Front Cover
 Part 1: The geology of the Lares...
 Part 2: The geology of the Humacao...
 Part 3: The geology of the Fajardo...

Title: Scientific survey of Porto Rico and the Virgin Islands
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00091487/00004
 Material Information
Title: Scientific survey of Porto Rico and the Virgin Islands
Alternate Title: Scientific survey of Puerto Rico and the Virgin Islands
Physical Description: 19 v. : ill. ; 24 cm.
Language: English
Creator: New York Academy of Sciences
Jay I. Kislak Reference Collection (Library of Congress)
Publisher: The Academy,
The Academy
Place of Publication: New York N.Y
Publication Date: 1923-1931
Frequency: completely irregular
Subject: Scientific expeditions -- Periodicals   ( lcsh )
Natural history -- Periodicals -- Puerto Rico   ( lcsh )
Natural history -- Periodicals -- Virgin Islands of the United States   ( lcsh )
Natuurlijke historie   ( gtt )
Geologie   ( gtt )
Expedities   ( gtt )
Genre: bibliography   ( marcgt )
Spatial Coverage: Puerto Rico
United States Virgin Islands
Summary: Includes bibliographies.
Ownership: Provenance: Gift of Jay I. Kislak Foundation.
Statement of Responsibility: New York Academy of Sciences.
Dates or Sequential Designation: Vol. 1, pt. 1-
Dates or Sequential Designation: Ceased with vol. XIX, pt. 1.
General Note: Latest issue consulted: Vol. 18, pt. 4 (1952).
General Note: Kislak Ref. Collection: Vol. 18, pt. 2 (1941)-pt. 4 (1952).
 Record Information
Bibliographic ID: UF00091487
Volume ID: VID00004
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 01760019
lccn - 2002209050

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Part 1: The geology of the Lares District, Porto Rico
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    Part 2: The geology of the Humacao District, Porto Rico
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    Part 3: The geology of the Fajardo District, Porto Rico
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Full Text



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Porto Rico and the Virgin Islands

The Geology of the Lares District, Porto Rico- Bela Hubbard



N~ coil




Introduction..................... .... ........ ............. ......... 2
Nature and purpose of the work................................... 2
Acknowledgments ................................................ 3
.Previous work................................................. 4
General description .................................................. 4
The Antilles ................................................... 4
Porto Rico ............................................................. 5
The Lares District ................. .............................. 6
The Cretaceous formations.............................. ............ 6
Petrography ........... ........................................ 7
Igneous rocks................................................ 7
Sedimentary and plastic rocks ............................... 15
Stratigraphy........................ ........................... 25
The Rio Culebrinas Series................... ................ 25
The Rio Blanco Series....................................... 26
The Rio Yauco Series ....................................... 29
Structural features....................... ................. 30
Age..................................... ........................ 34
Correlation....................... ..... .... ..................... 35
Lithogenesis..................................................... 36
Geologic history ............................... .................. 37
The Tertiary formations ............................................... 38
Stratigraphy........................... ......................... 39
The San Sebastian shale ..................................... 39
The Lares formation .......................................... 42
The Cibao limestone ............................ ............... 44
The Los Puertos limestone................................... 45
The Quebradillas limestone ................................... 46
Summary of formations....................................... 49
Structure ........................................................ 49
Age and correlation............................................... 51
The San Sebastian fauna.................................... 53
The Lares fauna .............................................. 57
The Cibao fauna.............................................. 58
The Los Puertos fauna ....................................... 59
The Quebradillas fauna ....................................... 60
Summary of the faunas ................. .................... 64
Correlation with the south coast.............................. 64


Comparison with important Antillian localities. ................ 71
Oligocene or Miocene ................. .......... 72
Tertiary historic y.................. .................. .......... 75
Magnitude of the submergence.............................. 76
The uplift ........................................ .......... 77
Plhysiography ........ ................. .. ........................... 77
The Complex Mountainous Oldland ................................. 78
The peneplane surface ........................................ 79
Drainage features............................................ 81
The Elevated Coastal Plain..................................... 83
The Coastal Plain belts ............. ........................ 86
Drainage..................................................... 88
Origin of the Pepino Hills ..................................... 91
The Playa Plains............... ................ ........... 93
Features of the coast line ........................................... 95
Terraces............................................. 95
The San Juan formation ......................................... 98
The consolidated dune salnds................. ................. 98
The consolidated beach deposits ............................. 99
Evidence of uplift ........................................... 100
Age of the San Juan formation ........ ....................... 100
Conclusions ............................. ..................... 103
The Playa deposits............................................... 103
Summary of Pleistocene-recent events ........................... 104
Mineral resources .................................................. 105
Iron.............................................................. 105
Copper............................................... 105
Kaolin.......................................................... 108
Brick clay ...................................................... 108
Lime ............................................................ 109
Building stone.................... ................................ 109
Road metal....................................................... 110
Lignite ........................................................ 110
Guano ........................................... .............. 111
Oil.................... ........................................ 111
Summary......................................................... 111
Bibliography .............................. ........................ 111



The present paper gives the results of a survey of the geology of the
Lares Distiict, Porto Rico, made during the summer of 1916 under the
auspices of the New York Academy of Sciences and the Insular Govern-
ment of Porto Rico. It is one of a series of reports, each covering the


geology of a portion of the island.' The Lares District comprises the
northwest corner of Porto Rico and Desecheo Island, a total area of about
500 square miles. The survey of this district involved the study of the
rock formations, mineral resources and the making of a geologic map.
As the field work was done in three months time, methods of the recon-
naissance type had to be used. Before entering the field, a base map
(3 inches 1 mile) was prepared from available sources and this was
divided into quadrangles of 5 x 2 miles. The field work consisted chiefly
in running traverses across the general strike of the rock formations. All
traverses were made by pacing or by the time elapse method. Elevations
were obtained by aneroid. In recording the data, two methods were used:
1. Cavalry Sketching Case. The blue print quadrangles of the base
map were used on the sketching case. Contours were sketched in for a
distance of one-half mile or more on either side of the line of traverse.
2. Note Book Traverse, using Brunton compass and protractor. This
method was found best for the more detailed work in limited areas, and
during rainy weather. Photographs were used repeatedly as a means of
gathering data for the topographic map. In portions of the area not
covered by photographs or traverse, the topographic features shown on
the map have been generalized.
The first two months of the field work were spent among the Tertiary
rocks of the district, while the final month was spent in the area of Cre-
taceous rocks. Subdivision and correlation of the Tertiary formations
was the most important problem to be worked out, and hence required
the expenditure of more time than a well balanced survey would ordi-
narily warrant. A large number of fossils were collected in the Tertiary
area, which have been described in Volume III, Part 2, of these reports.


Acknowledgment is made particularly to Dr. C. P. Berkey for assist-
ance and advice in almost every phase of the work. Dr. A. W. Grabau,
Dr. D. W. Johnson and other members of the Columbia University Geol-
ogy Department have given valued .suggestions and aid. In the field
work, every courtesy and assistance was given by the government officials
at San Juan, and acknowledgment is made especially to Colonel George
R. Shanton, Chief of Insular Police, Major Basil H. Dutcher, U. S. A.,
and Judge Bonner, Auditor. In Mayaguez, Mr. D. W. May, Director of
the Agricultural Experiment Station, took very great interest in the work

For an account of the plans and progress of the geological survey of Porto Rico,
see Scientific Survey of Porto Rico and the Virgin Islands, New York Academy of Sci-
ences, vol. I, part 1, pp. 1-10, 24-29, 1919.


and assisted in every way. Sefior Narciso Rabell, of San Sebastian, a
student of the geology of the island, aided in collecting many of the
fossils, in giving valuable information, and in showing a lively interest in
the progress of the work. The aid of the Insular Police in every town
in the Lares District was a big factor in carrying on the survey. Resi-
dents of the district not only displayed the warmest hospitality, but
showed a keen and intelligent interest in the purposes and results of the
field work.

The list of articles dealing with Porto Rico is given in the Bibliog-
raphy. Prior to 1916, little geological work of a detailed nature had
been done in northwestern Porto Rico. R. T. Hill made a reconnaissance
in 1898, and published several articles. He visited the Lares District,
and made observations along the Lares Road, where he collected many
fossil corals, later described by T. W. Vaughan. In 1914, in the course
of their reconnaissance of the island, C. P. Berkey and C. R. Fenner
covered the main routes of travel in the district, and noted many of the
geologic features characteristic of this part of the island. During the
same year, certain fossil localities in the district were visited by C. A.
Reeds, where a large amount of material was collected. In 1915, E. T.
Hodge made a brief reconnaissance of certain localities in the southern
half of the district, to investigate the reported presence of oil shales.



Porto Rico, forming the easternmost member of the Greater Antilles,
is an almost completely submerged complex mountain chain having a
maximum elevation of about 28,000 feet above the adjacent lowlands or
submarine valleys of the Atlantic and Caribbean. This mountain chain
is more or less continuous in an east-west direction, its highest portions
forming the islands of Cuba, Haiti-Santo Domingo, Jamaica, and others.
Lateral branches of this chain form the Lesser Antilles and minor groups
of islands.2 The rocks of this Antillean chain are chiefly of Mesozoic
age, and comprise volcanic flows, intrusives, and sediments of marine and
continental origin. These rocks, as a whole, show evidence of having
been formed during a more or less continuous period of volcanic activity,
which reached a climax in extensive orogenic movements at the close of
2 An excellent illustration of the sub-sea relief of the Antillean region is given in Bull.
103, U. S. Nat. Mus., Plate 73, 1919.


Mesozoic time. With the Tertiary, began a differential submergence, not
affecting some portions of the region until Oligocene time, and resulting
in a deposition of reef and shell limestones. Subsequent uplift in Mio-
cene time, has exposed the fringing beds of Tertiary limestones on nearly'
all the islands of the Antillean group. Finally, there have been minor
crustal movements and fluctuations of sea level during the Pleistocene
Epoch. The crustal movements have continued to the present.

PonTo Rico

As a result of the major geologic events just outlined, modified by
minor events peculiar to the vicinity of Porto Rico, the following are the
chief geologic and physiographic elements or units found on the island:
1. Complex mountainous oldland area, which makes up the core of the
island. The rocks so far as known are all of Cretaceous age and com-
prise intrusives, flows, tuffs, ash, shales and other sedimentary rocks.
The igneous rocks are predominantly andesitic, though other types occur.
The structure is highly complex, marked by many thrust faults and over-
turned folds. The predominant strike of the structure is northwest-
southeast, as it is in most places throughout the older rocks of the An-
tilles. This oldland area is characterized by its mountainous aspect,
steep soil-covered slopes, with marked absence of cliffs or other exposures
of unweathered rock. The present cycle of erosion has reached maturity.
The existence of at least two previous cycles of erosion has been shown
by Lobcck (1922).
2. Elevated Coastal Plain. Marked by nearly horizontal deposits of
reef and shell limestones of Oligocene age, deposited along the north and
south coasts of the island (see Berkey, 1915). These limestones with
basal shale members rest unconformably upon the Upper Cretaceous
rocks, the time interval covering the Eocene period and the early Oligo-
cene. The total vertical uplift reached a maximum of 1500 feet. The
uplift on the north coast was differential, accompanied by gentle warping.
On the south coast it was accompanied by local faulting and considerable
tilting of the beds to seaward. Thus the north coast beds dip at angles
averaging less than 50 in a seaward direction, while the average for the
south coast is considerably greater.
Erosion of the present cycle (post-Oligocene) has affected these Ter-
tiary limestones, chiefly by surface and subterranean solution, and has
stripped the oldland area of a portion of its Tertiary covering.
3. Pleistocene to Recent Coastal Deposits. These include alluvial
plains or playas at mouths of the large streams, elevated beach gravels,


and indurated dune sands. They represent the effects of Pleistocene
variations of sea level, accompanied and followed by local crustal move-
ments, which, over most of the island, have resulted in a series of uplifts
totalling at least 200 feet.


This district contains a portion of almost every major geologic and
physiographic unit to be found on the island. The Older Series, or
Upper Cretaceous rocks, occupy the mountainous area south of the Lares
Road. In this area there are remnants of the earliest post-Cretaceous
or late Cretaceous peneplane, the first erosion cycle. Evidence of the
second cycle, however, is not found within the district. The entire area
north of the Lares highway is occupied by the overlapping, relatively
undisturbed limestones, marls and shales of the Younger Series or Oli-
gocene, resting upon an irregular surface carved in the highly disturbed
Cretaceous rocks, and dipping gently seaward. This is the most complete
development of these rocks to be found on the island. Post-Oligocene
uplift has raised these massive reef limestones to a present maximum
elevation of 1300 feet. Solution with extensive development of under-
ground drainage has produced in this limestone belt a peculiar type of
Karst topography with its pepino hills. The Pleistocene and more recent
uplifts are well shown along the extensive coastline of the district. They
are marked by marine terraces and elevated fossil beaches. The typical
playas are developed at the mouths of the largest streams, particularly
on the west coast.


As a whole, the rocks are more nearly similar to those of the Ponce
District to the south than to the rocks elsewhere on the island. When
compared with eastern Porto Rico, the most striking feature in the Lares
District is the predominance of sales and volcanic elastics, with an ab-
sence of intrusives of the large batholithic type. The strike of the beds
is northwest-southeast, as is the case throughout the rest of the island.
In the Lares District, the average strike is north 500 west, although there
are variations from this over considerable areas. The prevailing dip is
to the southwest, and the northernmost beds are therefore the oldest.
except locally in cases of repetition by folds and faults. In the eastern
portion of the area, volcanic tuffs predominate, while in the western half,
sales predominate. Of igneous rocks, none of the deep seated, batho-


lithic types occur, although some coarse diorite was found in stream
gravels near Aiiasco. The prevailing igneous rock in the district is
andesite porphyry, occurring as sills, laccoliths, and irregular intrusive


The igneous rocks are characterized by a small petrographic range.
Andesite porphyry makes up about nine-tenths of all the igneous material
in the district. The textural range is from felsites to porphyries.
Glasses are rare or not easily recognizable because of the devitrification
and weathering. The following types have been determined, and their
distribution shown on the geologic map:


Name of Rock Usual Occurrence

Quartz diorite porphyry ..............

Andesite ...........................

Andesite porphyry ...................

Augite andesite ......................

Augite andesite porphyry ............

Gabbro porphyry ...................

S erpeiitine ..........................

Irregular intrusives.

Surface flows, usually amygdaloidal.

Surface flows, sills, dikes, and lacco-
lithic or elongate intrusive bodies.

Surface flows, amygdaloidal.

Surface flows, sills, dikes, and irreg-
ular intrusive bodies grading into
Gabbro porphyry.

Irregular intrusives, grading into
Augite andesite porphyry.

Dikes and irregular intrusives of
small size.

Quartz Diorile Porphyry. The only locality in which this rock is
found is in the Rio Blanco valley south of Lares. It occurs as a boss of
considerable size, and is associated with one of the most conspicuous
centers of former volcanic activity in the district. The rock is compara-
tively resistant to erosion, and forms the prominent peak of Mt. Torre-
cillo (Fig. 1).


The principal minerals are plagioclase (andesine-oligoclase, andesine,
labradorite), hornblende, and quartz. These minerals form the pheno-
crysts, many of them large and in some cases occupying much more space
than the groundmass. The plagioclase occurs abundantly as microlites
in the felsitic groundmass. The quartz crystals are not idiomorphic, but
occur as sub-round grains, in some cases quite conspicuous in hand speci-
mens. The accessory minerals are magnetite, apatite, rutile, and prob-
ably some orthoclase. The apatite crystals occur characteristically in the
feldspar grains, and the rutile is confined to the quartz. Quartz is only

FIG. 1.--Tiew looking solth from near K. 1,7. Lares-Rio Blanco Road
Elevation, 1.500 feet. Mt. Torrecillo on the left. Cordillera Central of the Ponce Dis-
rict in the background, with low-hanging clouds marking the valley of the Rio Blanco
in the middle ground.

an accessory constituent in some portions of the rock. The chief altera-
tions are, hornblende to chlorite and serpentine; plagioclase to kaolin,
saussurite, and carbonate; and magnetite to limonite.
The rock grades into andesite porphyry, and locally has a high content
of tuffaceous material, and resembles a true tuff. Portions which are
exceedingly high in tuffaceous material are generally highly kaolinized.
One of the pockets of white kaolin is shown in figure 2.
The typical phases of the rock are readily recognized in the field by
the freshness, resistance to erosion, light gray color mottled with black
hornblende phenocrysts, and the conspicuous grains of quartz. It is
quite different in general appearance from the monotonous andesite
porphyries found throughout the district.


Andesite. Andesites of the type found in the Ponce District are rather
rare. They occur in limited areas along the Rio Blanco east of Afiasco.
The most typical occurrence is on the north side of the Rio Blanco valley,
one mile east of the terminus of the sugar railroad. This rock is locally
amygdaloidal, and undoubtedly a surface flow. It is one of the few large
exposures of andesite in the Lares District which is not porphyritic. In
thin section, it shows micro-diabasic structure, produced by microlites or
minute lath-like crystals of plagioclase (apparently oligoclase), with an
interstitial groundmass of ferromagnesian mineral, probably entirely

FIG. 2.-AK. !;7.. Lares-Rio Blanco IRoad at Mt. Torrecillo
View looking across the road, showing pocket of white kaolin in quartz diorite porphyry.

hornblende, though some of this may be secondary from augite. The
rock has undergone considerable alteration from weathering.
Andesite Porphyry. This is by far the most abundant type of igneous
rock found in the district. Its chief occurrences as intrusive bodies are
shown on the geologic map. It occurs as surface flows at other points,
notably south of Aguada, and at K. 10.7 on the Mayaguez-Las Marias
Road. Its extrusive origin in these instances is shown by the abundance
of amygdaloidal cavities. In a few localities typical palisade structure
is developed, as, for example, on the Rio Blanco, 11/ miles east of the
end of the Lares-Rio Blanco Road (Fig. 3). Dikes of andesite porphyry
intruded in massive tuffs are found at various points along the Rio


Blanco sugar railway, east of Afiasco. This rock also occurs as sills in
the tuffs and ash beds, but many of these are small and not shown on the
geologic map.
Plagioclase is always the chief mineral and most persistent phenocryst-
forming constituent. Of the plagioclases, andesine and andesine-oligo-

FIIo. 3.-Palistidc struttutre in atrdesite porplhyiry
Rio Blanco one and a half miles upstream (east) from the end of the Lares-Rio
Blanco Road.

clase are by far the most common. In many cases, labradorite is present
as a principal mineral with the andesine. Oligoclase rarely occurs as a
principal mineral. Hornblende is the ever present ferromagnesian con-
stituent. Where it is not abundant, the rock is pink or light gray in


color due to the preponderance of feldspars, but where the ferromagne-
sian element is present in abundance, as is more commonly the case, the
rock is gray or dark gray if labradorite is present. The accessory min-
erals are augite, magnetite, apatite, pyrite, and rarely ilmenite, albite,
and quartz. The common alterations are, hornblende to chlorite, ser-
pentine, epidote, limonite, calcite, and quartz; plagioclase to calcite,
kaolin, quartz and saussurite.
The plagioclases occur also as microlites in the groundmass, and ex-
hibit a marked parallelism in their arrangement as a result of flowage
of the magma during crystallization. Fractured or fragmental pheno-
crysts are very common, also indicating flowage. Strain effects can in
many cases be observed in the phenocrysts, as might be expected. The
zonal effects of the plagioclase phenocrysts is one of the most typical
features of nearly all of the andesite porphyries. These zones of growth
are of variable composition, as shown by their slightly different extinc-
tion angles and differential alteration. In some crystals the interior is
completely altered, while the border is fresh. In other cases the altera-
tion has affected alternate zones, so that the concentric structure is vis-
ible in hand specimens of the rock. The abundance of carbonate as an
alteration product of the plagioclases is illustrated by the fact that many
of the weathered andesites effervesce with acid.
From a study of slides of the andesite porphyries certain associations
of minerals are evident. Either andesine or andesine-oligoclase is always
present as a principal mineral, and in some cases both varieties occur.
With these either oligoclase or labradorite may be present, but oligoclase
apparently does not occur with the labradorite. Where augite is present
as an accessory mineral, labradorite usually occurs as a principal mineral.
Ilmenite is associated with magnetite but does not occur unless mag-
netite is present. It is not nearly as common in the andesites of the
Lares District as in similar rocks of the Ponce District.
Quartz and calcite are frequently found filling joints and small cracks.
In a few localities pyrite and chalcopyrite occur in the quartz veins. The
best example of this is found on the Rio Blanco near Alto Sano. The
quartz veins occur in andesite porphyry dikes cutting massive tuffs.
With the chalcopyrite are minor quantities of bornite. The veins are
nowhere more than 3 inches thick, and most of them much less. In the
amygdaloidal andesite porphyry in the area south of Aguada, and on the
Rio Blanco sugar railroad, 3 miles east of Afiasco, introduced matter is
found in the form of amygdules, partially or wholly filling the cavities.
The amygdules are chiefly of amorphous silica and zeolites. In the


locality south of Aguada, massive veins of banded agate with core of
crystalline quartz, occur abundantly, cutting the andesite porphyry, and
apparently associated with nearby intrusive bodies of serpentine. The
amygdaloidal cavities are elongate parallel with the flowage alignment
of the plagioclase phenocrysts, and are lined with banded agate or other
amorphous silica, the interior being filled with crystalline quartz. Where
they are only partially filled, the quartz pyramids form small geodes,
with incrustations of prehnite and other zeolites.
More or less tuffaceous material is present in practically all the ande-
site porphyries. In thin sections, this included matter is seen to consist
mainly of broken crystals of feldspar, hornblende, and other minerals.
It is often difficult to determine whether these are merely phenocrysts
broken up by flowage during crystallization, or whether they are tuffa-
ceous materials of extraneous derivation. Not infrequently fragments of
devitrified glass, or lithlic fragments of more than one crystal are encoun-
tered, and these indicate the true nature of the broken crystals associated
with them. In some cases the rock is so crowded with fragmental ma-
terials that it is impossible to tell whether it is of igneous or of plastic
origin. In examining thin sections, one may be considerably surprised to
find fragmental foraminiferal shells in what otherwise appears to be a
typical andesite porphyry. The difficulty is increased by the fact that
many of the tuffaceous crystalline fragments, especially the plagioclase,
are so regular in form that they are not distinguishable from phenocrysts.
It may truthfully be said that there are all gradations between a tuffa-
ceous andesite and an andesite tuff.
Augite Andesite. The only known occurrence of this rock is in the
southern half of the area of lava flows shown on the geologic map south
of Aguada. It is a dark massive amygdaloidal to vesicular lava, in
some places very ropy and scoriaceous. The amygdaloidal cavities have
no regularity of occurrence which might indicate successive flows. In
some places, the cavities are filled with amorphous silica, zeolites, and
native copper with associated copper minerals. Elsewhere the amygda-
loidal cavities are empty, and the rock is light in weight due to its ex-
treme porosity. In some exposures it resembles a coarse pumice. The
mineralized areas are associated with points and small fault crush zones.
The latter are filled vith calcite, stilbite, native copper, malachite, and
other minerals associated with the copper. This mineralization fills both
the crush zones and adjacent cavities in the rock. This locality is de-
scribed more fully in the chapter on mineral resources.
In thin section, this lava shows a dark ferromagnesian groundmass


with fine lathes or microlites of plagioclase (chiefly andesine) exhibiting
perfect flow structure. Occasional small grains of ferromagnesian min-
eral show the presence of augite, but whether hornblende is present also
was not determined.
Augite Andesite Porphyry. Next to andesite porphyry, this is the
most abundant type of igneous rock in the Lares District. Like the
former, it occurs characteristically as elongate intrusive bodies, appar-
ently sills or laccoliths. It is also associated with the larger and more
massive bodies of gabbro porphyry, of which it seems to form lateral off-
shoots. Examples of this relationship are found on the Mayaguez-Las
Marias Road between K. 8 and K. 9, and south of the Lares Road near
K. 37. In some other localities it is associated with, and seems to grade
into andesite porphyry, as in the Rio Blanco valley south of Lares, and
in the same valley farther west, near east terminus of the sugar railroad
from Afiasco.
This rock is distinguished in the field from andesite porphyry by its
darker gray to greenish color, the latter being due to alteration of
augite. The large phenocrysts of augite are another distinctive charac-
teristic. There are all gradations between augite andesite porphyry and
gabbro. porphyries, depending merely upon the relative prominence of
phenocrysts and groundmass. Both'varieties of rock may frequently be
found in the same outcrop. The principal minerals are augite, andesine-
oligoclase, labradorite, andesine, and oligoclase. It is seldom that more
than two of these are found occurring together as principal minerals.
The commonest association is labradorite with andesine. The usual
accessory minerals are magnetite, hornblende, one or more of the above
named plagioclase varieties, apatite, and pyrite. Hornblende is present
as a principal constituent in a few of the specimens. Occasionally what
appears to be primary quartz is found in small quantity. The common
alterations are, augite to chlorite, uralite, serpentine, and carbonate;
plagioclases to saussurite, carbonate, kaolin and quartz; magnetite to
limonite. Of introduced material, calcite occurs in minor quantities in
some specimens.
Texturally, the rock is a typical augite porphyry, with the plagioclase
phenocrysts generally smaller than the augite, though considerable of
the latter is disseminated as small grains in the groundmass. The
plagioclases are characterized by strain effects, zonal growth, and usually
by parallel arrangement due to flowage. They are in many instances
fractured, with the cracks filled by groundmass. A poikilitic habit is
occasionally seen, with inclusions of ferromagnesian minerals in the
plagioclase phenocrysts.


The outcrops of this rock are, as a rule, fairly fresh, but thin sections
show considerable alterations. Secondary carbonate is often so abundant
that the rock will effervesce with acid. This is especially true where
there is a considerable quantity of included tuffaceous material.
Gabbro Porphyry. This rock is found on the Mayaguez-Las Marias
load between K. 8 and K. 9, and south of the Lares Road near K. 37.
As previously noted, it is associated with the less porphyritic augite
andesite porphyry. It forms rather large intrusive bodies which should
be classed as bosses or volcanic necks. They do not exhibit the elongate
form characterizing the andesite porphyries. In the case of the occur-
rence on the Mayaguez-Las Marias Road, the intrusive relationship to
the adjacent shale beds is evident.
Mineralogically, the rock is about the same as the augite andesite
porphyry, the chief difference being in texture. The commonest prin-
cipal minerals are augite, andesine, and labradorite. As accessories,
magnetite is present in every specimen, with usually some hornblende,
apatite, and andesine-oligoclase. The characteristic alteration products,
as in the augite andesite porphyries, are chlorite, secondary hornblende,
serpentine, epidote, kaolin, carbonate, saussurite, quartz, and limonite.
Strain effects, and evidences of fracturing and flowage during crystal-
Slization are characteristic. Likewise, the usual zone effects are seen in
the plagioclases, with apparently the more acid variations making up the
outer zones of the crystals. The groundmass is characteristically dark,
due to the predominance of ferromagnesian constituents, and is usually
marked by microlites and plagioclase. Quite often very little ground-
mass is present. In some specimens, the augite phenocrysts are very
large, and the augite is greatly in excess over the feldspar. These phases
are typical augite porphyrites, like those found near Maricao, in the
Ponce District.
Serpentine. No outcrops of this rock were found in the Lares District,
but its presence is shown by its surface weathering product, the typical
red limonite soil with limonite concretions, like that covering the Mesa,
southeast of Mayaguez. It is presumably derived from an olivine-bearing
rock. The distribution of this material is shown on the geologic map in
the volcanic area south of Aguada. It occurs in small irregular patches,
lying wholly within the area of andesite lava flows. The serpentine prob-
ably occurs as (likes in the andesite. The limonite ore is similar in
general character and origin to the Mayaguez ore, which has been de-
scribed by Fettke (1918) and by Mitchell (1922) and shows that the
Mayaguez serpentine body is undoubtedly intrusive into the adjacent


shales, and hence represents one of the later phases of volcanic activity
affecting Porto Rico. The Aguada serpentine is doubtless the contem-
porary of the Mayaguez intrusive.


The sediments are almost entirely of elastic make-up, and derived in
large part from volcanic sources. There is a total absence of sandstones
and conglomerates, and very few of the fragments in the tuffs show even
the slightest suggestion of roundness. Limestones are also lacking except
in a few isolated instances of no importance, and even these contain
minor quantities of tuffaceous material. The following types of elastic
or sedimentary rock seem worthy of separate description:


Name of Rock

Massive tuffs and agglomerate........

Bedded tuffs ........................

A sh ......................... .......

Ashy shale ......................

Black shale ........................

Lim e shale ..........................

Limestone ..........................

Chert ..............................


Chiefly in the eastern part of the
district, and in the central zone
called the Rio Blanco series.

Interbedded with shales and ash.

Interbcdded with shales and tuff,
beds usually not as thick as the


Southwestern portion of the district.

Wide spread, but characteristic of
Pt. Jiguero region and the Atalaya
Range north of Aiiasco.

South of Las Marias and on Desecheo
Island. As lenses of small extent
in shale or tuff.

North of Mayaguez and on Desecheo
Island, as lenses in shale.

Massive Tuffs and Agglomerate. These rocks are so deeply weathered
that very little can be found out about their structure and composition.


In many places weathered exposures will show the lithic fragments ap-
parently well preserved, but with the entire rock so badly decayed that
it can be cut into slices with a knife as easily as clay. In the more

FIG. 4.-Volcanic agglomlerate, Rio Blanco near mouth of Rio Prieto
The view shows two large slabs of lime shale imbedded In a matrix of finer pyroclastic
material. This rock marks one of the centers of volcanic activity during late Cretaceous

massive exposures, not the slightest trace of bedding or assortment of
fragments can be seen. In composition, .they are for the most part made
up of fragments of andesite porphyry. Kaolin and iron oxides are the
chief weathering products. The more reddish areas of clay soil on the


mountainous slopes in the southeastern portion of the Lares District are
derived chiefly from these massive tuffs. The coarse agglomeratic phases
are of more limited occurrence, usually being found close to the centers
where volcanic activity was particularly strong. One of the best ex-
posures of this type of rock is found in the Rio Blanco valley southwest
of Lares, close to the mouth of the Rio Prieto. Large slabs of lime shale
are found imbedded in a massive, compact agglomerate, made up largely
of andesite fragments (Fig. 4). The lime shale occurs nearby in a rela-
tively undisturbed condition, and alternates with thin layers of tuff.

Fia. 5.-Volcanic agglomerate, with blocks of andcsite porphyry
South of K. 8, Lares Road.

Apparently the lime shale consolidated soon after deposition, leaving the
interbedded tuff beds still unconsolidated, so that when the volcanic dis-
ruptive action took place, large slabs of the shale were thrown up and
imbedded in the resulting ;. _.1..!I. r i The tuff layers, being uncon-
solidated, were completely broken up and distributed as groundmass in
the agglomerate. This groundmass is composed of fragments of feldspar,
quartz, calcite, magnetite, ferromagnesian minerals, pieces of andesite
porphyry, and lime shale. Another type of agglomerate, in which the
fragments are chiefly large blocks of andesite porphyry is well exposed
south of K. 8, Lares Road (Fig. 5). This material lies immediately


adjacent to a small boss or volcanic neck of augite andesite porphyry,
and in turn is surrounded by thin beds of tuff and shale which have been
differentially indurated and altered in proximity to the intrusive body
and agglomerate (Fig. 6). A short distance from the area of volcanic
activity, they are relatively undisturbed and are unaltered.
Bedded Tuffs. These are, for the most part, fine grained, and grade
into typical ash beds. They occur characteristically interbedded with
shales and ash, and are distributed throughout the district, though more
abundant in the western part. While the coarseness of the material is

Fit. 6.-Shales and thin-bedded tuff, showing alteration effects produced by
inltrutsil body
K. 8. Lares Road.

a nearby

quite variable, even in short distances, few of the fragments are larger
than one-half inch in diameter. They are composed of broken pheno-
crysts or crystalline particles derived from andesite porphyries, and hence
contain the usual minerals, such as plagioclase, ferromagnesians, and
magnetite. Where finely interbedded with shales, they contain a consid-
erable percentage of calcite and quartz grains. Some of the quartz grains
show slight traces of rounded edges, as though subjected to current action.
Wherever the tuff is interbedded with black shale, the black shale forms
a matrix for the angular grains of the tuff. Where interbedded with
lime shale, they have a high content of calcite and fragments of the lime


shale, and have many veins of calcite in localities of close folding or
faulting. The bedded tuffs can usually be recognized in the field by their
characteristic spheroidal weathering (Fig. 7) which distinguishes them
from the occasional thin sills of andesite. This type of weathering is
also to be found in the more massive tuffs (Fig. 8). It is sometimes
difficult to distinguish such occurrences from conglomerate.
As has been pointed out by Berkey, Mitchell, and others, the tuffs are
much more subject to surface weathering than igneous rocks of the same
mineral composition. It is only where the tuff has been indurated, due

FIG. 7.--Sphicroid(al rcatherinif in bedded tuff
Near K. 12. Aguada-Rincon Road.

to the action of a nearby intrusive body, that it is more resistant to ero-
sion than the shale with which it is interbedded (Fig. 5). The tuff beds
are characteristically thicker than the adjacent beds of shale. They
range from 6 inches to several feet, and have no minor stratification,
although some beds show a slight assortment of coarse and fine material.
Ash. Beds of ash occur with the shales and bedded tuffs. They re-
semble the shales, and where the outcrop is weathered the two are not
distinguishable. Typical ash beds are not abundant. The most common
occurrence is shale high in ashy material, and there are all gradations
between a typical shale and a typical ash. Where the rock is exceedingly
fine grained, it is sometimes impossible to determine the composition.


Any fine particles of volcanic glass which may have been present are
'usually not recognizable in thin sections, owing to devitrification and
alteration. In some cases, minute rod-like particles are present, sug-
gestive of fibrous glass or pele's hair, but these are invariably found badly
altered by weathering. The ash beds grade, in texture, to fine grain
tuffs which do not differ otherwise from typical ash. In thin sections,
ash may usually be distinguished from shale by its lack of assortment of

- ~ P t

7- '7

w ~ -

~i~C ^ "

Fie. 8.-Spheroidal weathering in massive tuff
Exposure in ravine just west of the plaza at Lares.

particles and absence or scarcity of foraminiferal shells. In hand speci-
mens it is usually distinguishable by its fracture. Berkey has pointed
out the fact that the sales generally break with a blocky fracture, espe-
cially in the case of weathered black shale. The lime shales have a
smooth, curved or almost conchoidal fracture. Typical ashes, however,
have a rougher, more irregular fracture, due to the unevenness of the
weathering which they have undergone. As a rule, the typical ashes are
of slightly coarser texture than most of the shales, but this difference is
not evident in hand specimens.


The ashes, and ashy beds in general, are divisible into two types,-
calcareous and non-calcareous. The former are chiefly shales high in
ash content, and carrying a few foraminiferal remains and grains of
calcite. The calcite grains are in some cases slightly rounded and pitted
as though by solution. The non-calcareous variety is made up chiefly of
andesitic material, minute angular crystalline particles, and an indeter-
minate groundmass, containing possibly devitrified .glass and altered
crystalline fragments. They contain no foraminiferal shells and but very
little calcite, except as introduced matter filling veins. Of the two types
of ash, the calcareous is the more common in the southwestern part of
the district, while in the northwest portion of the Cretaceous area, the
non-calcareous variety predominates except in the area south of Moca.
Ashy Shales. Shales high in the ash content can usually be classified
only by the use of thin sections. The depth to which they are weathered
increases the difficulty. The weathered ashy shales are generally non-
calcareous, and resemble the weathered black shale, but are higher in
kaolinized material. They are typically reddish brown to gray in color,
and at some places contain white patches or streaks of kaolin (Fig. 9).
Black Shales. These have been described by Berkey and Mitchell, and
similar shales have been described by Semmes. Mitchell shows that the
red, blocky shales exposed around Mayaguez are the weathering product
of a black shale, found exposed as such, only in quarries or stream chan-
nels. Fresh exposures of this shale occur south and northeast of Con-
sumo and at K. 3.7, Mayaguez-Consumo Road. There are all gradations
between a pyritiferous black shale and a very dark lime shale. The black
shale exposed near Consumo shows in thin sections, a dark opaque
groundmass, apparently in part carbonaceous matter, in which are im-
bedded many foraminiferal shells, a few minute angular fragments of
quartz and feldspar, and comparatively large well formed cubes of pyrite.
The pyrite in crystallizing out has crowded the surrounding carbona-
ceous groundmass. Carbonaceous films, apparently small plant remains,
are seen in some hand specimens of the rock. The only calcareous ma-
terial visible in thin sections consists of foraminifera, but these are
sufficient to cause the rock to effervesce with acid. A microscopic strati-
fication or assortment is visible in thin sections, but is not evident in
hand specimens.
A more calcareous phase of this shale occurs at K. 3.7, Mayaguez-
Consumo Road. The rock in this locality is an alternation of thin bands
of the dark foraminiferal shale with bands of fine grained andesitic tuff.
The contact between the two is very sharp. In thin sections, the shale


shows a dark carbonaceous groundmass with foraminiferal shells in abun-
dance. The adjacent tuff layers are composed of angular fragments of
andesite and fragmental crystals, all of which are imbedded in the same

FIG. 9.-Ashy shale showing kaolinized portions
Iilo Yauco Series, three miles east-northeast of Itincon.

dark groundmass which forms the bulk of the shale. Foraminiferal
shells are found in the tuff layers with the carbonaceous matrix. In
addition to foraminifera, the shale contains an abundance of radiolarian


remains, marked by rounded bodies of amorphous or spherulitic silica.
However, a few unaltered radiolarian shells occur.
Lime Sliales. These are much commoner than the black shale. Typ-
ical exposures of lime shale are shown in figures 10 and 36. Many of the
layers are highly calcareous, and have the character of a thin-bedded or
banded, hard, compact limestone, dark bluish gray in color. Exposures
of this type are to be seen in quarries along the automobile road crossing
the Atalaya Range northwest of Afiasco.
In thin section, this rock proves to be made up chiefly of calcite grains
with a variable amount of volcanic matter, such as angular fragments of

FIG. 10.-Typical lime sales
Rio C'lebrinas Series, one and a quarter miles south of K. 5, Lares Road.

feldspar, hornblende, and magnetite. Considerable recrystallization of
the calcite is evident, and this gives the rock its high density and com-
pactness. Foraminiferal shells occur, often in considerable quantity, but
in some specimens they are absent. Thus this rock differs from the
pyritiferous black shale in that its calcareous content is not dependent
upon the presence of foraminifera. The foraminiferal shells are all of
microscopic size, and include indeterminate forms resembling Globigerina
and Rotalia. In some cases their structure has been entirely destroyed
by subsequent crystallization to aragonite. Some of the shells are frag-


mental, as though having been transported before deposition, but this
feature is not common.
Owing to its high lime content, lime shale is more resistant to erosion
and weathering than any other rock occurring abundantly in the Lares
District. Along the Atalaya Range, north and northwest of Afiasco, their
influence upon the topography is rather marked. Other, less extensive,
developments of the same type of shale occur on Desecheo Island, in the
Rio Blanco valley near the mouth of the Rio Prieto, at certain points in
the hills south of Moca, and along the automobile road northeast of
Limestone. True limestone is almost unknown in the Cretaceous rocks
of the Lares District, although one might class certain portions of the
lime shale as a limestone.
A small but prominent inlier of limestone occurs southeast of the
Mayaguez-Las Marias Road opposite K. 22. This rock is a hard, compact
fine grained crystalline limestone, gray to pink in color, and white on
weathered surfaces. The structure is massive, with no bedding visible in
the outcrop. Bedding, however, may have been obscured, since the out-
crop shows faulting and fracturing to an extreme degree, with numerous
calcite veins. In thin section, the rock appears as a fairly pure limestone,
made up almost entirely of recrystallized calcite grains with a very small
admixture of tuffaceous material. It contains many foraminiferal shells
(chiefly Nummulites and Globigerina) and fragments of Radiolites.
Lithologically, the rock resembles the San German limestone of the Ponce
District. A few thin beds of similar rock occur with the lime shales on
Desccheo Island. These exhibit distinct stratification, and are banded by
zones of tuffaceous material. The fossils are chiefly foraminiferal shells
(Globigerina and others), in great abundance, and a few fragments of
Ch ert. Chert beds are of very local extent. They occur as lenses in
the shales north of Mayaguez. Mitchell found an outcrop on the north
side of Mayaguez Bay. Another exposure is to be seen just west of K. 2,
Mayaguez-Afiasco Road. Chert is also found in small quantity, inter-
bedded with shales on Desecheo Island. In its field occurrence, this rock
is conspicuous because of its relatively fresh, unweathered condition.
The color varies from gray to pink, or reddish brown where stained by
limonite. It is extremely hard throughout, but extensively fractured,
and breaks into sharp, very irregular blocks, not having the conchoidal
or curved fracture surfaces characteristic of most cherts. In thin section,
under crossed nicols, it appears as a fine aggregate of amorphous silica,


crowded with microspherulitic portions, which are very probably the re-
mains of radiolarian shells. Tuffaceous material is present in very small

Owing to difficulties of field work and'the limited time spent in the
field, it is impossible to correlate rocks of similar lithologic habit occur-
ring in various portions of the district. It is possible, however, to make
larger subdivisions, each doubtless containing several formations. The
geologic map shows two belts of shales, with a central belt of tuffs, intru-
sives, and extrusives. The northern or oldest belt of rocks comprises
tuffs and intrusives in the southeast, which grade into shales to the north-
west. This belt is called the Rio Culebrinas Series because of its develop-
ment along the Rio Culebrinas valley. The central belt, predominantly
tuffs and andesitic rocks, is called the Rio Blanco Series, from its ex-
posure along the Rio Blanco. The Rio Yauco Series, in the southern
portion of the district, is named from the Rio Yauco Shale, a term ap-
plied by Mitchell to the black shales which form a prominent part of this
series between Mayaguez and Consumo. The relationship of these three
series, as well as the complexity of the structure, is shown in the cross-
sections (Plate I, Figs. 7-10). It should be noted that the boundaries
of these subdivisions are marked more or less by gradational change from
one prevailing type of rock to another, hence the boundary lines as drawn
on the geologic map are somewhat arbitrary. On this map no attempt
has been made to show all the occurrences of each type of rock. Thus
where shale is the predominant rock, the entire area is mapped as shale,
and only the larger tuff beds or intrusives are shown.


In the region south of Lares, this series is represented by a wide area
of tuffs. Exposures of unweathered rock are so few that the true char-
acter of the underlying formation cannot be determined in most places.
At least one small area of shale occurs east of Lares, and there are doubt-
less many others. Intrusive bodies of andesite porphyry, augite andesite
porphyry, and gabbro porphyry are found, chiefly in the vicinity of Lares.
To the northwest, the tuffs grade into shales. This transition is well
shown by the alternation (interfingering) found in the section south of
San Sebastian (Plate I, Fig. 7).
Still farther to the northwest, shale is the predominant material, but
is interbedded throughout with ash and tuff. A small area of intrusive,


chiefly augite andesite porphyry, occurs southeast of Moca. The sales
are highly weathered wherever found, and their occasional red color indi-
cates that they may be derived in part from a black shale like the typical
Rio Yauco shale to the south. Most of these weathered shales are non-
calcareous, except in the section south of Moca, where both shales and
tuffs are calcareous throughout. The best exposures of shale in the series
are to be found south of San Sebastian, south of Moca, and near Aguada.
Complex folding and faulting is typical of the series, but does not seem
to be as intense as in the Rio Yauco Series. In many places, as southeast

FIG. 11.-Rio Culebrinas sales, two miles south of Moca, showiing overturned syncline
Strike S. 85 W., pitch 17 S. W. View looking west.

of Moca, the beds are relatively undisturbed and dip at low angles. About
two miles south of Moca, the shales are highly folded and faulted. Fig-
ure 11 shows some shale beds in this locality, folded into an overturned

This belt is made up principally of tuffs with intrusives and surface
flows. It contains most of the volcanic rocks exposed in the Lares Dis-
trict. Most of the tuffs are massive and structureless, though in fresh
exposures, faint traces of bedding can often be seen. Here, as elsewhere,


the prevailing dip is to the southwest. Most of the old volcanic vents are
in this belt. Among these, the two most important ones occur south of
Aguada and south of Lares in the Rio Blanco valley. The former is
marked by the largest area of surface flows in the district, consisting of
amygdaloidal rocks andesitee and augite andesite). It is in the augite
andesite that native copper occurs. In this large area of surface flows
are small intrusives of serpentine, marked on the surface by patches of
iron ore like that occurring on the Mesa at Mayaguez. Another surface
flow of augite andesite is found in the Rio Afiasco valley about three miles
east of Afasco. There is no native copper, however, in this rock.

Flr. 12.-Fault in Rio Blanoo Series, Rio Blan o near mouth of Rio Prieto
View looking south. The thin-bedded lime sales are seen on the left, dipping south-
west. The massive thick-bedded tuffs are seen on the right, dipping steeply to the east.
This discordance in dip and strike was found upon examination to be the result of a
thrust fault of considerable magnitude.

The area south of Lares in the Rio Blanco valley is an old volcanic
complex and probably represents the location of several former volcanoes.
Only one area of undoubted surface flows is found in the Rio Blanco
Series, but andesite porphyry showing columnar structure occurs along
the river about 11/, miles east of the end of the Lares-Rio Blanco Road
(see Fig. 3). The columns exhibit radial arrangement and probably
mark an old volcanic throat. The size of the phenocrysts seems to show
that this rock was not a surface flow. Northwest of this point is a rather
large included area of shales, tuffs, and lime shales, lithologically similar


to those of the Rio Yauco Series. This inlier of sediments has been badly
faulted and cut up by the surrounding intrusives. In the Rio Blanco
valley at the mouth of the Rio Prieto, the lime shales are faulted (Fig.
12) and broken up, forming in places a volcanic agglomerate of andesitic
content, in which large slabs of the shale are imbedded (Fig. 4). The
intrusion of quartz diorite porphyry, shown on the map in this volcanic
area, is best exposed in a high peak known as Mt. Torrecillo (Fig. 1),
located at K. 48-4 9 on the Lares-Rio Blanco Road.
The best exposures of the Rio Blanco Series are found along the sugar
railroad east of Anfasco in the Rio Blanco valley. Going upstream, one


FIG. 13.---Spheroidal weathering il fine-graoiedi tIuffs, alternating with slinle
Rio Yauco Series. K. 12.2, Aguada-Rincon Road.

passes from the belt of Rio Yauco Series shales into a succession of tuffs,
andesite flows and andesite porphyry intrusives, throughout which very
little of the structure can be seen. Occasionally obscure bedding may be
found in the tuffs, or the dip of a flow or sill of andesite may be distin-
guished. Here and there are small included patches of shale of very
limited extent. The most curious and unexpected feature of the Rio
Blanco Series is the presence of an isolated outcrop of limestone, rising
from a monotonous area of deeply weathered rock, all apparently tuff.
As shown on the map, this limestone lies 112 miles southwest of Las
Marias, and may be seen from a point near K. 22 on the Mayaguez-Las
Marias Road. It is similar lithologically to the San German limestone


of the Ponce District and contains Radiolites sp. and many foraminifera,
including indeterminate species of Globigerina, Nummulites and an
Orbitoides or similar form belonging to the Orbitoides group.


This series, as previously noted, is predominantly an area of shales,
often interbedded with ash and tuff, and with occasional andesitic flows
and intrusives. Most of the shale is much less weathered than that of
the Rio Culebrinas Series. This is due to the relative abundance of the
hard gray-blue lime shales which are much more resistant to erosion and
weathering than are the more ashy, easily kaolinized shales of the series
to the north. It would seem that the Rio Yauco beds have undergone
greater folding and faulting than the underlying rocks of the other two
series, but this difference is probably only apparent, not real. The details
of structure in the other two series are usually obscured by weathering.
The northern, or stratigraphically lowest, portion of the Rio Yauco
belt contains much shale of the ashy type alternating with tuffs. Excel-
lent exposures of these are found along the automobile road northeast of
Rincon. A very characteristic feature of these beds is the spheroidal
weathering of the tuffs (Figs. 7 and 13). Southeast from Rincon, and
forming the range of hills known as Atalaya Peak, are developed the
blue-gray, hard lime sales, often banded, and resembling the Pefiuelas
shale of the Ponce district In the almost total absence of true lime-
stone in the Lares District these lime shales are among the ridge-form-
ing types of rocks to be found. Southeast from Aflasco these lime shales
grade into the pyritic black shale of the vicinity of Consumo, and its
weathered product, the red kaolinized shales, are well exposed in the
vicinity of Mayaguez.
Tuffs of the more massive type are more abundant southeast of Afiasco
Plava. An intrusive body of considerable size composed of gabbro
porphyry, grading into augite andesite porphyry, is well exposed at vari-
ous points from K. 8 to K. 10 on the Mayaguez-Las Marias Road. It is
clearly intrusive into the sales. North of Mayaguez, near K. 2 (Maya-
guez-Afiasco Road), are several small areas of chert, occurring as lenses
in the surrounding shale. They are apparently of radiolarian origin.
Desecheo Island, lying 15 miles west of Pt. Jiguero, contains more
features of geologic interest than any area of equal size on the mainland.
It is a continuation of the belt of lime shales of the Rio Yauco Series
which form the back-bone of the Cordillera Central, known on the west
coast of Porto Rico as Atalaya Peak. The arid climate of Desecheo has


preserved the rocks in a relatively unweathered state, hence the structure
can be well seen. The enormous amount of faulting and fracturing is
well shown in figures 14 and 15. Among the special features should be
mentioned the presence of chert beds like those north of Mayaguez, and
of a few thin beds of limestone of the San German type, containing
Radiolites and an abundance of foraminifera.


The strike of the beds is, as previously stated, northwest-southeast.
On the map the lines designating shale have been drawn so as to indi-

FG. 14.---Fault in Rio Yluco Series, northirest side of Desecheo Island

cate the strike wherever known. The dip is to the southwest, at variable,
but almost universally high angles. Variable or northeast dips denote
folding or faulting, or both.
The characteristic structure of the series is shown in the four.sections
(Plate I, Figs. 7, 8, 9, 10). Figures 9 and 10 are generalized or ideal
sections, and illustrate the difference between the east and the west parts
of the Lares District. Figures 7 and 8 are based on traverses made across
the series. Wherever the course of the traverse has deviated from the
line of the section, the data has been projected on the section. The loca-
tions of the sections are not shown on the map, but may readily be seen
from the designations of towns through which they pass.
Structurally, the rocks of the district are of two kinds,-massive and
bedded. The massive rocks include the andesite intrusives, of laccolithic


or irregular form, some of the thicker surface flows, such as those south
of Aguada, and the massive tuffs. The latter cover large areas, and are
almost universally too deeply weathered to show their real structure.
The few unweathered exposures show either very obscure bedding of
massive proportions or else none whatever. The bedded rocks comprise
chiefly the shales, with interbedded tuffs, the smaller andesite sills and
flows. It is chiefly in these bedded rocks that the highly disturbed con-
dition of the rocks can be seen.
Intrusives. These are characteristically of the laccolithic type, or oc-
curring as sills in the bedded formations or massive tuffs. Their origin
is indicated by their occurrence as elongate bodies following the general
strike of the surrounding beds. Many of them may be surface flows, but.
their porphyritic habit is so nearly universal, and the phenocrysts of such
considerable size that it seems safer to interpret them as intrusives. It
is nearly everywhere impossible to solve this problem from the nature of
the contact with adjacent sediments and volcanic plastics. The reasons
for this are chiefly because of the deep surface weathering and absence
of extensive exposures of fresh rock. Another factor is the similarity in
mineral make-up of the igneous and elastic rocks, and the consequent
absence of contact metamorphic effects. Some of the large and irregular
igneous bodies, such as the gabbro porphyries, are obviously intrusive
into the sediments. They cut across the beds and are not elongate parallel
to the strike.
Folds. At various localities in the shale areas are zones of exception-
ally strong folding and faulting, marked by crush zones and extreme
variability of dips and strikes. Wherever the folds are exposed to view,
they are seen to be overturned folds, usually pitching at considerably
high angles. A typical example is shown in figure 11. These minor
folds are superposed on major folds which form the largest and most
continuous structural units of the shale areas. The best example is the
Ailasco Synclinoriunm, shown in the section on Plate I, figure 10. This
great syncline in the Rio Yauco shales underlies the Afiasco Plava. Its
northeastern limb and a portion of its southwestern limb are exposed
along the south side of Atalaya Range, northwest of Afiasco. From
Afiasco Playa it extends southeast, across the Mayaguez-Consumo Road
between K. 6 and K. 12. Other large synclines and anticlines occur
northeast and southeast of Rincon. The axial planes of most of the
minor folds dip with the adjacent and less disturbed beds, hence to the
southwest. The pitch of the folds is to the northwest, though in some
instances in the opposite direction.


In addition to this northwest-southeast system of major and minor
folds, there is evidence of another, and probably later system, more of the
nature of broad, extensive flexures, whose axes seem to extend in approxi-
mately a north-south direction, nearly normal to the strike. Evidence
of this may be seen in the very gradual and progressive change of strike

F1L'. 15.--l'racturiUng with alhitc reins. Rio Y auo ,Sr'ries shal's, northwest side of
Dc.e heo Island

of the sales over certain areas of considerable size. This feature is
shown on the map in numerous places, notably in the area of Rio Yauco
sales lying south of Afiasco and north of Mayaguez. The'same move-
ments which produced these north-south flexures probably were respon-
sible, at least to some extent, for the plunging of the overturned anti-
clines and synclines above noted.


Faults. Both normal and thrust faults occur in great number.
Usually, evidence of faulting is to be found only in the presence of crush
zones. In such cases very little can be determined as to the nature or
extent of the movement. They are probably for the most part thrust
faults of considerable extent. Most of the normal faults seem to be of
relatively slight movements, though very numerous, and in aggregate,
doubtless represent displacement to be measured in thousands of feet.
While the data are insufficient to make a positive statement, observations
seem to show that the normal faults more commonly cut across the gen-
eral strike of the formations, and hence are to be associated with the
north-south flexures previously described. The thrust faults also cut
across the strike of the beds, but on the average at lower angles, and


... .. .......

/- / ,/

FIG. 16.-Tirust fault in Rio 0Yaliro shale
K. 13.85, Mayaguez-Consumo Road. Strike of beds, N. 75 W.. dip 45 S. W. Displace-
ment. several feet.

hence are to be associated with the earlier, or mountain-building move-
ments, which produced the intensive folding.
Except in the cases of minor faults, the amount of displacement could
not be determined in any of the faults observed. Figure 12 shows a
thrust fault which probably indicates a movement of some hundreds of
feet. The view shows massive tuff beds almost normal to the adjacent
shales, against which they have been faulted. No marked folding occurs
in the vicinity, thus the discordance shown in the illustration could not
have been brought about by any minor displacement. Thrust faults are
well exposed on Desecheo Island, as shown in figure 14. The faults are
accompanied by extensive fracturing, and the numerous joints thus pro-
duced have benen.filled with quartz or calcite. The extent to which the
fracturing has gone is well shown in figure 15, illustrating an exposure
on the northwest coast of Desecheo Island.


A minor thrust fault of special significance is exposed at K. 13.85 on
the Mayaguez-Consumo Road. As shown in the illustration, figure 16,
the fault plane dips at a very low angle and shows a displacement of only
a few feet. It is located on the north limb of a large syncline, and is
apparently a break thrust, produced during the folding. That it is not
a normal fault is shown by the very slight dip of the plane. It, there-
fore, serves to emphasize the point previously made, that the thrust fault-
ing was probably contemporaneous with the intensive northwest-southeast
Thickness. The complexity of structure, absence of continuous out-
crops, and limited time spent in the field make accurate determinations
of thickness out of the question. In the section of Rio Culebrinas Series
south of Moca the sales and tuffs measure over 3000 feet in thickness.
This probably is near the maximum figure for this series. In the Rio
Yauco Series, about 6000 feet of sales and tuffs are exposed along the
Mayaguez-Las Marias Road from K. 13 to K. 19. A traverse across the
series northeast from Rincon shows a total thickness of over 8000 feet.
This measurement makes considerable allowance for duplication of strata
by folding and faulting, and probably represents the maximum thickness
of the entire Rio Yauco Series. The thickness of the Rio Blanco Series
cannot be determined, owing to the massive character of the tuffs and the
many intrusive bodies.
These estimates of thickness include the interbedded flows and intru-
sives, which, although individually small in the sections measured, doubt-
less make up a large portion of the total. The measurements take account
of the larger folds and allow for many of the minor folds. The effect
that faulting may have had in duplicating the succession of beds is, how-
ever, impossible to estimate, since the amount of movement along fault
planes could in no case be determined. It should be pointed out that
many faults, both thrust and normal, occur along the sections measured,
and their effect has doubtless been to greatly increase the apparent thick-
ness in each case. Nevertheless, it may be stated that the total thickness
of elastic rocks exposed in the Cretaceous series of the Lares District is
to be measured in thousands of feet. A conservative estimate for the
entire group would be about 10,000 feet.


The age of the Rio Yauco Series is Upper Cretaceous, as shown by the
presence of Radioliles, and the same foraminifera characteristic of the
Upper Certaceous beds in the Ponce District. The Rio Blanco Series is


also probably Upper Cretaceous, though the presence in it of Radiolites
is not as reliable evidence as in the case of the Rio Yauco Series, since
the limestone which contains this fossil may be an in-faulted remnant
from another series of rocks. The Rio Culebrinas Series is the oldest
group in the Lares District, but there is no evidence to show that it is
older than Upper Cretaceous in age. It has beds of lime shale which
contain the same foraminifera as the Rio Yauco lime shales, but these
minute organisms are hardly determinable specifically. There is no evi-
dence of unconformity between or within the three series of the district.
Mitchell has found the same to be probably true in the southward con-
tinuation of these beds in the Ponce District. There is no evidence of
Eocene sediments in the Older Series of the Lares District, and likewise
none in the Ponce District, as Mitchell has shown.


The Rio Yauco Series is probably the equivalent of most of the shales
in the Ponce District. Mitchell states that the Pefiuelas shale is probably
equivalent to the upper portion of the Rio Yauco shales. The lime shales
of the Rio Yauco Series of the Lares District are lithologically similar
to the Pefiuelas shale, and may be equivalent. The presence of Radiolites
cannot be used to correlate different members, since all the limestones as
yet found in both the Ponce and Lares Districts carry this fossil, and
have similar foraminifera. From stratigraphic and structural consider-
ations, the following statements seem probable:
1. The Rio Yauco Series is equivalent to most of the sediments occur-
ring in the Ponce District.
2. Other formations in the Ponce District of doubtful correlation will
very likely prove to be younger than the Rio Yauco shales.
3. The Rio Culebrinas shales are older than anything in the Ponce
District, but are probably not older than Upper Cretaceous in age.
4. The entire range of sediments from the lowest members of the Rio
Culebrinas to the uppermost members of the Rio Yauco Series represent
a continuous succession, unbroken by any unconformities or disconformi-
ties of appreciable magnitude.
Further correlation with areas of Porto Rico to the east is practically
impossible at the present stage of investigation. The strike of the forma-
tions in the Coamo District, if projected, would seem to show that they
pass northwestward across the northern half of the Lares District, cov-
ered by Tertiary formations. The presence of Radiolites in the Coamo


tuff limestone is significant in showing that at least a portion of the
formations exposed in the Coamo District are of Upper Cretaceous age.

The origin of the various types of igneous rocks has been pointed out
in the discussion of the petrography of these rocks. The origin of the
elastic rocks has likewise been referred to, but deserves further elabora-
tion before attempting to outline the Cretaceous history of the district.
It has been noted that practically all of the elastic rocks contain ma-
terial derived from volcanic sources. From the tuffs, which were entirely
derived from such sources, to sales and limestone, which contain minor
amounts of this material, the entire succession of beds show that they
were deposited during an almost uninterrupted period of vulcanism. As
Mitchell has pointed out, the massive tuffs, which show no bedding, were
doubtless accumulated on land surfaces, while the shales and stratified
tuffs and ashes were accumulated under water. The lime shales and
black pyritic sales contain so many foraminifera that their marine
origin seems highly probable. In the case of the limestones, interbedded
with these sales, and carrying Radiolites and abundant foraminifera,
their marine origin cannot be questioned. The alternation of massive
unfossiliferous tuffs with shales and limestone containing marine organ-
isms thus shows a frequent oscillation of level during deposition, in which
arms of the sea frequently encroached on the land area. The sparsity of
marine life, and the singularly small number of types making up the
fauna, illustrate the very hazardous and unfavorable conditions for the
development of living organisms. A glance at the geologic map will at
once suggest, in view of thd above interpretations, the significance of the
predominance of massive tuffs in the eastern portion of the district. The
land mass apparently lay to the east, and the invasions of the sea came
from the west and southwest. The predominance of calcareous beds on
Desecheo Island, and their relatively greater foraminiferal content, sup-
port this view. The greater abundance of limestone in the Ponce District
to the south is also significant. The lens-like habit of the tuffs inter-
bedded with the shales seems to indicate that rivers played an important
part in the deposition of both the shales and bedded tuffs.
In regions subjected to periodic violent volcanic eruptions, we should
expect the animal organisms inhabiting the adjacent sea to be killed 'off
periodically in great numbers. Consequently, we should look for occa-
sional strata crowded with fossil remains, with intervening beds contain-
ing exceedingly few fossils due to the rapid accumulation of elastic sedi-


ment and the scarcity of marine forms living under such continuously
unfavorably conditions. This is precisely what is found in the sediments
both in the Lares and Ponce Districts, as the following evidence shows:
1. The limestones of the San German type are in places crowded with
the remains of Radiolites.
2. Certain beds of the lime shale show an abundance of foraminifera,
while others show very few or none.
3. The black pyritic shale is indicative of the rapid accumulation of
organic remains, chiefly foraminifera, and possibly plant matter.
4. The occasional chert beds seem to be derived from sudden and rapid
accumulations of Radiolaria, and possibly other silicious organisms.
Foraminifera do not thrive in impure, sediment-laden waters. Their
abundance in some of the layers of calcareous ash and lime shale con-
taining ashy material, and the evenly banded character of these rocks,
points to the following special conditions of deposition:
1. Quiet waters, free from strong currents, and presumably not very
2. Pure water, except during showers of volcanic ash, which resulted
in a sudden pollution of the water, killing off of the foraminifera and
other planktonic forms, and the rapid settling of both ash and shells on
the quiet bottoms.

From the above data, including the descriptions of the various types
of rock which occur in the Older Series, the conclusions regarding their
age, and the interpretations of conditions under which they were formed,
the following steps are given in the Cretaceous and early Tertiary history
of the Lares District:
1. Pre-Cretaceous.-Unknown. The basement of the Upper Creta-
ceous volcanic flows and elastics has not been found in the Lares District,
and probably will not be found.
2. Upper Cretaceous. Period of almost continuous volcanic eruptions
of the explosive type, marked by the ejection of enormous quantities of
lithic fragments and frequent showers of ash, resulting in the rapid ac-
cumulation of great thicknesses of elastic deposits. This action was
accompanied throughout by lava flows of andesite and augite-andesite,
and intrusives of the same material into the surrounding elastic rocks
during their formation. There were frequent oscillations of level with
invasions of arms of the sea covering portions of the western part of the
district, and on one occasion, at least, extending as far east as Lares.


3. Late Upper Cretaceous. Final phases of vulcanism, marked by
minor intrusions of magma of extreme composition, such as peridotite,
now altered to serpentine. These final intrusives were accompanied by
the beginnings of great orogenic movements.
4. Close of the Cretaceous. Orogenic movements, comprising extreme
folding and thrust faulting, with the maximum forces applied in a north-
east-southwest direction. This was probably the local phase of wide-
spread crustal movements which outlined the present sub-sea mountain
chain, the basement upon which the Antillean islands rest.
5. Close of the Cretaceous or Beginning of Eocene. Further crustal
movements, involving north-south warping, normal faulting, and pos-
sibly accompanied by further uplift.
6. Eocene. Continuous erosion, and reduction to oldland surface of
comparatively slight relief.
7. Early Oligocene. Renewed uplift, with dissection of the oldland
surface to a region of considerable relief. Deposition of coarse gravels in
valleys of torrential streams (seen southeast of Moca).
8. Middle Oligocene. The beginning of a partial submergence of the
island with deposition of the San Sebastian shales in embayments along
the north coast of that time. Further events are given in the discussion
of the Tertiary and Pleistocene.


The most complete development of the Tertiary formations in Porto
Rico is in the Lares District, and occupies the area between the Lares
Road and the north coast. These formations are a structural unit
(Berkey, 1915, p. 12), resting unconformably upon the highly disturbed
Cretaceous rocks, and overlain unconformably by Pleistocene and Recent
consolidated dune sands and beach gravels, limited to the coast line. The
Tertiary beds are a series of limestones, in small part of coral origin,
underlain nearly everywhere by basal sales, clays, marls, or conglom-
erates. The maximum thickness of the entire group is about 3800 feet,
with the strata having an average dip of about 4 seaward. The range
in age is from Middle Oligocene to Upper Oligocene.
Table 3 shows the subdivisions and names applied to the Tertiary for-
mations of the north coast by previous writers and in the present paper.
Hill's term Pepino Formation, although having priority over the term
Arecibo of Berkey, will not be used in the present paper, because (a) the
corals collected by Hill from his Pepino formation occur also in the


Upper San Sebastian shale, and (b) Pepino refers to the lithologic and
topographic character of the formation, and is therefore an undesirable

The existence of a basal shale was first pointed out by Hill (1899 c),
who did not apply a formational name to it, but merely mentioned its
position relative to the Pepino Formation. Berkey (1915) was the first
to suggest a formational name (San Sebastian shale). Subsequently,
Reeds gave the name Collazo shale, because of the excellent exposures
and numerous fossils found at Callazo Falls, east of San Sebastian. Fol-
lowing Reeds, Miss Maury has used the name Rio Collazo Shales. In
the present paper the name suggested by Berkey will be used, because
(a) Berkey's name has priority, and (b) it is a more appropriate name
than Collazo. A glance at the cross-section (Plate I) or columnar sec-
tions (Plate II) will show that the maximum development of this basal
shale is near San Sebastian.
The columnar sections (Plate II) show the various types of material
of which the formation is made. Although it is predominantly a shale,
it consists in large part of dark bluish clay carrying seams of lignite with
pyrite and marcasite, conglomerate and pebble beds (in most places
unconsolidated), red calcareous sand or lime sand, green marl, and im-
pure limestone. Many of the lignitic clays contain fossil leaves and
fresh or brackish water molluscs. Sharks' teeth and vertebrate bones
occur in many of the beds. The marls and red lime sands contain marine
fossils, and as shown in the Collazo section (Plate II) alternate with the
brackish or fresh water beds. The marls at the top of the formation
carry an abundance of large coral heads. Limestone argillaceouss) is
the only material in the San Sebastian formation which is firmly con-
solidated. In other strata, the lower the content of lime the less consoli-
dated the material. Most of the "conglomerates," which predominate in
the basal part of the formation, are merely loose gravels or pebble beds.
Ground water has easy access in all strata except the dark blue clays, and
it has leached out large quantities of calcium carbonate. Most of the
fossil molluscs are found as molds with but small portions of the shell
As previously noted, the San Sebastian shale is thickest in the vicinity
of San Sebastian. From this point it pinches out on the west and thins
to 100 feet at the east end of the Lares District. Farther east, in the


Rio Arecibo canyon, it is absent (Berkey, 1915, p. 16). Thus, as com-
pared with overlying formations, the San Sebastian is of local occurrence.
That its relationship with the Cretaceous rocks is ulconformable has
been shown by Berkey, and is too evident to require further demonstra-
tion. The actual basal contact cannot be seen in any of the localities
visited, but exposures close to the contact can be seen in several places,
the most significant of which are in the immediate vicinity of Lares.
From a survey of these exposures, the following features are to be noted:

FIG. 17.-Boulder clay and coarse grarcl at base of the Tertiary
K. 8.9, Lares Road. near Moca. This deposit lies unconforniably on the Cretaceous
rocks, which in this locality consist of shales, ash, and augite-andesite porphyry.

1. The surface upon which the San Sebastian shale was deposited was
one of considerable relief (probably two or three hundred feet).
2. The San Sebastian rests not upon fresh, cleanly eroded Cretaceous
rock, but upon badly decayed rock or residual soil. Portions of the basal
San Sebastian at Lares are made up of reworked residual soil, differing
in no respects from modern residual soil of the interior mountains, except
that it contains, here and there, the molds of Tertiary fossils.
At K. 8.9, Lares Road, there is an interesting deposit of boulder clay
and coarse gravel (Fig. 17) resembling glacial till in its general appear-
ance. This material is unconsolidated, and composed of boulders of
Cretaceous rocks imbedded in a kaolinized matrix. It contains absolutely


no material which could have been derived from any of the Tertiary for-
mations. To the northwest this deposit pinches out and is replaced by
basal Tertiary pebble beds and shale (Lares formation) near Moca.
These large boulders probably mark the channel of an Eocene early Oli-
gocene river. Such coarse gravel would probably not be common in
channels of old streams flowing on a peneplane. On the other hand, such
deposits should be common in a region of considerable relief, such as is
shown by the relationship of the basal Tertiary at Lares. At the top of
the San Sebastian shale, the contact with the overlying Lares limestone
is a gradational one. The change from marl to limestone by an alter-

FIG. 18.-Interfingering or alternation of limestone and marl, marking gradational
(ontact of San Sebastian shale and Lares limestone
Bridge at Collazo Falls, Lares Road. The more resistant beds of Lares limestone are
seen just above the road and again near the top of the view.

nation (interfingering) of strata of the two types or rock is shown in
figure 18.
From the sections (Plate II) it will be noted that pebble beds and
lignitic clays predominate in the lower part of the San Sebastian shale.
These individual beds, however, cannot be correlated in the different
sections. In the section at Lares, the upper member is a red lime sand
containing Peclen laresense. This bed is traceable as far east as the
Camuy River, but cannot be correlated with certainty with anything in
sections to the west. The most continuous member in the formation is
the zone of green marls with abundant corals, which forms the top'of the
San Sebastian formation at Collazo. This same zone, with the same


corals, is found at the top of the San Sebastian in the Hato arriba sec-
tion, west of San Sebastian. Some of the same corals occur in the basal
portion of the Lares limestone in the cuesta just north of IJares, hence
this portion of the Lares may possibly represent a limestone facies equiv-
alent to the upper San Sebastian marl beds. In the Collazo section, im-
mediately below this coral zone (zone C of Vaughan, 1919), is a zone of
marl containing Clementia dariena and Turritella tornata. This zone
likewise underlies the coral zone in the Hato arriba section, but the rock
is much more calcareous than the same horizon at Collazo. To sum-
marize, there are only three zones, of any considerable continuity in the
San Sebastian shale:
1. Upper Zone (zone C of Vaughan). Marls with abundant corals,
grading eastward into limestone and red fossiliferous lime sand.
2. The Middle Zone. Marls and other types of material marked by
abundance of ( l, n,, ,t; dariena and Turritella tornata var. portarico-
3. Lower Zone. Predominantly lignitic clays and gravel beds, with
some marine marl and limestone, the latter containing numerous tests of
a small Scutella (like S. mississippiensis). The extremely local nature
of most of the beds and fossil occurrences makes it inadvisable to attempt
a finer subdivision of the formation.


The term Lares Formation was first proposed by the writer (see Table
3) for the massive limestone overlying the San Sebastian shale, and most
conspicuous north of the town of Lares, where it forms the cuesta and an
extensive area of haystack or pepino hills. The distribution and bound-
aries of the formation were definitely shown in a paper presented before
the New York Academy of Sciences in 1917. In a paper on the Porto
Rican Tertiary, Dr. Maury (1919) uses the term Lares limestone for
beds overlying the San Sebastian shale, but no distribution or strati-
graphic limits are given. The chief horizon marker of the formation,
according to Maury, is a large cerite shell (Campanile (Portoricia)
larica Maury). This shell was not found by the writer in any of the
numerous localities studied, but a very similar shell (Cerithium (Cam pa-
nile) collazum Hubbard) was found in abundance in the San Sebastian
shale, and sparingly in the lower part of tile Lares formation. It should
be stated in this connection that these shells are poorly preserved, and
that the above two designations may represent the same species.
In the type locality, north of Lares, the Lares formation is a massive,


cavernous, white limestone, with scattered portions of thin bedded, hard,
or chalky limestone. As a whole, the Lares limestone is of shell and
foraminiferal origin, although many massive, reef-like portions are made
up in large part of corals. The coral limestone facies is so intimately
mixed with the well bedded portions that it is not possible to estimate in
what proportion the formation is of coral origin. However, the limestone
should be classed as a reef formation. It shows the lateral gradation
from stratified to massive structure, characteristic of reef formations in
general. The massive portions are either largely of coral origin or else
made up chiefly of foraminifera (Lepidocyclina and others) and micro-
scopic shell fragments. In most localities, this massive rock is hard,
compact, and partially crystalline, and extremely cavernous. The thin
bedded portions are chiefly chalky and soft, or alternate hard and soft
layers, many of which are yellowish from limonite stains. These strata
are not true chalk, but finely ground limestone, practically a rock flour,
derived probably from the adjacent reef structures. Most of the mol-
luscan shells occur in these well stratified portions of the formation.
So far the description applies to the eastern part of the district. From
the Rio Guajataca westward there is a change in facies. The white lime-
stone, with its massive exposures and pepino hills, grades into a softer,
more argillaceous limestone. The topographic expression of this differ-
ence in character of the rock is brought out in the geologic map. In the
Hato arriba section, west of San Sebastian, the formation consists of
alternating beds of white chalky limestone, argillaceous limestone, and
marls with abundant oyster shells, (0. virginica). Still farther west, in
the vicinity of Moca, the San Sebastian shale is missing, and the Lares
formation is the basal member. It might easily be mistaken for the San
Sebastian shale in this locality, because of its lithologic similarity, as
shown in the columnar section (Plate II). The presence of typical Lares
fossils, such as Cardium cinderellce alternatum and Pecten grabani, and
the absence of nearly all of the most characteristic San Sebastian species,
is sufficient evidence to show that these basal beds are equivalent to the
Lares limestone of the eastern part of the district. South of Aguadilla,
the formation is largely buried by the recent playa deposits of the Rio
Culebrinas. However, the Lares formation occurs southwest of the Rio
Culebrinas in a narrow coastal belt extending to Pt. Jiguero. Through-
out this area the rock is a limestone, grading from soft white or yellowish
chalky material to a hard, semi-crystalline limestone, predominantly
white, but locally red in color. At the base of the formation, a small
thickness of shale or gravel beds is present in most of the localities. At


Pt. Jiguero, however, the limestone apparently rests directly upon the
upturned Cretaceous beds, though the actual contact is not exposed to
The contact with the San Sebastian shale has already been described
as conformable. Similarly the contact with the overlying Cibao lime-
stone is a conformable, somewhat gradational contact, though very defi-
nitely marked topographically. There is no evidence of a discordance of
dip, of an erosion interval, or of a faunal hiatus between the Lares for-
mation and Ciboa limestone. The same was shown to be true of the
contact with the San Sebastian shale. The position of the Lares as basal
formation in the west shows a progressive overlap during a gradual sub-
mergence of the oldland. No other interpretation of the above data is


This name is taken from the barrio of Cibao, north of Lares, where the
formation is best developed. In the type locality (barrio of Cibao), the
Cibao is essentially a soft, white, chalky limestone with an abundant but
poorly preserved molluscan fauna. Interbedded with this white chalky
limestone are:
1. Beds of marl with abundant oyster shells (0. sellaformis porto-
2. Beds of hard pink or white limestone with Lepidocyclina and
The predominant softness of the material explains the rolling prairie
lowland developed on this belt. The high content of argillaceous ma-
terial in the rock is well shown by the extensive covering of red and black
residual clay soils.
From its maximum thickness in the barrio of Cibao, the formation
thins westward, and south of Aguadilla it is only 300 feet thick. The
character of the rock is different in this locality, the white chalky facies
containing a greater abundance of hard intercalated beds, many of which
are red in color. No evidence was found of an unconformity, discon-
formity or faunal hiatus between the Cibao and the overlying formation.
It is not possible to distinguish zones within the Cibao limestone which
continue laterally throughout the formation. A white chalky zone, strati-
graphically near the top, contains an abundance of an echinoid (Echiio-
lampus resembling E. aldrichi). This echinoid zone can be traced from
the Rio Camuy westward for at least six miles.



The name is taken from the barrio of Los Puertos, north of Lares,
where the formation is best exposed.

FII. 19.-Los Puertos limestone, massire reef type
Exposed in vertical cliff, nearly 150 feet high, and in which there is very little bedding.
Rio Camuy Canyon, five miles south of HIatillo.

The rock is predominantly a series of massive, reef-like beds, alternat-
ing with thin bedded chalky strata. It is a reef formation essentially


like the Lares limestone, but with pepino hills developed on a greater
scale with greater relief. Excellent exposures of this formation may be
seen in the canyons of the Rio Camuy and Rio Guajataca (Fig. 19).
The massive reef-like portions are hard, semi-crystalline, and very cav-
ernous. The prevailing color is pink. Fossils are not well preserved and
are chiefly small forms, including foraminifera (Orbitoli!es and Lepi-
docyclina). The thin bedded portions of the formation are soft chalky
strata, alternating with harder strata. The soft layers are in many places
stained with limonite, giving a varied coloring to the canyon walls.
These thin strata are in some localities intercalated with other soft chalky
beds containing angular fragments of hard reef-type limestone. The
chalky beds arc usually deeply weathered and contain very few fossils
preserved well enough to recognize. Corals occur in the massive zones,
but apparently for only a fraction of one percent of the entire formation.
Foraminifera and molluscs are the chief contributors.
The Los Puertos limestone is entirely conformable with the overlying
Quebradillas limestone, and shows no faunal hiatus. No zones, either
fossil or lithologic, cain be established in the Los Puertos limestone. Ex-
cept for the variations above noted, the formation is lithologically a unit,
just as it is a physiographic unit, with its distinctive belt of pepino hills.
The columnar sections (Plate II) illustrate the differences found. In
the west coast section, the formation contains a great deal of the hard,
red, semi-crystalline limestone which is characteristic of portions of all
the limestone formations of the Arecibo group in this part of the district.
It will be noted (Plates I and II) that the Los Puertos limestone, while
variable in thickness, does not thin to the west, as do the underlying
As shown in Table 3, the name was first proposed by Berkey. It is
named after the town of Quebradillas on the north coast, where the best
exposures of the formation are to be found. The type fossil locality is
near Quebradillas at the mouth of the Rio Quajataca (Fig. 20). This
formation consists for the most part of a hard cavernous reef limestone,
which has a flinty appearance. It is made up chiefly of minute shell
fragments (molluscan and foraminiferal) and may be in part chemically
deposted lime carbonate. A few corals occur, but these are not of the
reef building type. In places, the formation is well bedded, with the
hard flinty limestone alternating with soft white chalky limestone, or
with layers of limestone breccia. Fossils occur in all of the beds, but


are never found abundantly except in small areas or "pockets." These
"pockets" are numerous, and do not all contain the same species. Some
of them are extremely crowded with molluscan shells. Almost invariably
the shell structures have been dissolved away, leaving only the external
and internal molds. This characteristic, together with the general
aspect of the rock, is so typical of the Quebradillas limestone that it
affords a reliable means of recognizing the formation in the field. A
typical exposure of the hard fossiliferous beds is shown in figure 20,
which also illustrates the massive character. Other phases of the forma-

FIC. 20.-Quebradillas limestone at the type fossil locality
Showing sea cliffs and east entrance of American railroad tunnel. Mouth of the Rio
Guajataca, west of Quebradillas.

tion of local extent occur at Isabela and on Pt. Borinquen. The sea cliffs
at Isabela are made up in large part of a thin bedded red limestone,
alternating with thin beds of indurated red lime sand, the entire exposure
totalling 150 feet thick. The rock is largely fragmental in origin, rela-
tively free from argillaceous matter, like all the rest of the formation,
and almost unfossiliferous. On Pt. Borinquen, in the sea cliffs northeast
of the lighthouse, is a 90-foot exposure of a white to yellowish chalky
limestone in which are interbedded vast numbers of large oyster shells
(0. antiguensis), well shown in figure 21. This zone of 0. antiguensis
can be traced as far east as Isabela, and apparently occurs in the San
Juan District.


The Quebradillas limestone is conformable upon the underlying Los
Puertos limestone and the boundary between the two is somewhat arbi-
trary. The Quebradillas is much more uniform in thickness than the
lower formations (see sections, Plates I and II). It is the most wide-

FIc. 21.-Quebradillas limestone at Pt. Borinquen
Showing shells of Ostrea antiquensis Brown imbedded in a soft, chalky limestone.
The total thickness exposed is ninety feet. most of which is made up in large part of
these giant oysters.

spread Tertiary formation in Porto Rico, and it marks the final deposi-
tion of a transgressing sea at the period of maximum submergence. The
Quebradillas limestone rests upon Cretaceous rocks in the San Juan Dis-
trict and on Vieques Island and shows clear evidence of a progressive


Like the lower formations, the Quebradillas limestone is not readily
divisible into zones or horizons, as an inspection of the columnar sections
(Plate II) will show. Lithologically, there can be no subdivision. From
a study of the distribution of the fossils, however, two zones of rather
doubtful validity may be distinguished:
1. Upper Zone, including uppermost 200 to 300 feet, characterized by
extremely abundant molluscan shells (Bowden fauna) and 0. antiguensis.
2. Lower Zone, marked by relative scarcity of fossils and presence of
Orbitolites mixed with Bowden fossils.


Table 4 (page 50) gives briefly the general make-up of the Tertiary
of the Lares District, with a few of the important features by which the
different formations are recognized.

It has already been pointed out that the Tertiary formations are a
structural unit. There are no breaks anywhere in the series. Although
there is no discordance of dip denoting an unconformity, it will be no-
ticed from the dips recorded on the cross-sections (Plate I, Figs. 3-6)
that those of the San Sebastian shale are somewhat erratic and in most
cases at higher angles than those of the overlying formations. The sec-
tions further show that there is, on the whole, a gradual decrease in dip
as one passes from the base to the top of the series. Thus, while the
average dip of all formations is 40, the San Sebastian averages over G6
and the Quebradillas between 2 and 3. The erratic dips recorded in
the San Sebastian shale are due in most cases to cross-bedding which
cannot always be distinguished as such in the field. Frequently abnormal
dips are noted in the limestone formations, but these were almost in-
variably found to be due to slumping resulting from underground solu-
tion. Such abnormal dips are in every case very local in extent. From
the San Juan District west to Lares, the normal dip of the beds is to the
north, but from Lares to the west coast it is to the northwest. The
nearer the west coast the more westerly the direction of dip. The signifi-
cance of this will be taken up later in discussing the Tertiary history.
Except for the gentle warping and local slumping above noted, no
faults, folds or other structural features were observed anywhere in the
Tertiary formations of the district. There are no dikes or other evi-
dences of igneous action in any of the Tertiary rocks, hence all volcanic
activity had died out before middle Oligocene time.






Los Puertos





San Sebastian shale

Middle Oligocene


Metis trinitaria
Orthaulax portoricocnsis
Ostrea antiguensis
Bullaria granosa
Chione woodwardi


Plateau, broad valleys, low
rolling hills, small pcpiao
hills in isolated groups or
in ridges.

Same as for Quebradillas Highest pepino hills in Porto
limestone except 0. (n- Rico

KEhinolnmpas sp. like E.

Cardium cinderella' alter-
Ostrea virginica

Corbula collazica
Ostrea collazica
Gerithium (Campanilc)
Clementia dariena

Rolling prairie, moderate re-
lief, low cuestas, sink
holes, no pcpino hills.

Pcpino hills in east, dis-
sected cuesta in west.

Iissected cuesta


Hard, pink, semi-crystalline, cavernous
to soft, chalky limestones, low in
argillaceous material. Fossils char-
acteristically in isolated "pockets."

Massive, hard, white or pink, semi-
crystalline. very cavernous limestone;
in many places thin-bedded, with al-
ternate hard and soft layers. Fossils

Soft, white or yellow, chalky, or argil-
laceous limestone. Fossils abundant
but poorly preserved.

Massive reef type limestone, locally
thin-bedded and chalky. Limestone
grades into shales, clays, and gravels
in the west.

Green marl, lime sand (marine), fluvia-
tile gravels, lignitic blue clays, shaly
limestone, and carbonaceous shale.


700 to 875

550 to 1000

250 to 1000

350 to 1275

100 to 700


The various speculations and conclusions regarding the age of the
north coast Tertiary formations has been shown in Table 3. The diver-
sity of opinion is explained by the fact that not until the summer of
1919 has a detailed study of the fossils been made. A large collection of
fossils made by Dr. C. A. Reeds in 1914 was described by Maury (1919),
and her conclusions regarding the age of the formations are given in
Table 3. During the .survey of the Lares District in 1916, the writer
made a very large collection of Tertiary fossils, all of which were recorded
as accurately as possible as to locality and stratigraphic position. These
fossils were studied during the summer of 1919, and the final conclusions
regarding the age of the formations based on this study are shown in
Table 3. It will be noted that the writer's conclusions agree with those
of Dr. Maury with the exception of the Miocene or Oligooene age of the
Quebradillas limestone. Dr. Semmes and the writer recognized in 1917
that the Quebradillas limestone is approximately equivalent in age to the
Bowden marl of Jamaica. The remaining point at issue is whether the
Bowden is Upper Oligocene or Lower Miocene. Evidence will be pre-
sented to show that it is Upper Oligocene.
About nine-tenths of all the fossils collected are molluscs.3 Of these,
140 species or varieties have been described or recorded by the writer
(Hubbard, 1921). A study of these fossils by horizons brings out some
interesting results. In figure 22, the stratigraphic range of the fauna of
each formation is shown graphically, the ordinate, or height of the curve
at each formational horizon from left to right, denoting the total number
of species of the fauna appearing in that horizon. Thus the curve of the
Los Puertos fauna shows that a total of 17 species were recorded from the
Los Puertos limestone. Of these, only 1 was present in the San Sebastian
shale, and about 13 survived in the Upper Quebradillas limestone. These
faunal curves illustrate the following facts:
1. All the faunas are transitional. There is no hiatus anywhere in the
series. This corroborates the stratigraphic evidence.
2. The faunas of the San Sebastian shale, Lares formation, and Que-
bradillas limestone are somewhat distinctive. The faunas of the other
two formations are mixed or transitional.
3. The San Sebastian fauna is almost wholly distinct from the Que-
bradillas fauna, and had almost died out in this region at the close of
Middle Oligocene (Cibao) time.
3 Wherever the term "fauna" is used in the present paper it applies to the molluscan
element, except where noted otherwise.


4. The Lares fauna contains three elements: (a) Species limited to
the Lares formation, about 40 percent of the total. (b) Species left over
from the San Sebastian. (c) A few species typical of the Quebradillas,
marking the beginning of the invasion of the Bowden fauna.

San Lares Cibao Los Queb-
Sebastian formation limestone Puertos radillas
shale limestone limestone

San 3s
Sebastian ,;
fauna ,

Lares 355
fauna aa

Cibao a
fauna 15-

Loes for
fauna 15 -

radillas -
fauna -


FIG. 22.--Graphic representation of the range of the Tertiary fa noas
The figures at the left show the number of species.

5. The Cibao fauna has more in common with the Lares than with the
Los Puertos, and hence the Cibao limestone should be grouped with the
two lower formations in the Middle Oligocene.
6. The Los Puertos fauna is most like the Quebradillas fauna, and
properly belongs in the Upper Oligocene. It marks a further increase
of the invading Bowden species.


7. In general, the invasion of the region by new species was a more
sudden process than the decline of the species already established. The
only exception to this is the early appearance and gradual increase in
number of a portion of the Bowden (or Quebradillas) fauna. The ap-
pearance of the bulk of the Bowden fauna, however, was relatively sud-
den. The term Bowden fauna, as used here, includes that characteristic
group of molluscan types common to the Bowden marl of Jamaica and
the Aphera-Sconsia formations of Santo Domingo.
8. It should be pointed out that a further collection and determination
of species from these formations would undoubtedly alter the curves given
above. Owing to the poor state of preservation of fossils in the Cibao
and Los Puertos limestones, only a small percentage of their faunas could
be determined and recorded. Further work in the Los Puertos limestone
will probably increase the number of known Quebradillas (or Bowden)
species in that formation, and show that the appearance of the Quebra-
dillas fauna was not so sudden as the curve indicates.
9. The decline of the San Sebastian fauna was due to the change in
bionomic conditions. Many of the San Sebastian species are brackish
water forms, none of which survived the change to pure marine condi-
tions, but some of which survived through Lares time. The latter are
found chiefly in the western part of the district, where the Lares is to a
large extent a brackish water or near shore deposit.


The following is a list of the molluscs found in the San Sebastian shale
and described by the writer (Hubbard, 1921). Those forms which do
not occur above the San Sebastian shale are indicated by an asterisk:
Pectcn (Pcctcn) laresensc n. sp.
Pectcn ((hlanms) collazocnsis n. sp.
Pecten (Chhl mys) portoricoenisis i. sp.
Pecten (Chlanmys) portoricocnsis var. reticulatis.
Pectcn (Chlamys) portoricoensis var. grandis.
Pectcn (Plagioctenium) rabelli n. sp.
Amusium papyraccum Gabb.
Amusium mauryi n. sp.
Amusium (Propeamusium) hollicki Maury.
Spondylus bostrychites Guppy.
Spondylus gunmanomacon B. and P.
Ostrea collazica Maury.
Area dariensis IB. and P.
Area sp. indet. (n. sp.).
Area (Scapharea) collazica Maury.


Glycimncris collazocnsis n. sp.
Lucina collazoensis n. sp.
Phacoides (Pseudomniltha) laresensis n. sp.
Clementia dariena Conrad.
Pitaria (Hyphamtosoma) carbascea Guppy.
Solen (Plectosolcn) collazoewn.is.
Corbula collazica Maury.
Tcredo incrassata Gabb.
Dentalium sp. indet.
Neritina (chipolana var.?) collazocnsis n. sp.
Crucibulum (Dispotaca) collazum n. sp.
Natica (Ampulina?) collazoensis n. sp.
Epitoniimr (Cirsotrcmna) collazoensis n. sp.
Turtrtella tornata var. portoricocnsis n. var.
Turritclla planigyrata Guppy.
Turritella ifitchelli n. sp.
Petaloconwhus? collazocnsis n. sp.
Cerithium (Campanile) collazum n. sp.
Cypraea sancti-sebastiani Maury.
Cassis sp. indet.
Turbinella chipoluna var. precursor n. var.
Bullaria paupercrula Sowerby

This list brings out the following important facts:
1. A large part of the fauna consists of new species or new varieties.
2. There are many brackish water and near shore forms. Among these
are Neritina, Ampulina, Cerithium (Campanile) and a large number of
pelecypods of fresh or brackish water type, most of which are specifically
indeterminate and not listed above.
The San Sebastian fauna as a whole is unique among the Antillean
Tertiary faunas so far described. It has, however, certain elements show-
ing relationship with other faunas.
The fauna has a very strong resemblance to the Gatmu (Upper Oligo-
cene). In most cases, however, the species are similar, but not identical.
It seems probable that a portion of the Gatun fauna was derived from
the San Sebastian fauna. The following is a list of the San Sebastian
species with their.Gatun analogues:

1. Glyicjricis collazoensis n. sp. Glycimesis gatunensis Toula.
2. Lucina collazoensis n. sp. Lucina sp., an undescribed shell in
the Kenip collection, at Columbia
3. Natica (Ampulina) collazoensis Natica sp. indet. B. and P.
n. sp.
4. Turritella planigyrata Guppy. Turritella gatunensis Conr.


Turritclla mitchelli n. sp.
Spondylus gumanomocon B. and P.
Amusium mauryi n. sp.
Area dariensis B. and P.
Clementia dariena Conrad.

Turritella altilira Conr.
Spondylus scotti B. and P.
Amusium luna B. and P.
Area dariensis B. and P.
Clemoetia dariena Conr.

There are many forms showing a general Flint River aspect. Many
of the indeterminate fossils found in the San Sebastian, and not listed
or described in the present report, show that the fauna resembles the
Flint River Oligocene fauna more closely than the species described
would indicate. The two most notable Flint River types are:

1. Turritella mitchelli n. sp.
2.. Cerithium (Campanile) colla:zum
n. Sp.

Turitella halensis Dall.
Cerithium halensis Dall.

There is a fairly large representation of Bowden and Aphera-Sconsia
species, which migrated from Porto Rico before Bowden time, since they
do not occur in the Quebradillas fauna. These are:

1. Amusium papyraccum Gabb.
2. Spondylus bostrychites Guppy.

3. Spondylus gumanomocon B. and P.
4. Iitaria carbasea Guppy.
5. Turritella tornata var.
6. Turritclla planigyrata Guppy.

St. Domingo, Bowden.
St. Domingo, Bowden, Tampa, Chi-
pola, Anguilla.
St. Domingo.
St. Domingo, Bowden.
St. Domingo (type), Cumana.
St. Domingo, Caroni Ser., Gatun.

1. Turbinella chipolana var. precursor n. var.
2. Ncritina (Chipoluna var.?) collazocnsis n. sp.

The variety of T. chipolana is more primitive than the Chipolan shell,
and hence supports the evidence that the San Sebastian shale is older
than the Chipola formation.
The Eocene aspect is only superficial, and there are actually few fossils
closely related to American Eocene species. Solen (Plectosolen) colla-
zoensis is strongly suggestive of Eocene species, such as S. plagiaulax
Cossmann, S. laversinensis Lef. and Wat., and S. obliquus Sowerby, all
from the Paris Basin. Corbula collazica Maury and Ostrea collazica
Maury (especially the small variety with regular, divaricate ribs) might
be considered to have an Eocene aspect. Cerithium (Campanile) colla-
zum resembles some of the large Cerite shells from the Paris Basin Eo-


cene, but is really much closer to the Flint River Oligocene forms, as
previously pointed out.
The possible European connections of Solen collazoensis and Cerithium
collazum have been referred to. One of the most interesting shells found
in the San Sebastian shale is a Cirsotrema, described as Epitonium (Cir-
sotrema) collazoensis n. sp. Apparently, its nearest relatives are found
in the Paris Basin. It is closest to C. subspinosum (Grat.) of the Aqui-
tanian and C. bourgeois de Boury of the Helvetian. In Europe, the genus
Cirsotrena ranges from the Eocene to the Pliocene, but those of the
peculiar type represented by the above-named species are limited to the
later Tertiary, and are particularly characteristic of the Aquitanian and
Helvetian of the Paris Basin region.
In the discussion of the stratigraphy, it was noted that the San Sebas-
tian shale is divisible into three zones:
1. Upper or Coral Zone (Zone C of Vaughan), 150'-200'.
2. Middle or Clementia dariena Zone, 50'.
3. Lower Zone, lignitic clays and gravels.
The corals are confined to the Upper Zone, and many of these same
corals are found in the lower part of the Lares limestone. Vaughan
(1919) lists the following species from the Pefino Formation, giving the
other occurrences of each species:
1. Astroco(nia porloricensis Vaughan, Antigua, and Canal Zone.
2. Orbiclla costata (Duncan), Antigua, Anguilla, Canal Zone.
3. Antiguastrea cellulosa (Duncan), Antigua, Florida, Georgia, etc.
4. 1Mcandra portoricensis Vaughan.
5. Lcptoscris portoricclsis Vaughan.
(i. Pirouastrea anguillensis Vaughan. Anguilla.
7. Sidcrtstrea confcrta (Duncan), Antigua, Canal Zone, Anguilla.
8. Cyathl Unirpha antiygensis (Duncan), Antigua. Cuba, Mexico.
9. C'uythomorpha tennis (Duncan). Antigua, Cuba.
10. Diploastrea c(russolaminlluta (Duncan), Antigua, Cuba, Georgia.
11. Astreopora portoricensis Vaughan.
12. Gon ioporu portoriccusis Vaughan, Antigua.

These corals are from the collections made by R. T. Hill, and a few
collected by the writer. They are probably all from Zone C or Upper
San Sebastian shale, with possibly a few from the basal Lares limestone.
Vaughan points out that 8 of the 12 species occur in the Middle Oligo-
cene of Antigua, and he concludes that this portion of the Pepino For-
mation is of Middle Oligocene age.
At Collazo, near the base of the San Sebastian shale, occurs a small
echinoid Scutella resembling S. mississippiensis Twitchell of the Clai-


borne, but apparently a new species. While it resembles the Eocene spe-
cies superficially, it also resembles some of the small Scutellas from the
Pacific coast Miocene, and hence does not indicate an early Tertiary age
for the basal portions of the San Sebastian shale.
The following foraminifera were found in the San Sebastian shale:

1. Orbi>olitcs cf. amcriclna Cushman, Zones 2 and 3.
2. Polystoimella sp., Zone 3.
3. Lcpidocyclina (Several species), Zones 2 and 3.

Of these, the first is by far the most abundant, and is found in all of
the Tertiary formations of the Lares District. It is very similar in size
and structure to 0. am ericana Cushman, from the Culebra and Em-
perador formations of the Canal Zone. Lepidocyclina is also abundant.
On the whole, these foraminifera indicate the Oligocene age of the San
Sebastian shale, but they will require further study before anything more
definite can be said.
Conclusion. The corals listed by Vaughan are undoubtedly conclusive
evidence of the Antiguan, or Middle Oligocene age of the San Sebastian
shale. The evidence of the other kinds of fossils is not so conclusive, but
does not disagree with the evidence of the corals. The molluscan fauna,
while somewhat mixed in its aspect, shows that the San Sebastian shale
cannot be as old as Eocene. It is more nearly similar to the recognized
Upper Oligocene than to the Lower Oligocene faunas.


The following is a list of the molluscan fossils found in the Lares
formation :4
I'cctcn (C'hlaimys) portoricoensis n. sp.
P'ccltc (Chlatnys) portoricocnsis var. reticulatis.
Pectcn (Chlamils) portoricoensis var. grandis.
Pecten (Chlanmys) grabaui n. sp.
Pecten (Chlatnys) grabami var. agunadtnsis.
Pecten (Chlamiys) Urabanli var. halticsis.
I'cetcn (Chilanys) grabiiii var. gnuayabnsis.
Pecten (Acquipcctcl) lobccki n. sp.
Pcctcn (Plagioctenioun) cercadica Maury.
Ostrca haitcnsis Sowerby.
Ostrca scllacformis var. portoricocusis n. var.
Ostrea collazice Maury.
Ostrca virginica Gmelin.
Ostrea cahobascusis var. portoricana 1. var.

SIndex of the Lares formation is indicated by an asterisk.


Lithophaga nigra d'Orbigny.
Area yaquensis? Maury.
Arca (Scapharca) collazica Maury.
Venericardia scabricostata? Guppy.
Phacoides (Miltha) riocancnsis Maury.
Cardium (Laevicardiun) cf. serratum Linn(.
Cardium (Trachycardium) muricoides n. sp.
Cardium (Trachycardium) cinderelle var. alternatum n. var.
Pitaria (Hyphantosoma) carbasca Guppy.
Chione woodwardi Guppy.
Chione hendersoni Dall.
Semele sp. indet.
Cyathodonta reedsi Maury.
Corbula collazica Maury.
Teredo incrassata Gabb.
Xenophora conchyliophora Born.
Turritella tornata var. portoricoensis n. var.
Cerithium (Canipanile) collazum n. sp.
Xancus n. sp.?
Mitra symmietrica Gabb.
Bullaria granosa Sowerby.

In addition to these species, many indeterminate fossils, chiefly gastro-
pods, were collected. Some of these are apparently limited to the Lares
formation. The presence of some of the Antiguan corals in the basal
Lares limestone indicated that the formation is Middle Oligocene in age.
Other fossils indicating Oligocene age are:
1. Agassizia sp., a small species resembling A. conradi (Bouv6), Oli-
gocene of Bainbridge, Georgia. It is apparently limited to the basal
portion of the Lares limestone.
2. Orbitoid foraminifera, including Orbitolites cf. ainericana (Culebra
and Emperador), and Lepidocyclina cf. mantelli, a large flat form. The
same species occurs abundantly in the Juana Diaz shale and basal Ponce
formation of the south coast of Porto Rico, and is listed by Mitchell
(1922) as Orbitoides mantelli.


The following list represents probably not more than a third of the
molluscan species in this formation. Nearly all of the material is poorly
preserved and largely indeterminate:
Pcctcn (Envola) reliquus var. porloroiensis n. var.
Pecten (Chlanmys) portoricoensis var. grandis.
Pectcn (Chamys) grabani var. hatocnsis.
Pecten (Chlamys) gt/ibaui var. gualabensis.
Spondylus bostrychites Guppy.


Spondylus guinanomocon Brown and Pilsbry.
Ostrea haitensis Sowerby.
Ostrea haitcnsis8 var.?
Ostrea sellaeformis var. portoricoensis n. var.
Area sp. Maury.
Cardium cf. serratum LinnE.
Pitaria (Hyphantosoma) Carbasca Guppy.
Chione hendersoni Dall.
Teredo incrassata Gabb.
Hipponyx portoricocnsis n. sp.

This list illustrates clearly the mixed or transitional character of the
molluscan fauna. The only distinctive index fossil of the Cibao limestone
is an echinoid, Echinolampas sp., which is very similar to E. aldrichi
Twitchell of the Vicksburg Oligocene. This fossil is found! in great
abundance in the upper portion of the formation, and the zone in which
it occurs can be traced from the Rio Camuy westward for a distance of
5 or 6 miles.
The basis for including the Cibao limestone in the Middle Oligocene
has already been given in discussing the faunal curve (Fig. 22).


A comparison of the following list of the Los Puertos fossils with the
list of the Quebradillas species will show the close relationship of the two
Ostrea scllaeformis var. portoricoensis n. var.
Glycinmeris portoricoensis n. sp.
LucLina cf. chrysostoma (Meuschen) Ihilippi.
Phaeoidcs (Miltha) riocanensis Maury.
Cardiun m mricoides n. sp.
Chione wood.wardi Guppy.
Chione hcndcrsoni Dall.
Metis trinitaria Dall.
Teredo incrassata Gabb.
Xcnophora conchyliophora Born.
Hipponyx portoricoensis n. sp.
Orthaulax portoricocnsis n. sp.

The poor state of preservation, as previously noted, is the reason for
the small number of species listed. Future work should increase the
total of Bowden types in this formation, but the above list is sufficient to
determine the age. Orbitolites cf. americana is the most abundant fossil
throughout the formation and in the lower part of the Quebradillas lime-

5 Index of the Cibao limestone.



The following list includes all the species identified and described, but
probably represents less than half of the entire molluscan fauna of this
formation. Those forms which are an index of the Quebradillas lime-
stone are designated by an asterisk:

Atrina rabelli n. sp.
Pccteni (Euvola) rcliquus var. portori(ccnsis n. var.
Pccten (Nodipcctcn) iodosus Linn6.
Pecten (Chlamys) hodgii n. sp.
Pcctcn (Plagioctieium?) borijqucnelnse n. sp.
Ostrea antigucnsis Brown.
Lcda pcltclla Dall.
Area yaquecsis Maury..
Arca (Scapharca) cf. don(cia Dall.
Barbatia rcticulata, (hmelin.
Barbatia cf. bonaczyi Gabb.
Glycimeris portoricocvsis n. Sl).
Chama involuta Guppy.
Chama portoricana n. sp.
Lucina cf. chrysostoma (Mleusch.) Philippi.
Phacoides (Miltha) rioicanensis iMaury.
Phacoides (Mliltha) sp. indlet.
Phaeoides (.iiltha) sp. indet.
Phacoides (Lucinisca) calhoinensis Dall.
Divaricella prevaricataf Guplpy.
Codakia magnoliana var. borinqucnense n. var.
Cardium ef. serraltm inn6 '.
Cardiium (Trigonocardia) sambaicumn var. porioricocusis n. var.
Cardium (Trigonocardiia) n. sp.? aft. C. sambaicum Maury.
Cardium (Trigonocardia) haitense var. WercadicaU Maury.
Cardium (Trigonocardia) haitense var. areciboense, var.
Cytherea (Cythcrea) berkyci n. sp.
Chione woodwardi Guppy.
Telling strophoidea n. sp.
Tellina portoricocusis n. sp.
Tellina (Scissula) grubaui n. sp.
Tellina aft. T. (Angalus) alossa )all.
lMetis trinitaria Dall.
'Psammosolen saicti-dominici Maury.
Cyathodonta reedsi Maury.
Tcredo incrassata Gabb.
Calliostoma portoricocnsis n. sp.
Turbo fettkii n. sp.
Liotia (Arcne) coronata var. portoricoenis n. var.
Xenophora conchyliophora Born.
Ilipponyx portoricocnsis 11. sp.


Calyptraea cf. centralis Conrad.
Crucibulum auricula var. portoricoensis n. var.
Crucibulum auricula? var.
Natica canrcna? (Linne) Moerch.
Turritella portoricoensis n. sp.
Turritella bcrklcyi n. sp.
Petaloconchus domingensis Sowerby.
Pyramidella portoricocnsis n. sp.
Bittium sp. indet.
Cerithium portoricoensis n. sp.
Cerithium quebradillensis n. sp.
Modulus modulus var. basileus Guppy.
Strombus proxinms? Sowerby.
Strom bus bifrons? Sowerby.
Orthaulax gabbi? Dall.
Orthaulax portoricocnsis n. sp.
Cypraea spurcoides? Gabb.
Malea camera Guppy.
Strombina portcricana n. sp.
Phos costatus Gabb.
I'hos elcgans var. portoricocnsis n. var.
Murex (Phyllonotus) cornurectus Guppy.
Alectrion gurabensis var. portoricocnsis n. var.
Alectrion gurabensis var. varicnti n. var.
Fusus henekeni Sowerby.
Turbinella chipolana var. areciboense n. var.
Xancus validus Sowerby.
Mitra hcnckeni Sowerby.
Olivclla muticoides var. portoricoensis n. var.
Olivella portoricoensis n. sp.
Cancellaria laevescens Guppy.
Turris albida var. haitensis Sowerby.
Turris albida var. cf. virgo Lamarck.
Drillia consors var. portoricoensis n. var.
Drillia grabaui n. sp.
Drillia portoricoensis n. sp.
Drillia seimiesi n. sp.
Cythara cf. elongata Gabb.
Terebra quebradillensis 1. sp.
Conus catenatus Sowerby.
Conus cf. marginatus Sowerby.
Bullaria paupercula Sowerby.
Bullaria portoricocnsis n. sp.
Bullaria granosa Sowerby.

The most striking feature of this list is the large representation of
Bowden and Aphera-Sconsia species. The approximate age equivalence
of the Quebradillas limestone with Bowden horizons in the West Indies
is too evident to require further discussion.


A-Number of identical species in common.
B--Number of similar species or varieties in common.
Data based on corals and foraminifera as well as
molluscs. Elsewhere the data includes only

South Coast Ponce limestone*.................
Porto Rico JJuana Diiaz shale ..............
--------- -- - - - - - - - - -
Caloosahatchie Pliocene......................

Jamaica (Bowden) .........................

St. Domingo (Sconsia) .......................

St. Iomingo (Aphera) ........................

St. Domingo (Samba Hills) ...................

Total-Aphera. Sconsia, Bowden..............

Alum Bluff, Chipola, Oak Grove...............

Panama (Gatun) ............................

Trinidad (Caroni)............................

Sombrero, W I ...............................



3 1
..- .. --

2 6(

2 3

'3 5

2 4

2 8

1 ..
- - -- ---- I



4 1 4 1

4 1 2 1

1 4

3 5 3

5 3 4

8 7 '2
------ ------ ------
2 4 1

S ------------






















1 ..


16 5

1 0

1 3

12 11
_------ -- _---
13 10

20 17

2 3

27 28

3 15

2 14

4 2

9 2


Duplin County, N. C. (Miocene) ...............

Panama (pre-Gatun) ..........................

Tam pa Silex ................... .............

Haiti (Las Cahobas) ........................

Georgia (Flint River) ........................

Antigua (Anitigua formation)................

Vicksburg Oligocene.........................

Claiborne Eocene .............................
Europe (Acquitainian)........................
Europe (Eocene)............................

Miscl. Pleistocene, Recent.....................

Antillian Region, Tertiary to Recent...........

New species not closely related to any known
species in the Antillian Region ..............

Total number of species and varieties recorded.


------- -


3 !

--- 2






37 35

.. l1


1 1

1 1


4 3

13 16


.. 1







1 2

5 7



.. .. [
---- -- --- ------ ------

1 .. .. ..
---- .---- -- --- - -...
1 i 1

-- ----- ---- --_-- .----- t-
1 : 1 a

--- ---- ---- ----
' . . - -. - - --. -. .

1 .

---- ----.............

1 4 3 9
.------ ------ ---...
6 5 29. 47
- ---- ---- ------._ ^

1 9 Y

- - - -




Foraminifera are abundant and include Orbitolites cf. americana Cush-
man (lower Quebradillas) and Polystomella? sp. (upper Quebradillas).
In the uppermost horizon and fragments of Scutella sp. and Cidaris sp.
and a Schizaster resembling S. scherzeri Gabb, a species from the Sapote,
Costa Rica and the Emperador limestone, Canal Zone. It also resembles
S. floridanus Clark, of the Vicksburg Oligocene. Corals are not common
in the Quebradillas, and none of the chief reef-building types occur.
Semmes (1919) illustrates a Fungid coral, indeterminate, from the
Quebradillas limestone of the San Juan District. This fossil occurs in
the Lares District, but is not abundant. The commonest coral in the
Quebradillas limestone (and equivalent horizons on the south coast and
on Vieques Island) is a Balanophyllia. There seem to be two species
(or varieties), each resembling species found in the Bowden or equivalent
formations. Another abundant coral is a Stylophora like S. affinis

The molluscs, which make up about nine-tenths of the fauna, show
that the uppermost Tertiary formation of the Lares District is of Bowden
age. The basal member is shown by the corals, and less certainly by the
molluscs, to be of Antiguan or Middle Oligocene age. There are only
two distinct faunas, the San Sebastian-Lares and the Quebradillas. The
others are mixtures of San Sebastian-Lares and Quebradillas (Bowden)
Table 5 (pages 62-63) is based upon the molluscan species above de-
scribed6 and shows the relative numerical distribution of the Lares Dis-
trict fossils in other localities and horizons of the West Indies and else-
where. It shows the close relationship of the Quebradillas fauna with
the Aphera-Sconsia faunas of Santo Domingo. Another feature brought
out is the effect of geographical position upon the similarity of the
faunas. Thus formations of the same age as the Quebradillas limestone,
such as the Gatun formation and the Chipola-Oak Grove series, but situ-
ated at considerable distance from Porto Rico, have few identical species
in common, but a considerable number of similar species or varieties.


The question of the correlation of the north and south coast Tertiary
formations has been awaiting the collection and comparison of fossils.
6 In the case of the two south coast formations (Juana Diaz and Ponce) the data
include other types of fossils besides molluscs.


Berkey made no attempt at a correlation because the fossils then collected
(1914) did not warrant it. Semmes (1919, p. 59) pointed out that the
Ponce formation is "in part practically equivalent to the Arecibo forma-
tion of the northern coast," but he did not make any subdivisions of the
south coast series. Maury (1919, p. 214), after a study of fossils col-
lected by Reeds, made a correlation of the north and south coast Tertiary
(Table 6). This correlation was advanced by Maury as a tentative one
and is the first attempt of a detailed character. However, the evidence
at hand seems to warrant certain departures from this correlation, as
shown by the following considerations:


Quebradillas limestone
with Bowden fauna
and Metis trinitaria.

Aguadilla limestone
with Orthaulax
Lares limestone with
Campanile (Portori-
cia) larica.


Ponce chalky beds with
Ostrea cahobasensis.

a Rio Collazo shales Guanica shaly lime-
with Clementia ra- stone with Ostrea
io belli. antiguensis.
Middle Oligocene -a

1. Mitchell (1922) shows that the beds at Guanica are stratigraphic-
ally at or near the top of the Ponce formation. In speaking of the Ponce
chalky beds, Berkey (1915, p. 14) says: "It is judged that the portion
of the formation seen at Guanica is a still higher horizon, but the exact
age values have not been worked out." From this, it seems evident that


Middle Miocene

Lower Miocene

Upper Oligocene


the Guanica beds overlie the Ponce chalky beds, and are at the top of the
column, not at the bottom as Maury stated.
2. The Guanica beds (together with the upper part of the Ponce beds
to the east) carry a typical Quebradillas (Bowden) fauna, as will be
shown later. Ostrea I,,;.,. ; o which Maury lists from the Guanica
beds is one of the best index fossils of the Quebradillas limestone.
Regarding the Juana Diaz, the basal shale member of the south coast
group, Maury (1919, p. 215) says: "The Juana Diaz shales furnished
very few molluscan shells. . The evidence at hand is too scanty
for any definite stratigraphic conclusion regarding these beds." As
shown by Table 5, and as will appear later, the Juana Diaz shale carries
several of the best index fossils of the San Sebastian shale, among which
Clementia dariena (= C. rabelli of Maury) is the most significant.
From a careful comparison of south coast fossils collected by Berkey,
Lobeck, and Mitchell, with material from the Lares District, the correla-
tion table (7) is proposed and is believed to be essentially correct. A
more detailed comparison is impossible because the south coast series has
not been definitely subdivided in the field.



San Juan formation


Quebradillas limestone

Los 'uertos limestone

Cibao limestone


San Juan formation

_------- Disconformity.___.__

Guanica beds and Upper

Ponce limestone.

- - - - - - ? - - - - - - -

____________ Lower Ponce limestone
Lares formation

San Sebastian shale

---_-_ Unconformity__ _____

Upper Cretaceous Older Series

- - - - - - - - ? - - - - - -

Juana Diaz shale


Older Series


Upper Oligocene-
Upper Oligocene

Middle Oligocene


The essential points in the basis of correlation have been referred to,
but the following lists of fossils from the south coast formations are
offered as further evidence. The localities recorded are taken from notes
by Berkey, Lobeck, and Mitchell:



Turitella halensis Dall

Turitclla halensis var.
alpha Mitchell

Scala? sp.

PIccte (Chlamnys) por-
toricocnlsis 11. sp.

C('lcentia a i d ena
Courad (=. rabelli

Solch (Plcctosolen coll-
(IZOCltsis 11. sp.

Natica sp. indet.

Cyprra sancti-sebas-
tiani Maury

PI'ten rabelli n. sp.

Strombus sp. indet.

Vencricardia cf. scabri-
costata Guppy


Juana Dinz. Yauco

Juana Diaz

Juana I )iaz

Near Juana Diaz

Juana Diaz

Juana IDiaz, Yauco.

JuTna I)iaz

Juana Diaz

Juana Diaz, Yauco
and K. 25, I'once-
Adjuntas road.


Juana Diaz, Yauco


Closely related to T. colla-
zocnsis of the San Sebas-
tian shale.

An internal mold. A simi-
lar or identical fossil
occurs in the upper San
Sebastian shale.

Index of San Sebastian and
Lares formations.

Index of

San Sebastian

Index of San Sebastian

i nt"rnal mold. Similar
molds are found in the
San Sebastian shale and
range through the Cibao

Index of

Index of

San Sebastian

San Sebastian

Resembles some internal
molds from the San Se-
bastian shale.

Occurs in Lares formation.




Teredo incrassata Gabb. R a n g e s throughout the
series, north and south
coasts and Vieques Is-
land. Of no value as an
index fossil.

Lepidocyclina sp. Juana Diaz, Yauco. A large foraminifer, ap-
parently the same as the
one occurring abundantly
in the Upper San Sebas-
tian shale, Lares forma-
tion, and Cibao limestone.



Orbilolites cf. ameri-
cana Cushman

Balanophyllia sp.

Stylophora sp. (like S.
affinis Duncan)

Strombus proximus

Bullaria cf. paupercula

Turritella cf. gatunen-
sis Conrad

Cardium cf. muricoides
n. sp.

Cardium cf. lingualeonis


Ponce, Guanica

Widespread, along

Culebrinas Pt.,
and along coast.

Mona Island

Mona Island

Mona Island

Near Guanica

Near (Gunica


Quebradillas limestone.

Extremely abundant in both
the Quebradillas and the
Upper Ponce limestones.

Characteristic of Quebra-
dillas and Upper Ponce

Quebradillas limestone.

Quebradillas limestone.

Apparently related to T.
portoricoensis of the Que-
bradillas limestone.

Ranges from Lares forma-
tion to Los Puertos lime-

Bowden, Jamaica.





Cardium haitense var.
cercadicum Maury

Cardium haitense var.
areciboense n. var.

Corbula sp. indet.

Chione woodwardi

Glycimeris cf. portori-
coensis n. sp.

Lucina cf. chryostoma
(Meusch.) Phil.

Phacoides (Llcinisca)
calhoiun(sis Dall

Cytherea (Cyttherea)
bcrkeyi n. sp.

Teredo incrassata

Ostrea antigucnsis

Terebra cirrus Dall

Tcrcbra cf. quebradil-
lensis n. sp.



Ponce, Culebrinas Pt.

Culebrinas Pt., Ponce,
Guanica, Rio Yauco
21/2 miles S. E. of


Vieques Is., Culebrinas
Pt., Ponce, N. W. of
Ponce, near Guan-
ica, K. 2.4, Ponce-
Adjuntas road.

Near Ponce

Ponce, West of Ponce,
N. E. of Ponce.

Vieques, Ponce

Guanica ?

Near Guanica



Quebradillas limestone.

Quebradillas limestone.

A small species, not found
in any of the north coast

Abundant in Quebradillas

May be the Quebradillas

The most characteristic fos-
sil of the Ponce forma-
tion. It occurs in the
Quebradillas and Los
Puertos limestones.

Quebradillas limestone.

Quebradillas limestone.

Quebradillas limestone. No
index value.

Quebradillas limestone. Re-
ported by Maury from the
Guanica shaly limestone.

Found in the lower Alphera
formation of Santo Do-

Resembles the Quebradillas
species and may be the




Area (Scadph/arca) cf.
riocanucntsis Maury

Bulltria grnJosa?



Culebrinas Pt.

Olivella muticoidcs var. K. 75.2, Ponce-Peflue-
prrtoricoi'rsis n. var. las Road, Aguila Pt.

Lcdti cf. p(itella Dall

Metis trinitaria Dall

Ostrca calhobascnsis
var. portolri ctt n.

Boring Sponge?

K. 75.2, Ponce-Peilue-
las Road

Culebrinas Pt.


Culebrinas Pt.


Upper Aphera of Santo
Domingo. Not found on
the north coast of Porto

Quebradillas limestone.

Quebradillas limestone.

Resembles this species,
which is very typical of
the Quebradillas lime-

Quebradillas limestone. Ap-
parently rare oil the
south coast.

The casts of the burrows
of some organism like
('hirma. These are so
widespread and charac-
teristic of the Quebradil-
las limestone that they
may be considered as in-
dex of this hor'zon. Their
burrows are chiefly in the
large gastropod shells.

It is believed that the last list includes fossils from all horizons of the
limestone overlying the Juana Diaz shale, and hence ranging from Lares
to Quebradillas in age. It is inadvisable with the data at hand to attempt
listing the fossils from the limestone as Upper or Lower Ponce. Never-
theless, it is evident that most of the more typical Quebradillas species
occur near the coast and therefore in the upper part of the Ponce lime-


Antigua. The Island of Antigua, in the Lesser Antilles, southeast of
Porto Rico, is considered the type locality of the Middle Oligocene
(Antiguan) of the Caribbean region. The island consists of an igneous
basement upon which rests a series of tuffs with interbedded marine
strata, and finally an uppermost limestone series known as the Antigua
formation, and considered by Vaughan as the type section of the Middle
Oligocene. The evidence given by Vaughan (1919, p. 259) is based upon
the fossil corals which he finds chiefly in a 60-foot fossil reef at or near
the base of the Antigua formation. The evidence of these corals shows
that the Antigua formation is equivalent to the lower members of the
Tertiary series in the Lares District of Porto Rico. On the basis of the
corals, Vaughan puts the entire Antigua formation in the Middle Oligo-
cene, and estimates its thickness at 350 feet. These conclusions appar-
ently do not agree with evidence furnished by others who have studied
the geology of this island. Of a list of 10 molluscan species from the
Antigua formation, collected and described by Brown (1913, p. 598),
6 are identical with, and 3 are similar or related to species occurring in
Antillean formations of Bowden age, while another species is character-
istic of the Anguillan, or so called Upper Oligocene. The thickness of
the Antigua formation is estimated by Spencer (1901) as "at least many
hundred feet," and by Brown as "upwards of 1 00 feet at least." It
forms a belt about 5 miles in width, and Vaughan estimates the seaward
dip to range from 10' to 15". Taking the average at 10, the thickness
must be several times greater than Vaughan estimated, and hence more
nearly comparable with the north coast series of Porto Rico. Consider-
ation of the above conflicting evidence seems to show that more than one
formation is represented in the Antiguan formation, and detailed strati-
graphic work will have to be done before the Antigua formation can be
regarded as the type section of the Middle Oligocene in the West Indies.
Santo Domingo. The highest Tertiary formations are the Aphera-
Sconsia formations described by Maury (1917). These formations are
approximately equivalent to the Bowden marl of Jamaica and to the
Quebradillas limestone of Porto Rico. Correlation of the Orthaulax
Zone of Santo Domingo and the pre-Quebradillas formations of Porto
Rico cannot be made with certainty because no detailed stratigraphic
work has been done in the older Santo Domingo formations.
Iaili. In Haiti, the Maissade beds (uppermost horizon) correspond
with the Quebradillas limestone. The underlying formation (Las
Cahobes) is probably equivalent to the San Sebastian shale, and overlying


limestones below the Quebradillas, as indicated by its stratigraphic posi-
tion. Among the fossils listed by Jones (1918, p. 738) from this forma-
tion are:
Turritella planigyrata Guppy.
Turritella tornata Guppy.
Venericardia scabricostata Guppy.

which in the Porto Rico section are limited to the San Sebastian and
Lares formations.
Other localities. It is quite likely that future work will greatly in-
crease the known number of localities in which the uppermost portion of
the Tertiary is of Bowden age. The small key of Sombrero, 140 miles
east of Porto Rico, is built up of a white limestone of Bowden (or Que-
bradillas) age. The following is a partial list of the fossils:7
Strombus proximus Sowerby.
Xenophora conchyliiphora Born.
Bullaria granosa Sowerby.
Tellina cf. strophoidea n. sp.
Cyathodonta cf. reedsi Maury.
Cardium haitense Sowerby.
Cardium cf. sambaicum Maury.
Chione woodwardi Guppy.

In addition to these, the casts of the Sponge (?)' burrows, character-
istic of the Quebradillas limestone, occur abundantly in the Sombrero
limestone. The proximity of Sombrero to Anguilla, Vaughan's type
locality of the Upper Oligocene, suggests the need of detailed strati-
graphic work in this vicinity.
The conclusions regarding the correlation of the Porto Rican with
some of the other Antillean localities is given in the accompanying table
(8). This table is based on those by Vaughan (1919a, p. 595), Maury,
and Jones, with minor changes. The chief departure from the authori-
ties above named is the placing of the Bowden formation and its equiv-
alents in the Upper Oligocene.


The question of the Miocene or Oligocene age of the formations equiv-
alent or approximately equivalent to the Bowden marl of Jamaica is one
on which authorities do not agree. Within the last few years Vaughan,
Maury and others have maintained that the age of these formations is

'Determinations by the writer from material In Paleontologlcal Museum, Columbia


Lower Miocene. On the other hand, Dall and some other authorities still
take the opposite view and place them in the Upper Oligocene. The
writer believes that the position taken by .Dall is correct. The evidence
favoring the Miocene age of the Bowden is given by Vaughan and need
not be reviewed here. The evidence favoring the Oligocene age may be
summarized as follows:
1. The genus Orthaulas has until recently been considered an index
of Oligocene age. It is now regarded by Cook and Vaughan as indicative
of either Oligocene or Lower Miocene. This change in the status of
Orthaulax is not due to any new discoveries of that genus in horizons
higher than it has previously been known to occur, but is due to the
placing of previously recognized Upper Oligocene formations in the'Mio-
cene nomenclature by Vaughan and others. The genus Orthaulax is one
of the most abundant fossils in the Quebradillas limestone, as well as in
the Los Puertos limestone. In the former horizon, there are at least two,
and very likely as many as four species of Orthaulax present. According
to Vaughan the Quebradillas limestone is Lower Miocene in age.8
If this view be accepted, then we must admit that the Lower Miocene
fauna of Porto Rico is largely Oligocene in its general aspect. Thus one
of the strongest arguments of the Miocene advocates is weakened, namely,
that the Chipola fauna (and presumably its time equivalents) in its aspect
"looks forward to the later Tertiary and Recent, rather than backward"
(see Vaughan, 1919a, p. 573). It is true that Orthaulax has been noted
as an exception to this statement, but every new discovery of abundant
Orthaulax in strata of Bowden-Chipola age tends to increase the signifi-
cance of this exception, and the Quebradillas fauna with its myriads of
Orthaulax shells mixed with the host of Bowden-Aphera-Sconsia types
furnishes one more" difficulty in the way of any argument based upon the
above quotation.
2. The presence of Ostrea antiguensis in great numbers at the top of
the Quebradillas limestone. Maury regards this fossil as so excellent an
index of the Oligocene that its presence in the Guanica shaly limestone
is sufficient evidence for correlating this formation with the Antiguan.
3. The gradual appearance of some of the Quebradillas species, start-
ing with the Lares formation (Antiguan).
4. The absence of any unconformity or marked faunal hiatus between
the Quebradillas limestone and the lower limestones. The entire series
is a structural unit.
5. Erosion interval and faunal break occurs in Porto Rico, as else-
where, at the end of the Bowden time.
s Personal communication.


6. Large representation in the San Sebastian shale of Gatun, Chipola,
and Bowden types shows relationship with the Bowden horizon.
7. Two species in the San Sebastian shale (Glycimeris collazoensis and
Turbinella chipolana precursor) apparently have closely related deriva-
tives, showing slightly greater specialization in the Quebradillas lime-
8. The marked difference of aspect between the San Sebastian and
Quebradillas faunas is due chiefly to change in bionomic conditions.
Thus the San Sebastian fauna, typically a lagoon or brackish water
facies, was forced to migrate because of changing conditions. It reap-
peared in Bowden time in the Canal Zone (Gatun fauna), with consid-
erable changes in species, but with the same general aspect it had in San
Sebastian time. The San Sebastian and Gatun faunas have in common
such characteristic species as Clemn entia dariein and Area dariensis.
9. Inspection of the fossils listed from the Canal Zone shows that the
Gatun formation has many species in common with the underlying lime-
stones. Most of the change in faunal aspect may be explained by change
of conditionsof deposition. In Antigua, Bowden molluscan fossils are
apparently so intimately mixed with Oligocene corals that no T'pper
Antiguan or Bowden horizon was differentiated th;:re by Vaughan. In
Florida, the Alun Bluff series is so closely associated with the underlying
Tampa and Chattahoochee horizons that authorities are still not in agree-
ment regarding the existence of a physical and faunal break, even after
considerable stratigraphic work has been done. All are agreed, however,
that there is a break at the top of the Alum Bluff group (= end of
Bowden time).
10. In the San Sebastian shale is found species of Cirsotreima which
is hardly distinguishable from certain species in the Ilelvetian (Miocene)
of Europe. Species of'Campanile and Plectosolen in the San Sebastan
shale have allied forms in the European Eocene. Facts such as these
suggest caution in attempting to decide the age of an American fauna
by comparing it, in its general aspect, with European faunas.
The only infallible criterion for correlation with European or other
distant sections is the recognition of a world wide crustal movement or
change of sea level. If the elevation of the South Atlantic-Antillean
region at the close of the Bowden-Alum Bluff time can be definitely
correlated with movement in other parts of the world, a definite and
logical division line can be drawn between the Oligocene and the Miocene.
In the American-Antillean Province this line of division is distinct, as
Dall (1898, p. 329) long ago pointed out. The evidence obtained in


studying the sections in the Lares District supports the early conclusions
of Dall rather than the recent conclusions of Vaughan.


The entire group of Tertiary formations of Porto Rico were deposited
during a continuous period of gradual submergence, which began in
Middle Oligocene time. The formations are therefore all conformable
with one another, and show a progressive overlap over the Older Series
rocks. The relationship is brought out in the ideal section (Fig. 23).
With the initiation of submergence, the sea encroached in the valleys of
the old land surface, forming embayments, as for example at San Sebas-



FIG. 23.-Ideal scction alonll hle north (oist at the tine of ut,.rimnll slubmergenrJ(lce
(late Oligoteee)
Showing transgressive overlap of the Tertiary formations on the old land surface.
The San Sebastian shale is confined to the Lares-San Sebastian embayment. The San
Sebastian shale and basal shale faces of the higher horizons are shown by the broken
lines. The formations are numbered consecutively : (1) San Sebastian shale, (2) Lares
formation, (3) Cibao limestone, (4) Los l'uertos limestone. (5) Quebradillas limestone.

tian and Juana Diaz, in which the chief deposits of basal shales accumu-
lated, under an alternation of brackish, freshwater and marine condi-
tions. At San Sebastian coral reefs formed across the mouth of the
embayment. The large "heads" or colonies of corals, broken from this
reef by the surf, are found imbedded in the marls of the San Sebastian
formation, exposed along the Lares Road. In connection with the
theories of origin of coral reefs, it is interesting to note that several
hundred feet of shale, in part marine, were deposited before the actual
reef itself was formed. Comparison of sections at Collazo and Lares
would seem to show that the Upper shales and coral marls of Collazo
grade into coral-reef limestone when traced toward Lares. This would
indicate that the green coral-bearing marls at Collazo were formed
behind an off-shore or fringing reef.


With continued submergence corals ceased to be the dominant reef-
building organisms, molluscs and foraminifera taking their place.
Throughout the entire series, the deposition kept pace with subsidence,
as shown by the fact that shallow water organisms predominate in all
the formations. In the eastern part of the Lares District, there was
apparently frequent emergence, with oxidation of the newly deposited
strata. The frequent occurrence of red limestone at various horizons in
the eastern part of the district may be evidence of this. The chief
evidence, however, is to be found in the thinning of the Cibao limestone
and Lares formation toward the west, indicating near shore deposition,
with a peninsula in the longitude of Moca. The submergence was prob-
ably accompanied by a seaward down-warping movement illustrated by
the fact that the Quebradillas limestone dips at lower angles than the
lower formations. This statement is true even where the Quebradillas
limestone is the basal formation, as in the San Juan District.


One of the unsettled problems has been the extent to which the island
was sugmerged in Upper Oligocene time. Lobeck concludes (1922) that
the island was not entirely submerged, as shown by:
1. Presence of gravel and clay at various horizons in the Tertiary beds.
2. Abrupt termination of Tertiary beds against the upland slopes of the
old land surface.
The evidence found in the Lares District may be summarized as
1. The faunal difference between the lower, formations in the Lares
District and the lower formations of the south coast is considerable, and
indicates a land mass quite extensive in an east-west direction, with no
connecting passages close at hand.
2. The Bowden faunas of the Quebradillas and Upper Ponce show
much greater similarity than do the older faunas, but still show greater
difference than the present beach faunas of the north and south coasts.
The extreme abundance of certain shells (like Metis trinitaria) in the
Quebradillas limestone, and their rare occurrence in the Ponce limestone
is very significant.
3. The great thickness of the Tertiary series (nearly 4000 feet) would
seem to show that the island was completely submerged. It has been
shown, however, that the formations near the old (Oligocene) shore line
are only a fraction of their maximum or off-shore thickness. The beds
were undoubtedly deposited with a slight initial dip to seaward, and


hence are of the nature of fore-set beds. The maxinium accumulation of
limestone was in an outward direction, rather than upward. This may
be one reason why the enormous thicknesses recorded for Oligocene reef
limestones like those of Haiti and'Porto Rico are not comparable with
measurements of beds the same age deposited in Florida and elsewhere
on the continental shelf.
4. A carful search was made for outliers of Tertiary rock in the higher
portions of interior mountains. The only evidence found was a single
boulder of Tertiary limestone (probably Lares) in the channel of the Rio
Guayaba, two miles southeast of Aguada. However, this does not show
that the Tertiary beds covered the Cordillera Central to the south.
From the above evidence, it may be stated that Porto Rico was not
completely submerged in Upper Oligocene time.


The uplift was differential. It amounted to at least 1300 feet in the
vicinity of Lares, and less than 100 feet in the extreme eastern part
of the island. This differential uplift resulted in the truncation of the
west end of the island, chiefly by warping. The northwest dip of the
Tertiary strata along the west coast has already been referred to in dis-
cussing the structure. It is probable that there was some faulting in
connection with this truncation of the island, and that the zone of fault-
ing lies somewhere to the west of the island, where faulting is now taking
place. This was shown by the 1918 earthquake.


There are three major physiographic provinces represented in the
Lares District:
1. The Complex Mountainous Oldland.
2. The Elevated Coastal Plain.
3. The Playa Plains.

These are the most important physiographic units of the island, and
have been described by Hill, Berkey, Lobeck, and the authors of the
geological reports on the different districts. The geologic map brings
out the contrasts of relief and topographic characteristics of the three
provinces as they occur in the Lares District.
The Complex Mountainous Oldland comprises the central mountain
chain; the core or backbone of the island. In the Lares District it is
the mountainous area south of the Lares Road; that is, the area making


up the southern half of the district. The maximum elevation (2000
feet) is in the southeast corner of this area, and the relief here is 500
to 700 feet.
The Coastal Plain comprises the area north of the Lares Road. It is
a plateau in a youthful or submature stage of dissection. From a maxi-
mum elevation of about 1500 feet (east of Lares), the plateau surface
slopes very gradually to the north coast, terminating there in sea cliffs
50 to 100 feet high. The rocks are Oligocene limestone with some basal
shale beds, the entire series lying nearly horizontal, or with a slight dip
to the north and northwest. These Oligocene formations overlap the
mountainous oldland as far as the Lares Road, where they culminate in
a more or less distinct cuesta, in places more than 300 feet high. This
cuesta marks the boundary between the Coastal Plain and Mountainous
Oldland provinces, and its position can readily be seen on the geologic
The Playa Plains are the nearly flat alluvial plains at or near sea
level, occurring along the coast at the mouths of the rivers. They are
especially large on the west coast of the Lares District, as for example,
the Culebrinas and Afiasco Playas. Smaller ones occur on the north
coast of the district, as for example, the Guajataca and (amuv Playas.
The Playa Plains (locally known as Playas) are of comparatively recent
origin, and are found onlv along the coast. Some occur adjacent to the
mountainous oldland; others adjacent to the coastal plain.


This province in the Lares District, includes practically all the area
south of the Lares Road (see map). The relief is very considerable
throughout the greater part, especially in the southeast corner of the
district. The central mountain range of Porto Rico, known as the Cor-
dillera Central, can be traced westward across the island to the vicinity
of Adjuntas. West of Adjuntas it divides into a southern range and
a northern range. The southern range extends along th. southern
border of the Lares District, through Maricao and Consumo, to Maya-
guez: The northern range passes south of Lares, where it is indistinct,
to Atalaya Peak, north of Afiasco. Northwest of Point Jiguero, it con-
tinues as a distinct submerged range, of which the highest summit forms
Desecheo Island, 15 miles from the west coast at Rincon.
The Mountainous Oldland in the Lares District is drained by two
master streams, the Rio Afiasco-Rio Blanco, and the Rio Culebrinas.
These two rivers flow west and northwest through the area to the west


coast, following in a general way, the rock structure. The Rio Afiasco-,
Rio Blanco system forms the parting valley between the north and south
branches of the Cordillera Central. The Rio Culebrinas forms the part-
ing valley (or inner lowland) between the oldland on the south, and the
overlapping Tertiary formations,of the coastal plain on the north.
The Mountainous Oldland is maturely dissected. All of the streams
are in the stage of youth, and are characterized by narrow, steep-sided
valleys, and numerous falls and rapids. As compared with the playas,
or lowlands of the coast, the climate of these interior mountains is cool
and humid. Rains are of almost daily occurrence, but are typically of
short duration. The mountain slopes, almost everywhere developed on
clay or laterite, are exceedingly steep, and as a rule, are covered with
forest trees. All of the trees are second growth, utilized as shade for
coffee. Thus what appears as wild forest land, is in reality highly cul-
tivated coffee land.
Travel in this mountainous area is very difficult. Automobile roads
are rare, and the native cart roads (ccaminos) are often impassable after
showers. Grades of 30' or more are commonly met with in travelling
these roads, and in the areas of greatest relief, coffee, bananas, and all
other products must be transported by pack animals. Some of the
richest coffee districts of Porto Rico are in the most inaccessible parts
of the mountains.

The summits of the Cordillera Central mark the remnants of a
formerly continuous surface of moderate relief (Fig. 24). This old
surface is the upper peneplane described by Lobeck. Above the pene-
plane surface rise a few scattered monadnocks of quartz diorite, or other
relatively resistant rock (Fig. 1). The peneplane has been maturely
dissected in the Lares District, and is not a striking feature except
where viewed from certain points of advantage. In the southeast corner
of the district, where it has an elevation of 1700 to 1900 feet, it slopes
gradually to the west. Near the west coast it is preserved only on por-
tions of the Atalaya Range north of Afiasco, and possibly on the Mesa at
Mayaguez. In central and eastern Porto Rico, Lobeck has distinguished
a lower peneplane, marking a second erosion cycle. Notraces of this
lower peneplane are found in the Lares District, probably because it is
buried here by the overlapping Tertiary formations.
The upper peneplane was formed some time after the close of the
Cretaceous Period and before the beginning of the Oligocene Period.
This is proven by the fact that late Cretaceous formations make up a


portion of the folded Older Series rocks on which the peneplane is
developed,, and are the youngest formations known to exist in the Older
Series. That the peneplanation took place before the Oligocene Period
is shown by the fact that the earliest marine formations deposited on
the peneplaned area are of middle Oligocene age. As previously noted,
Lobeck has shown the existence of two peneplanes in Porto Rico. Both
of these must have been made during the interval between the close
of the Cretaceous Period and the beginning of middle Oligocene time.
The most probable date for the upper peneplane is the Paleocene Period;
for the lower peneplane, the Eocene Period. The formation of the

FIG. 24.--Ac(orda now maturely dissected
View looking N. 64 E. from K. 0.0, Consumo-Maricao Road.

lower peneplane seems to have been interrupted by an uplift, initiating
a third cycle of erosion, and resulting in the dissection of both pene-
planes. This is shown by the fact that the basal formations of the
Tertiary Series lie upon an extremely irregular surface, in some places
filling old valleys in the oldland surface. From their location, these
buried valleys seem to have been cut in the lower peneplane. Coarse
gravels occupying one of these buried valleys near Moca, and immediately
underlying the basal marine Oligocene formations in this locality, point
to the existence of youthful streams in this region just preceding the
invasion of the sea in middle Oligocene time. Thus the third erosion
cycle belongs to the late Eocene or early Oligocene Periods, or possibly


both. This cycle had apparently reached a stage of late youth or early
maturity before it was interrupted by the middle Oligocene submergence.
The following are the important physiographic events which took
place during the time interval between the folding of the Cretaceous
formations of Porto Rico and the deposition of the Tertiary coastal
plain strata:
1. First erosion cycle, ending in the formation of the upper pene-
plane. (Late Cretaceous to end of the Paleocene.)
2. Uplift, with dissection of the upper peneplane, and the formation
of a second or lower peneplane. (Eocene.)
3. Uplift, with dissection of the lower peneplane and further dissec-
tion of the upper peneplane. (Late Eocene to early Oligocene.)
4. Subsidence, resulting in partial submergence of the island, and
interruption of the third erosion cycle. (Middle Oligocene to early
The geological dates given in this outline are to be considered as the
probable approximate dates of these events. The date of the close of
the third erosion cycle, however, is based on good stratigraphic evidence.
At the close of the Oligocene Period, the entire island was uplifted, the
amount of vertical movement being differential, but reaching a maximum
of 1500 feet in the eastern part of the Lares District. Thus erosion of
the present cycle was initiated in early Miocene time, and has continued
to the present. Those portions of the interior mountains which were
not submerged in Oligocene time have been subjected to erosion since
the close of the Cretaceous Period. Other portions nearer the coast
have been stripped of some of their mantle of Oligocene strata during
the present cycle. This is true of most of the area drained by the Rio
Culebrinas and its tributaries.


All the streams of the mountainous oldland are in the stage of youth.
Falls and rapids occur in abundance in streams of all sizes. The only
outcrops of fresh, unweathered rock in the oldland area are found in the
stream channels. This, in a region where rock decay is extremely rapid,
is a good indication that the rivers are still actively deepening their
River terraces occur in many places along the Rio Aiiasco, Rio Blanco,
and Rio Culebrinas valleys. None of theme are continuous, and they
occur at all elevations above river level, from 10 to 100 feet. Most of
them are built of river gravel; some are rock cut. In most places they


are obscured by forest trees which cover the valley walls, and their true
nature is therefore not evident.
A special feature of the drainage of the oldland area is the presence
of hanging valleys where small tributaries enter the valleys of trunk
streams, such as the Rio Blanco. In the upper Rio Blanco valley in
the southeast part of the Lares District, hanging valleys are very
numerous. The small streams enter the deep, narrow valley of the
main stream with rapids and in many cases falls of considerable height.
The same feature may be seen in the upper Rio Culebrinas valley, and,
in fact, seems to be the rule wherever small tributaries enter a trunk
stream of large volume.
Any attempt at interpretation of the origin of the streams of the
mountainous oldland is made difficult because the relationship of stream
to rock structure is in most places not apparent, and the structure,
where not obscured by soil and vegetation, is as a rule so complex that
a long and careful study would be required to unravel it. This state-
ment applies especially to the southeastern part of the Lares District.
A glance at the map will show that in the western part of the oldland
area, the main streams follow the strike of the Cretaceous formations
more or less closely. Such streams are developed in belts of the less
resistant rocks, especially tuffs and sales, and should be classified as
subsequent streams. Examples of these are the Rio Casey, Rio Cafias,
Rio Santiago, Rio Grande, Rio Culebrinas (in part), and Rio Blanco
(in part).
The Rio Afasco-Rio Blanco is a stream of complex origin, and being
the largest stream in the district, deserves further description. The
upper portion, known as the Rio Blanco, follows the structure where it
leaves the areas of massive igneous rock and enters belts of stratified
rock. In these portions, it may be regarded as subsequent. The lower
portion, known as the Rio Afiasco, cuts across the strike of the forma-
tions without regard to the relative resistance of the different types of
rock. That part of the mountainous oldlqnd drained by the Rio Aliasco
was probably submerged during the Oligocene Period. If so, the de-
posits formed at that time have been removed by post-Oligocene erosion,
and the area is to be classed as a pseudo-oldland. This theory is sup-
ported by the fact the nearest outliers of Tertiary strata (at Pt. Jiguero)
are of middle Oligocene age, indicating that some 2000 feet of upper
Oligocene strata have been removed by erosion. If this portion of the
area is indeed a pseudo-oldland, then the Rio Afiasco is probably a super-
imposed stream. Additional evidence favoring this view may be obtained
from a study of two other streams flowing to the west coast. These


are Calvache Creek and Pueblo Creek, both southeast of Rincon. The
map shows that the upper portions of these streams follow the belts of
weak rock, and are thus adjusted to the structure. In each case, the
stream disregards the rock structure in the lower part of its course,
exactly as the Rio Afiasco does.
As a whole, the mountainous oldland area in the Lares District pre-
sents two contrasting types of drainage pattern (see map). In the
eastern part, the pattern is dendritic; in the western part, trellis. The
explanation of this is apparent. In the eastern half, the country rock is
largely igneous intrusive bodies and massive tuffs and agglomerates. In
the western half, stratified rocks and interbedded lava flows and sills
predominate, and the beds are nearly everywhere folded and tilted at
high angles. The influence of hard and soft beds on the drainage lines
is very marked.
This province includes the area between the Lares Road and the
north coast, and a narrow strip along the west coast from Aguadilla
to Point Jiguero. The Coastal Plain is developed on a belt of nearly
horizontal Oligocene strata, mostly limestone, which overlap the old-
land. It has a topography wholly distinct from the topography of the
complex mountainous oldland on the south. Because of its elevated
position and nearly horizontal strata, the coastal plain belt may be
classed as a plateau. It is marked off from the oldland by a more or
less distinct cuesta, which faces south and overlooks an inner-lowland.
The inner-lowland is developed on the Older Series rocks, and in the
Lares District, owes its existence to the erosive action of the Rio Cule-
brinas and its tributaries.
From the summit of the cuesta, which marks the highest elevation of
the Tertiary coastal plain strata, the plateau surface slopes seaward
(north and northwest) at an average angle of less than 10. This
plateau surface is in a youthful or submature stage of dissection. Only
two master consequent streams cross the belt, from the oldland area to
the north coast. These are the Rio Guajataca and the Rio (amuy.
They have cut narrow canyons in the limestone to a maximum depth
of more than 300 feet. River erosion, however, has not been the only
destructive agency at work in this limestone area. Surface solution and
underground solution have been exceedingly active, and have produced
a peculiar type of Karst topography, characterized by sink holes and
conical mounds or hills of limestone known as pepinos (Spanish for
cucumbers), or haystack hills. This sink-hole-pepino hill topography


is so rugged that travel across it is possible only by a few favorable
routes. Thus, while the plateau is in a youthful stage of dissection, as
far as stream erosion goes, the topography in certain belts is as rough
as that of a maturely dissected country, because of the activity of under-
ground solution.
The pepino hills are the most interesting and unique feature of the
coastal plain. Illustrations of them are shown in figures 28 and 29,
and their distribution in east-west belts across the plateau surface is
brought out on the geologic map. They have been shown on this map

FIG. 25.--The Tertiary r cesta on the right, with inner lowland valleyy of the Rio
C'lebrinas) on the left
Town of San Sebastian in the distance. View looking west from the Lares Road at
Collazo. The cuesta is developed on Tertiary formations; the Iares limestone at the
top. and the San Sebastian shale underlying the talus slope below. The inner lowland
is developed on the Rio Culebrinas sales and tuffs of the Older Series (Cretaceous).

by hachures, because to contour them would be an endless task, and a
contour interval of 100 feet would fail to bring out the extremely rugged
topography of these belts of hills.
As may be seen from the illustrations, these hills are roughly conical
or mound-shaped. In size, they range from small mounds less than 20
feet high, to hills at least 300 feet high. They are, where best developed,
closely crowded, the intervening spaces being occupied by sink-holes of
elongate or irregular pattern. In these belts of pepino hills, one can
often not find level spaces large enough to pitch a tent on. All trails
and cart roads meander around the steep sides of the hills, avoiding the


sink-holes. The trails are dangerous to travel after showers, when the
residual clay soil is made extremely slippery. Soil, however, is not thick
or widespread in these areas, the outcrops and talus of white limestone
covering much of the surface.
In viewing these hills from a distance, or from the summit of one of
them, it is noticeable that the summits of all the higher hills have a fair
accordance in level, presumably marking the plateau surface as it was
before being cut up into this rough topography by underground solution.

FIG. 26.-Tertiary cuesta east of Lares
View looking north from Rio Blanco Road south of Lares. On the left are seen the
white cliffs of the Lares limestone. On the right a spur of the old land surface is over-
lapped by the Tertiary limestone, and the cuesta at that point is not a distinct physio-
graphic feature.

On the plateau surface, adjacent to the belts of pepino hills there are
belts of relatively smooth, undissected country. Typical views of this
type of topography are shown in figure 27. The prairie belts are covered
with black or red residual clay soils, excellent for growing sugar cane and
tobacco. Where not cultivated, they make good grazing land. They are
naturally grass-covered, and the trees are widely scattered, and consist
chiefly of the Royal Palm. The smooth or rolling country is broken here
and there by canyons, sink-holes, and low cuestas which mark the outcrop
of relatively resistant limestone beds dipping 3' or 4 to the north.
The principal cuesta occurs along the Lares Road, overlooking the


inner-lowland, as already noted. It marks the southern limit of the
Tertiary coastal plain strata which overlap the complex mountainous
oldland. Figures 25 and 26 show the variations in the topography of
this cuesta. Where the basal shale beds of the Tertiary Series are capped
by resistant limestone, the cuesta is prominent and not maturely dis-
sected. Where only Tertiary shale is present, however, the cuesta is
so thoroughly dissected by the tributaries of the Rio Culebrinas that it is
not a prominent topographic feature. The inner-lowland is developed
throughout on shales, tufts, and other rocks of the Older Series, which in
general, are more easily eroded than the Tertiary limestone forming
the summit of most of the cuesta. At some points along the cuesta, the
basal shale of the Tertiary Series is absent, and the higher horizons of
limestone lie directly on half buried spurs of the rugged oldland surface.
At such places, there is no inner-lowland, and the cuesta is low and indis-
tinct. Other cuestas, of minor size and importance, occur in various
parts of the plateau surface north of the main cuesta. They occur where
resistant reef limestone alternate with soft, chalky or argillaceous beds.
The regional dip is north or northwest at low angles; thus all of these
cuestas face south or southeast.


The belted character of the coastal plain has been referred to in
describing the belts of pepino hills, and it is brought out on the geologic
map. Each belt owes its topographic characteristics to the rock forma-
tion on which it is developed, hence the boundaries of the different belts
are almost the same as the boundaries of the different Tertiary forma-
tions. A description of the Tertiary formations has already been given
and the topographic expression of each was summarized in Table 4.
The belts, named in stratigraphic order, from the lowest to the
highest, are:
1. The Lares Pepino Belt. The belt of pepino hills developed in the
Lares limestone, on the plateau surface north of the cuesta in the eastern
half of the Lares District.
2. The Cibao Prairie Belt. Adjacent to, and north of the Lares
Pepino Belt. Developed on the Cibao limestone.
3. The Los Puertos Pepino Belt. North of the Cibao Belt, and
developed on the Los Puertos limestone.
4. The Quebradillas Plateau Belt. Developed on the Quebradillas
limestone. Extends from the Los Puertos Pepino Belt to the north


To obtain a clear conception of the relationship of these belts, and the
obvious reason for the belted character of the coastal plain, the reader
is referred to the geologic cross-sections (Plate I), showing the north-
south profiles across these topographic belts.
The Lares Pepino Belt. The best development of this belt is the area
north of the town of Lares. From the Guajataca River eastward be-
yond the east border of the Lares District, the plateau surface. just back
of the cuesta is characterized by high and densely crowded'pepino hills.
These hills are formed in the pure white and pink Lares limestone, both

FIG. 27.--Typical sink hole in the Cibao limestone prairie
View looking E. N. E. from field station 156, Bario of Cibao.

of the massive and thin-bedded types. West of the Guajataca River, the
Lares formation grades from pure limestone into soft, chalky and argil-
laceous limestones, and finally into sales and gravel beds. It is there-
fore significant to note that the pepino hills do not extend very far west
of the Guajataca River in the Lares Belt.
The Cibao Prairie Belt.. This belt is a relatively flat, rolling, grass-
covered prairie, situated between the two belts of pepino hills (see map),
and at a slightly lower elevation than the adjacent pepino hill country
on either side. The Cibao limestone is chiefly a soft, chalky rock, with
intercalated beds of hard limestone, which form low cuestas. Figure
27 shows typical views of the topography of this belt.


The Los Puertos Pepino Belt. This belt is essentially like the Lares
Belt, but the pepino hills are larger, and the topography correspondingly
more rugged and difficult to traverse. The Los Puertos Belt is much
more continuous east and west than is the Lares Belt. The striking
contrast between this pepino hill topography and the Cibao prairie
topography is shown in figures 28 and 29.
The Quebradillas Plateau Belt. This belt is for the most part a
plateau in a youthful stage of dissection, sloping gradually seaward,
and terminated by sea cliffs along most of the coast line. The surface

Fil. 28.-View looking V. .'. W. from field station 25. barrio of Cibao
Showing Cibao limestone prairie in the foreground and pepino hills of the Los Puertos
limestone In the background.

is remarkably flat, especially in the northwest corner of the district.
Above this surface rise a few hills of the pepino type, grouped irregu-
larly, or in long, ridge-like chains some of which are quite continuous
east and west. Sink holes are common, but not as large or numerous as
those of the Cibao prairie.


The coastal plain is traversed by two master consequent streams, the
Rio Guajataca and Rio Camuy, each of which has its headwaters in the
oldland area to the south. With the exception of these two streams,
which cut deep canyons (Fig. 30) through the limestone belt, about


nine-tenths of all the drainage is subterranean. Even the Camuy takes
to a subterranean course in crossing the Cibao belt (Fig. 31). In
many places, particularly in the Cibao prairie, the low rumble of under-
ground rivers can be heard; in other places they are seen in the bottoms
of the large sink-holes, where the water comes briefly to view, boiling
as though in some giant cauldron. After a continuous 24 hours of rain,
it was noted that there was practically no surface run-off on the Cibao
prairie, not even the smallest rivulet being in evidence. The rain water
had escaped almost immediately into the underground channels.

FIG. 29.-View looking south from field station 153, barrio of Cibao
Showing Cibao limestone in the foreground, pepino hills of the Lares formation in the

It is possible that the Rio Guajataca and Rio Camuy were at one time
largely subterranean in their courses through the limestones, and that
the present canyons have been produced in many places by caving of the
surface. The following considerations support this theory:
1. Most of the present drainage lines are subterranean.
2. Sink-holes are evidently forming at the present time above these
subterranean channels, and given time enough, should result in the com-
plete caving in of the rock overlying such channels.
3. Large masses of limestone, many of them more than 100 feet in
diameter, occur in the Rio Camuy canyon, more or less obstructing the
stream. They are especially numerous in that part of the canyon
adjacent to the subterranean route of the river. They indicate that at


one time, a larger portion of the Rio Camuy was subterranean, and that
the present subterranean course is in process of being destroyed.
The future drainage history of the coastal plain will very probably be
an opening up of underground water courses by caving, until surface
drainage of the area is eventually established.


The topography of these hills has been described, but a discussion of
their origin has been left for the last, because it is a subject closely con-

FIG. 30.- ieuw looking N. IV. from field station 181
Showing canyon wall of Rio Camuy in the Los I'ertos limestone belt.

nected with the draining of the coastal plain. It is believed that the
following observations are of critical importance in formulating any
theory of the origin of this type of topography:
1. The Cibao and Quebradillas limestone belts do not have a typical
development of pepino hills. Likewise, these hills do not occur' in the
Lares formation in the western part of the Lares District. In analyzing
these associations, it is evident that certain types of formations are not
favorable to their development. These types are (a) shales, marls,
argillaceous or impure limestones, (b) continuous limestone strata of
the hard, fine-grained, flinty type, characteristic of the Quebradillas


2. In the most arid portion of the coastal plain, the northwest corner,
there are no pepino hills. This is illustrated by the even plateaus back
of Point Borinquen. The more arid the climate, the less the amount of
surface solution which has taken place.

FIG. 31.-"Blue Hole" of the Rio Camuy
The river flows through a deep canyon terminated abruptly by an almost vertical
wall, and there takes to a subterranean channel in passing through the Clbao limestone
belt. The "blue hole" is seen at the extreme right, nearly blocked by the timber jam
in the foreground.

3. The pepino hills are not individual reefs, since they are not limited
to the massive reef type of limestone. Some of them are developed in


well stratified limestone, and in such cases, the same strata can be traced
in detail from one hill to another (Fig. 32).
4. Pepino hills topography is best developed where the limestone is
the most cavernous; that is, in those limestones which have suffered the
maximum amount of solution by ground water. The hills are not invari-
ably found, however, in this type of limestone formation.
5. In many localities the hills show a north-south linear grouping,
with intervening lanes of sink holes (Fig. 33). This arrangement of

Fio. 32.-View looking cost from field station 6, Larcs limestone. north of Lares
Showing adjacent pepino hills developed on soft thin-bedded limestone, with the same
strata traceable from one hill to the other.

sink holes can be produced in no other way than by caving along sub-
terranean drainage lines.
G. Careful observation shows that the hills have a tendency to be steep-
est on the side toward the west. This feature is so noticeable in places
as to give a saw-tooth effect to the sky line (Fig. 34). Further observa-
tion shows that this asymmetry has no relation to slumping or tilting, no
relation to difference in structure or composition of the rock, and no
relation to vegetation covering the hills. The only apparent explanation
left is that of differential weathering and solution. The daily showers
occur usually in the afternoon, when the sun has been shining on the
hills from the west, and hence while the rock on the west is at a higher


temperature than that on the east or shaded sides. This increases the
rate of solution, and results in a more cavernous structure being devel-
oped on the west sides. Such differential surface solution would be
noticeable only where the process is extremely rapid.
From the above observations, it is believed that the pepino hills are the
byproduct of extensive underground solution and extremely rapid surface
solution, combined with the other necessary factors, lithologic and cli-
matic, above described. Under these conditions, pepino hills might be
developed anywhere in the tropical zone.


FIo. 33.--Tiew looking north fro fro eld station 151. in Los Pucrtos limestone belt
Showing one of the north-south lanes of sink holes passing through the pepinos, and
through which the Atlantic Ocean is visible on clear days.


The term Playa is used in the West Indies to denote the flat or gently
sloping alluvial plains which occur along the coasts of nearly all the large
islands at the mouths of rivers. Such playas are in many cases several
miles wide, but none is elevated more than a few feet above sea level.
In the Lares District, large playas occur at the mouth of the Rio
Afiasco and at the mouth of the Rio Culebrinas. There are many smaller
ones on the north and west coasts. The playas, or playa plains differ
chiefly in size and outline, and a description of the largest (the Afiasco
Playa) will include all the features to be found in any of the others.


The Afiasco Playa occupies the mouth of a drowned valley, and is
bounded on its inland sides by the mountains or hills of the oldland. In
ground plan, the playa is fan-shaped, which is the form of a typical delta
or alluvial fan. The surface in most places is smooth and apparently
level, but hand-level measurements show a gradual slope to seaward from
the interior portion, where the elevation is 20 to 30 feet above high tide
level. The coastal margin is marked by a broad sandy beach, sand dunes
back of the beach, and a zone of cocoanut palms back of the ridge of
dune sand. There is a narrow zone back of the zone of palms where the

FIG. 34.-View looking south from near K. h, Aguadilla-Isabela Road
Quebradillas limestone plateau in foreground, pepino hills of the Los Puertos lime-
stone in the background. The view illustrates the "saw-tooth" skyline, the steeper sides
of the hills being on the west.

land stands at or slightly below sea level, with small patches of salt
marsh, but no large or continuous lagoon: The remainder of the playa
is planted in sugar cane. The town of Afiasco is located in the interior
part of the playa, at an elevation of about 20 feet above sea level.
The Rio Afiasco, leaving the narrow flood plain between the mountains,
east of Afiasco, takes a meandering course across the playa to the sea.
In the interior part of the playa, the river is intrenched 10 to 15 feet
below the playa surface, exposing and actively eroding the layers of
unconsolidated material of which the playa is built. Numerous gullies
have been eroded into adjacent parts of the playa surface by small inter-
mittent tributaries of the main stream.

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