
within the reef?
What is the difference between a plain and a plateau?
A "mountain and a hill?
Define mountain-system. A chain. A knot.
What is the name of the highest plateau in the world?
Of the largest plain ?
In what different ways were plains formed?
Distinguish between a longitudinal and a transverse
valley. Explain the manner in which mountains were
-Give a short account of the surface structure, or the
arrangement of the high and low lands, of North America..
Of South America. Of Europe. Of Asia. Of Africa, and
of Australia. Which of these resemble each other? In
what respect do they all resemble one another?
Name the culminating points of each of the continents.
Name the predominant and secondary mpuntain-systems
of each of the continents.. "/
How many times larger is Asia than ustralia? Than
Europe? Africa; North America? South America?
North America. r
Name the principal mountains of the Pacific mountain-
system. Which contains the culminating point of the
Where is the Great Basin ? By what mountains is it
Name the principal mountains of the Appalachian sys-
Is the greater portion of the area of North America
above or below 1000 feet?
What rivers drain the great low plain of North Amer-
Name the principal plateaus of the Andes. Through
which does the equator pass? Which contains Lake Titi-
SWhere is the plateau of Guiana? Of Brazil?
What three large river-systems drain the great low plain
of South America? What resemblances can you find be-
tween the directions of these rivers and those which drain
Describe the chain of the Alps.
What river-systems divide its northern slope into three
divisions? Name the principal mountains of each division.
What three peninsulas project southward from the south-
ern slopes of the predominant mountain-system?
Name the principal mountains of each peninsula.
Name the great low plains of Europe.
What mountains form the northern boundary of the
plateau of Thibet? The southern boundary? The north-
ern boundary of the plateau of Mongolia? The eastern
boundary? What mountains extend through China?
What mountains form the boundaries of the plateau of
Iran? Is Arabia a plateau or a plain ?
Is the land north of the Sea of Aral high or low?
In which line of trend do the mountainous elevations
of Asia extend?
What portions of Africa are high? What portions are
Where is the predominant system? Where is the cul-
minating point? What part of the interior is low?
Where are the Mocambe Mountains? The Crystal Moun-
tains, the Cameroons, the Atlas, the Kong, the Lupata, and
Where is the predominant mountain-system? The sec-
Where is Mount Kosciusko? The Murray River?
BY contact of air with the water-areas, an immense quantity of invisible vapor passes into the
atmosphere, from which, when sufficiently cooled, it re-appears and descends as fog, dew, rain, hail, sleet,
or snow. It then, in greater part, drains through various lake- and river-systems into the ocean, where
it is either again evaporated, or carried about in waves, tides, or currents. This circulation of water
never ceases, and upon it depends the existence of all life on the earth.
Physical Properties of Water.
146. Composition.-Water is formed by the
combination- of oxygen and hydrogen, in the pro-
portion, by weight, of eight parts of oxygen to
one part of hydrogen; or, by volume, of one part
of oxygen to two parts of hydrogen.
147. Properties. -Pure water is a colorless,
transparent, tasteless, and inodorous liquid. It
freezes at 320 Fahr., and, under the ordinary
pressure of the atmosphere, boils at 2120 Fahr.
Water exists in three states: solid, liquid, and gaseous.
Under ordinary circumstances it freezes at 320. It evapo-
rates, or passes off from the surface as vapor, at all tempera-
tures, even at 320; but it is only at the boiling-point that
the vapor escapes from the mass of the liquid as well as
from the surface.
Heated in open vessels, under the ordinary pressure of
the atmosphere, its temperature cannot be raised higher than
2120, any increase of heat only causing it to boil more rap-
idly. Heated in closed vessels, which prevent the escape
58 PHYSICAL GEOGRAPHY.
of steam, its temperature can be raised very high. In
such cases great pressure is exerted on the walls of the
vessel. Conversely, on high mountains, where the pres-
sure of the atmosphere is lower than at the level of the
sea, water boils at temperatures lower than 2120 Fahr.
148. Maximum Density of Water.-A pint of
cold water is heavier than a pint of warm water,
because as water is cooled it contracts and grows
denser. The coldest pint of water, however, is
not the heaviest. -The heaviest pint of water is
water at the temperature of 39.20 Fahr. This
temperature is therefore called the temperature
of the maximum density of water. If water at
this temperature be heated, it becomes lighter, or
expands; if water at this temperature be cooled,
it also becomes lighter or expands until ice is
formed, which floats on the water. When at the
temperature of its maximum density, water is
7.20 warmer than the freezing-point.
149. Effect of the Maximum Density of Water
on its Freezing.-If water continued to contract
indefinitely while cooling until freezing began,
the ice first formed would sink to the bottom, and,
this process continuing, the entire mass would soon
become solid. In this manner all bodies of fresh
water, in times of great cold, might freeze through-
out; when, not even the heat of a tropical sun
could entirely melt them.
But for this curious exception in the physical properties
of water, at least three-fourths of the globe would be in-
capable of sustaining its present life.
The entire floor of the ocean, both in the tropics and in
the temperate and the polar regions, is covered with a layer
of cold, salt water at nearly the temperature of its maxi-
mum density. In the tropics the surface-water is warmer
and lighter than this dense layer, and in the polar re-
gions it is colder and lighter.
150. Specific Heat of Water.-Another re-
markable property of water-its specific heat-
enables it to play an important part in the
economy of the world.
The specific heat of a body is the quantity of
heat-energy required to produce a definite in-
crease of temperature in a given weight of that
Water has a very great specific heat; that is,
a given quantity of water requires more heat-energy
to warm it, and gives out more heat-energy on cool-
ing, than an equal quantity of any other common
The quantity of heat required to raise a pound of ice-
cold water to 2120, would heat a pound of ice-cold iron to a
bright red heat, or to about 16000 Fahr.; or, conversely, a
pound of boiling water cooling to the freezing-point, would
give out as much heat as a pound of red-hot iron cooling
to 32 Fahr.
The enormous capacity of water for heat is of
great value to the life of the earth. The oceanic
waters are vast reservoirs of heat, storing heat in
summer and giving it out in winter. The great
specific heat of water prevents it from either heat-'
ing or cooling rapidly. Large bodies of water,
therefore, prevent great extremes of heat and
151. Heat Absorbed or Emitted during Change
of State.-During the conversion of a solid into
a liquid, or a liquid into a vapor, a large quantity
of heat-energy is absorbed. This heat-energy does
not increase the temperature of the body, and
therefore cannot be detected by the thermometer.
The heat-energy is then in the condition of stored
or potential energy, sometimes called latent heat.
When the vapor condenses into a liquid, or the
liquid freezes, the stored heat-energy again becomes
sensible as heat.
In freezing, water gives out heat and raises the
mean temperature of the atmosphere.
In melting, ice takes in heat and lowers the mean
temperature of the atmosphere.
Water has a higher latent heat than any other
Stored Heat-Energy of Ice-Cold Water.-In
order to heat a pound of water 10 Fahr. an
amount of heat called a heat-unit, or a pound
degree is required. Before one pound of ice at
320 Fahr. can melt and form one pound of water
at 320 Fahr., it must take in 142 heat units; and
yet a thermometer plunged in the water from
melting ice will indicate the same temperature as
when entirely surrounded by lumps of the un-
The great latent heat of ice-cold water has an important
influence on the freezing of large bodies of water, since,
after the surface-layers have reached the temperature of
the freezing-point, they have still 142 heat-units to lose be-
fore they can solidify. Again, when ice reaches a tempera-
ture of 320 Fahr., it has still 142 heat-units to absorb before
it can melt. Were it not for this fact destructive floods
would often result from the rapid melting of the winter's
accumulation of snow and ice.
Stored Heat-Energy of Water- Vapor.-Before
one pound of water can pass off as vapor, it
must take in sufficient heat to raise nearly 1000
pounds of water 10 Fahr. The vapor which then
escapes is still at the same temperature as the
water from which it came. The 1000 heat-units,
or pound-degrees of heat, have been rendered latent,
and have no influence on the thermometer.
When the vapor in the air is condensed as rain,
snow, hail, fog, or cloud, the stored heat-energy
again becomes sensible. Much of the vapor
which is formed in the equatorial regions is car-
ried by the winds to high northern latitudes,
where, on condensing, it gives out its heat and
moderates the intense cold which would otherwise
152. Solvent Powers.-Water is one of the best
solvents of all common substances. During the
constant washings to which the continents are
subjected by the rains, their surfaces are cleansed
from decaying animal and vegetable matters,
which are partly dissolved and carried by the
rivers into the ocean. The atmospheric waters
in the same way cleanse the air of many of its
153. Water is the Main Food of Animals and
Plants.-By far the greater part of the bodies of
animals and plants is composed of water. With-
out large quantities of water no vigorous life can
be sustained in any locality.
Deserts are caused entirely by the absence of
154. Drainage.-The atmospheric waters, or
those which fall from the atmosphere as rain,
hail, or snow, either sink through the porous
strata and are drained under ground, or run
directly off the surface. Thus result two kinds
of drainage-Subterranean and Surface.
155. Subterranean Drainage.-The water which
sinks through the porous strata continues descend-
ing until it meets impervious layers, when it either
runs along their surface, bursting out as springs
at some lower level, where the layers outcrop, or
it collects in subterranean reservoirs. The origin
of all springs is to be traced to subterranean
Underground streams sometimes attain considerable size.
In portions of the Swiss Jura streams burst from the sides
of hills in sufficient volume to turn the wheels of moder-
ately large mills. In a few instances the subterranean
stream can be navigated for considerable distances, as in
the Mammoth Cave of Kentucky, or in the Grotto of
Adelsberg, near Trieste.
156. Surface Drainage--The water which is
drained directly from the surface, either runs
down the slopes in rivulets and rills, which,
uniting with larger streams, are poured directly
into the ocean, or it collects in the depressions of
basin-shaped valleys, where, having no connection
with the ocean, it can be discharged by evapora-
tion only. Thus arise two kinds of surface drain-
age-oceanic and inland.
157. Springs are the outpourings of subterra-
nean waters. The waters, having soaked through
the porous strata, again emerge at the surface,
(1.) By running along an inclined, impervious
layer of clay, hard rock, or other material until
Fig. 57. Origin of Springs.
they emerge at some lower level, where the strata
(2.) By being forced upward out of the reser-
voirs into which they have collected by the pres-
sure of compressed gas, highly heated steam, or,
more commonly, by the pressure of a communi-
cating column of water.
It is in the first way that most of the springs of moun-
tainous districts discharge their waters. The tilted and
broken condition of the strata is such as to favor the es-
cape along some of the many layers that crop out on the
mountain-slopes. The springs of plains, which are at some
distance from mountains, discharge their waters mainly by
the methods mentioned under the second heading.
When a well is dug in most porous soils, the water from
the porous strata on the sides runs in and partially fills
158. Classification of Springs,-Springs are
most conveniently arranged in different classes
according to peculiarities in the size, shape, and
depth of their reservoirs, and the nature of the
mineral substances composing the strata over which
the waters flow, or in which they collect.
The Reservoirs of springs are the places where
60 PHYSICAL GEOGRAPHY.
the waters that sink into the ground collect.
Reservoirs are sometimes large subterranean
basins, but more frequently are merely porous
strata, such as beds of sand or gravel, which lie
between impervious layers of clay or hard rock.
The water collects in the spaces between the par-
ticles of sand or gravel.
159. Size of Reservoir.-When the reservoir
is large, the spring is constant; when small, the
spring is temporary.
Constant Springs are those which flow continu-
ally, and are but little affected in the volume of
their discharge even by long-continued droughts.
Temporary Springs are those which flow only
for a short time after wet weather, drying up on
the appearance of even moderate droughts.
The quantity of water discharged by a spring depends on the
size of the orifice or outlet tube, and the depth of the outlet be-
low the surface of the water in the reservoir. The flow is
proportional to the square root of the depth. That is to
say, if with a given depth of orifice the velocity be one
foot per second, in order to make the water escape with
twice the velocity the depth must be increased fourfold.
The actual velocity is somewhat less than this, being di-
minished by friction.
Since the volume discharged by some springs
is very considerable, we must infer that their
reservoirs are of great size. Many springs prob-
ably receive the drainage from hundreds of
square miles of surface.
160. Shape of the Reservoir.-When the out-
let tube of the reservoir is siphon-shaped, the dis-
charge of the spring becomes periodical. The
Fig. 58. A Periodical Spring.
spring continues to discharge its waters for a
time, and then stops flowing, even during wet
weather. After a certain interval it again dis-
charges. The times during which the spring con-
tinues to discharge are always practically the
same. Hence the spring is called a periodical
The cause of periodical springs is due to the siphon-
shape of the outlet tube. A siphon is a tube so bent as to
have two vertical arms of unequal length. When filled,
it will continue to discharge as long as its shorter arm is
below the water and the longer arm free. If a large cav-
ernous reservoir be in connection with the surface of the
earth by a tube of this shape, it will begin to discharge its
water when, by infiltration, the level reaches the highest
bend of the tube, as at a, in Fig. 58, since the water will then
drive out the air and fill the entire tube. The discharge
will then continue until the water-level falls below the
mouth of the tube, or at b, in the figure. The time of the
discharge is always practically the same, since the same
quantity is discharged each time under exactly similar
Springs are common on the shores of the ocean. Their
waters are fresh because the outflow of the fresh water
prevents the inflow of the salt water. This is the case
even on coral islands, where the height of the land is
but ten or twelve feet above the sea. A comparatively
shallow well, on such islands, generally yields fresh water,
derived, of course, from the rainfall.
161. Depth of Reservoir.-According to the
distance the reservoir is situated below the sur-
face of the earth, springs are divided into Cold,
and Hot or Thermal.
Cold Springs are those whose temperature does
not exceed 600 Fahr. Their waters are sometimes
much colder than 600 Fahr.
Very cold springs owe their low temperatures
to the sources whence they draw their supplies.
In mountainous districts these can generally be
traced to the melting of huge snow-fields, or
masses of ice called glaciers. The temperature
in such cases is often nearly that of ordinary ice-
The reservoirs of all springs the temperature
of whose waters ranges from 500 to 600 are, in
general, comparatively near the surface. They
are colder than surface waters-
(1.) Because they are shielded from the sun;
(2.) Because evaporation occurs in their cav-
The temperature of springs of this kind is, in
general, but slightly affected by changes in the
temperature of the outer air. Since the reservoirs
of ordinary springs are shielded from the hot air
in summer and from the cold air in winter, their
waters are colder than river-water in summer, and
warmer than river-water in winter. Their waters
average, in their temperature, that of the strata
over which they flow in their subterranean course.
The mean annual temperature of the strata over
which the waters flow can, therefore, be ascertained
by plunging a thermometer into the water as it
comes out of the spring.
Hot or Thermal Springs range in temperature
from 600 Fahr. to the boiling-point. In geysers
the temperature of the water far down in the tube
is considerably above the boiling-point at the sur-
Hot springs which occur in the neighborhood
of active-volcanoes owe their high temperature to
the vicinity of their reservoirs to beds of recently-
Hot springs, however, are common in regions
distant from volcanic disturbance. In such cases
their high temperature must be attributed to the dis-
tance of their reservoirs from the earth's surface, the
heat being derived directly from the interior. *
In some cases the source of the heat is to be attributed
to chemical action in neighboring strata.
Thermal springs, whose reservoirs are at comparatively
moderate depths, may discharge their waters by ordinary
hydrostatic pressure; but where, from the great depth of
the reservoirs, this force would be insufficient, the waters
are probably raised, to the surface by the pressure of super-
heated steam or compressed gas.
Since the temperature rises 10 for about every 55 feet of
descent, in cases where the increased temperature is due
solely to depth, if the issuing waters have a tempera-
ture of 1490 Fahr., the reservoirs must be about one mile
below the surface, or fifty-five times the difference between
1490 and 600, the temperature of ordinary springs. In
many cases the waters probably rise from profound depths
as columns of steam, condensing in reservoirs that are less
Source of Deep-seated Waters.-Deep-seated waters
are probably derived by infiltration from the bed of the
ocean. The natural porosity of large areas is greatly in-
creased by the immense pressure of the water, which in
the deep ocean is equal to thousands of pounds per square
.^ .. --
Fig, 9. Artesian Well,
162. Artesian Wells differ from ordinary wells
in that their waters are discharged by natural
pressure on their reservoirs, so that pumping is
not necessary to raise the water. Such wells are
therefore true springs.
The reservoirs are basin-shaped, and generally
consist of several water-logged, porous strata, con-
tained between two, curved, impervious strata. If
the upper porous layer be pierced, the waters will
flow out by reason of the pressure of the liquid
in -the higher parts. The reservoirs of many
natural springs are of this kind, the upper im-
pervious strata being broken in one or more
places by some natural force.
Artesian wells have been sunk to great depths, and it is
a significant fact that the temperature of the issuing
waters is always proportional to the depth, showing a
nearly constant increase of 1 above the temperature of
ordinary springs-viz. about 600 Fahr.-for every 55 feet
of descent. In the case of the artesian well of Grenelle,
Paris, the successful boring of which was accomplished
only after many years of the most discouraging labor,
and which reached a depth of nearly 1800 feet, the tem-
perature of the water was 820 Fahr. A well at Neusalz-
werk, Prussia, has penetrated 2200 feet; its temperature
is 910 Fahr.
163. Geysers are boiling springs which, at in-
tervals more or less regular, shoot out huge col-
umns of water with great violence. They are
Fig, 60. Geyser in Eruption.
confined to the neighborhood of volcanic dis-
tricts, and, by some, are classed with subordinate
volcanic phenomena. The jets of water some-
62 PHYSICAL GEOGRAPHY.
times reach a height of more than two hundred
The geyser issues from the summit of a conical hillock
of silicious material deposited by the water. A broad,
shallow basin generally surmounts the hillock and forms
the mouth of a deep, funnel-shaped tube. The sides of
both tube and basin are lined with a smooth incrustation
of silica. In the Great Geyser of Iceland, the basin is 52
feet wide and the tube 75 feet deep.
Both the tube and basin are the work of the spring,
being deposited from the silica contained in the highly
heated waters. It is only when the tube has reached a
certain depth that the spring becomes a true geyser.
Wen the depth becomes too great the geyser eruptions
cease, the waters forcing their way through the walls of
the tube to some lower level. Hence, in all geyser re-
gions, numerous deserted geyser-tubes, and simple ther-
mal springs occur.
The waters of some geyser regions are calcareous. In
this case the tube of the geyser is, of course, formed of
164. Bunsen's Theory of Geysers.-Bunsen explains
the cause of geyser eruptions as follows: The heat of the
volcanic strata, through which the geyser-tube extends,
causes the water which fills it to become highly heated.
The water at the bottom of the tube, having to sustain
the pressure of that above it, gradually acquires a tem-
perature far above the boiling-point at the surface. The
temperature of the water in the tube will, therefore, de-
crease from the bottom to the surface.
If now, when the tube is filled, the water, near the mid-
dle, is brought to its boiling temperature, the steam thus
formed momentarily lifts the water if the upper part of
the tube, when the water in the lower part, released from
its pressure, bursts into steam and forcibly ejects the con-
tents of the tube.
Bunsen succeeded in lowering a thermometer into the
tube of the Great Geyser in Iceland just before an erup-
tion. At the depth of 72 feet he found the temperature
of the water to be 2610 Fahr.,'or 490 above the ordinary
1 165. Geyser Regions.-There are three exten-
sive geyser regions:
(1.) In Iceland, in the south-western part of
the island, where over one hundred occur in a
(2.) In New Zealand, about the centre of the
northern island, where, near the active volcano
Tongariro, over one thousand mud springs, hot
springs, and geysers burst from the ground.
(3.) In Yellowstone National Park, in Wyoming,
where numerous large geysers occur, mostly near
the head-waters of the Madison and Yellowstone
Rivers, at heights often as great as 8000 feet
above the sea-level. Here the boiling-point of
the water at the surface of the geyser, owing to
the diminished atmospheric pressure, is as low
as about 2000 Fahr.
A small geyser region is found in California,
near San Francisco.
166. Nature of the Mineral Substances form-
ing the Reservoir.-The subterranean waters dis-
solve various mineral matters either from the
strata over which they flow, or from their reser-
voirs; this is especially true of thermal springs,
owing to the greater solvent powers of the heated
The waters of mineral springs generally contain
a number of mineral ingredients. Mineral springs
are divided into various classes according to the
(1.) Calcareous Springs are those whose waters
contain lime in solution.
Thermal waters charged with carbonic acid usually con-
tain large quantities of lime, which they have dissolved
from subterranean strata. On reaching the surface the
waters cool and part with some of their carbonic acid, and
deposit layer after layer of hard limestone, called travertine.
In this way immense quantities of limestone are brought
to the surface from great depths, leaving huge subterra-
In portions of Tuscany, Italy, beds of travertine occur
more than 250 feet thick.
(2.) Silicious Springs are those whose waters
(3.) Sulphurous Waters are those whose waters
contain sulphuretted hydrogen and various metal-
lic sulphides or sulphates.
Sulphurous springs are found in Baden, near Vienna,
and in Virginia.
(4.) Chalybeate Springs are those whose waters
(5.) Salt Springs or Brines are those whose
waters contain common salt.
The springs of Halle, in the Alps of Salzburg, yield
15,000 tons of salt annually. The artesian well of Neu-
salzwerk, Prussia, yields about 28,000 tons annually. In
the United States the springs of Salina and Syracuse are
among the most important. The water in the springs of
Salina is ten times salter than ocean-water. The salt is
obtained from these springs by the evaporation of the
(6.) Acidulous Springs are those whose waters
contain large quantities of carbonic acid gas, as
the Seltzer springs in Germany, and those of
Vichy in France.
167. Petroleum and Bituminous Springs.-Be-
sides the springs above mentioned, there are two
others, closely connected, but which can scarcely
be included in any of the above classes. These
are petroleum and bituminous springs.
Petroleum Springs are those containing rock- or coal-
oil. They rise from large reservoirs containing oil instead
of water. The oil is derived from the slow decomposition,
in the presence of heat, of various animal and vegetable
matters which are found in the strata of nearly all the
geological formations. The reservoirs are of the same
nature as those of artesian wells, the oil being obtained
Petroleum springs are numerous. The most extensive
regions in the world are found in the great oil districts of
Western Pennsylvania and the neighboring States.
Bituminous Springs, or those from which pitch or
bitumen issue. Their origin is the same as that of oil
springs, the decomposition, however, occurring in a some-
what different way. The famous pitch lake on the island
of Trinidad, north-east of South America, probably owes
its origin to the large quantities of trees and other vege-
table matters, which have been rolled down the Orinoco
and buried in the delta formation on the eastern shores
of the island.
168. Definitions.-The water that issues from
the ground as springs, that is derived from the
melting of ice or snow, or that drains directly
from the surface after rainfall, runs down the
slopes of the land and collects in the depressions
formed by the intersection of the slopes, forming
rills or rivulets, which at last combine in larger
streams called rivers.
The source of a river is the place where it
rises; the mouth, the place where it empties; the
channel, the depression through which it flows.
Rivers generally rise in mountains, where the
rainfall is greater than elsewhere, and where
vast beds of snow and ice occur.
In reality, all rivers have three mouths, or places where
they discharge their waters:
(1.) Where the river empties directly into some other
body of water;
(2.) Where the river empties by evaporation into the
air; that is, its entire upper surface;
(3.) Where the river empties into the earth through the
porous strata of its bed or channel.
Since the downward motion of a river is caused by the
inclination of its channel from the source to the mouth, a
correct idea of the general inclination of any country can
be obtained by a careful study of a map in which the di-
rections of the rivers are represented. In studying the
various river-systems the student should endeavor to ob-
tain in this way clear ideas of the general directions of the
The River-System is the main stream, with all
its tributaries and branches.
The Basin is the entire area of land which
drains into the river-system.
The Water-shed is the ridge or elevation which
separates two opposite slopes. The streams flow
in opposite directions from the water-shed.
The Velocity of a river depends on the inclina-
tion or pitch of the channel and the volume or
depth of the water.
169. River-Courses.--The river-channel, from
its source to its mouth, is, for ease of description,
conveniently divided into three parts or courses:
the upper, middle, and lower.
The Upper Course of a river is that part which
is situated in the mountainous or hilly country
near its source. In this course the river has a
great velocity, and its channel is characterized by
sharp, sudden turns, alternating with long, straight
courses. In the upper course erosion occurs
almost entirely along the bottom of the channel,
so that the river runs between steep, and some-
times almost vertical, banks. In this way river-
valleys are formed, generally with narrow and
overhanging, precipitous sides. In the upper and
middle courses rapids and waterfalls occur.
Rapids and Waterfalls.-During the erosion of
the channel, where harder rocks occur in, the bed
of the stream, the softer strata, immediately adjoin-
ing them down stream, are rapidly worn away, and
the obstruction becomes at last the head of a
waterfall. The height grows rapidly from the
increased force of the falling water, and continues
until stopped by some similar obstruction below.
Fig, 61, Erosion of Waterfall.
Thus, suppose a a, Fig. 61, is the bed of a river, the di-
rection of flow of which is shown by the arrow. The softer
rock being worn away more rapidly, the bed reaches the
level 1, 1. A fall, and consequent increase in the velocity
of the river, soon causes the level of the bed to reach 2, 2,
3, 3, and 4, 4, successively. At the same time the falling
water eats away the vertical wall of the precipice, causing
the ivaterfall to move up stream. The water then cuts the
precipice away in steps, as shown at 5, 6, 7, thus changing
the fall into cascades. These are finally worn away, as
shown at 8, changing the cascades to rapids, when, finally,
the fall disappears entirely, or the erosion of the hard
rock is completed.
When the water falls perpendicularly-that is,
when it does not slip or slide-it forms a water-
fall or cataract; in all other cases of swift de-
scent it forms rapids.
The grandest falls in the world are those of the Niagara,
160 feet high. Though greatly inferior to many others in
height, yet their volume of water is so great that they
surpass all others in grandeur. The Victoria Falls of the
Zambezi in Africa nearly equal in volume those of the
Niagara. Their height is 360 feet.
The highest falls in the world are those of the Yosemite,
in California. Two projecting ledges break the sheet into
three falls, whose total height exceeds 2000 feet. One of
the highest falls in Europe is the Staubbach or Dust-brook,
in the valley of the Lauterbriinnen in Switzerland. The
water makes one sheer fall of 959 feet, and is lost in a
sheet of mist before it reaches the ground.
The Middle Course extends from where the
river emerges from the mountainous or hilly dis-
tricts to the low plains near the mouth. The
descent is comparatively slight, and the velocity
small. The erosion of the bottom of the channel
is insignificant, but at the sides, especially during
freshets, the river undermines its banks and thus
widens its valley. Here the river is divided into
two distinct portions: the channelproper and the
alluvial flats or flood-grounds.
The Lower Course extends from the middle
course to the mouth. The fall is slight, and the
170. Changes in River-courses.-During floods, when
the velocity and eroding power are greatly increased, ex-
tensive changes often occur in river-courses. After the
floods have subsided the water is found running through
new channels, its old ones being either completely filled
with deposits of mud, or occupied by slender streams.
Along the Mississippi these partially deserted channels
are called bayous, and, in places, widen out into large lakes.
(See Fig. 63.) The Red River appears to have formerly
emptied into the Mexican Gulf through a separate chan-
nel. In the basins of the Amazon, the Ganges, and the
Po, the old deserted channels are numerous on both banks
of the streams.
171. River Mouths.-A wide, open river-mouth
is called an Estuary; the accumulation of mud
or sand which occurs in the mouths of certain
rivers is called a Delta.
172. Inundations.-During certain seasons of
the year, the amount of water drained into the
river-channel is greater than it can discharge; it
then overflows its banks and inundates the sur-
Inundations of rivers are caused-
(1.) By excessive rainfall;
(2.) By periodical rains;
(3.) By the melting of ice and snow.
In the tropics, where the rainfall is more or
less periodical, the inundations of the rivers are
also periodical. The melting of the ice and snow,
which occurs regularly at the beginning of the
warm weather, also causes periodical inundations.
The Nile rises annually on account of the period-
ical rainfall of its upper sources; the Mississippi
semi-annually, once from the melting of snow,
and once from the winter rainfall.
When both the area of the river-basin and the rainfall
in inches are known, experience permits of a calculation,
by means of which the probable time and extent of rise of
water in a river can be approximately predicted. In times
of heavy rainfall, the Weather Bureau of the United
States is enabled to predict the probable rise of the im-
Influence of the Destruction of the Forests on In-
undations.-When the forests are removed from a large
portion of a river-basin, the rains are no longer absorbed
quietly by the ground, but drain rapidly off its surface into
the river-channels, and thus in a short time the entire
precipitation is poured into the main channel, causing an
overflow. It is from this cause that the disastrous effects
of otherwise harmless storms are produced. The inunda-
tions are most intensified by this cause in the early spring,
when the ice and snow begin to melt. The destructive
effects of the floods are increased by masses of floating ice,
which, becoming gorged in shallow places in the stream,
back up the waters above. The increased frequency of
inundations in the United States is, to a great extent, to
he attributed to the rapid destruction of the forests.
173. The Quantity of Water Discharged by a
River depends principally-
(1.) On the size of the basin;
(2.) On the amount of the rainfall.
The quantity of water in a river also depends-
(1.) On the climate of the basin, a dry, hot air diminish-
ing the quantity by evaporation;
(2.) On the physical features of the basin, whether wooded
(3.) On the nature of the bed or channel, whether leaky
It will be noticed that these three circumstances are
connected with the two additional river-mouths already
alluded to: the air-surface of the river, and the channel-
Keith Johnston estimates the daily discharge of all the
rivers of the world at 229,000,000,000 cubic yards, or over
2,620,000 cubic yards per second.
" CHAPTER IV.
Transporting Power of Rivers.
174. Silt or Detritus.-Rivers are ceaselessly
at work carrying the eroded materials, called silt
or detritus, from their upper to their lower courses.
Valleys are thus formed, miles in width and thou-
sands of feet in depth, and lofty mountains greatly
reduced in height.
The amount of silt transported by rivers is almost in-
credible. According to the careful estimates of Hum-
phreys and Abbot, the silt brought down every year by
the Mississippi and thrown into the Mexican Gulf, if
collected in one place, would cover a field one square mile
in area to the depth of 268 feet. According to Lyell, the
deposits, in the Bay of Bengal, of the Ganges and the
Brahmapootra, are nearly as great.
The rivers are carrying the mountains seaward,
and the continents are thus decreasing in mean
height and increasing in mean breadth.
175. Deposition of Silt.-Since the silt or
eroded mineral matter is heavier than water, it
will settle in all parts of the river-course. It will,
however, remain in those places only where the
velocity of the river is comparatively small.
These places are as follows:
(1.) In the channel of the river;
(2.) On the banks, over the alluvial flats or
(3.) At the mouth;
(4.) Along the coast near the mouth.
176. In the Channel.-In rivers that traverse
great plains, the inclination near the mouth is
slight, and the diminished velocity allows the ma-
terial to accumulate in the channel, thus raising
the general level of the stream. When the rivers
traverse settled districts, the inhabitants are com-
pelled to erect huge river-walls to prevent the
flooding of the adjacent lands; and, in some places,
the channel has been filled to such an extent that
the ordinary level of the river is higher than that
of the plains along its banks.
The levees or banks of the Mississippi are of this nature.
On the level plain of Lombardy the surface of the Po, in
some places, is higher than the tops of the neighboring
houses. When floods occur in such districts, the breaking
of a levee or river-wall is generally attended by much
177. Rafts.-Drift timber, thrown into the stream by
the undermining of the banks, is common in rivers that
traverse wooded districts. Portions of such- timber, be-
coming imbedded in shallow parts of the channel, form
obstructions which prevent the passage of subsequent
masses. The impediment so formed checks the velocity
of the stream, and mud deposits occur between the trees.
Such accumulations are called rafts. The raft of the Red
River, previous to its removal, was thirteen miles in length.
A large raft exists near the mouth of the Mackenzie River
in British America.
178. On the Alluvial Flats or Flood-grounds.
-The low flat plains on the sides of the river,
which are formed by the erosion of the banks in
the middle and lower courses, are covered by the
water when the river overflows its banks. In the
shallow water over these parts the velocity of the
water is slight, and the silt is deposited, thus
forming rich alluvial plains.
In large rivers the flood-grounds often attain consider-
able size. In. the Mississippi at Vicksburg the width of
the alluvial plain is over 60 miles.
In the lower courses of a river, the velocity
being small, comparatively slight obstacles suffice
to turn the waters from their course. The river-
channel is therefore characterized by wide bends
Fig. 63, Alluvial Flats of the Mississippi,
(Showing deserted courses and fluviatile islands and lakes.)
or curves. At the bend of a river the main cur-
rent is directed against one of the banks, where
rapid erosion takes place, the eroded material ac-
WER OF RIVERS.
cumulating lower down the river, in the bed of
the stream, where the velocity is small. The river
is thus continually damming
.-'. up portions of its old chan-
nel and cutting new ones.
The rapid excavation of
: these portions of the alluvial
'-"} plain is favored by the loose
materials which compose it.
/ l-- A''^ Sometimes the river cuts a
r new channel across the nar-
i' "l r row neck of a bend, part of
S .its waters running through
S the old channel and part
t ; through the new. In this
'7:'- way fluviatile islands are
S formed. One of the chan-
nels is sometimes separated
S from the other by a deposi-
SI t tion of mud or sand. The
water fills the old channel
Fig, 64. Formation of by soaking through the soil,
Fluviatile Islands and and thus fluviatile lakes are
formed. Numerous fluviatile
lakes occur near the banks of the Lower Missis-
sippi and the Red River.
The Delta of the Ganges and the Brahmapootra,
in the Bay of Bengal, is considerably larger than the Delta
of the Nile. Between the Hoogly and the main branch
of the Ganges, numerous streams force their way between
countless islands, called the Sunderbunds, inhabited by
tigers and crocodiles. The Po, the Rhone, the Rhine, and the
Danube in Europe, the Tigris, the Euphrates, the Yang-tse-
Kidng and Hoang-Ho in Asia, and the Senegal and the Zam-
bezi in Africa, have extensive deltas.
Thus, suppose the river flows in the direction of the
arrow at S, Fig. 64, and its channel has the bends shown.
A new channel may be formed at a, b, the river either
flowing through both channels, thus converting the neck
of land I, into a fluviatile island, or the old channel may
fill up and form a fluviatile lake, L, by bars forming in
the old channel at a and b.
S179. At the Mouth.-Delta Formations.-In
sheltered parts of the ocean, where the tides are
weak and the ocean-currents feeble, or in inland
seas and lakes, where they are entirely absent, the
eroded material accumulates at the mouth of the
river in large, triangular-shaped deposits, called
deltas, from their resemblance to the Greek letter
(A) of that name.
The Delta of the Mississippi is the largest in the
Western Continent. Its entire area is about 12,300 square
miles, though but two-thirds of it are permanently above
the water, the remainder being a sea-marsh. It begins a
little below the mouth of the Red River. The stream cuts
through the delta in one main channel, but near the ex-
treme end of the delta forms several mouths. On all sides
of the main stream, numerous smaller streams force their
way into the Gulf through the soft material.
The Delta of the Nile, at its outlet into the Mediter-
ranean, occupies an area of nearly 9000 square miles. A
large portion of the sediment of the river is deposited over
the flood-grounds during inundations. The fertility of the
land is largely dependent on these deposits.
180. Along the Coast, near the Mouth.-Fluvio-
Marine Formations are deposits of silt that form
along the coast near and opposite the mouths of
rivers, under the combined action of the river-
current and the tides of the ocean. A sand-bar
is formed at some little distance from the mouth
of the river, where the outflowing river-current
fig. 66. Fluvio-Marine Formations.
and the inflowing tide neutralize each other. The
impediment so formed permits of the rapid de-
position of silt, which fills up the portions of the
ocean so shut off, and converts them into shallow
bodies of water called sounds. These sounds, by
gradual rising of the land, are afterward con-
verted into river-swamps. According to Dana,
the eastern and southern coasts of the United
States, from Virginia to Texas, are an almost con-
tinuous fluvio-marine formation. Albemarle and
Pamlico Sounds and the Great Dismal, Alligator,
and Okefinoke Swamps are but different stages in
the formation of these deposits.
181. Continental Drainage is dependent on the
position of the mountain-systems and the direc-
tion of their slopes. The mountain-ridges or
peaks, or the high plateaus, form the water-sheds.
In some cases, from a single peak or plateau, the
water drains into distinct river-systems, emptying
into different oceans.
182. North America.-The central plain of
North America is drained by four large river-
systems: the Mackenzie into the Arctic Ocean;
the Saskatchewan and the Nelson into Hudson
Bay; the St. Lawrence into the Gulf of St. Law-
rence; and the Mississippi into the Gulf of Mex-
ico. The basin of the Mississippi occupies the
long slopes of the Rocky Mountains and the
Appalachians. The Missouri and the Ohio are
the principal tributaries of the Mississippi.
Numerous streams descend the eastern slopes
of the Appalachian system into the Atlantic.
Owing to the position of the predominant sys-
tem, the streams which empty into the Pacific are
comparatively small. The principal are the Yu-
kon, the Columbia, and the Colorado.
There are several remarkable isolated water-sheds or
drainage-centres in North America. These are-
(1.) In the central part of the Rocky Mountain system,
where the land drains in different directions into the sys-
tems of the Mississippi, the Columbia, and the Colorado
(2.) In the northern part of the Rocky Mountains,
where the drainage is received by the systems of the
Yukon, the Mackenzie, and the Saskatchewan Rivers.
183. South America resembles North America
in its drainage systems. The long, gentle slopes
of the Andes, and those of the systems of Brazil
and of Guiana, are occupied at their intersections
by the three great river-systems of the continent:
that of the Orinoco, in the north; that of the
Amazon, near the centre; and that of the La
Plata, in the south. Nearly the entire continent
is drained by these rivers and their tributaries
into the basin of the Atlantic.
The Pacific receives no considerable streams.
Only impetuous mountain-torrents are found.
The Magdalena, which drains north, corresponds to the
Mackenzie; the Orinoco and the Amazon, which drain
east, to. the Nelson and the St. Lawrence; and the La
Platte, which drains south, to the Mississippi.
184. Europe forms an exception to the other
continents as regards its drainage. Though some
of its large rivers rise in its predominant moun-
tain-system, yet the majority rise in the incon-
siderable elevations of the Valdai Hills. The
Alps are drained by four large rivers-the Rhone,
the Rhine, the Danube, and the Po. These all
have large deltas.
Although in this part of the continent the frequent in-
tersection of the two lines of trend produces numerous
basin-shaped valleys, yet, owing to breaks in the enclosing
mountains, none of any size have an inland drainage, but
discharge their waters through numerous tributaries into
one or another of the principal river-systems.
The Great Low Plain of Europe is drained
toward the north and west by the Petchora and
Dwina into the Arctic; by the Duna, the Nie-
men, the Vistula, and the Oder into the Baltic;
OCEAN ICAREAS & RIVER SYSTEMS
- Of THE lAR,4 h'
II I I I "
160 140 120 100
~MAl Pacitic system. Irctwc ysterrL.
861 60 40 20 0 20 4
and by the Elbe and the Weser into the North
Sea. It is drained toward the south and east by
the Ural and the Volga into the inland basin of
the Caspian; and by the Don, the Dnieper, and
the .Dniester into the Sea of Azov and the Black
All the peninsulas have streams traversing them. The
Seine, the Loire, and the Garonne from France, and the
Douro, the Tagus, and the Gaudiana from Spain and Por-
tugal, empty into the Atlantic. The Ebro from Spain,
and the Po from Italy, empty into the Mediterranean.
185. Asia possesses the most extensive inland
drainage of all the continents. The plateaus are
surrounded by lofty mountains containing but
comparatively few breaks, and their waters, there-
fore, can find no passage to the sea. The outer
slopes, however, are drained by some of the
largest rivers in the world.
The Great Northern Plain drains into the
Arctic, mainly through the Lena, the Yenisei,
and the Obe.
The Eastern Slopes drain into the Pacific
through the Amoor, the Hoang-Ho, the Yang-tse-
Kiang, and the Cambodia.
The Southern Slopes drain into the Indian
Ocean through the Irrawaddy, the Brahmapootra,
the Ganges, the Indus, the Tigris, and the Eu-
The principal drainage-centre in Asia is the Plateau of
Thibet, from which descend the Hoang-Ho, the Yang-tse-
Kiang, the Cambodia, the Irrawaddy, the Ganges, the
Brahmapootra, and the Indus.
186. Africa, being low in the interior, with
high mountain-walls on her borders, is charac-
terized, like the Americas, by the union of her
smaller river-systems into a few laire streams,
which drain nearly the entire continent.. These
embrace the Nile, emptying into the Mediterra-
nean; the Zambezi, into the Indian Ocean; and
the Orange, the Congo, the Niger, and the Senegal,
into the Atlantic.
187. Australia.-The Murray, which drains the
south-eastern part of the continent into the Indian
Ocean, is the only considerable stream.
188. Principal Oceanic Systems.-A careful
study of the river-basins of the- different oceans
discloses the following fact:
The Atlantic and Arctic Oceans receive the
waters of nearly all the large river-systems of the
The cause of this is as follows: The predomi-
nant systems being situated nearest the deepest
ocean, the long, gentle slopes descend toward the
smaller, shallower oceans (the Atlantic and the
Arctic), which thus receive the greatest drainage.
For details of the various river-systems-such as the
length, area of basin, etc.-see Table, page 174.
189. Lakes are bodies of water accumulated in
depressions of the surface of the land.
They are connected either with the systems of
oceanic or of inland drainage. The waters of
lakes draining into the ocean are fresh; those
having no connection with the ocean are salt.
Depth.-From their mode of formation lakes
which occur in mountainous districts are, as a
class, deeper than those found on the great low
plains, since the former occupy the basins of nar-
row but deep valleys, and the latter the depres-
sions of the gentle undulations of the plain.
In mountainous districts the depths of the depressions
are sometimes so great that the bottom of the lake is con-
siderably below the sea-level. Lake Maggiore in the Swiss
Alps extends about 2000 feet below the level of the sea.
0 feet. Lake Huron.
400 feet. /
Fig. 67. Elevations and Depressions of Lakes.
One of the most remarkable series of depressions in the
general land-surface of the world is that occupied by the
waters of Lakes Superior, Michigan, Huron, Erie, and On.
tario. Superior and Huron, though some 600 feet above
the level of the ocean, reach, in their greatest depths, far
below its surface; the former being 270 feet, and the latter
about 400 feet, below the general level of the Atlantic.
When a lake is connected with a river-system,
the place where the principal stream enters is
called the head of the lake; the place where it
empties is called the foot of the lake.
190. Geographical Distribution. -The large
70 PHYSICAL GEOGRAPHY.
lake-regions of the world are almost entirely
.confined to the northern continents.
191. Oceanic Drainage Systems.-North Amer-
ica contains the most extensive lake-system in the
world. The lake-region surrounds Hudson Bay,
and drains into the Arctic through the Mac-
kenzie; into Hudson Bay through the Sas-
katchewan; or into the Atlantic through the
St. Lawrence. To it belong the Great Lakes-
Superior, Michigan, Huron, Erie, and Ontario-
Fig. 68. View on Lake George, N. Y.
embracing a combined area of nearly 100,000
square miles-and the numerous lakes of Brit-
Athabasca, Great Slave, and Great Bear Lakes drain
into the Arctic through the Mackenzie; Lake Winnepeg,
into Hudson Bay through the Nelson; and the Great
Lakes, into the Atlantic through the St. Lawrence.
Europe contains ,two extensive systems of fresh-
water lakes. The larger region is in Low Europe,
and surrounds the Baltic Sea and its branches;
to it belong Lakes Ladoga and Onega in Russia,
Wener and Wetter in Sweden, with numerous
smaller lakes. The smaller region is found in
the Alps in High Europe.
Africa contains an extensive system of lakes
west of the predominant system. Victoria 'and
Albert Nyanzas, which drain into the Nile, Lake
Tanganyika, which drains into the Livingstone
or the Congo, and Lake Nyassa; which drains
into the Zambezi, are the principal lakes.
The remaining continents contain but few large
fresh-water lakes. In South America we find Lake
Maracaybo, with brackish water from its vicinity
to the sea; and in Asia, Lake Baikal.
7192. The Inland Drainage Systems are inti-
mately connected with that of inland rivers. The
term Steppe Lakes and Rivers is generally applied
to those which have no outlet to the ocean.
Cause of the Saltness of Inland Waters.-All river-
water contains a small quantity of common salt and other
saline substances. Since lakes which have no outlet, or,
as they are generally called, inland lakes, lose their waters
by evaporation only, the saline ingredients must be con-
tinually increasing in quantity; the water of such lakes
is therefore generally salt.
The Dead Sea in Syria is remarkable for the quantity
of its saline ingredients. In every one hundred pounds
of its waters there are over twenty-six pounds, or more
than one-fourth, of various saline ingredients.
North America.-The largest inland drainage-
system is in the Great Basin, containing Great
Salt, Walker, Pyramid, and Owen Lakes.
South America.-The largest region of inland
drainage includes the plateau of Bolivia, contain-
ing Lake Titicaca. The waters of this lake are
fresh, but have no outlet to the sea, the river form-
ing the outlet being lost in a salty, sandy plain.
Europe and Asi'a contain a vast region of in-
land drainage extending from the Valdai Hills
eastward to the Great Kinghan Mountains, em-
bracing most of the Asiatic plateaus.
The region contains Lake Elton in Russia, and the Cas-
pian and Aral Seas. The combined area of the last two
is 175,000 square miles. They receive the waters of the
Volga, the Ural, the Sir, and the Amoo, all large streams.
Numerous lakes occur on the plateaus. Lake Lop, in the
depression north of Thibet, receives the Tarim, and Lake
Hamoon, on the Iranian plateau, the Helmund River.
Africa contains Lake Tchad in the Soudan, re-
ceiving the Komadagu and the Shirwa, and Lake
Ngami in Southern Africa.
Australia contains Lakes Eyre, Torrens, Gaird-
ner, and Amadeo near the southern coast.
193. Utility of Lakes.-By offering extended basins
into which the rivers, when swollen, can disgorge them-
selves, lakes greatly diminish the destructive effects of
inundations, often checking them entirely. They afford
extended surfaces for evaporation, and, collecting the finer
sediment of the rivers when deserted by their waters,
form fertile plains.
Water is formed by the union of oxygen and hydrogen.
The waters of the earth may be divided into two classes
-the continental and the oceanic.
Water is a solid at and below 320 Fahr., a liquid from
320 to 2120, and a vapor above 2120. It passes off as vapor,
however, at all temperatures.
A pint of water is heaviest at the temperature of 39.20
Fahr. Hence in deep lakes, covered with ice, the lower
layers of water are 7.20 Fahr. above the freezing-point.
Large bodies of water moderate the extremes of tem-
perature, because water takes in more heat while warming
and gives out more.on cooling than any other common
During the freezing of a body of water, or the condensa-
tion of a mass of vapor, considerable stored heat-energy
appears, or latent heat becomes sensible and warms the
After a body of water has been cooled to the tempera-
ture of 320 Fahr., it has still 142 heat-units, or pound-de-
grees, to lose before it can turn into ice.
After a body of ice has been warmed to the temperature
of 320 Fahr., it has still 142 heat-units, or pound-degrees,
of heat to gain before it can turn into water.
Therefore, both freezing and melting are gradual pro-
The rains cleanse the surface of the earth and purify
Water is necessary for the existence of life. It forms
the main food of both animals and plants.
The atmospheric waters are drained into the ocean
either by surface or subterranean drainage.
Springs are the outpourings of the subterranean waters.
Springs may be classified according to peculiarities in
the size, shape, and depth of their reservoirs, and the
nature of the mineral substances composing the strata
over which the waters flow or in which they collect.
According to the size of their reservoirs, springs are
either constant or temporary.
If their reservoirs have siphon-shaped outlet tubes,
their discharges are periodical.
When their reservoirs are superficial, springs are cold;
when deep-seated, they are hot or thermal.
Springs whose waters are moderately cold have their
reservoirs near the surface. Their lower temperature is
due to their waters being shielded from the sun.
Springs with very cold waters have their sources in the
melting of large masses of ice or snow.
Hot or thermal springs owe their high temperature to
the heat they receive from the interior of the earth.
Geysers are boiling springs,.which, at irregular intervals,
shoot out huge columns of water with great violence.
The most extensive geyser regions are those of Iceland,
New Zealand, and Wyoming.
Calcareous springs contain lime; silicious, silex; sul-
phurous, sulphuretted hydrogen and metallic sulphides or
sulphates; chalybeate, iron; brines, common salt; acidu-
lous, carbonic acid; petroleum, coal oil; bituminous, pitch.
Rivers are fed both by surface and subterranean drain-
The main stream with all its tributaries and branches
is called the river-system. The territory drained into the
river-system is called the river-basin. The ridge or ele-
vation separating opposite slopes is called the water-shed.
In the upper courses of rivers erosion occurs mainly on
the bottom of the channel; in the lower courses, at the
In the lower courses of rivers extensive flats or plains
are found. They are caused by the erosion of the banks
and the subsequent deposition of fine mud during inunda-
Rivers are constantly at work carrying the mountains
toward the sea. Through their agency the mean height
of the continents is decreasing, and their mean breadth
The eroded material, or silt, may accumulate-1. In the
channel of the river; 2. Along the banks, on the alluvial
flats or flood-grounds; 3. At the river's mouth; and 4.
Along the coast, near the mouth.
The accumulations in the channel of the lower Missis-
sippi have so raised the bed of the stream as to necessitate
the erection of levees or embankments along the sides.
Where the tides are weak and the ocean currents absent
or feeble, the eroded material, or silt, accumulates at the
mouths of rivers in masses termed deltas.
The Alps are drained by the Rhine, the Rhone, the Po,
and the Danube; these rivers have extensive delta-forma-
The plateau of Thibet is drained by the Hoang-Ho, the
Yang-tse-Kiang, the Ganges, the Brahmapootra, and the
Indus; all these rivers have extensive delta-formations.
Among other extensive deltas are those of the Missis-
sippi, which drains the long slopes of the Pacific and
Appalachian mountain-systems; the Nile, the Tigris, the
Euphrates, and the Zambezi.
Fluvio-marine formations occur along the coasts; they
are caused by the combined action of the river and tides.
The destruction of forests, by increasing the rapidity of
drainage, increases the violence of floods. Lakes along
the river-courses decrease their violence, by allowing the
torrents to discharge their waters.
The direction of the drainage of a country is dependent
on the direction of its slopes.
The central plain of North America is drained north
into the Arctic Ocean through the Mackenzie; east into
the Atlantic through the Nelson and the St. Lawrence;
and south into the Gulf of Mexico through the Mississippi.
The central plain of South America is drained north
into the Caribbean Sea through the Magdalena, east, into
the Atlantic through the Orinbco and the Amazon, and
south, into the Atlantic through the Rio de la Plata.
The rivers draining the great low plain of Europe rise
either in the Valdai Hills or on the northern slopes of the
Asia possesses the most extended system of inland
drainage of tha continents. Extended systems are also
found in North America and Europe.
The Atlantic and the Arctic Oceans drain about three-
fourths of the continental waters.
The largest systems of fresh-water lakes occur in North
America and Europe.
The Great Lakes of North America occupy remarkable
depressions in the continent. The beds of some of them
are several hundred feet below the level of the sea.
Lakes without an outlet are salt, because the waters
they receive contain small quantities of saline ingredients,
while the waters they lose contain n
72 PHYSICAL GEOGRAPHY.
What is the composition of water?
Enumerate the physical properties which enable water
to play so important a part in the economy of the earth.
What effect has the temperature of the maximum den-
sity of water on the freezing of large bodies of fresh water ?
How do large bodies of water moderate the extremes of
heat and cold?
Why are freezing and melting necessarily gradual pro-
What effect has a heavy rainfall on the temperature of
Explain the cause of deserts.
Define subterranean drainage. Surface drainage.
Upon what does the quantity of water discharged by a
spring in a given time depend?
Explain the cause of periodical springs.
What is the temperature of cold springs? Of hot or
What is the probable cause of the high temperature of
How can the probable depth of the reservoir of an arte-
sian spring be ascertained from the temperature of its
What are geysers? Explain the cause of their erup-
What is the origin of the tube and basin of the geyser?
Name the three largest geyser regions of the world.
What is travertine? How is it formed?
Name some of the most important springs from which
large quantities of salt are obtained.
What is believed to be the origin of petroleum or coal
How are the precipices of waterfalls caused? In what
courses of a river are they most common?
Name the highest waterfall in the world. The grand-
Distinguish between an estuary and a delta.
How does the destruction of the forest increase the
severity of inundations?
Upon what does the quantity of water in a river de-
In what different portions of a stream may the silt or
detritus be deposited?
What are rafts? How are they caused ?
Explain the formation of fluviatile islands and lakes.
Name some of the most extensive delta-formations in
North America. In Europe. In Asia. In Africa.
What is the probable origin of the swamp-lands of the
How may a tolerably accurate notion of the direction
of the slopes of a country be obtained by a study of the
direction of its rivers?
In what respects do the drainage of North and South
America resemble each other?
Name the principal systems of inland drainage of the
Explain the cause of the saltness of inland waters.
Which ocean drains the largest areas of the continents?
Which the smallest?
Name the important rivers which drain into the Atlan-
tic from North America. From South America. From
Europe. From Africa.
Name the important rivers which drain into the Pacific
from North America. From Asia.
Name the important rivers which drain into the Indian
Ocean from Africa. From Asia. From Australia.
What two systems of inland drainage are there in North
America? What large region in South America?
Name an important steppe lake and river in each of the
Describe the region of inland' drainage of Europe and
Asia. What large lakes and rivers belong to this region?
Describe the regions of inland drainage of Africa. Of
Australia. Name the important lakes found in each
What South American river corresponds in the direction
of its drainage with the St. Lawrence? With the Mac-
kenzie? With the Mississippi?
Name the large rivers which drain the predominant
mountain-system of Asia. Of Europe. Of Africa. Of
North America. Of South America. Of Australia.
Describe the fresh-water lake-region of North America.
Of South America. Of Europe. Of Africa.
In which line of trend are most of the fresh-water lakes
of North America found?
Name the Atlantic rivers which have large deltas The
Pacific rivers. The Indian rivers.
THE OCEAN. 73
V'b- CHAPTER I.
194. Composition.--The water of the ocean
contains a number of various saline ingredients,
which give it a bitter taste and render it heavier
than fresh water in the proportion of 1.027 to 1.
Every hundred pounds of ocean-water contains
about three and one-third pounds of various
Chloride of sodium, or common salt, chloride of magne-
sium, sulphates and carbonates of lime, magnesia, and
potassa, and various bromides, chlorides, and iodides, are
the principal saline ingredients.
195. Origin of the Saltness of the Ocean.-The
rivers are constantly dissolving from their channels large
quantities of mineral matters, and pouring them into the
ocean. Besides this, fully three-fourths of the earth's sur-
face is covered permanently by the oceanic waters. In
this way immense quantities of mineral ingredients have
been dissolved out from the crust. The latter cause was
especially active during the geological past, when frequent
convulsions brought fresh portions of the crust into con-
'tact with the warm waters.
The ocean is salter in those parts where the evaporation
exceeds the rainfall, or at about the latitude of the tropics;
where the rainfall exceeds the evaporation, the water is
slightly fresher than at the equator.
In inland seas, like the Mediterranean or the Red Sea,
Which, though connected with the ocean, yet lose much
more of their waters by evaporation than by outflow, the
proportion of salt is slightly greater than in the ocean.
In such cases a current generally flows into the sea from
the ocean. In colder latitudes, inland seas, like the Bal-
tic, receiving the waters of large rivers, contain rather
less salt than the open sea, and a current generally flows
from them into the ocean.
196. Color.-Though transparent and colorless
in small quantities, yet in large masses the color
of sea-water is. a deep blue. The same is true
of fresh water. Over limited portions of the
ocean the waters are sometimes of a reddish or
a greenish hue, from the presence of numberless
Sometimes a pale light or phosphorescence,
visible only at night, and due to the presence of
animalcule, appears where the air comes into con-
tact with the water, as in the wake of a vessel or
on the crests of the waves.
197. Temperature. -The salts dissolved in
ocean-water lower the temperature of its freez-
ing-point. Ordinary ocean-water freezes at about
270 F. In places where the water is salter, the
temperature of its freezing-point is lower.
Ice formed from ocean-water is comparatively
fresh, nearly all the salt being separated as the
water freezes or crystallizes. The salt, thus thrown
out from the frozen water, is dissolved by the
water below, lowers the temperature of its freez-
ing-point, and thus increases its density. In this
manner the water below the ice may have a tem-
perature lower than that at which the surface-
water freezes, and yet remain liquid.
In the polar regions the water below the sur-
face is at a temperature lower than that of the
freezing-point of the surface-water. This cold
water, from its greater density, spreads over the
floor of the ocean in all latitudes, so that, except
where stirred by deep currents, the entire bottom
of the ocean is covered with a layer of dense,
heavy water, the temperature of which is nearly
The temperature of this water is about 350 F. Near
the poles it is somewhat lower: about 290, or a little higher
than its maximum density of the surface-waters.
The upper limit of this line of invariable temperature
varies with the latitude. Near the equator, where the
waters are heated to great depths, it is found at about
10,000 feet below the surface. Toward the poles, it comes
nearer the surface, reaching it at about Lat. 600, from
which point it again sinks, being found at Lat. 700 at
about 4500 feet below the surface.
In the tropics the temperature of the surface-water
is about 800 F.; in the polar regions it is near the
freezing-point. The ice which forms in the polar
regions collects in vast ice-fields or floes.
198. Shape of the Bottom of the Ocean.-The
bed of the ocean, though diversified like the sur-
face of the land, contains fewer irregularities.
Numerous soundings show that it extends for
immense distances in long undulations and slopes.
Its plateaus and plains, therefore, are of great
size, compared with those of the continents.
Submerged mountain-ranges occur both in the
deep ocean and along the shores. The latter
3) U I Y1 VhRL/ 2V;-LLJ
74 PHYSICAL GEOGRAPHY.
belong, properly, to the continental systems of
199. The Oceanic Areas.-The ocean is one
continuous body of water, but for purposes of
description and study it is generally divided into
five smaller bodies: the Pacific, Atlantic, Indian,
Arctic, and Antarctic Oceans. The'last two are
separated from the preceding by the polar circles;
Sthe others are separated mainly by the continents.
As the continents do not extend to the Antarctic
Circle, the meridians of Cape Horn, Cape of
Good Hope, and South Cape in Tasmania, are
taken as the ocean boundaries south of these
The following table gives the relative size of the oceanic
The Pacific occupies about the entire water-area.
"Antarctic a "
200. Articulation of Land and Water.-The
indentations of the oceans, or the lines of junc-
tion between the water and the land, may be
arranged under four heads:
(1.) Inland Seas, or those surrounded by a
nearly continuous or unbroken land-border; as
the Gulf of Mexico, Hudson Bay, the Baltic, and
the Mediterranean, in the Atlantic; the Red Sea
and the Persian Gulf, in the Indian; and the
Gulf of California, in the Pacific.
(2.) Border Seas, or those isolated. from the
rest of the ocean by peninsulas and island chains;
as the Caribbean Sea, the Gulf of St. Lawrence,
and the North Sea, in the Atlantic; and Bering
Sea, the Sea of Okhotsk, the Sea of Japan, and
the North and South China Seas, in the Pacific.
(3.) Gulfs and Bays, or broad expansions of
the water extending but a short distance into
the land; as the Gulf of Guinea and the Bay of
Biscay, in the Atlantic; and the Bay of Bengal
and the Arabian Sea, in the Indian.
(4.) Fiords, or deep inlets, with high, rocky
headlands, extending often from 50 to 100 miles
into the land. One of the best instances of this
form of indentation is off the Norway coast. Ac-
cording to Dana, fords are valleys that were ex-
cavated by vast ice-masses called glaciers, but
which have since become partially submerged by
the gradual subsidence of the land.
Fiord valleys occur on the Norway coast, on
the coasts of Greenland, Labrador, Nova Scotia,
and Maine, on the western coast of Patagonia
and Chili, and on the western coast of North
America north of the Straits of Fuca. On parts
of the coast of Greenland the glaciers are now
cutting out their partially submerged valleys,
and forming what will probably become fiord
The Atlantic Ocean is characterized by inland
seas; the Pacific, by border seas; the Indian, by
gulfs and bays; the Atlantic and the Pacific, by
201. Depth of the Ocean.-The mean depth of
the ocean is about 12,000 ft.,.or nearly 24 miles.
Recent soundings give the greatest depth of the
Atlantic, in the neighborhood of the island of
St. Thomas of the West Indies, as-27,000 feet.
The greatest depth in the Pacific, as reported by
recent careful soundings, a,
iag .. These give a depth of about
5- miles, or less than the greatest elevation of the
land. It is probable, however, that some portions
of the ocean are much deeper.
The greater depressions of the ocean are called
deeps, the shallower portions are called rises.
202. The Pacific .Ocean.-The shape of the
shore-line of the Pacific is that of an immense
oval, nearly closed at the north, but broad and
open at the south.
As indicated by the island chains, a number of shallow
places, or rises, extend in the direction of the north-west
trend: the summits of those on the north form the Sand-
wich Islands, and the summits of those on the south form
the Polynesian Island chain.
203. The Atlantic Ocean.-The shape of the
shore-line of the Atlantic is that of a long,
trough-like valley, with nearly parallel sides.
The Atlantic has a broad connection with both
the polar oceans, and forms the only open chan-
nel for the intermingling of the warm and cold
Shape of the Bed.-Recent soundings in the Atlantic show
the presence of a submarine plateau extending in mid-
ocean parallel to the coasts of the continents from the lati-
tude of the southern point of Africa to Iceland, thus di-
viding the basin into eastern and western valleys. The
western valley is the deeper; the average depths of the two
being respectively 18,000 and 13,000 feet. A remarkable
Feet. Level of the sea.
Fig. 69. The Telegraphic Plateau.
plateau extends across these valleys, from Newfoundland
to Ireland. Its depth ranges from 10,000 to nearly 13,000
feet. It is called the Telegraphic Plateau, and bears a
number of telegraphic cables. The eastern and western'
OCEANIC MOVEMENTS. 75
valleys, though less marked in this region, are still dis-
The true bed of the ocean begins at a considerable dis-
tance from the eastern coast of North America. For dis-
tances of from 75 to 100 miles, the depth scarcely exceeds
600 feet; but from this point it descends, by steep terraces,
to profound depths.
The British Isles are connected with the continent of
Europe by a large submerged plateau, which underlies
nearly the whole North Sea, and extends for considerable
distances off the western and southern coasts. The depth
of this part of the ocean is nowhere very great.
204. The Indian Ocean.-The shape of the
shore-line is, in general, triangular. This ocean
has no connection with the Arctic, but is entirely
open on the south, where it merges into the great
water-area of the globe: the basins of the Ant-
arctic and Pacific.
Shape of the Bed.-A submarine plateau extends to the
south off the western coast of Hindostan. Its summits
form the Laccadive, Maldive, and Chagos Islands, and pos-
sibly extends in the same direction as far as Kerguelen
205. The Antarctic and Arctic Oceans.-The
shore-line of the Arctic has the shape of an ir-
regular ring. The shore-line of the Antarctic is
probably of the same shape.
But little is known concerning the beds of these
oceans. From the very limited land-areas south
of lat. 500 S., the bed of the Antarctic is presum-
ably deeper than that of the Arctic, except toward
the south pole, where it is probably shallower.
206. Ooze Deposits.-Foraminiferal Land.-
The reef-forming coral polyps are not the only
animalcule the accumulation of whose bodies
after death add to the land-masses of the earth.
Deep-sea soundings show that over extended areas
Fig. 70, Foraminifera,
the floor of the ocean is evenly covered with a
creamy layer of mud or ooze, which, like the
deposits of the coral animalcule, is composed
principally of carbonate of lime. This ooze con-
sists almost entirely of microscopic skeletons of a
group of animalcule known as the Foraminifera,
from the great number of perforations or open-
ings in their hard parts. These animalcule are
so small that 1,000,000 are equal in bulk to only
one cubic inch. They appear to live in the layers
of water near the surface, and after death to
fall gradually to the bottom of the sea. Sound-
ings show their presence over very extended
Many of the very deep parts of the ocean's bed
are covered, not with foraminiferal deposits, but
with a layer of red mud composed of finely-di-
vided clay. Its origin is probably as follows:
In very deep parts of the ocean before .the fora-
miniferal deposits reach the bottom their limey
matters are dissolved, and the undissolved parts
form the deposits of fine red mud.
207. The Oceanic Movements can be' arranged
under three heads: waves, tides, and currents.
Waves are swinging motions of the water,
caused by the action of the wind. Their height
and velocity depend on the force of the wind, and
the depth of the basin in which they occur. The
stronger the wind, and the deeper the ocean, the
higher the waves and the greater their velocity.
Fig, 71, Ocean Waves.
Height of Waves.-Scoresby measured waves in the
North Atlantic 43 feet above the level of the trough.
Waves have been reported in the South Atlantic, off the
Cape of Good Hope, between 50 and 60 feet high. Navi-
76 PHYSICAL GEOGRAPHY.
gators have occasionally reported higher waves, but the
accuracy of their measurements is, perhaps, to be doubted.
In the open sea, with a moderate wind, the height of
ordinary waves is about 6 feet.
The distance between two successive crests varies from
10 to 20 times their height. Waves 4 feet high have
their successive crests 40 feet apart; those 33 feet high,
about 500 feet apart.
208. No Progressive Motion of Water in
Waves.-In wave motion, the water seems to be
moving in the direction in which the wave is ad-
vancing, but this is only apparent; light objects,
floating on the water, rise and fall, but do not
move forward with the wave. In shallow water,
however, the water really advances. The for-
ward motion of the wave is retarded, so that the
waves following reach it, thus increasing its
height. The motion at the bottom is lessened,
and the top curls over and breaks, producing
what are called breakers.
On gently sloping shores, the water which runs down
the beach, after it has been thrown upon it by the breakers,
forms, at a little distance from the shore, the dreaded
"undertow" of our bathing-resorts.
Force of the Waves.-When high, and moving
in the direction of the wind, the waves dash
against any obstacle, such as a line of coast, with
great force, and may thus cut it away and change
the coast-line. This action occurs only on ex-
posed, shelving coasts. The wave-motion is, in
general, very feeble at 40 feet below the surface.
The eroding action of the ocean waves is, there-
fore, far inferior to that of the continental waters.
209. Tides are the periodical risings and fail-
ings of the water, caused by the attraction of the
sun and moon. The alternate risings and falling
succeed each other with great regularity, about
every six hours. Unlike waves, in which the
motion is confined practically to the surface
waters only, tides affect the waters of the ocean
from top to bottom.
The rising of the Water is called flood tide; the
falling, ebb tide. When the waters reach their
highest and lowest points, they remain stationary
for a few minutes. These points are called, re-
spectively, high and low water. Corresponding
high or low water, at any place, occurs fifty-two
minutes later each successive day.
210. Theory of the Tides.-If the earth were
uniformly covered with a layer of water, the pas-
sage of the moon over any place, as at a, Fig. 72,
would cause the water to lose its globular form,
become. bulged at a, and b, and flattened at c,
and d. In other words, the water would become
deeper at a, and b, at the parts of the earth near-
est and farthest from the moon, and shallower in
S d ,
Fig, 72, Lunar Tide,
all places 900 or at right angles to these points,
such, for example, as at c, and d.
This deepening and shallowing of the water is
caused by the attraction of the moon. As the
moon passes over a, the water is drawn toward
the moon, thus deepening the water directly under
the moon, and shallowing it at c, and d.
The cause of the deepening of the water at b,
on the side farthest from the moon, is as follows:
the solid earth being, as a whole, nearer the moon
than the water at b, but farther from it than that
at a, must take a position which will be nearly
midway between a, and b, leaving a protuberance
at b, nearly equal to that at a.
The protuberances a, and b, mark the position
of high tides. At all points of the earth 900 from
the protuberances, as at c, and d, the depression is
Greatest. These mark the position of low tides.
High tides, then, occur at those points of the
earth's surface which are cut by a straight line,
which passes through the centre of the earth and
that of the attracting body, as the sun or moon.
Low tides are found at right angles to these
Had the earth no rotation, the tidal waves, so
formed, would slowly follow the moon in its mo-
tion around the earth. But, by the rotation of
the earth, different parts of its surface are rapidly
brought under the moon, and the tidal waves,
consequently, move rapidly from one part of the
Had the moon no motion around the earth, there would
be two high tides and two low tides every 24 hours.
While, however, the earth is making one complete rota-
tion, the moon, in its motion around the earth, has
changed its position, and the earth rotates for 52 minutes
longer before the same point again comes directly under
Since the uniformity of the water surface is
broken by the elevations of the land, the progress
of the tidal wave is greatly affected by the size,
shape, and depth of the oceanic basin, and the
OCEANIC MOVEMENTS. 77
position of the continents. Owing to the obstruc-
tions offered by the continents, and by inequalities
in the bed of the ocean, a very considerable re-
tardation of the tidal wave is effected, so that a
high tide may not occur at a place until long
after the moon has passed over it.
Solar Tides.-The sun also produces a system
of tidal waves, but owing to its greater distance
from the earth, the tides thus produced are much
smaller than those of the moon, upon which, there-
fore, they exert but a modifying influence. The
tide-producing power of the moon is greater than
that of the sun, in about the proportion of 800
to 355. That is, the tide produced by the moon
is about 2f times greater than that produced by
The tidal wave moves, in general, from east to west, or in
the opposite direction to the rotation of the earth. The motion
of so large a mass of water thus opposed to the earth's ro-
tation, must gradually diminish the axial velocity, and,
eventually, entirely stop the rotation of the earth; in this
way an increase in the length of day and night should be
produced, but so far, however, no increase has been de-
tected, although astronomical observations extend back-
ward for long periods. The increased axial velocity, pro-
duced by the contraction of the globe, probably balances
the retarding influence of the tides.
In the deep ocean, and near the mouths of rivers, the
duration of the flood and ebb are about equal; but in most
rivers, at some distance from the mouth, the ebb is longer
than the flood. The cause is to be found in the fact that
the outflowing river current meets and temporarily neu-
tralizes the inflowing flood tide, thus diminishing its dura-
tion, and afterward, adding its motion to the ebb, makes
the difference between the two still greater.
The tidal wave often ascends a stream to a much greater
elevation above the level of its mouth than the height of
the tide at the river's mouth. In large rivers, like the
Amazon, the tidal wave advances up the river as much as
100 feet above the sea-level.
Some of the proofs of the connection between the tides
and the attraction of the moon and sun are as follows:
(1.) The interval between corresponding high tides at
any place is the same as the interval between two succes-
sive passages of the moon over that place: 24 hours, 52
(2.) The tides are higher when the moon is nearer the
(3.) The tides are higher when the sun and moon are
simultaneously acting to cause high tides in the same
Fig. 73. Cause of the Phases of the Moon.
Phases of the Moon.-An inspection of Fig. 73 will
show, that during new and full moon, the earth, moon,
and sun are all in the same straight line, but, that during
the first and last quarters, they are at right angles. The
portions of the earth and moon turned toward the sun are
illumined, the shaded portions are in the darkness. To
an observer on the earth, the moon, at a, appears new,
since the dark part is turned toward him; at b, however,
it must appear full, since the illumined portions are toward
him. At c, and d, the positions of the quarters, only one-
half of the illumined half, or one quarter, is seen.
flood and ebb
Fig. 74. Position of the Earth, Moon, and Sun during Spring and Neap Tides.
211. Spring and Neap Tides.-When the sun
and moon act simultaneously, on the same hemi-
sphere )of the earth, as shown in Fig. 74, the tidal
wave i6 higher than usual. The flood tides are
then h'hl,-.'t, and the ebb tides lowest. These
are called spring tides. They occur twice during
every revolution of the moon-once at full, and
once at new moon. The highest spring tides oc-
cur a short time before the March and the Sep-
tember equinoxes, when the sun is over the equa-
When, however, the sun and moon are 900
78 PHYSICAL GEOGRAPHY.
apart, or in quadrature, each produces a tide on
the portion of the earth directly under it, dimin-
ishing somewhat that produced by the other body.
High tide, then, occurs under the moon, while the
high tide caused by the sun, becomes, by compari-
son, a low tide. Such tides are called neap tides.
During their prevalence, the flood is not. very
high, nor the ebb very low. They occur twice
during each revolution of the moon, but are low-
est about the time of the June and December
The average relative height of the spring tide to that
of the neap tide is about as 7 to 4.
212. Birthplace of the Tidal Wave.-Although
a tidal wave is formed in all parts of the ocean
where the moon is overhead, yet the Cradle of
the Tides" may properly be located in the great
southern area of the Pacific Ocean. Here the
combined attraction of the sun and moon origin-
ate a wave, which would travel around the earth
due east andwest, with its crests north and south;
but, meeting the channels of the oceans, it is
forced up them toward the north. Its progress is
accelerated in the deep basins, and retarded in the
shallow ones. On striking the coasts of the con-
tinents, deflected or secondary waves move off in
different directions, thus producing great com-
plexity in the form of the parent wave.
213. Co-Tidal Lines.-The progress of the tidal
wave, in each of the oceans, is best understood by
tracing on a map, lines connecting all places
which receive the tidal wave at the same time.
These are called co-tidal lines. The distance be-
tween two consecutive lines represents the time, in
hours, required for the progress of the tidal wave.
In parts of the ocean where the wave travels rap-
idly the co-tidal lines are far apart; when its prog-
ress is retarded, they are crowded together.
Fig, 75, Co-Tidal Chart,
Since it is only possible to. take the height of the tide
on the coasts of islands and of the continents, the tracks
of the co-tidal lines must be to a considerable extent con-
214. The Pacific Ocean.-Twice every day a
tidal wave starts in the south-eastern part of the
Pacific Ocean, west of South America, somewhere
between the two heavy lines marked xII on the
chart. It advances rapidly toward the north-
west in the deep valley of this oceai;, reaching-
Kamtchatka in about 6 hours. Toward the west
its progress is retarded by the shallower water,
and by the numerous islands, so that it only
reaches New Zealand in about 6 hours an1 enters
the Indian Ocean in about 12 hours. /
215. The Indian Ocean.-The 12-hour-old tidal
OCEAN CURRENTS. 79
wave from the Pacific, meets and moves along
with a wave started in this ocean by the moon,
and advances in the direction indicated by the
co-tidal lines entering the Atlantic Ocean about
12 hours afterward.
216. The Atlantic Ocean.-The tidal wave from
the Indian joins two other waves, one formed by
the moon in this ocean, and the other a deflected
wave that has backed into the Atlantic from the
Pacific. The tidal wave thus formed advances
rapidly up the deep valley of the Atlantic, reach-
ing Newfoundland 12 hours afterward, or 48 hours
after it started in the Pacific. It then advances
rather less rapidly toward the north-east, reach-
ing the Loffoden Islands 12 hours afterward, or
60 hours after leaving its starting-place in the
217. Tides in Inland Seas and Lakes are very
small and, consequently, difficult to detect. In
the Mediterranean Sea the tides on the coasts
average about 18 inches. The tide in Lake
Michigan is about 1 inches.
218. Height of Tidal Wave.-Ocean tides are
lowest in mid-ocean, where they range from two
to three feet. Off the coasts of the continents,
especially when forced up narrow, shelving bays,
deep gulfs, or broad river mouths, they attain
great heights. The cause of these unusual heights
is evident. When the progress of the tidal wave
is retarded, either by the contraction of the chan-
nel or by other causes, the following part of the
wave overtakes the advanced part, and thus, what
the wave loses in speed it gains in height, from the
heaping up of the advancing waters. Where the
co-tidal lines, therefore, are crowded together on the
chart, high tides are likely to occur; for example,
the Arabian Sea -and Bay of Bengal, the North
and South China Seas, the eastern coasts of Pata-
gonia, the Bay'of Fundy, the English Channel,
and the Irish Sea, have very high tides.
Near the heads of the Persian Gulf and China
Seas, the tides sometimes rise about 36 feet. At
the mouth of the Severn, the spring tides rise
from 45 to 48 feet; on the southern coast of the
English Channel, 50 feet; and in the Bay of
Fundy, near the head, the spring tides, aided by
favoring winds, sometimes reach 70 feet, and, oc-
casionally, even 100 feet.
A strong wind, blowing in the direction in which the
tidal wave is advancing, causes an increase in the height
of the tide.
A low barometer is attended by a higher tide than
usual; a high barometer, by a lower tide.
219. Other Tidal Phenomena.
The Bore or Eager.-On entering the estuary of a
river, the volume of whose discharge is considerable, the
onward progress of the tidal wave is checked; but, piling
up its waters, the incoming tide at last overcomes the re-
sistance of the stream, and advances rapidly, in several
huge waves. The tides of the Hoogly, the Elbe, the
Weser, and the Amazon, are examples. In the latter
river, the wave is said to rise from 30 to 50 feet.
Races and Whirlpools.-When considerable differ-
ences of level are caused by the tides, in parts of the ocean
separated by narrow channels, the waters, in their effort
to regain their equilibrium, move with great velocity, pro-
ducing what are called races. At times, several races meet
each other obliquely, thus producing whirlpools. Near the
Channel Islands, and off the northern coasts of Scotland,
races are numerous. The Maelstrom, off the coasts of Nor-
way, is an instance of a whirlpool, though the motion of
the waters is not exactly a whirling one. The main phe-
nomenon is a rapid motion of the waters, alternately back-
ward and forward, caused by the conflict of tidal currents
off the Loffoden Islands.
220. Constant Ocean Currents.-Besides tidal
currents, the waters of the ocean are disturbed
to great depths, by currents, moving with consid-
erable regularity to and from the equatorial and
polar regions, and thus producing a constant in-
terchange of their waters. These movements are
called constant currents, and, unlike waves, con-
sist in a real, onward movement of the water.
Constant currents resemble rivers, but are im-
mensely broader and deeper. As a rule, their
temperature differs considerably from that of the
waters through which they flow. They are not
confined to the surface, but exist as well at great
depths, when they are called under or counter cur-
rents, and flow in a direction opposite to that of the
221. The Principal Cause of Constant Ocean
Currents is the difference of density of the water
produced by the differences of temperature be-
tween the equatorial and the polar regions.
As the waters of the polar regions lose their
heat they become denser, and, sinking to the bot-
tom, form a mountain-like accumulation of dense,
cold water, which, as rapidly as formed, spreads
over the floor of the ocean underneath the lighter
waters. The consequent lowering of the level of
the polar waters causes an influx of the surface
waters from the equatorial regions. In this man-
ner a constant interchange is effected between the
80 PHYSICAL GEOGRAPHY.
equatorial and polar regions, which, for the greater
part, takes place along the bottom from the poles
to the equator, and along the surface from the
equator to the poles. Since, however, the pole is
a mere point, this interchange occurs mainly be-
tween the equator and the polar circles.
Fig. 76. Currents caused by Difference of Temperature.
Thus in Fig. 76, the mountain-like accumula-
tion is shown as having its crest at about the lati-
tude of the polar circle. The arrows show the
direction of the currents. At the equatorial re-
gions, the surface water is warmer and lighter,
and at the polar regions, probably, colder and
As a rule, the warm currents are on the surface, and the,
cold currents, from their greater density, are underneath
them. In shallow oceans, however, the cold currents come
to the surface, thus displacing the warm currents and de-
flecting them to deeper parts of the ocean.
Had the earth no rotation on its axis, this in-
terchange would be due north and south, or would
take place directly between the equatorial and
polar regions. On account of the earth's rota-
tion, however, and a variety of other causes,
these north-and-south directions are consider-
ably changed. The principal of these deflecting
(1.) The earth's rotation;
(2.) The position of the land masses
(3.) The winds;
(4.) Differences of density caused by evapora-
(5.) Differences of level caused by evapora-
The changes in direction caused by the earth's rotation
and the position of the land masses are as follows: as the
waters are in constant motion, the polar waters reach the
equatorial regions with an eastward motion less than that
of the earth. In the equatorial regions, therefore, the
waters are unable to acquire the earth's motion toward the
east, and are left behind; that is, the earth, slipping from
under them, causes them to cross the ocean at a, a', Fig.
77, from east to west, although they are in reality moving.
with the earth toward the east.
Reaching the western borders of the oceans, near b, b',
the continents prevent their going farther west, and de-
flect them into northern and southern branches, and they
begin to move toward the poles.
,r Frum c, to d, and from c', to d', the poleward-moving
waters are deflected toward the east in both hemispheres.
The waters on reaching c, from a, and 6, still retain the
eastward motion they acquired while moving with the
Fig, 77. Deflections of Ocean Currents.
earth. This motion is greater than that of the earth be-
tween c, and d. Between these points, therefore, the water
is acted on by two forces, one tending to carry it toward
the poles, and the other tending to carry it eastward.
The resultant of these forces carries the water from c, to
d, and from c', to d', or toward the north-east in the North-
ern, and toward the south-east in the Southern Hemisphere.
Between d, and e, and d', and e', the waters still retain
this excess of eastward motion, and, therefore, move in
the directions shown.
Between e, and a, and e', and a', the waters in both hemi-
spheres are deflected toward the west because they are
unable to acquire the earth's motion toward the east.
Another, and perhaps the main, cause of this westward
deflection is the depression caused by the westward move-
ment of the equatorial waters at a, and a'.
The action of the winds is to tend to move the surface
waters in the direction in which they are blowing. This
action is by some authorities regarded as the principal
cause of constant currents.
The difference in the density of the water, caused by
evaporation, leaving the water salter and denser in some
parts, and fresher and lighter in others, probably acts to
some extent as a deflecting cause. For example, the water
evaporated near the equator, and precipitated, for the
greater part, in regions near the borders of the tropics,
renders the regions salter and denser from which it was
evaporated, and fresher and less dense where it is precipi-
The difference in level caused by the greater evapora-
tion in the equatorial regions north of the equator than
in corresponding latitudes in the Southern Hemisphere
Shas been ascribed as one of the causes of the flow of Ant-
arctic waters toward the equator.
222. General Features of Constant Currents.-
The following motions of the surface currents are
common to all the three central oceans:
(1.) A movement of the equatorial waters, a, a,
from east to west;
(2.) Their deflection into northern and south-
ern branches (b and c), on reaching the western
borders of the ocean;
(3.) A movement of the waters beyond the
equator from west to east (d, e);
160 140 120 100 80 / 60
40 20 0 20 40 60 80 100 120 140 160 180
A. P '_&A .-. \, ..gE,
. -.-- t "'.-
.-- t- --
SMAP OF THE WORLD
showing the direction
O EAN CURRENTS.
I y'* i
SC -C -* Ll
.... i. 1
'~ : :c. ....,; 7!
--b- n ,-- -. .
r~~~~'ei ~ ~ ~J I .~rYt' 4-4p- 4 I U
! :// J:
'- -f i
. "" ,*.. IIJ ,/
---"-_ <.. I / ,I I
A V T A
C TI Ci 0c A-
180 240 120 122 88 20 40 20 22 40 60 80 IQO 220 240 280 180
160 1 140 120 100 80 60 40 20
20 40 o 80so lo 120 140 160 s180
-~-I--~Lr~--LL-' I:------~--~~-- -LC~
-' Ill"l-L -I I iii
c f OLC9ToL 0Cou0r1TE
I! ~- -- I I~
I'-~-I~~ Il:~;~il~ ~
Lir-'-~~--sc~'- -~~------~-CI-. iC~-L-~-M~L
,. ,, . ..- .V
(4.) A separation of these latter currents into
two branches (f, g and h, i), one continuing toward
Fig.78. Chart of Constant Currents.
the poles, and the other toward the equator, where
they join with the equatorial currents, thus com-
pleting a circuit in the shape of a vast ellipse;
Fig, 78, Chart of Constant Currents.
the poles, and the other toward the equator, where
they join with the equatorial currents, thus com-
pleting a circuit in the shape of a vast ellipse ;
(5.) A flow of the Arctic waters along the
western border of the ocean (j), and of the Ant-
arctic along the eastern (k).
Since the Indian Ocean is completely closed on the
north, only part of the above movements are observed.
In the Pacific, an equatorial counter-current crosses the
ocean from west to east.
223. Currents of the Atlantic.-The equatorial
current crosses the ocean, from east to west, in
two branches: a south equatorial current, which
comes from the Antarctic, and a north equatorial
current, which comes mainly from regions north
of the equator.
The north equatorial current flows along the
northern coast of South America, and, separating,
part of it enters the Caribbean Sea and Gulf of
Mexico, and part flows north, passing east of the
The Gulf Stream flows along the eastern coast
of North America, with a velocity of from four to
five miles per hour, and in mid-ocean, between
Newfoundland and Spain, divides, one branch
flowing toward Norway, Spitzbergen, and Nova
Zembla, the other flowing southward, down the
coasts of Africa, where it forms the main feeder
Sof the north equatorial current.
The south equatorial current, after crossing the
ocean, flows south along the Brazilian coast, and
divides near Rio Janeiro, the main part flowing
eastward and mingling with the Antarctic cur-
rent, and the remainder continuing down the east-
j atadadmnln ihteAtrtccr
ern coast of South America. Cold currents from
the Arctic flow down the coasts of Greenland and
Labrador. A broad polar current sweeps from
the Antarctic Ocean, and forms the main feeder
of the south equatorial current, but passes in
greater part eastward, south of Africa:
A small elliptical current flows near the equator,
between the north and south equatorial currents.
224. Currents of the Pacific.-North and south
equatorial currents flow from east to west, and
between them a smaller, less powerful equatorial
counter-current, from west to east. The south
equatorial current, fed by the broad Antarctic
current, is the larger of the two.
The -north equatorial current, on reaching the
Philippine Islands, divides into northern and
southern branches; a portion of its southern
branch returns with the equatorial counter-cur-
rent, while the northern branch, the main por-
tion, flows north-east along the Asiatic coast as
the Kuro Sivo, the counterpart of the Gulf
Stream. At about Lat. 500, this flows east-
wardly as a North Pacific current, and off the
shores of North America it returns, in an ellip-
tical path, southerly to the north equatorial cur-
rent, forming its main feeder. A small current
flows through the eastern side of Bering Strait,
into the Arctic Ocean.
The south equatorial current of the Pacific is
broken into numerous branches during its passage
through the islands in mid-ocean. Reaching the
Australian continent and the neighboring archi-
pelagoes, it sends small streams toward the north,
but the main portion flows south, along the Aus-
tralian coast, when, flowing eastward, it merges
with the cold Antarctic current.
The Antarctic current moves as a broad belt
of water toward the north-east, when, flowing up
the western coast of South America, it turns to
the west, and forms the main feeder of the south
equatorial current. A part of the Antarctic cur-
rent flows eastward, south of South America, and
enters the Atlantic as the Cape Horn current.
A small cold current from the Arctic flows
through Bering Strait, down the Asiatic coast.
225. Currents of the Indian Ocean.-Only a
south equatorial current exists, which flows down
the eastern and western coasts of Madagascar, and
down the African coast to Cape Agulhas, when,
turning eastward, it merges with the Antarctic
current, and flows up the western coast of Aus-
tralia, where it joins the equatorial current.
The north equatorial current in this ocean is indistinct-
(1.) Because the ocean has no outlet to the north;
(2.) Powerful seasonal winds, called the monsoons, move
the waters alternately in different directions, as huge drift
Sargasso Seas.-Near the centre of the ellip-
tical movement in each of the central oceans,
masses of seaweed have collected where the water
is least disturbed. These are called sargasso seas.
226. Utility of Currents:
(1.) They moderate the extremes of climate by
carrying the warm equatorial waters to the poles,
and the cold polar waters to the equator;
(2.) They increase materially the speed of ves-
sels sailing in certain directions;
(3.) They transport large quantities of timber
to high northern latitudes.
Ocean water contains about three and one-third pounds
of various saline ingredients, in every one hundred. 'Chlo-
ride of sodium; sulphates and carbonates of lime, mag-
nesia, and potassa; and various chlorides, bromides, and
iodides, are the principal saline ingredients.
The salt of the ocean is derived either from the
washings of the land, or is dissolved out from the por-
tions of the crust which are continually covered by its
The ocean is salter in those parts where the evaporation
exceeds the rainfall. Seas like the Mediterranean, which
are connected with the ocean by narrow channels, and in
which the evaporation is greater than the rainfall, are
salter than the ocean. Others, like, the Baltic, in which
the rainfall exceeds the evaporation, are fresher than the
Most of the bed of the ocean is covered with a layer of
dense water, at about the temperature of its maximum
The Pacific and Atlantic Oceans occupy about three-
fourths of the entire water-area of the earth.
South of the southern extremities of South America,
Africa, and Australia, the meridians of Cape Horn, Cape
Agulhas, and South Cape in Tasmania, are assumed as
the eastern boundaries of the Pacific, Atlantic, and Indian
The articulation of land and water assumes four distinct
forms: Inland Seas, Border Seas, Gulfs and Bays, and Fiords.
Inland Seas characterize the Atlantic; Border Seas, the Pa-
cific; Gulfs and Bays, the Indian Ocean; and Fiords, the
Atlantic and Pacific.
The telegraphic plateau lies between Ireland and New-
foundland. Its average depth is about two miles.
The bottom of the ocean is not as much diversified as
the surface of the land. Its plateaus and plains are be-
lieved to be much broader than are those of the land. The
profound valleys of the ocean are called deeps, its shallow
The greatest depth of the ocean that has as yet been
accurately sounded is about 5J miles. It is probably deeper
than this in some places.
Over extended areas, the floor of the ocean is'uniformly
covered with a deposit of fine calcareous mud or ooze,
formed of the hard parts of the bodies of minute animal-
The movements of the oceanic waters may be arranged
under the three heads: waves, tides, and currents.
The height and velocity pf a wave depend upon the
force of the wind and the depth of the oceanic basin.
In ordinary wave motion, the water rises and falls, but
does not move forward.
Tides are the periodical risings and falling of the water,
caused by the attraction of the sun and moon.
The rising of the water is called flood tide; the falling,
If the earth were uniformly covered with a layer
of water, two high tides would occur simultaneously;
one on the side of the earth directly under the sun
or moon, the other on the side farthest from the sun
The tidal wave crosses the ocean from east to west, fol-
lowing the moon in the opposite direction to that in which
the earth passes under it while rotating. Its progress is
considerably retarded by the projections of the continents,
and the shape of the oceanic beds. Had the moon no real
motion around the earth, there would be two high and
two low tides every twenty-four hours, or the high and
low tides would be exactly six hours apart.
Spring Tides are caused by the combined attractions-of
the sun and moon on the same portions of the earth. Neap
tides by their opposite attractions.
The parent tidal wave is considered as originating in
the great water-area of the Pacific on the south.
Co-tidal lines are lines connecting places which have
high tides at the same time.
When the progress of the tidal wave is retarded by the
shelving coast of a continent, what the tide loses in speed,
it gains in height. The highest tides, therefore, occur
where the co-tidal lines are crowded together.
Bores, Races, and Whirlpools are tidal phenomena.
Oceanic currents are either temporary, periodical, or
The heat of the sun and the rotation of the earth are
the main causes of constant oceanic currents.
The followiLg peculiarities characterize the constant
currents in the three central oceans:
(1.) A flow in the equatorial regions from the east to
(2.) A flow in extra-tropical regions from the west to
(3.) A division of the eastwardly flowing extra-tropical
waters in mid-ocean into two branches; one of which
flows toward the poles, and the other toward the equator,
where it merges into the equatorial currents.
84 PHYSICAL GEOGRAPHY.
The principal cause of constant ocean currents is the
difference in the density of the equatorial and polar
waters, produced by differences of temperature.
The cold, dense waters of the polar regions tend to mix
with the warm, light waters of the equatorial regions
along due north-and-south lines. This tendency to north
and south direction is prevented by the following causes:
(1.) The rotation of the earth;
(2.) The position of the continents;
(3.) The direction of the winds;
(4.) The difference in the saltness of the water;
(5.) The inequality of the evaporation and rainfall.
In the Pacific, a counter-current crosses the ocean in the
equatorial region, from west to east.
In the Indian Ocean, the directions of the currents are
modified by the land masses, which surround the northern
part of its bed.
In the northern hemispheres, the western borders of the
oceans are colder than the eastern borders in the same lati-
tude, because the former receive the polar currents and the
latter the equatorial.
Currents moderate the extremes of climate, by carry-
ing the warm equatorial waters to the poles, and the cold
polar waters to the equator.
How much heavier is salt water than fresh water?
What is the freezing-point of ocean water?
Explain the origin of the saltness of the oceanic waters.
In the equatorial region, where is the water the colder,
at the surface or near the bottom of the ocean ?
How do the areas of the Pacific and Atlantic compare
with each other in size? Of the Antarctic and Arctic?
Define inland sea; border sea; gulf or bay; fiord; give
examples of each.
Define deeps; rises.
What, most probably, is the shape of the bed of the At-
lantic? Of the Pacific? Of the Indian Ocean?
Describe the Telegraphic Plateau.
How does the greatest depth of the ocean compare with
the greatest elevation of the land?
Upon what does the height of a wave depend ? On what
does its velocity depend?
What proof is there that during wave motion in deep
water there is no continued onward motion of the water?
Distinguish between ebb and flood tides.
What proofs have we that tides are occasioned mainly
by the attraction of the moon?
What are spring tides? Neap tides? During what
phases of the moon do they each occur?
Why should the moon, which is so much smaller than
the sun, exert a more powerful influence in producing
Where does the parent tidal wave originate ?
What are co-tidal lines?
Why does the tidal wave progress from east to west?
Explain the nature of the influence which the tidal
wave exerts on the rotation of the earth.
In what parts of the ocean will unusually high tides
By what are races and whirlpools occasioned?
Distinguish between temporary, periodical, and constant
Explain the origin of constant currents. How are the
directions of constant currents affected by the rotation of
the earth and the shapes of the continents?
What features of constant currents are common to each
of the three central oceans?
On which side of the northern oceans do the polar cur-
rents flow? On which side of the southern oceans?
What are sargasso seas? How are they formed?
What effect is produced by ocean currents on the ex-
tremes of climate?
Of what value are ocean currents to navigation?
Point out, on the map of the river-systems, the inland
seas of the Atlantic; of the Pacific; of the Indian Ocean.
Point out the border seas of the Atlantic; of the
Point out the gulfs or bays of the Atlantic; of the In-
Point out the principal regions of fiords.
How many hours does it take the tidal wave to progress
from Tasmania to the Cape of Good Hope? From Tasma-
nia to Newfoundland? From Tasmania to the British
Isles? (See map of the co-tidal lines.)
In what parts of the Atlantic does the tidal influence
progress most rapidly?
If the velocity of any kind of wave motion in water in-
creases with the depth of the basin, what parts of the At-
lantic appear to be the deepest? What portions of the
Pacific? What portions of the Indian Ocean?
Trace on the map of the ocean currents, the motion
of the Antarctic currents in each of the three central
Where is the Cape Horn current? Is it hot or cold?
What points of resemblance exist between the north
and south equatorial currents in the Atlantic and Pacific
Trace the progress of the Gulf Stream.
What points of resemblance exist between the Gulf
Stream and the Japan current?
How far to the north-east do the waters of the Gulf
What distant shores are warmed by the waters of the
Gulf Stream? By those of the Japan current?
Why do not the heated waters of the Gulf Stream exert
a more powerful influence on the climate of the eastern
sea-board of the United States?
Point out the, principal cold currents; the principal
Which currents would aid, and which would retard, the
progress of a vessel in sailing from New York to San Fran-
cisco? From America to Europe? .From America to India
WE live at the bottom of a vast ocean of air, which, like the ocean of water, is subject to three
general movements-waves, tides, and currents. By means of waves, its upper surface is heaved in
huge mountain-like masses in one place, and hollowed out in deep valleys in another. By means of
currents, circulatory movements are set up, which effect a constant interchange between the air of the
equatorial and the polar regions. By means of tides, the depth of the atmosphere is increased in some
places and decreased in others.
Of these three movements of the atmosphere, currents are of the greatest importance. Aerial cur-
rents, or winds, are similar to oceanic currents, but are more extensive and rapid, owing to the greater
mobility of air.
By retaining and modifying the solar heat, absorbing and distributing moisture, supplying animals
with oxygen and plants with carbonic acid, the atmosphere plays an important part in the economy
of the earth.
Meteorology is the science which treats of the atmosphere and its phenomena.
General Properties of the Atmo-
227. Composition.-The atmosphere is a me-
chanical mixture of nitrogen and oxygen, in the
proportion, by weight, of nearly 77 per cent. of
nitrogen to 23 per cent. of oxygen. To these
must be added a nearly constant quantity of car-
bonic acid, about 5 or 6 parts in every 10,000
parts of air, or about a cubic inch of carbonic acid
to every cubic foot of air, and a very variable pro-
86 PHYSICAL GEOGRAPHY.
portion of watery vapor. The gaseous ingredients,
though of different densities, are found in the
same relative proportions at all heights, owing
to a property of gases called diffusion.
The oxygen and carbonic acid are the most important
of the gaseous constituents. Oxygen supports combustion
and respiration, and is thus necessary to the existence of
animal life. Carbonic acid, composed of carbon and oxy-
gen, is the source from which vegetation derives its woody
fibre, and is thus necessary to the existence of plant life.
In respiration, animals take in oxygen and give out car-
bonic acid; in sunlight, plants take in carbonic acid and
give out oxygen. In this way the relative proportions of
the substances necessary to the existence of animal and plant
life are kept nearly constant.
228. Elasticity.-The atmosphere is eminently
elastic; that is, when compressed, or made to oc-
cupy a smaller volume, it will regain its original
volume on the removal of the pressure. Air also
expands when heated and contracts when cooled.
229. Pressure.-So evenly does the atmosphere
press on all sides of objects that it was long be-
fore it was discovered that air possesses weight.
The discovery was made by Torricelli, an Italian
philosopher and pupil of the famous Galileo. The
instrument Torricelli employed is called a Ba-
ig, 79. Barometer,
230. The Barometer.-The principle of the barometer
is as follows: A glass tube, about 33 inches in length, is
closed at one end and filled with pure mercury. Placing
a finger over the open end, the tube is reversed and dipped
below the surface of mercury in a cup or other vessel.
On removing the finger, a column of mercury remains in
the tube, being sustained there by the pressure of the at-
mosphere. Near the sea-level this column is about 30
inches high; on mountains it is much lower; in all cases,
the weight of the mercurial column being equal to that
of an equally thick column of air, extending from the
level of the reservoir to the top of the atmosphere.
Any variation in the pressure of the atmosphere is
marked by a corresponding variation in the height of the
mercury in the barometer, the column rising with in-
creased, and falling with diminished, pressure.
The entire atmosphere presses on the earth
with the same weight as would a layer of mer-
cury about 30 inches in depth. A column of
mercury 30 inches high, and one square inch in
area of cross section, weighs about 15 pounds.
Therefore, the pressure which the atmosphere exerts
on the earth's surface, at the level of the sea, is equal
to about 15 pounds for every square inch of surface.
The entire weight of the atmosphere, in pounds,
is equal to 15 times the number of square inches
in the earth's surface.
The atmospheric pressure is not uniform on all parts of
the earth at the same level. From a few degrees beyond
the equator the pressure increases in each hemisphere up
to about lat. 350, where it reaches its maximum, decreasing
in the northern hemisphere to lat. 650, when it again in-
creases toward the poles.
231, Height of the Atmosphere.-If the air
were everywhere of the same density, its height
could be easily calculated; but, on account of its
elasticity, the lower layers are denser than the
others, because they have to bear the weight of
those above them. The density must, therefore,
rapidly diminish as we ascend.
If by pressure on a gas we diminish its volume one-
half, its density will be doubled; conversely, if the den-
sity be diminished one-half, the volume will be doubled.
The following table, calculated from the law of increase
in volume with diminished pressure, gives the barometric
height, the volume, and the density of the air at different
elevations above the sea. The elevation of 3.4 miles is the
result of observation; the other distances are estimated.
Barometric Vol. of Given Density Estimated Distance
Height in Inches. Weight of Air. ab. Sea, in Miles.
30.00 1 1 0.0
15.00 2 3.4
7.50 4 4 6.8
3.75 8 4 10.2
1.87 16 A 13.6'
.93 32 A 17.0
It appears from the above table that by far the
greater part of the air by weight lies within a few
miles of the surface, nearly three-fourths being
below the level of the summits of the highest
The height of the upper limit of the atmosphere
has been variously estimated. Calculations based
upon the diminution of pressure with the height,
place it at from 45 to 50 miles above the level of
the sea; others, based on the duration of twilight,
place it at distances varying from 35 to 200 miles.
The form of the atmosphere is that of an ob-
late spheroid, the oblateness of which is greater
than that of the earth.
By carefully observing the decrease in pressure with the
elevation, at different altitudes, and making proper correc-
tions, the heights of mountains can be readily determined
by the barometer. The measurement of heights by the
barometer, or similar means, is called Hypsometry.
232. The Climate of a country is the condi-
tion of its atmosphere as regards heat or cold.
The climate of a country also embraces the con-
dition of the air as regards moisture or dryness,
and healthiness or unhealthiness, which are de-
pendent on the temperature.
233. Temperature.-The temperature of the
atmosphere is determined by means of an instru-
ment called a thermometer.
The thermometer consists of a glass tube of very fine
bore, furnished at one end with a bulb. The tube is care-
fully dried and the bulb filled with pure mercury and
heated in the flame of a spirit-lamp; the mercury expands,
and, filling the fine capillary tube, a portion runs out
from the open end, thus effectually expelling the air. A
blowpipe flame is then directed against the open end and
the tube hermetically sealed. As the bulb cools, the mer-
cury contracts, and leaves a vacuum in the upper part of
the tube. The instrument will now indicate changes in
temperature; for, whenever the bulb grows warmer, the
column of mercury expands and rises; and when it grows
colder, it contracts and falls.
In order to compare these changes of level they are
referred to certain fixed or standard points: the freezing-
and boiling-points of pure water. These are obtained by
marking the respective heights to which the mercury rises
when the thermometer is plunged into melting ice and
into the steam escaping from boiling water. In Fahren-
heit's scale the freezing-point is placed at 320, the boil-
ing-point at 2120, and the space between these two points
divided into 180, (212 -32) equal parts, called degrees. In
the Centigrade scale the freezing- and boiling-points are re-
spectively 0 and 1000. Fahrenheit's degrees are repre-
sented by an F., thus, 2120 F.; Centigrade's by a C., as
234. Astronomical and Physical Climates.-
Astronomical climate is that which would result
were the earth's surface entirely uniform and of
but one kind: all land or all water.
Physical climate is that which actually exists.
Since the physical climate is only a modification of the
astronomical, we shall briefly review the causes which
tend to produce a regular decrease in temperature from
the equator to the poles.
Astronomical Climate.-The sun is practically
the only source of the earth's heat. On account
of the earth's spherical shape, those portions of
the surface are most powerfully heated which re-
ceive the vertical rays, and these are confined to
a zone reaching 230 27' on each side of the equa-
tor. Beyond these the rays fall with an obliquity
which increases as we approach the poles.
235. Causes of the greater heating power of
the vertical rays of the sun than of the oblique
/ i i
SI I /
I I I
i i i f i i
Pig, 80, Causes of the Greater Heating Power of the Vertical
than of the Oblique Rays.
(1.) The vertical rays are spread over a smaller
area. Equal areas of the sun's surface give off
equal quantities of heat. If, therefore, the bun-
dle of rays a b, and c d, come from equal areas,
the amounts of heat they emit will be equal; but
while the heat given off from a b, the more ver-
tical rays, is spread over the earth's surface from
f, to g, that from c d, is spread over the greater
area h i; the area f g, therefore, which receives
the more vertical rays, is much warmer than h i,
where the obliquity is greater.
(2.) The vertical rays pass through a thinner
layer of air. Only a part of the sun's heat
reaches the surface of the earth; about 28 per
cent. of the vertical rays are absorbed during their
passage through the atmosphere. The amount of
this absorption must increase as the length of
path increases. In the figure, the light shading
represents the atmosphere. It is clear that the
oblique rays pass through a thicker stratum of
air than the more direct ones, and, therefore, are
deprived of a greater amount of heat.
According to Laplace, the thickness of the stratum of air
traversed by the rays when the sun is at the horizon is
35.5 times greater than when it is directly overhead. A
similar absorption of light affects the comparative bright-
ness of daylight in different latitudes.
(3.) The vertical rays strike more directly,
and, therefore, produce more heat. The heating
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NLIP OF THE WORILD c- .1 REFERENCES ...................... ..........
1$- WOTHERMIAL U!NIES' C Torrid Zone. c te. The iurvs after te. names of Cities,gve the average
and the bor ndaries of the i Temperate Zones. temperates for Januaryand JcUthus, Quebec /O.O didicates that
PHYSICAL ZONES. Frigid ons. the avoe teratre for Jan a yi0D.' adfbrJ aly 6
180 I O 140 120 100 80 6 4O0 20 0 20 0 80 100 120 nO m w .O 160 .
180 160 140 120 100 80 63 40 20 0 20 40 60 80 100 120 140 160 180
17. wy cld ^^
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180ISO 160 140 o 120 / 100
1 o80 j 60
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o 80 100 \ 120 140
power of the more nearly vertical rays is greater
than that of the rays which strike obliquely.
236. Variations in Temperature.-The differ-
ences in the heating power of the vertical and ob-
lique rays of the sun cause the temperature of
the earth's surface to decrease gradually from the
equator toward the poles. The differences of tem-
perature thus effected are further increased by the
difference in the length of daylight and darkness.
While the sun is shining on any part of the earth
the air is gaining heat; when it is not shining the
air is losing heat.. When the length of daylight
exceeds that of the darkness, the gain exceeds
the loss; when the darkness exceeds the day-
light, the loss exceeds the gain.
The excessively low temperatures that would
result from the oblique rays in high latitudes are
prevented by the great length of daylight during
the short summers, thus allowing the sun to con-
tinue heating the surface during longer periods.
The warmest part of the day in high latitudes
sometimes equals that in the equatorial regions.
During the long winters, however, the continued
loss of heat makes the cold intense.
Hence in the tropics we find a continual sum-
mer; in the temperate-zones, a summer and winter
of nearly equal length; and in the polar zones,
short, hot summers, followed by long, intensely cold
The true temperature of the air is ascertained by hang-
ing a thermometer a few feet above the ground, so as to be
shielded from the direct rays of the sun, and yet be in free
contact on all sides with the air.
237. Manner in which the Atmosphere re-
ceives its Heat from the Sun.-The atmosphere
receives its 'heat from .the sun-
(1.) Directly. As the sun's rays pass through
the air, about 28 per cent. of the vertical rays
are directly absorbed, thus heating the air: The
remainder pass on and either heat the earth, or
are reflected from its surface.
(2.) From the heated earth. The sun's rays
heat the earth and the heated earth heats the air.
It does this in three ways:
(a.) By the air coming in contact with the
(b.) By the heated earth radiating its heat, or
sending it out through the air in all directions.
After the sun's heat has been absorbed by the
earth and radiated from it, a change occurs.which
renders the rays much more readily absorbed by
(c.) By the heat being reflected from the earth
and again sent through the air. But little heat
is imparted to the air in this way.
It is mainly the aqueous vapor the atmosphere
contains that absorbs the sun's heat. Dry air
allows the greater part of the heat to pass through
it; therefore variations in the quantity of vapor in
the air must necessarily produce corresponding
variations in the distribution of heat.
238. Isothermal Lines are lines connecting
places on the earth which have the same mean
The Mean Daily Temperature of a place is ob-
tained by taking the average of its temperature
during twenty-four consecutive hours.
The Mean Annual Temperature of a place is
the average of its mean daily temperature
throughout the year.
If the physical climate were the same as the
astronomical, the isothermal lines would coincide
with the parallels of latitude.
An inspection of the map of the isothermal lines shows
that their deviations from the parallels, though well
"marked in all parts of the earth, are greatest in the north-
ern hemisphere. Wherever, from any cause, the mean tem-
perature of a place is higher, the isothermal lines are found
nearer the-poles; when lower, nearer the equator. The former
effects are noticedvparticularly in portions of the ocean
traversed by warm currents; the latter, in crossing por-
tions of the ocean traversed by cold currents. In the map
of the isothermal lines the influence of elevation is re-
moved by adding 10 for every 1000 feet of elevation.
239. Physical Zones.-The Physical Torrid
Zone lies on both sides of the equator, between:
the annual isotherms of 700 Fahr.
The Physical Temperate Zones lie north and
south of the Physical Torrid Zone, between the
annual isotherms of 700 and 300 Fahr.
The Physical Frigid Zones lie north and south
of the Physical Temperate Zones, from the an-
nual isotherms of 300 Fahr. to the poles.
The greatest mean annual temperature in the
eastern hemisphere is found in portions of North
Central Africa, and in Arabia near the Red Sea,
in the southern part of Hindostan, and in the
northern part of New Guinea and the neighbor-
ing islands; in the western hemisphere, in the
northern parts of South America and in CentRal
"240. Modifiers of Climate.- The principal
causes which prevent the isothermal lines from
coinciding, with the parallels of latitude are:
(1.) The Distribution of the Land and Water
Areas.-Land heats or cools rapidly, absorbing or
emittiIg but little heat. This is because the land
90 PHYSICAL GEOGRAPHY.
has a small capacity for heat, and also because
the heat passes through but a comparatively thin
layer. Therefore, a comparatively short exposure
of land to heat produces a high temperature, and
a comparatively short exposure to cooling, a low
temperature. Water heats or cools slowly, ab-
sorbing or emitting large quantities of heat. This
is because water has a great capacity for heat.
The heat penetrates a comparatively deep layer,
and then, too, as soon as slightly heated, the warm
water is replaced by cooler water. Therefore, the
water can be exposed to either long heating or
long cooling without growing very hot or very
cold. Hence, the land is subject to great and
sudden changes of temperature; the water, to
small and gradual changes.
Places situated near the sea have, therefore, a
more equable, uniform climate than those in the
same latitude in the interior of the continent.
The former are said to have an oceanic climate;
the -latter, a continental climate.
In the polar regions, a preponderance of moder-
ately elevated land areas causes a colder climate than
an equal area of water, because land loses heat
more rapidly than water.
In the tropics, a preponderance of land areas
causes a warmer climate than an equal area of water,
because land gains heat more rapidly than water.
(2.) The Distribution of the Relief Forms of
the Land Masses.
(1.) Elevation.-The temperature of the atmo-
sphere rapidly decreases with the elevation. The
decrease is about 30 Fahr. for every 1000 feet.
The increased cold is caused as follows:
(1.) Since the air receives so much of its heat indirectly
from the earth's surface, the farther we go upward from
the surface, the colder it grows.
(2.) In the upper regions of the atmosphere the de-
creased density and humidity of the air prevent it from ab-
sorbing either the direct rays of the sun, or those reflected
or radiated from the earth. The effect of elevation is so
powerful that on the sides of high tropical mountains the
same changes occur in the vegetation that are observed in
passing from the equator to the poles.
(2.) Direction of the Slopes.-That slope of
an elevation on which the sun's rays fall in a di-
rection the more nearly at right angles to its sur-
face will be the warmest.
In the northern hemisphere the southern slope of a hill
is warmer in winter than the northern slope, because the
rays fall more nearly at right angles to its surface. -
(3.) Position of the Mountain-Ranges.-A
mountain-range will make the country near it
warmer if the wind from which it shields it is
cold; it will make it colder if such wind is
The position of the mountain-ranges of a country also
greatly affects the distribution of its rainfall. Thus, the
tropical Andes are well watered and fertile on their east-
ern slopes, but dry and barren on their western. The pre-
vailing moist trade winds, forced to ascend the slopes,
deposit all their moisture on them in abundant showers,
and are dry and vaporless when they reach the other side.
(4.) Nature of the Surface.-The temperature
of a tract of land is greatly affected by the nature
of its surface. If covered with abundant vege-
tation, like a forest, or if wet and marshy, its sur-
face heats and cools slowly, and has a compara-
tively uniform temperature; but if destitute of
vegetation, and dry, sandy, or rocky, it both
heats and cools rapidly, and is subject to great
extremes of temperature.
(3.) Distribution of Winds and Moisture.-The
principal action of the winds, and their accom-
panying moisture, is to moderate the extremes of
temperature by the constant interchange between
the heat of the equatorial and the cold of the
polar regions. Both wind and vapor absorb and
render latent large quantities of heat in the equa-
torial regions, and give it out, in higher latitudes,
on cooling. In cold countries the climate is ren-
dered considerably warmer by the immense quan-
tity of heat thus emitted by the condensed vapor.
(4.) Ocean Currents.-Since the warm waters
move to the polar regions, and the cold waters to
the equatorial regions, the general effect of ocean
currents on climate is to reduce the extremes of
The combined effects of the action of the winds,
moisture, and ocean currents are seen in the north-
ern continents, whose western shores, under the in-
fluence of the prevailing south-westerly winds,
copious rains, and tropical currents, are consider-
ably warmer than the eastern shores in the same
The coasts of Great Britain are warm and fertile, while
Labrador, in the same latitude, is cold and sterile. The
island of Sitka, in the Pacific, is warmer than Kamtchatka
from similar causes.
--------o~ o ,--- -
241. Origin of Winds.-Winds are masses of
air in motion. They resemble currents in the
ocean, and result from the same causes-differ-
THE WINDS. 91
ences of density caused by differences of tem-
/ 1/ A, 1| ~ I b 7
Fig, 81. Origin of Winds. '-, *" ,-
The equilibrium of the atmosphere is disirbed
by differences of temperature as follows: When
any area becomes heated, as at a a, Fig. 81, the
air over it, expanding and becoming lighter, is
pressed upward by the colder air which rushes
in from all sides. Thus result the following
currents: ascending currents, b b, over the heated
area; lateral, surface currents, c c, from all sides
toward the heated area; upper currents, d d, from
the heated area; and descending currents, e e.
It is the lateral currents which flow toward or
from the heated area that are felt mainly as
winds. The ascending currents rise until they
reach a stratum of air of nearly the same den-
sity as their own, and then spread laterally in
all directions toward the areas where the air
has been rarefied by the movements of the lat-
eral surface currents, until they finally descend,
and recommence their motion toward the heated
area. These circulatory motions continue as long
as the heated area remains warmer than surround-
In speaking of winds, reference is always made to the
surface currents, unless otherwise stated.
242. Origin of the Atmospheric Circulation.-
The hottest portions of the earth are, in general,
within the tropics; hence in the equatorial regions
ascending currents continually prevail. To sup-
ply the partial vacuum so created, lateral sur-
face currents blow in toward the equator from
the poles, while the ascending currents, after
reaching a certain elevation, blow as upper cur-
rents toward the poles. Thus result currents by
which the entire mass'of the atmosphere is kept in
constant circulation, and an interchange effected
between the air of the equator and the poles.
The most important of these currents are the
(1.) Polar currents, or the lateral surface cur-
rents, which flow from the poles to the equator;
(2.) Equatorial currents, or the upper currents,
which flow from the equator toward the poles.
It will be noticed that wherever the surface
wind blows in any given direction, the upper
wind blows in the opposite direction.
In several instances the ashes of volcanoes have been
carried great distances in directions opposite to that in which
the surface wind was blowing. The smoke from tall chim-
neys at first takes the direction of the surface wind, but
rising, is soon carried in the opposite direction by the
upper currents. The clouds are often seen moving in a
direction opposite to that indicated by vanes placed on
the tops of the houses.
A current of air is named according to the di-
rection from which it comes; a current of water,
according to the direction in which it is going.
Thus, a north-east wind comes from the north-
east; a north-east current of water goes toward
243. Effect of the Earth's Rotation on the
Direction of the Wind.-Were the earth at rest,
the equatorial and polar currents would blow due
north and south in each hemisphere; but by the
rotation of the earth they are turned out of their
course in a manner similar to the oceanic currents
The polar currents, as they approach the equa-
tor, where the axial velocity toward the east is
greater, are left behind by the more rapidly mov-
ing earth, and thus come, as shown in Fig. 83,
from the north-east in the northern hemisphere,
and from the south-east in the southern.
The equatorial currents, under the influence of
the earth's eastward motion, are carried toward
the east as they approach the poles, and thus
come, as shown in Fig. 83, from the south-west in
the northern hemisphere, and from the north-west
in the southern.
Wherever the polar winds prevail, their direc-
tion, when unaffected by local disturbances, will
be north-east in the northern hemisphere, and south-
east in the southern. Near the equator their di-
rection is nearly due east.
Wherever the equatorial currents prevail, their
direction will be south-west in the northern hemi-
sphere, and north-west in the southern.
In Fig. 82, the equatorial currents are repre-
sented as continuing to either pole as upper cur-
rents, and the polar winds as surface currents to
the equator. If this were so, constant north-east-
erly winds would prevail in the northern hemi-
92 PHYSICAL GEOGRAPHY.
Fig. 82. Direction of Wind as Affected by Rotation. Fig, 83, Interohange of the Equatorial and Polar Currents. Wind Zones,
sphere, and constant south-easterly winds in the
southern. Several causes, however, exist to pre-
vent this simple circulation of the air between the
equatorial and polar regions.
The equatorial currents do not continue as upper
currents all the way to the poles, but fall and become
surface currents, replacing the polar winds, which
rise and continue for a while toward the equator as
244. Causes of Interchange of Surface and
Upper Currents.-The causes which produce this-
shifting of the equatorial and polar currents are:
(1.) The equatorial currents become cold-
(a.) By the cold of elevation;
(b.) By expansion;
(c.) By change of latitude.
The equatorial currents therefore fall and are
replaced by the polar currents, which have been
gradually growing warmer by continuing near
the surface of the earth.
(2.) As the equatorial currents approach the
poles they have a smaller area over which to
spread, and, being thereby compressed, are caused
to descend and become surface currents.
This interchange between the equatorial and polar cur-
rents takes place at about lat. 300. It varies, however,
with the position of the sun, moving toward the poles
when the sun is nearly overhead, and toward the equator
when the sun is in the other hemisphere.
The interchange in the position of the equatorial and
polar currents is represented in Fig. 83.
As the equatorial currents fall, they divide,
part going to the poles, and part returning to the
The general system of the aerial circulation
thus indicated is more regular over the oceans
than over the land. Over the continents the
greater heat of the land during summer causes
a general tendency of the wind to blow toward
the land; similarly, the greater cold of the land
during winter causes a tendency of the wind to
blow toward the sea.
245. Classification of Winds.-Winds are di-
vided into three classes:
(1.) Constant, or those whose direction remains
the same throughout the year.
(2.) Periodical, or those which, for regular pe-
riods, blow alternately in opposite directions.
(3.) Variable, or those which blow in any di-
\ 246. Wind Zones.-The principal wind zones
are the zone of calms, the zones of the trades, the
zones of the calms of Cancer and Capricorn, the
zones of the variable winds, and the zones of the
Zone of Calms.-In parts of the ocean near the
equator the ascending currents are sufficiently
powerful to neutralize entirely the inblowing
polar currents, and thus produce a calm, which,
however, is liable at any moment to be disturbed
by powerful winds. The boundaries,of the zone
vary with the season; they extend from about 20
to 110 north latitude.
Zones of the Trades.--From the limits of the
zone of calms to about 300 on each side of the
equator the polar currents blow with great steadi-
ness throughout the year. The constancy in their
direction has caused these winds to be named
"trade winds," from their great value to com-
merce. Their direction is north-east in the north-
ern hemisphere, and south-east in the southern.
Zones of the Calms of Cancer and Capricorn.
-Between the zones of the trades and the vari-
ables, where the interchange takes place between
the equatorial and polar currents, zones of calms
occur. Their boundaries are not well defined,
and are dependent on the position of the sun.
Zones of the Variable Winds.-Beyond the
limits of the preceding zones to near the latitude
of the polar circles, the equatorial and polar cur-
rents alternately prevail. Here the equatorial
and polar currents are continually striving for
the mastery, sometimes one and sometimes the
other becoming the surface current. During
these conflicts the wind may blow from any
quarter; but when either current is once estab-
lished it often continues constant for some days.
This is especially the case over the ocean, where
the modifying influences are less marked.
Though the winds in these zones are variable,
still two directions predominate: south-west and
north-east in the northern hemisphere, and north-
west and south-east in the southern. Westerly
winds, however, occur the most frequently in
nearly all parts of these zones.
The equatorial currents are sometimes called the Return
Trades, or the Anti-trades, because they blow in the oppo-
site direction to the trades.
Between about lat. 250 and 400, N. and S., over parts of
the ocean, the winds are nearly periodical, blowing during
the hotter portions of the year in each hemisphere from
the poles, and during the remainder of the year from the
equator. This zone is often called the Zone of the Sub-
Polar Zones.-From the limits of the zones of
the variables to the poles, there are regions of pre-
vailing polar winds. These winds are most fre-
quently north-east in the northern hemisphere,
and south-east in the southern.
247. Dove's Law of the Rotation of the Winds.-
The equatorial and polar currents usually displace each
other, and become surface winds in a regular order, first
discovered by Prof. Dove of Berlin.
In the northern hemisphere, before the polar current is
permanently established from the north-east, the wind
blows in regular order from the west, north-west, and north.
.The displacement of the polar by the equatorial currents
occurs in the opposite direction: from the east, south-east,
and south, before the general south-west current is perma-
In the southern hemisphere these motions are reversed.
This rotation of the winds, together with the effects
produced on the thermometer and barometer, is indicated
in the following diagram. Since the equatorial currents
are warm, moist, and light, when they prevail the ther-
mometer rises and the barometer falls. On the establish-
ment of the polar currents, however, the thermometer
falls and the barometer rises.
--..A -- "'.
Fig. 84, Rotation of the Winds (after Dove).
The warm waves" of the zones of the variable
winds are caused by the prevalence of the equa-
torial currents. Similarly, the "cold waves" are
caused by the prevalence of the polar currents.
248. Land and Sea Breezes.-During the day
the land near the coast becomes warmer than the
sea. An ascending current, therefore, rises over
the land, and a breeze, called the sea breeze, sets
in from the sea. At night the land, from its
more rapid cooling, soon becomes colder than the
water; the ascending current then rises from the
Fig. 85, Land and Sea Breezes.
water, and a breeze, called the land breeze, sets in
from the land. The strength of these winds de-
pends upon the difference in the temperature of
the land and water; they are, therefore, best de-
fined in the tropical and extra-tropical regions,
though they may occur in higher latitudes during
the hottest parts of the year. Land and sea
breezes are periodical winds.
, 49. Monsoons are periodical winds, which dur-
ng part of the year blow with great regularity in
one direction, and during the remainder of the
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THE WINDS. 95
year in the opposite direction. They are in real-
ity huge land and sea breezes, caused by the dif-
ference in temperature between the warmer and
colder halves of the year. They occur mainly in
the regions of the trades, and are in reality trade
winds which have been turned out of their course
by the unequal heating of land and water.
During winter, in either hemisphere, the oceans,
being warmer than the land, cause a greater
regularity in the trades; but during summer, the
tropical continents become intensely heated, and
their powerful ascending currents cause the equa-
torial currents to blow toward the heated areas
as surface winds, and thus displace the trades.
The interval between the two monsoons is gener-
ally characterized by calms, suddenly followed by
furious gales, that may blow from any quarter.
250. Monsoon Regions.-There are three well-
marked regions of monsoons-the Indian Ocean,
the Gulf of Guinea, and the Mexican Gulf and
Caribbean Sea. The first is the largest and most
Monsoons of the Indian Ocean.-Here the
trades are deflected by the overheating of the
continents of Asia, Africa, and Australia.
In the northern hemisphere the north-east trades prevail
with great regularity over the Indian Ocean during the
cooler half of the year: from October to April, but during
the warmer half: from April to October, the heated Asiatic
continent deflects the trades, and the equatorial currents
prevail from the south-west. The same winds also pre-
vail south of the equator, on the western border of the
ocean, along the eastern coast of Africa as far south as
In the southern hemisphere, in the south-eastern portion
of the ocean, the south-east trade is similarly deflected by
the Australian continent. Here the winds blow south-
east during the southern winter, and north-west during
Monsoons of the Gulf of Guinea.-Here the
north-east trades are deflected by the intensely
heated continent of Africa. The south-west sum-
mer monsoon blows over the land as far inland
as the Kong Mountains.
Monsoons of the Mexican Gulf and Caribbean
Sea.-In this region the north-east trade winds
are deflected by the overheating of the Missis-
sippi Valley. The Northers of Texas, which are
cold winds blowing for a few days ata time over
the Texan and Mexican plains, may be considered
as connected with the winter monsoons.
Besides the preceding well-marked regions, nearly all
the coasts of the continents in and near the tropics have
small monsoon regions, as, for example, the western coasts
of Mexico, the eastern an'd western coasts of South Amer-
ica, and the western and northern coasts of Africa.
251. Desert Winds.-The rapid heating and
cooling of deserts make them great disturbers
of the regular system of winds. Currents al-
ternately blow toward and from the heated area.
The latter are intensely hot and dry.
The Etesian Winds.-During summer the barren
soil of the Desert of Sahara, becoming intensely
heated, causes strong north-east winds to blow
over the Mediterranean Sea. These are called
the Etesian winds, and continue from July to
September; they are strongest during the day-
Hot Desert Winds.-From the Sahara a period-
ical wind, called the HIarmattan, blows on the south-
west, over the coasts of Guinea; on the north, the
Solano blows over Spain, and the Sirocco blows
over Southern Italy and Sicily. Though some-
what tempered during their passage across the
Mediterranean, these winds dre still exceedingly
hot and oppressive.
From the deserts of Nubia and Arabia in-
tensely hot, dry winds blow in all directions over
the coasts of Arabia, Nubia, Persia, and Syria.
These winds are known under the general name
of the simoom or samiel. From their high tem-
perature and the absence of moisture, they often
cause death from nervous exhaustion.
During the prevalence of the simoom, particles of fine
sand are carried into the atmosphere and obscure the light
of the sun. Becoming intensely heated, these particles,
by their radiation, increase the temperature of the air,
Y-> U1. I
Fig. 86. Sand Storm in the Desert.
which sometimes rises as high as 1200 or 1300 Fahr. When
powerful winds prevail, dense clouds of sand are carried
about in the atmosphere, producing the so-called sand
storms. The sand-drifts which are thus formed constantly
change their position.
The Khamsin blows at irregular intervals over
Egypt from the south; but when established,
generally continues for fifty days. It is intensely
hot and dry, like the simoom, and is loaded with
252. Mountain Winds.--During the day the
elevated slopes of mountains heat the air over
them hotter than at corresponding elevations over
the valleys. Currents, therefore, ascend the val-
leys toward the mountains during the day. During
the night, however, the air near the summits be-
comes colder than that near the base. Currents,
therefore, descend the valleys from the mountains
during the night.
253. Storms are violent disturbances of the
ordinary equilibrium of the atmosphere by wind,
rain, snow, hail, or thunder and lightning.
During storms the wind varies in velocity from
that of a scarcely perceptible breeze to upwards
of 200 miles per hour.
VELOCITY AND POWER OF WINDs.
elocity of Wind in Common Names of Winds.
Miles, per hour.
1 Hardly Perceptible Breeze.
4 to 5 Gentle Wind.
10 to 15 Pleasant Brisk Gale.
20 to 25 Very Brisk.
30 to 35 High Wind.
40 Very High.
60 Great Storm.
100 Violent Hurricane.
80 to 200 Tornado.
254. Cyclones are storms of considerable ex-
tent, in which the velocity of the wind is much
greater than usual, and the air moves in eddies or
whirls, somewhat similar to whirlwinds, but of
vastly greater power and diameter.
In all such storms the wind revolves around a
calm centre; over the calm centre the barometer
is low, but on the sides, and especially on that side
toward which the storm is moving, it is high.
Besides the rotary motion of the wind, there is
also a progressive motion, which causes the storm
to advance bodily, moving rapidly in a parabolic-
path. The general term Oyclone has been ap-
plied to these storms on account of their rotary
motion. They have also various local names.
Cyclones originate in the tropical regions, but
frequently extend far into the temperate zones.
Pig, 87, A Storm at Sea,
255. Regions of Cyclones.-The following are
the most noted regions:
The West Indies, where they are generally
The China Seas, where they are known as
The Indian Ocean.
In each of these regions the storms occur about
the time of the change of the regular winds, and
have their origin in marked differences of tem-
perature; thus in the Indian Ocean and the China
Seas, they generally occur at the change of the mon-
soon, after the great heat of summer. They are at-
tended with the condensation of moisture and in-
tense electrical disturbance.
256. Cause of Cyclones.-Cyclones originate in
an area of low barometer caused by the ascending
current of air that follows the overheating of any
region. As the air rushes in from all sides it is
deflected by the earth's rotation, and assumes a
rotary or whirling motion around the heated area.
The centrifugal force generated by this rotation
causes the barometric pressure of the area to be-
come lower and the area to grow larger. Mean-
while the inflowing air, ascending, is chilled by
the cold of elevation and by expansion sufficiently
to condense its vapor rapidly. The heat energy,
previously latent in the vapor, is now disengaged,
and causes the air to mount higher and condense
still more of its vapor. It is to the energy thus
rapidly liberated by the condensation of the vapor
that the violence of the cyclone is due. Cyclones,
therefore, acquire extraordinary violence only
when an abundance of vapor is present in the
As the inblowing winds come near the heated
area, they must blow with increased violence in
order to permit the same quantity of air to pass
over the constantly narrowing path.
Besides the rotary motion of the wind, the
storm moves or progresses over a parabolic path,
which in the tropics is generally toward the west,
and in the temperate zones toward the east. This
progressive motion of the storm is like the similar
\ -1 450 N.
Fig. 88. Chart showing Path and Direction of Cyclone.
motion often noticed in a rapidly spinning top.
It is due to the combined influences of the inrush
of air, the earth's rotation, and centrifugal force.
257. Peculiarities of Cyclones.-Cyclones rage
most furiously in the neighborhood of islands and
along the coasts of continents. They are most
powerful near their origin. As they advance the
spiral increases in size and the fury of the wind
gradually diminishes, because the amount of moist-
ure in the air is less. The rotary motion varies
from 30 to 100 miles an hour. The progressive
motion of the calm centre is more moderate-
from 20 to 50 miles an hour. This progressive
motion is least in the tropics and greatest in the
The wind invariably rotates in the same direc-
tion in each hemisphere; in the northern, it ro-
tates from right to left, or in the direction oppo-
site to that of the hands of a watch; in the south-
ern, from left to right, or in the same direction as
the hands of a watch. The cause of the regu-
r A ....'o---
gig. 89. Cause of the Rotation of the Wind,
clarity of rotation is seen, from an inspection of
Fig. 89, tb be due to the rotation of the earth.
The wind, blowing in from all sides toward the
heated area, is so deflected by the rotary motion
of the earth as to move in vast circles, from right
to left in the northern hemisphere, and from left
to right in the southern.
The force of the wind in these storms is tremendous.
So furiously does the wind lash the water that its tem-
perature is often sensibly raised by the friction.
The intelligent navigator always endeavors to avoid the
centre of the storm, since it is the most dangerous part.
This he can do by remembering the direction of the rota-
tion of the wind in the hemisphere he may be in; for if,
in the northern hemisphere, he stands so that the wind
blows directly in his face, the calm centre is on his right,
while in the southern hemisphere it is on his left; and in-
stead of running with the storm, hoping to outsail it, he
will boldly steer toward its circumference.
258. Tornadoes and Whirlwinds are the same
as cyclones, except that they are more limited in
area. Their violence, however, often exceeds that