• TABLE OF CONTENTS
HIDE
 Title Page
 Table of Contents
 List of Illustrations
 Errata
 Part 1
 History of the survey
 Introduction to the geology of...
 New base map of Porto Rico
 Geology of the San Juan Distri...
 Part 2
 Geology of the Coamo-Guayama...
 Part 3
 Geology of the Ponce District
 Part 4
 The physiography of Porto Rico
 Index
 Maps














Title: Scientific survey of Porto Rico and the Virgin Islands
ALL VOLUMES CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00091487/00003
 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: 1919-1922
Frequency: completely irregular
 Subjects
Subjects / Keywords: 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 )
Periodicals   ( lcsh )
Spatial Coverage: Puerto Rico
United States Virgin Islands
 Notes
Summary: Includes bibliographies.
Statement of Responsibility: New York Academy of Sciences.
Ownership: Provenance: Gift of Jay I. Kislak Foundation.
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: VID00003
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
    Title Page
        Page i
        Page ii
    Table of Contents
        Page iii
    List of Illustrations
        Page iv
        Page v
        Page vi
    Errata
        Page vii
        Page viii
    Part 1
        Page ix
        Page x
    History of the survey
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Introduction to the geology of Porto Rico
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
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        Page 26a
        Page 27
        Page 28
        Page 29
    New base map of Porto Rico
        Page 30
        Page 30a
        Page 31
        Page 32
    Geology of the San Juan District
        Page 33
        Page 34
        Page 35
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    Part 2
        Page 110a
        Page 110a
    Geology of the Coamo-Guayama District
        Page 111
        Page 112
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    Part 3
        Page 228a
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    Geology of the Ponce District
        Page 229
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    Part 4
        Page 300a
        Page 300b
    The physiography of Porto Rico
        Page 301
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    Index
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    Maps
        Page 385
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Full Text


NEW YORK ACADEMY OF SCIENCES


SCIENTIFIC SURVEY
OF

Porto Rico and the Virgin Islands


VOLUME I


NEW YORK:
PUBLISHOY THE ACADEMY
19-1922









































3i
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,,














CONTENTS OF VOLUME I

Page
T itle-page ....................................................... .. i
Contents .. ........................... .. .... ..... ............ iii
Dates of Publication of Parts.................. ...... .................. iii
List of Illustrations ................................................ iv
Errata ..................................... ........................ vii
History of the Survey. By N. L. BRITTON .............................. 1
Introduction to the Geology of Porto Rico. By CHARLES'P. BERKEY..... 11
New Base Map of Porto Rico. By CHESTER A. REEDS. .................. 30
Geology of the San Juan District. By DOUGLAS R. SEMMES............. 33
Geology of the Coamo-Guayama District. By EDWIN T. HODGE ........... 111
Geology of the Ponce District. By GRAHAM J. MITCHELL................ 22)
The Physiography of Porto Rico. By ARMIN K. LOBECK ................ (
Index ............................................... ................ :81


DATES OF PUBLICATION OF PARTS

Part 1, September 26, 1919.
Part 2, May 25, 1920.
Part 3, March 1, 1922.
Part 4, December 20. 1922. r








iv SCIENTIFIC SURVEY OF PORTO RICO



LIST OF ILLUSTRATIONS

PLATES
Page
I. Block Iiagram of the San Juan District, P. R...................... 42
II. Fauna of the Arecibo Formation ................................. 5s
III. Foram inifera ..................... ................. ............ 74
IV. Geologic Cross-section................................ .. ......... 1(6
V. Generalized Geological Cross-section of Ponce District............. 262
VI. Generalized Geological Cross-section of Ponce District ............. 264
VII. Generalized Columnar Section of Ponce District................... 26S

FIGURES
Pa;ge
Diorite porphyry sills ................................................ 20
Detail of the double structure in the San Juan formation at Arecibo..... 21
Haystack (pepino) hills ........................................... 22
Valley of the Plata,.just below the power dam at K 19, Comerio Road.... 39
San Juan consolidated dune sand forming fringing islands at Vega Baja
Playa .............................................................. 43
The playa plains near Arecibo ......................................... 44
Typical haystack topography as developed in the western portion of the
San Juan District .................. ............................. 45
Section through the Arecibo formation....... .................... 47
Inface of cuesta and inner lowland near Corozal ...................... 48
The inner lowland'and the inface of the cuesta at Corozal ............... 49
Morro castle and a portion of the city of San Juan, built upon a promon-
tory composed of the San Juan consolidated lime sand ............... 53
San Juan consolidated lime sand..................................... 54
Exfoliation producing concentric arrangements in ash beds south of
Bayamon .......................................... ....... ....... 62
Crumpling due to solifluction in the Juan Ascencio chert................. 66
Typical foraminiferal shale........................................... S
Chart showing the relative positions of the principal foraminiferal beds.. 73
Diorite porphyry sills intruded between layers of shales and ashes at K 20.
Comerio Road .................................... .............. 77
Faulted trachyte dike crossing the La Plata River ............'. ......... 78
Photomicrograph of vitrophyr occurring north of Cidra ................. SO
Rhyolite porphyry............... ...................... .............. 82
Soda granite, showing poikilitic structure .............................. 8.
Soda granite occurring south of Naranjito............................. 84
Quartz diorite ................... ........... ............ ........... 86
Trachyte. showing poorly developed trachytic structure, crossed nicols... 87
Latite. showing veins filled with epidote and iron oxides................ SS
An andesite porphyry with large phenocrysts of plagioclase surrounded by
a groundma'ss. showing the structure of a natural slag............... R9
Amlesite porphyry impregnated with chrysocolla. Barrio Pasto......... 90








CONTENTS OF VOLUME I


Puge
Iiorite porphyry with pyroxene phenocrysts in a groundmass of lath-
shaped feldspars (plagioclase)............... ........ ..................... 1
Olivine basalt........ ............................................... 92
Cross-bedded structure in the San Juan formation at Arecibo........... 97
Block diagram of Coastal Plain................. ..................... 117
Guayama upon the upper coastal terrace............................... 118
Cayey in the Barranquitas-Cayey Lowland.............................. 121
Tuff beds........................ ....................... ........... 131
Lithocampe sp. in shales of the Barranquitas-Cayey series.............. 133
Shells worn, oxidized, and cemented together to form one of the lime-
stones of the Barranquitas-Cayey series............................. 135
Topography of the Barranquitas-Cayey Lowland ........................ 136
Character of the unconformity between the Barranquitas-Cayey and the
Sierra de Cayey -series ........................................... 13S
Conglomerate with an igneous matrix................................ 140
Ash shales containing Orbitolites.................. ................. 144
Chert with desiccation fractures...................................... 145
Crystal tuff........... ............................................... 14(
Lithic tuff .................................... ...................... 151
Coamo Springs limestone exposed in the Descalabrados Gap............. 154
Lithothamnion thallus in cross-section in the Coamo Springs limestone... 155
Lithothamnion thallus in longitudinal section in Coamo Springs limestone 156
Coamo Springs Range......... ................... ................ 157
Lithothamnion in Coamo Springs limestone............................ 158
Amphistcgina in Coamo Springs limestone............................. 160
Porodiscus concentricus in shale. ... ................................. 162
Shales of the Rio Descalabrados series........ ..................... 16:
Valley of the Guamani................... .......................... 16S
Cerro Gavias. Coastal Terrace, Coamo River.......................... 169
Felspathic andesite............... .............. 172
Augite andesite................. ................. .................. 174
Augite andesite showing feldspars completely altered; augites unaltered.. 175
Glassy-augite andesite ............................ ................ 176
Olivine andesite... .............. ........................... 177
Olivine free basalt...... ..................... ..................... 178
Olivine basalt...................................... 179
Lava flow containing pyroclasts of calcite which are surrounded by garnets 181
Flow breccia ........................................................ 182
Igneous structure of the Sierra de Cayey conglomerate matrix.......... 183
Relation of isothermal surfaces to andesitic columns..................... 184
Hornblende granite............... ............................... 18(6
Biotite granodiorite ............................... ..... .............. 187
Hornblende diorite.................. ................... ............. 188
Hornblende-augite diorite........................... ..................... 189
Hornblende andesite............................................... 190
Hornblende-augite andesite ........................... ............... 191
IPorodiscus conncr'tricus in tuffaceous shale................... ......... 195
"lobigeiina and algae in the Coamo Springs limestone.................. .195








SCIENTIFIC StURVEY OF PORTO RICO


l'Pge
(hlorite developing from feldspars which are associated with fresh augites 203
Epidote replacing feldspars .. ............ ........................ .... 204
Minerals named in order from the center outwards: stained antigorite.
stilbite, laumontite, and calcite................... .................. 205
Intimate mixture of quartz, epidote, and actinolite....................... 206
Quartz and epidote replacing feldspar................................. 207
Spherulitic growths of specular hematite and chalcedony................ 208
Coamo Springs................. ..................... ............. 210
Barrio del Carmen Prospect ......................................... 221
Portion of Complex mountain province as seen from the Tertiary hills
south of Yauco......................... .. .. ...................... 235
Cuspated beach, west coast, Porto Rico, just south of Melones Point..... 246
Fluted Guayabal limestone of Juana Diaz............................. 256
Rio Yauco shale, as seen from point one mile north of kilometer 15.0,
Yauco-Lares road............................................... 263
Faulted shale at kilometer 14.3, Yauco-Lares road...................... 266
Augite porphyrite showing zoned augite crystal set in fine-grained ground-
m ass ..................................... ........................ 274
r'lp, 'itl' rosaceus, Lamark......................................... 284
Hemiaster berceyi n. sp., Jackson. x 3/2 .............................. 286
"Montalva" Salina, west of Guanica, on south coast ................... 296
Map showing the average annual rainfall in Porto Rico.................. 303
Sketch map of Porto Rico showing regions visited....................... 304
Map of Porto Rico showing the chief physiographic regions............. 304
The main stages in the physiographic development of Porto Rico........ 305
Block diagram of the southwest corner of Porto Rico................... 311
Appalachian ridge structure in the oldland rocks between Boqueron and
Parguera ..................... ........... .... ................. 312
Valley of the Manati River above Ciales............................... 313
The valley and flood plain of the Rio Grande de Loiza near Carolina..... 317
North-south profile through Porto Rico................................ 318
Diagram of the Guanica district ...................................... 318
The inner lowland at Guanica. ...................... ........... .... 321
The inner lowland at Guanica ........................................ 322
Basal Tertiary south of Yauco, along the Guanica road................. 326
The gorge of the Manati, incised below the surface of the coastal plain... 330
Open haystack belt between Barceloneta and Arecibo................... 335
Apparent wave-cut bench at Quebradillas.............................. 336
The inner lowland of the south coast, looking eastward from cuesta face
back of Ponce.......................................................... 338
The Tertiary coastal plain lapping upon southern flank of Mariquita Hill. 339
The surface of the coastal plain between Ponce and Penuelas............ 340
Beds of the coastal plain dipping slightly toward the oldland............ 341
Road cut at K 9 H 9 on the Ponce-Peiluelas road, showing relation of Ter-
tiary to older series............................................... 342
Section of hill along Jacaguas River one mile southwest of Juana Diaz.. 343
The drowned coast of eastern Porto Rico.............................. 345
Lower course of the Descalabrado River during the dry season.......... 350


















V. I

CONTENTS OF VOLUME I vii

P'alge
The high terrace of the Plata River at K 13 on the Comerio road........ 351
Alluvial terraces of the Pefia Pobre Creek, west of Rio Blanco crossing... 352
Tidal deltas at Luquillo............................................. 354
Sketch map of the coast of Luquillo................................... 354
Tidal flats just east of Point Embacaderos............................. 355
Wave action along joint plane, San Juan Point ........................ 356
The Caribes Islands .................................................. 357
The low southward sloping coastal plain east of Cape Rojo.............. 358
The marine cliff forming the edge of the Tertiary coastal plain on the
north coast west of Isabela........................................ 359
North-south profiles through the margin of the coastal plain at Quebra-
dillas and Isabela............. ........................ .............. 360
River mouths at the north coast. ................... ................. 362
The coast east of Quebradillas ....................................... 365
The uplifted beach on the south side of Desecheo Island................. 367
The uplifted beach on Muertos Island.................................. 369
East coast of Mona Island .......................................... 372
Sketch of Culebra and Vieques Island................................. 74
Muertos Island from the north..................... ........... 375

MAPS

Outline Map of Porto Rico showing distribution of district studies....... 2(
New Base Map of Porto Rico, Compiled from the latest Government
Sources ........................................ .. ............... 30
Geologic Map of the San Juan District, Porto Rico.
Geologic Map of the Coamo-Guayama District, Porto Rico.
Geologic Map of the Ponce District, Porto Rico.
Physiographic Map of Porto Rico.

ERRATA

Page 58, line 33. and page 59, line S, read Mctis for Metris






NEW YORK ACADEMY OF SCIENCES


SCIENTIFIC SURVEY
OF

Porto Rico and the Virgin Islands


VOLUME I-Part 1
History of the Survey-N. L. Britton
Geological Introduction-C P. Berkey
Geology of the San Juan District-D. R. Semmes


NEW YORK:
PUBLISHED BY THE ACADEMY
1919










HISTORY OF THE SURVEY


BY N. L. BRITTON
CIAIRMAl OF TIIE COMMITTEE

In the latter part of the year 1913, subsequent to a discussion in the
Council of the Academy relative to proposed extensions of the Acad-
emy's ordinary activities, a proposition f6r a scientific survey of Porto
Rico was approved and referred to a committee consisting of Professors
Boas, Britton, Crampton, Kemp and Poor, at the meeting of the Council
in November of that year, at which meeting the Council appropriated
$500 annually for five years for the purposes of this investigation. Dr.
E. O. Hovey and Dr. R. W. Tower, Recording Secretaries of the Acad-
emy, have acted with the Committee. Professor Charles P. Berkey suc-
ceeded Professor Kemp as a member of the Committee in 1917. Mr.
Emerson McMillin, at that time President of the Academy, kindly offered
to assist the work, and has contributed $3,000. The Porto Rico Govern-
ment was also interested in the investigation, and has since made two
appropriations of $5,000 for prosecuting the work. The cooperation of
The American Museum of Natural History, The New York Botanical
Garden, scientific departments of Columbia University and other institu-
tions was secured, and both field-work and the study of the collections
made has progressed almost continuously. The cooperating institutions
already had some collections from Porto Rico. The two agricultural ex-
periment stations of Porto Rico, one at Mayaguez and one at Rio Piedras,
have also aided the investigation. A number of preliminary papers deal-
ing with special phases of the work have been published. The publica-
tion of the final reports of the survey is herewith commenced and will be
continued in the following sequence: geology, paleontology, botany,
zoology and anthropology.
Professor Crampton proceeded to Porto Rico at the end of 1913 and
carried out a zoological reconnaissance, traveling over a large part of the
island, and discussed the plan with officials of the Insular Government.
He subsequently published an account of Porto Rico.'
In January, February and March, 1914, Dr. J. A. Shafer prosecuted
botanical exploration on the island Vieques, one of the small islands
Am. Mus. Jour., XV, pp. 59-70.






SCIENTIFIC SURVEY OF PORTO RICO


politically a part of Porto Rico, and made the first representative col-
lection of plants obtained on that island.2
During February and part of March, 1914, Dr. Britton, accompanied
by Mrs. Britton, Mr. John F. Cowell and Dr. Frank E. Lutz, made bo-
tanical and entomological collections in the northern and western parts
of Porto Rico and in the small islands Desecheo and Mona, which yielded
several thousand specimens and much information. Dr. Britton con-
tributed to the twenty-fifth anniversary celebration of the Missouri
Botanical Garden a paper on The Vegetation of Mona Island.3
In the summer and autumn of 1914, zoological collections were made
over considerable portions ol'.the island by Messrs. Roy W. Miner in
marine invertebrates and myriopods, John T. Nichols in fishes, and
Frank E. Watson and II. G. Barber in insects and arachnoids. Many
thousand specimens were obtained and important additions were made
to the knowledge of these groups of animals. The spiders of this col-
lection, taken with those previously obtained by Professor Crampton and
Dr. Lutz, yielded important information, which was utilized by Dr. Lutz
in his List of Greater Antillean Spiders, with Notes on Their Distribu-
tion.4 Nichols has published some of the results of his work under the
title Fishes New to Porto Rico."
The geological investigation was commenced by Dr. Charles P. Berkey
and Dr. Clarence X. Fenner, who spent parts of August and Septem-
ber, 1914, in a reconnaissance and laid a substantial foundation for sub-
sequent studies. Professor Berkey has published an illustrated account
of this reconnaissance under the title Geological Reconnaissance of Porto
Rico.6
In July and August, 1914, Dr. J. A. Shafer devoted about six weeks
to the study of the forests of the Sierra de Naguabo, and added a num-
ber of species of plants to the known flora of Porto Rico.7
Under the auspices of the New York Botanical Garden, Professor N.
Wille, Director of the Botanical Garden at Christiania, Norway, spent
the period from the latter part of December, 1914, until the end of
March, 1915, in collecting and studying the fresh-water and aerial alga'
over considerable areas of the island. He obtained many thousand speci-
mens of these organisms, which proved to be more abundant there than

"Jour. N. Y. Bot. Gard., XIV, pp. 103-105.
a Ann. Mo. Bot. Gard., II, pp. 33-58, Pls II. (Contributions from N. Y. Bot. Gard.,
No. 175.)
Ann. N. Y. Acad. Sci., XXVI. pp. 71-148.
Bull. Am. Mus. Nat. Hist., XXXIV, pp. 141-146.
Ann. N. Y. Acad. Sei., XXVI, pp. 1-70. (Contributions from Dept. of Geology. Co-
lumbia Univ., Vol. XXV. No. 20.)
7 Jour. N. Y. Bot. (ar(d.. XVI, pp. 23:-33.







BRITTOX, HISTORY OF THE SURVEY


had hitherto been supposed. This collection has not yet been critically
studied, but Professor Wille is confident that it contains some unde-
scribed genera and numerous undescribed species. Since his return to
Norway, subsequent to the expedition, Professor Wille has been much
occupied with administrative duties, but it is hoped that he will yet find
time to enumerate and describe the elements of this rich collection, the
first comprehensive one that has been made of these minute alge within
the American tropics. He has published a narrative report of his field-
work.8
The greater portions of the months of February and March, 1915, were
spent by Dr. Britton, Mrs. Britton, Mr. John F. Cowell and Mr. Stew-
ardson Brown in continuing botanical exploration, especially in western
and southwestern portions of Porto Rico, including a study of the coasts
from Mayaguez south to Morrillos de Cabo Rojo and westward to Ponce
and Cayo Muertos. Large collections were obtained, which added much
to the knowledge of the geographical distribution of the flora, and a
series of rock specimens were collected and turned over to the geologists
of the survey for examination.9
Based on the observations made on this and preceding trips, Dr. Brit-
ton outlined a plan of forest policy for the Porto Rican Government,
which he presented at the spring meeting of the National Academy of
Sciences, held at Washington in April, 1915, and it was transmitted to
the Governor of Porto Rico. The reforestation of Porto Rico is acutely
necessary. A recent report on the forests of Porto Rico by Mr. Louis S.
Murphy, Forest Examiner of the United States Forest Service, is of
great value in this connection.10
Dr. Chester A. Reeds and Mr. Prentice B. Hill devoted the months of
June and July, 1915, to paleontological studies and collections in the
western part of the island, obtaining over 10,000 specimens rich in fossil
corals and fossil mollusks. They detected a stratum containing large
quantities of fossil leaves; this noteworthy discovery, supplemented by
further collections subsequently made by Mr. Bela Hubbard, gives us the
first knowledge of the Tertiary flora of the West Indies. The collections
of fossil plants made by Messrs. Reeds and Hubbard are being studied
by Dr. Arthur Hollick. Dr. Reeds has published three maps. He discov-
ered remains of a new fossil Sirenian, which has been described by Dr.
W. D. Matthew."

8 Jour. N. Y. Bot. Gard., XVI, pp. 132-146.
SJour. N. Y. Bot. Gard., XVI, pp. 103-112.
1o Bull. U. S. Dept. Agric.. 1916. No. 354, p. 99.
1 Ann. N. Y. Acad. Sci., XXVII, pp. 23-29.






SCIENTIFIC SURVEY OF PORTO RICO


Mr. Edwin T. Hodge spent the summer and early autumn of 1915,
first, in an investigation of the supposed petroleum oil shales in the west-
ern part of the island, obtaining samples from 26 different localities, and
later in the year submitted a report upon them, which gave negative re-
sults, Mr. Hodge concluding that oil-carrying shales do not exist in Porto
Rico. Mr. Hodge subsequently made a detailed geological study of the
Coamo region, and his results have been made ready for publication. He
also made a study of the thermal springs of the island, especially those
of Bafios de Coamo.
Mr. D. R. Semmes prosecuted geological investigations of the section
of the north central portion of the island, which, taken with the work
of Mr. Hodge to the south, furnishes data for a complete geological
cross-section from coast to coast. The studies of Mr. Semmes are pub-
lished herewith.
Mycological studies and collections were made from June to August,
1915, by Professor F. L. Stevens, in continuation of similar investiga-
tions prosecuted by him while Dean of the College of Agriculture at
Mayaguez. He visited nearly all parts of the island and collected many
thousand specimens; his trip was under the auspices of the University of
Illinois and of the New York Botanical Garden.12 Professor Stevens
described and illustrated the species of the large genus Meliola as repre-
sented in Porto Rico, under the title The Genus Meliola in Porto Rico,
which includes descriptions of sixty-two new species and varieties.13
Professor Crampton continued zoological field-work during January,
1915, especially in the northwestern portion of the island, at which time
he supplemented previous collections by obtaining large numbers of
specimens in the various groups of invertebrates.
In the spring and early summer of 1915, Dr. Frank E. Lutz and Mr.
A. J. Mutchler made extensive entomological collections in various parts
of the island, adding some 15,000 specimens of insects and large num-
bers of spiders to collections previously made. Mr. Roy W. Miner and
Mr. H. Mueller spent several weeks in the summer in the study and col-
lection of marine invertebrates, especially in the vicinity of Guanica,
some 8,000 specimens being obtained at that time from this region.
Professor Raymond C. Osburn carried on studies of the organisms of the
deeper waters in the vicinity of Guanica during the summer, obtaining
over 2,000 specimens, principally by dredging.
Professor Franz Boas prosecuted anthropological and archmologieal
investigations at several points during May and June, 1915, aided by Dr.
12 Jour. N. Y. Bot. Gard., XVII, pp. 82-85.
13 Illinois Biological Monographs II, No. 4, pp. 1-86, Pls. I-V.






BRITTON, HISTORY OF THE SURVEY


H. K. Haeberlin, Dr. J. Alden Mason and Mr. Robert T. Aitken. Pro-
fessor Boas obtained data which caused him to doubt the general assump-
tion that children reach maturity earlier in tropical regions than in the
temperate zone, and other data which are important as regards the hygi-
ene of childhood in the tropics. His observations on the teeth of Porto
Rican children have been published by Mr. Leslie Spier.14 He obtained
cordial cooperation in this investigation from the Department of Public
Education of the island. He also concluded from his observations at this
time that the Porto Rican race has been influenced in its development by
environment.
Professor Boas referred the collection of folk-lore data to Dr. J. Alden
Mason, who prosecuted this study from June, 1915, until 1916, and
accumulated an immense collection of folk tales, riddles, ballads and
songs, which was subsequently referred for editing to Professor Aurelio
M. Espinosa, of Leland Stanford University, whose work on this impor-
tant contribution is now approaching completion, and who has reported
to the Committee that the documents are the most important contribu-
tion of its kind made to literature. Professor Espinosa and Dr. Mason
have published Porto Rico Folk Lore-1. Riddles."
Dr. Haeberlin made excavations in the Jobo district, including that of
one stone enclosure and a cave, which contained ceremonial objects and
large numbers of bones of an extinct mammal. A large cave in the
vicinity of Utuado, excavated by Messrs. Mason and Aitken, proved to be
the burial ground of an ancient community, and here also large numbers
of bones of an extinct mammal were found.16 One of these mammals
proved to be new to science, and has been described by Dr. J. A. Allen
under the title An Extinct Octodont from the Island of Porto Rico, West
Indies.7 Dr. Haeberlin has published a paper entitled Some Archaeo-
logical Work in Porto Rico."8
An ancient village site at Capk, located by Professor Boas and subse-
quently investigated by Messrs. Mason and Aitken, proved to be the most
important of all archeological localities thus far discovered in Porto
Rico. Efforts were made by the Committee to induce the Porto Rico
Government to purchase and preserve this site as a national monument,
but hitherto without success. A preliminary description of the site has
been given by Dr. Mason.19
Am. Anthropologist, XX, pp. 37-48.
15 Jour. Am. Folk-Lore, XXIX, pp. 423-504.
1a XIX Congress of Americanists, pp. 224-228.
17 Ann. N. Y. Acad. Sci., XXVII, pp. 17-22, Pls. I-V.
Is Am. Anthropologist, XIX, pp. 214-238.
19 XIX Congress of Americanists, pp. 220-223.







SCIENTIFIC SURVEY OF PORTO RICO


Dr. Marshall A. Howe spent parts of June and July, 1915, in con-
tinuation of his previous studies of marine algaw, principally along the
southwestern coast from Ponce and Cayo Muertos to Cabo Rojo. During
this trip he obtained about 3,000 specimens, including some species addi-
tional to the known flora of the island.20
Professor Bruce Fink, of Miami University, proceeded to Porto Rico
in November, 1915, and spent part of the winter there in making exten-
sive collections of fungi and lichens. He has generously contributed
information obtained by him to the Committee.
Dr. Herbert J. Spinden prosecuted archaeological work at several points
during the spring and summer of 1916 and made extensive collections,
the study of which yielded interesting apd valuable information. He
excavated certain shell heaps and obtained specimens and information
therefrom, which he regarded as throwing light upon the origin of Porto
Rican aborigines. He also studied the aboriginal petroglyphs or rock-
carvings.
Mr. Bela Hubbard carried out a geological survey of the northwestern
section of the island in the summer of 1916, and his description of this
work is in process of completion for publication. Dr. Charles R. Fettke
studied the southwestern districts of the island in an areal geological
survey, which included a detailed study of the extensive areas of eruptive
rocks. The study of the large collections made by him is progressing.
Mr. A. K. Lobeck studied, during the latter part of 1916 and January,
1917, the physiography of the entire island, and the results of this inves-
tigation are prepared for publication.
During the summer of 1916, a very important study of the parasitic
fungi of Porto Rico was made by Professor H. H. Whetzel, of Cornell
University, and Dr. E. W. Olive, of the Brooklyn Botanic Garden, aided
by a grant from the New York Botanical Garden, and several thousand
specimens of these plants were obtained, which are under investigation
by a number of different experts.
Mr. H. E. Anthony prosecuted studies of recent and fossil mammals
in Porto Rico during the summer of 1916, and made some highly inter-
esting and important discoveries, which have been described by him in
three illustrated papers:

Preliminary Report of Fossil Mammals from Porto Rico.21
Preliminary Diagnosis of an Apparently New Family of Insectivores.22

Jour. N. Y. Bot. Gard.. XVI, pp. 219-225.
n Ann. N. Y. Acad. Scl.. XXVII, pp. 193-203.
22 Bull. Am. Mus. Nat. Hist., XXXV. pp. 725-728.







BRITTOX. HISTORY OF THE SURVEY


New Fossil Rodents from Porto Rico, with Additional Notes on Elas-
modontomys obliquus Anthony and Tieteropsomys insulans Anthony.23

During the progress of the survey, Dr. John A. Stevenson, of the
Insular Experiment Station at Rio Piedras, has made botanical collec-
tions in various parts of the island, which have contributed much to our
knowledge of the flora; he is now preparing a list of the fungi of Porto
Rico. Dr. Basil Hicks Dutcher, TT. S. A., stationed at San Juan, has
given valuable aid to nearly all our field parties by advice, information
and personal assistance.
A number of reports of progress and other presentations of parts of
the work have been made at meetings of.the Academy from time to time.
An outline of the proposed survey was presented by members of the Com-
mittee in the form of a symposium at the meeting of April 13, 1914."4
Another symposium was given at the meeting of December 14, 1914.25
At the meeting of the Section of Geology and Mineralogy on November 1,
1915, preliminary reports on field-work were given by Messrs. Semmes,
Hodge and Reeds.26 At the meeting of the Section of Biology Novem-
ber 8, 1915, communications were presented by Messrs. Reeds, J. A. Allen
and Matthew.27 The Committee submitted a report of progress at the
annual meeting of December 20, 1915.2s
At the meeting of the Academy held January 24, 1916, Dr. Boas, Mr.
Aitken and Dr. Haeberlin29 presented communications on ethnology and
archeology, and at the meeting of May 15, 1916, geological communica-
tions were presented by Messrs. Hodge, Semmes, Reeds, Fettke, Hub-
bard,30 Lobeck and Professor Berkey."8
During several months of 1917, Mr. Graham J. Mitchell prosecuted
an areal geological survey of the southwestern portion of Porto Rico, and
his report of this work is in progress of preparation for publication. Dr.
Charles R. Fettke has recently published, with the approval of the Com-
mittee, a paper entitled The Limonite Deposits of Mayaguez Mesa, Porto
Rico.32
The final geological reports are being written, for the most part, by the

*a Bull. Am. Mus. Nat. Hist., XXXVII, pp. 183-189.
2 Ann. N. Y. Acad. Scl., XXIV, pp. 369.
2- Ann. N. Y. Acad. Sci., XXIV, pp. 398, 309.
26 Ann. N. Y. Acad. Sci., XXVI, pp. 433-436.
7 Ann. N. Y. Acad. Sci., XXVI, pp. 436-430.
28 Ann. N. Y. Acad. Set., XXVI, pp. 456-462.
Ann. N. Y. Acad. Sci., XXVII, pp. 250, 251.
3 Bull. Am. Inst. Mining Engineers, No. 135.
Ann. N. Y. Acad. Sci., XXVII, pp. 277-282.
*P Bull. Am. Inst. Mining Engineers, No. 135, pp. 661-676.







SCIENTIFIC SURVEY OF PORTO RICO


several geologists who have participated in the field-work, and, as already
stated, considerable portions of them are ready for publication. Dr.
T. W. Vaughan, of the United States Geological Survey, has kindly con-
sented to study the collections and prepare the descriptions of the fossil
corals, and those obtained up to the present time have already been sent
to him.
The final botanical reports are being prepared by the following investi-
gators:

Flowering Plants.-Dr. N. L. Britton and Mr. Percy Wilson, the
grasses by Professor A. S. Hitchcock.
Ferns and Fern Allies.-Preliminary work by Miss Margaret Slosson.
Mosses.-Mrs. N. L. Britton and Mr. R. S. Williams.
Hepatics.-Professor A. W. Evans.
Lichens.-Professor Lincoln W. Riddle.
Fungi.-Dr. W. A. Murrill, Dr. F. J. Seaver, Professor J. C. Arthur,
Professor F. L. Stevens, Professor H. H. Whetzel, Dr. E. W. Olive, Dr.
John A. Stevenson.
Algce (marine).-Dr. Marshall A. Howe.
Algce (fresh-water and aerial).-Professor N. Wille.

No arrangement has yet been made for the study of Porto Rican
diatoms.
The zoological collections are being studied and final reports prepared
as follows:

Mammals.-Mr. H. E. Anthony.33
Birds.-Mr. Alex. Wetmore has published an exhaustive report on the
birds which'may be taken as a basis for our document.34
Reptiles.-Preliminary work by Miss M. C. Dickerson.
Amphibians.-Preliminary work by Miss M. C. Dickerson.
Fishes.-Mr. J. T. Nichols and Professor C. F. Silvester.
Insects:
Orthoptera.-Mr. J. A. G. Rehn and Mr. Morgan Hebbard.
Coleoptera.-Mr. C. W. Leng and Mr. A. J. Mutchler.
Hemiptera.-Mr. H. G. Barber.
Lepidoptera.-Mr. F. E. Watson.
Hymenoptera.-Mr. J. Bequaert and Dr. F. E. Lutz.
Myriopods.-Mr. Roy W. Miner.

SMem. Am. Mus. Nat. Hist., N. S., II, Part 2.
Bull. U. S. Dept. Agric., 1916, No. 326, p. 140.







BRITTON, HISTORY OF THE SURVEY


MolluskAs.-Preliminary work by Dr. L. P. Gratacap.
Ascidians.-Dr. Willard Van Name.
Crustaceans.-Mr. Roy W. Miner and Dr. Willard Van Name.
Annelides.-Professor A. E. Treadwell.
Echinoderms.-Preliminary work by Mr. D. M. Fisk.
Bryozoans.-Dr. R. C. Osburn.
Coelenterates.-Mr. Roy W. Miner.
Representative collections of Porto Rican protozoans have not yet been
made.
The anthropological and archeological documents are being written
by the several investigators who have conducted the field-work. Pre-
liminary papers, in addition to those already noted in this report, are
being prepared by Professor Boas on anthropometrical observations in
Porto Rico, by Dr. Mason on the archaeological investigation of the very
important site at Capa, by Dr. Aitken on the cave sites near Utuado, and
by Dr. Mason and Professor Espinosa on Porto Rican folk tales.
As to the general progress of the survey, it may be stated that over
two-thirds of the necessary geological field-work has been completed.
Except for an investigation of the diatoms, which has not yet been ar-
ranged for, no additional botanical field-work is really essential, although
the final reports would be more complete if additional collections were
made, but this is of course true all over the world. Considerable addi-
tional zoological field-work and additional collections in several of the
groups of animals are highly desirable, and, as before remarked, the pro-
tozoa have not been collected. Anthropology and archaeology still offer
widely attractive fields for further examination. No oceanographic work
has as yet been possible.
In cooperation with the Insular Government, portions of the collec-
tions made have been returned to Porto Rico to form the basis of a Nat-
ural History Museum, and other portions will be forwarded to Porto Rico
as the study proceeds. The remainder is to be divided among the cooper-
ating institutions and investigators.
The islands St. Thomas, St. Jan and St. Croix, recently purchased by
the United States from Denmark, lie close to Porto Rico, to the east and
southeast. It is highly desirable, before our survey is brought to a close,
that these should be studied geologically, zoilogically and archaeologically.
Their botany is fairly well known. I have recently contributed a cata-
logue of their known flora to the Brooklyn Botanic Garden, and this has
been published in the volume of papers presented on the occasion of the



























10 SCIENTIFIC SURVEY OF PORTO RICO

opening of the new laboratories of that institution." The survey will
thus be made to include all the Caribbean insular possessions of the
United States as they stand at-present, and be entitled A Scientific Sur-
vey of Porto Rico and the Virgin Islands.
The Chairman of the Committee gratefully acknowledges aid from the
other members in the preparation of this historical sketch of the survey.

35 Mem. Brooklyn Bot. Gard.. I. pp. 19-118. (Contributions from N. Y. Bot. Gard.,
No. 203.)










INTRODUCTION TO THE GEOLOGY OF
SPORTO RICO

BY CHARLES P. BERKEY

CONTENTS
Page
Introductory statement ................................................ 11
Geological reconnaissance.............. ............................ 14
Major geological features. .................................... ........ 15
Rock formations...................... .......... ... ............ 15
The Younger Series....................... ................... 16
The Older Series .................................... ........ 17
Petrographic range ................................. ....... 18
Structure..................................... ............. 19
Relief features ................ ................. ...... .. ....... 22
Geologic history.................... .............................. 23
Problems of the Survey. .............................................. 24
Expeditions ..................................................... 24
Classes of studies................................. .............. 25
General studies.............................................. 25
District studies, with outline map ............................. 26
Special investigations..................... .................. 27
Bibliography ........................................................ 29
New base map of Porto Rico. By CHESTER A. REEDS .................... 3



INTRODUCTORY STATEMENT

The Island of Porto Rico lies between latitude 1700 54' and 180 30'
north and longitude 65 13' and 67 15' west. It is the most eastern and
southern of the Greater Antilles. Thus it lies within the trade-wind belt,
which is an important factor in the local variations of its tropical climate.
In the Atlantic immediately north of Porto Rico lies Brownson Deep,
which reaches the profound depth of 24,000 feet, and in the Caribbean
farther to the south lies Tanner Deep, reaching 15,000 feet. Although
its loftiest mountains do not exceed 4000 feet elevation above the sea
level, the whole island appears to be just a portion or segment of a great
mountainous mass or chain that rises from a submerged platform above
which its true relief might well be measured. A relief difference of
28,000 feet such as is given from the bottom of Brownson Deep to the
(11)






SCIENTIFIC SURVEY OF PORTO RICO


top of El Yunque is not exceeded in many places the world over within
such moderate distances, but it must not be assumed that this is the
elevation above the submarine platform.
Porto Rico may thus be regarded to some advantage in the present
consideration as a badly eroded uppermost portion of a great mountain
mass which probably at one time formed a part of a chain with general
east-west trend, most of which is now submerged. Certain very elevated
segments of this chain still stand above sea level and form the islands of
the Greater Antilles. Whether or not the structural relations of these
exposed segments are of fault-block type and what relation such an island
as Porto Rico may have to a former continental mass extending toward
and possibly connecting North and South America, however interesting
or suggestive they may be, are questions that might be better discussed in
the summary at the conclusion of this survey rather than in this pre-
liminary statement. Such questions are involved in the final solution of
the geologic history of the island, on which a whole projected series of
district and special studies have a bearing.
Porto Rico is very mountainous. A central cordillera extends nearly
east and west, starting abruptly at the sea near Rincon on the west coast
and continuing to El Yunque, the great mountain summit which forms
the central feature of the eastern end of the island. It is not by any
means as simple, however, as this statement would suggest, for there are
branches of the main range both east and west, but especially to the east,
where the Sierra de Luquillo, culminating in El Yunque, and the Sierra
de Cayay, 10 miles farther south, form very distinct features. At either
end of the island the chief mountain relief is centrally located, but
throughout the whole of the middle portion and for two-thirds of the
total length of the island, including the Sierra de Cayay, and thence
westward for 60 miles, the chief mountain range lies only about 10 miles
from the south coast. Almost two-thirds of the total area, therefore,
belongs to the northerly slopes of the chief mountain range.
Although the topography on the whole is very rugged and the island
has a decidedly mountainous aspect, there is almost everywhere a very
moderate relief along the coast and at mnny places the coastal lands are
almost flat. These very flat areas lie always at the mouths of rivers and
undoubtedly have been made in large part by deposition of the silts of
the streams. Such alluvial tracts are called playas in Porto Rico.
All relief forms except these playas are of erosional origin. Exceed-
ingly steep slopes are the rule, suggesting early maturity in physiographic
history. This phase of the geologic history of Porto Rico has been made
the subject of a special study by Mr. Armin K. Lobeck and will soon be






BERKEY, INTRODUCTION TO THE GEOLOGY


published in this series. One of the most surprising things is the way the
soils cling to these steep hillsides, and it is possible to repeatedly see slopes
under cultivation that measure from 30 to 40 degrees.
Numerous small islands lie along the coast. Two of comparatively
large size, Vieques and Culebra, which are respectively 20 and 7 miles in
length, mark the continuation of the Porto Rican land mass toward the
east. All are included as a part of the studies of Porto Rico.
Streams are numerous and surprisingly large. Mr. H. M. Wilson
(1899, 1900) has published an excellent account of the hydrology of the
island, and to this reference should be made for details of this nature.
He credits this small island with 39 large and 1300 small streams.
Rainfall is very unevenly distributed. At the east end, near El Yunque,
there is an annual rainfall of 123 inches. On the other hand, at Cabo
Rojo, at the extreme southwest corner, there is seldom any rain at all.
Because of the high mountain ranges, there is an abundance of rain on
the windward side, which includes the east end and most of the north
side of the island, and since the principal range stands far south of the
middle, the larger portion of the island is fairly well watered. A com-
paratively narrow strip along the south side, especially to the west from
Guayama, and again a strip at the northwest corner, are comparatively
arid. Extensive irrigation works -have been constructed along the south
side.
In most districts the bed rock is compact enough to discourage under-
ground circulation and the run-off is correspondingly responsive to rain-
fall. But in the northwest corner of the island, on a broad limestone belt
reaching from Aguadilla to Camuy, and less prominently even farther
eastward, there are many caves and sinks, and a most remarkable minor
topography,1 where underground water circulation is locally of much
importance. In some cases large streams rising on other ground farther
inland are completely lost here for long distances, following subterranean
channels in these limestones.
There are no inland lakes and but few coastal lakes. All of these
appear to be related either to playa development or to comparatively
recent elevation and subsidence. One near Guanica still retains internal
evidences of its former marine relations.
The climate is strictly tropical, but it is so tempered by reason of the
trade-wind breezes and by the mountainous aspect and elevation of much
of the country that it is usually very agreeable indeed. The entire lack
of great changes of temperature and the prevailingly moist conditions
'For discussion of this feature in much more detail, see the following article by
Douglas R. Semmes on The Geology of the San Juan District, Porto Rico.






SCIEXT1IFIC SURVEY OF PORTO RICO


over most of the island have a direct influence on the character of rock
decay and disintegration and also on the quality of the soil.
The average daily temperature is 80 degrees, and it rarely rises above
90 or drops below 70, while the maximum is said to be 99 degrees.
In area Porto Rico covers 3,(670 square miles, and is therefore about
three-fourths the size of the State of Connecticut. It is roughly rectan-
gular in outline, with the longer axis east and west, and in actual dimen-
sions is approximately 35 miles in average width and 105 miles long.
It is the fourth in size of the West India Islands. Porto Rico is very
productive and is said to be the most densely populated country in the
world.

GEOLOGICAL RECONNAISSANXCE

The Island of Porto Rico has never had a systematic survey, and com-
paratively few articles have been written on its geology. A few papers,
however, have described certain special features and general physical con-
ditions in a very instructiveway. The writings of R. T. Hill (1898,
1899), especially, contain a comprehensive outline of the geologic history
of the Island of Porto Rico, and those of H. M. Wilson (1899, 1900) and
W. Dinwiddie (1899) give as good general description of its physical
features as is anywhere to be found.
In the summer of 1914 the writer, in company with Dr. Clarence N.
Fenncr of the Geophysical Laboratory, Washington, made a geological
reconnaissance of the island, with the object of determining the chief
lines of its geologic history and outlining the major problems to receive
further study. This reconnaissance was undertaken as a part of the
Natural History Survey planned by the New York Academy of Sciences
and was intended to lay the foundations of a complete geological survey
which it was expected would become the work of several subsequent
expeditions and might extend over 4 or 5 years. Observations made at
that time, together with additional studies of material secured on this
first expedition, served as the basis of a report entitled Geological Recon-
naissance of Porto Rico (Berkey, 1915), which was published the fol-
lowing year, and has been the guide for several special expeditions and
district studies since that time. This paper still constitutes the fullest
statement of the structure and origin and geological history of the whole
island; but it has now been supplemented by several district studies which
have been made in much greater local detail, and also by a special investi-
gation of the physiography of the island: all of which, with yet others to
be made, form a part of the plan for this Survey.2 Seven such districts
2 A detailed list of these topics and studies is given at the end of this article.






BERlKEY, INTI'ODUCTION TO THE (;EOLOGY


and areal studies and fqur general investigations were originally planned.
Five of the district or areal studies have been made and one general
investigation has been completed.

MAJOR GEOLOGIC FEATURES

Determination of the principal rock types and larger structural units,
together with their relations and the principal problems deserving fur-
ther investigation, may be considered the chief work of a general geologic
reconnaissance. To this task the energies of the first expedition was
directed with sufficient success, as it now appears with respect to its
major lines, to stand the test of the more detailed subsequent work. A
brief outline of the more important geologic features characterizing the
whole island follows.

ROCK FoIr MAT IONs_

There is an exceedingly large number of formational rock units in this
field and a very great number also of rock types or varieties, and the
confusion as a consequence is so great, at first sight, that it all appears a
hopeless muddle. This is especially true of the interior of the Island
rather than the coastal margins. In the higher interior regions igneous
rocks and elastics derived more or less directly from igneous action such
as tuffs and ashes prevail, although formations of simpler sedimentary
(water laid) habit are very abundant and sedimentation structures are
at least as prominent as are the massive structures of the true igneous
rocks.
Along the coastal margin, on the other hand, especially along the north
coast west of San Juan and along the south coast west of Juana Diaz, the
rock formations are much simpler in aspect. Soon one comes to realize,
therefore, that two distinct formational series are represented and, judg-
ing from the very marked differences in habit, one would expect them to
be separated by a more or less profound break or unconformity.
To this extent at least the first suggestion in an attempt to organize
the complex structure of the island is correct. There are undoubtedly
two very distinct formational series which, on more extended inspection,
prove to be readily distinguished and separated. The one referred to as
characterizing the interior is much the more complex in make-up and
structure and most of its members have been extensively deformed and
modified since their deposition. The simpler series is made up of a suc-
cession of sedimentary and organic accumulations, including sales,
mnarls. chalks and limestones, little disturbed. As should be expected,






tCIESXTFIC SURVEY OF PORTO RICO


there is a very distinct unconformity between .these two major series
represented by an erosion plane which bevels the upturned members of
the older complex and seems to have all of the earmarks of a peneplain.
The complexity of the older of these two series suggests again the possi-
bility of a subdivision of it, but all studies so far have failed to show a
major line of separation. The formational groupings are therefore in
approximately the order suggested in their original reconnaissance, essen-
tially as follows:
THE YOUNGER SERIES

This includes shale and limestone beds, reef limestones and occasional
sandstone and pebble accumulations. These are readily recognized as
belonging to the Tertiary period, but the greater detail of age relation is
a matter necessarily left for the more elaborate studies of those districts
where these formations are prominent or to the results of general strati-
graphic and paleontologic study.
The Younger Series as used in these studies includes at least two very
readily differentiated but very unevenly developed members which were
referred to in the reconnaissance under names taken from their most
typical occurrence.
(a) The San Juan Formation is the most recent consolidated repre-
sentative in the island. It is confined to the immediate coast margins,
especially the north coast, and the adjacent small islands, the most strik-
ing occurrence forming the promontory on which the city of San Juan
is built.
(b) The Arecibo Formation is a thick series of limestones, marls and
shales. This is a much more extensive and geologically important mem-
ber than the San Juan. Subdivision is doubtless possible with further
study, and it has been convenient from the very first to distinguish cer-
tain particular facies or the beds of particular localities by special locality
names; but these do not necessarily represent subdivisions of wide appli-
cability. They have been, however, of distinct usefulness in all studies
thus far made. The most commonly used are the following:

1. The Arecibo limestone
upper limestone member on the north side of the island.
2. The San Sebastian sales
lower, shale member on the north side of the island.
3. The Juana Diaz marls
south side shales and marls near Juana Diaz.
4. The Ponce Chalk beds
verve white chalks near Ponce on south side of the island.







BERKEY, INTRODUCTION TO THE GEOLOGY


The greater part of all these beds are satisfactorily determined to be of
Oligocene age, but the latest studies indicate the presence of representa-
tives of both Eocene and Miocene age as well.

THE OLDER SERIES

This includes a complex association of tuffs, ashes, shale, conglomerate,
limestones and a very great variety of intrusives and occasional lava flows.
These are readily seen to be enough older than the other series to have
suffered much additional modification of petrographic character and
deformation, and it is comparatively simple also to determine that the
members of the series as far as they bear fossil evidence seem to be con-
fined to late Mesozoic age, perhaps wholly to the Cretaceous period.
The Older Series thus far has failed to show even as distinct lines of
subdivision as the Younger. Perhaps there are breaks as great as that
between the San Juan and the Arecibo formations of the Younger Se-
ries, but the structures are much more obscure, by reason of deformation
and other changes, and are less fully exposed. There is a suggestion of
a possible structural break marked by one belt of limestones found by
Dr. Semmes in the San Juan district (see San Juan District geologic
map) and perhaps in the La Muda Limestone of the original reconnais-
sance account, but no other supporting evidence is yet at hand, even after
a study of more than half of the island in detail.
The Older Series is apparently a fairly continuous accumulation of no
greater complexity and structural difficulty than should be expected from
a long and almost continuous volcanic history.
Although still regarded as a single series of formations, it has been
found very convenient in this case also to designate certain well marked
units by locality names. No assumption is made as to their extent or the
possibility deserving recognition in attempts at correlation. The Recon-
naissance expedition made free use of such terms to much advantage in
the field, and later still others have been used by the investigators of
special districts. As a matter of fact, the great difficulty found in the
beginning in following any single field unit very far or identifying it at
other places made the use of many locality and arbitrary names particu-
larly serviceable at that time. Subsequent studies have not successfully
eliminated them, and it may well be that the accumulating proof of the
great variability of all these formations as well as other units will compel
the retention of such local names. Among the terms that belong to the
reconnaissance stage and that bid fair to be retained are the following:






SCIENTIFIC SURI/EY OF 'PORTO RICO


1. The Fajardo sales, a very widespread type of blocky shale.
2. The Mayaguez shales, similar to Fajardo shales in general struc-
tural habit.
3. The Coamo Tuff-Limestone, a heavy limestone carrying abundant
tuff fragments.
4. The Trujillo Alta Limestone, a very limited occurrence of dense
limestone.
5. The "Shred" Limestone, a name suggested by the appearance of the
chief fossil form.
6. The La Muda Limestone, a formation that seems to mark a break
of more than average consequence.
T. The Mountain Limestone, of Hill-thin-bedded, shaly limestones
and limy shales of very extensive development along some of the
mountain ridges.
S. The Corozal Limestone, a peculiar local occurrence of limestone
breccia at Corozal.

PETROGRAPHIC RANGE

The petrographic variety of this older series is very great, but the types
of large consequence are not numerous. They may nearly all be included
within such group terms as limestone, marls, shales, sandstones, conglom-
crates, tuffs, ashy tuffs, ashes, and ash shales among the organic sedimen-
tary, and elastic rocks; andesitcs, andesitic porphyries, diorite, syenite
and granite as the chief crystalline igneous rocks; and serpentines and
graywackes and rarely some contact products as the chief badly modified
types.
Although the range of different types when stated in this form is not
at all remarkable, there is almost every conceivable variation of minor
composition, quality, structural or textural peculiarity, physical condi-
tion, degree of modification and state of preservation represented in
each one.
Most of them, furthermore, occur in an almost countless succession of
beds and sills or other field units of all sizes from masses of many square
miles in areal extent or hundreds of feet in thickness to thin layers or
stringers or dikes of only a few inches. As a matter of fact, the most
striking thing about these occurrences is the exceedingly great number
of small units which alternate or change so many times in short distances
that detailed mapping of them, or even an attempt to represent this detail
by the most carefully constructed geologic cross-sections, seems hopeless.
All practicable representations of the geologic structure are necessarily
generalizations.






BERKEY, lX'I'RODUCTION TO THE GEOLOGY


Maps of any kind covering portions oC districts must be done on the
basis of a grouping together of great numbers of individual units in rec-
ognition of the domination of some particular type. Thus it seems to be
possible to mark out belts or areas of dominant limestone, shales, tuffs
or conglomerates, some of which are fairly satisfactory as a basis for a
geological map. But even in some of these.cases the smaller individual
constituent units vary so rapidly along their strike that the average
quality is wholly different within distances of a few miles. This char-
acter makes detailed geologic mapping a difficult and most discouraging
undertaking.
The Younger Series, on the other hand, is very much simpler, since no
igneous rocks are represented at all. But even here, within the limits set
by the shales and sands of one extreme and the marls, chalks, reefs and
massive limestones at the other, there is every gradation, the character
being evidently under the control of such local conditions, and so poorly
marked off in succession by fundamental differences, that here again a
subdivision for mapping purposes is extremely difficult to apply. A
strictly paleontologic subdivision, however, is being more successfully
applied as the investigation of certain districts proceeds.

STRUCTURE

The general geologic structure of the Younger Series of formations is
simple. The beds lie, for the most part, dipping gently toward the sea.
This is especially true of the broad belt of these rocks forming the north
side of the island. On the south side there is some deformation, chiefly
due to faulting and crowding. In many places also there is slumping
due to sink-hole development.
The Older Series has, on the contrary, a very complex structure. Only
rarely do the bedded rocks have their original attitude. They commonly
stand much tilted and even vertical, or in rare cases overturned, and
faulting is common. But the most prominent structural feature is the
association literally of multitudes of thin-bedded ashy or limy shales and
equally thin igneous intrusive sheets or sills. It is not at all rare to see
an alternating series of such units, a few inches or a few feet thick, so
regular that when well weathered the whole succession looks simply like
a series of alternating shaly and more massive beds. The number and
complexity of the igneous intrusions are surprisingly great and at many
places the intrusive members dominate in the formation. In most cases
there are no noticeable contact effects.
Much larger intrusive sills, which are almost always dioritic in com-
position, are also very common and there are a few large bosses several






SCIENTIFIC SURVEY OF I'ORTO RICO


miles across of granite, syenite and serpentine. In a few places the local
interruption of the beds by massive tuffs cut by intrusive complexes seems
to mark old volcanic vents which must have been active after the adjacent
strata had formed and perhaps had attained something of the quality and
attitude that they now have. One large old vent of this kind is now the
site of the Jacaguas Reservoir. The occurrence of beds of limestone
breccia, including great amounts of tuff fragments, and a continuation
of the same regular beds beyond such complexes, are further supports to
this interpretation.


FIG. 1.-Diorite porphyry sills
These sills are intruded between layers of calcareous shales and ash beds on the road
near Comerio. The streaked or banded layers are shales. the massive portions, seen best
at the left side of the print, are sills. A transgressive relation can be seen between the
two layers at the extreme left.

Minor structural features such as xenolithic inclusions in the intrusives
are extensively developed. Sometimes a sill or dike is literally crowded
with tuff fragments so closely packed together that the whole has more
the general outward appearance of a tuff than a bona fide intrusive. Re-
peated inspection of the structure, however, has removed every doubt as
to the essential correctness of the observation. A rather remarkable
minor structure in the San Juan formation, which is believed to be a
consolidated dune sand, is illustrated in the accompanying figure. It









BERKEYI. INTRODUCTION TO THE GEOLOGY


FIG. 2.-Detail of the double structure in the San Juan formation at Arecibo
This photograph was taken to bring out the horizontal structure crossing the inclined
layers. There is no doubt whatever that the dark layer of less consolidated sand in the
lower third of the photograph is a primary bedding structure, but the horizontal marks
crossing the inclined layers in the upper part of the view are believed to be of secondary
origin.






SCIENTIFIC SURiEY OiF PI'OTO RICO


shows nearly horizontal bedding at the base, with. highly inclined thin
beds (cross-bed structure) above, on which or in which a less pronounced
horizontal structure is also preserved completely crossing the inclined
layers. This formation has not been deformed at all, so the structures
must be connected with the original sedimentation and perhaps the indu-
ration processes in some way. No better explanation has been offered
than that of the Reconnaissance report, which regards the cross-bed struc-
ture as a wind-laid deposit and the horizontal structure, crossing it, as a
cementation or induration structure developed in standilig water during
successive steps of subsidence and elevation.




















FIG. 3.-Huystack (pepino) hills
A characteristic view, showing the small soil-covered flats and associated haystack
hills found in the region of the Arecibo formation. Photograph taken on the road be-
tween Arecibo and Barceloneta.

RELIEF FEATURES

The present relief features of Porto Rico are all those of simple erosion.
In the interior the structure is so complex and variable that surface forms
seem to show little structural control. The mountains are rugged, steep
and irregular in outline. Mountain tops are almost everywhere knife-
edge divides, no matter whether they cross or follow the rock structure.
The features of the coastal margins, however, are much more influ-
enced by formational attitude. The younger series of beds lie on the
rather evenly eroded (peneplained) surface of the complex older series.
Erosion has stripped back much of this younger cover from the old land,






BERKEY, INTRODUCTION TO THE GEOLOGY


thus developing the usual inner lowland and cuesta forms with traces of
the old peneplain well preserved on the adjacent ground on the inner
side. A full treatment of these features is included in a physiographic
study of the island which has already been made by Mr. Armin K. Lobeck,
and whose report will soon be published as a part of this Survey.
On the cuesta and the outer slopes, wherever limestones are the surface
rock, great numbers of caves have developed. A most striking surface
feature is thus produced with sink-holes as the first distinct stage which
by enlargement develop fiats separated one from the other by remnants
of the old walls. In some districts these last remnants form great num-
bers of small conical hills, referred to by Hill as the pepino hills and by
Berkey as haystack hills. This is the most unusual and remarkable of
the surface features of the island.
Immediately along the coast one may find occasional traces of a step-
like form which has attracted much attention. They were taken at once
to be sea terraces, and subsequent studies seem to confirm that conclusion.
Such forms, therefore, indicate that the island has been elevated in com-
paratively recent time. Other related features, among them the occur-
rence of marginal islands, suggest that at one time the island may have
stood even higher than now. Probably there has been a succession of
elevations and subsidences in later as well as in the earlier geologic
history of the island.
GEOLOGIC HISTORY

In barest outline the geologic history of Porto Rico is simple. A com-
plete or even a reasonably full account of the geologic history of Porto
Rico cannot be written at this step of the investigation.
There is no record of very ancient rocks at all. But in Cretaceous time,
or perhaps immediately before, this region was the scene of violent and
long-continued explosive volcanic activity which resulted through many
vicissitudes in the accumulation of thousands of feet of volcanic elastic
matters, partly iri massive unassorted condition, partly worn, transported
and assorted, partly mixed with weathered detrital matters and organic
growths, all together forming a great succession of tuffs, ashes, shales and
limy rocks.
The summary of the major historical items formulated in the original
Reconnaissance report is still serviceable:
(1) A long geologic period of volcanic activity, accompanied by mar-
ginal attempts at assorting of fragmental and detrital material and
organic accumulation disturbed from time to time by renewed or extended
igneous activity.







SCIENTIFIC SURVEY OF PORTO RICO


(2) A dying out of volcanic energy, greater stability of the mass with
respect to elevation and subsidence, and erosional attack continued long
enough to result in extended planation and partial baseleveling with final
extensive submergence.
(3) The development of an unconformable overlying series of shales,
reef limestones and related deposits chiefly of organic origin, brought to
an end by final re-emergence.
(4) The development of present surface features under stream erosion
and marine marginal attack, with modifications arising from oscillation
of level.

PROBLEMS OF THE SURVEY

The problems that were suggested in the Reconnaissance report have
engaged the endeavors of several expeditions. Altogether seven have been
made and four studies have been completed. Others are yet to be made.
The whole plan with its accomplishments may be listed as follows:

EXPEDITIONS

(a) Expedition of 1914 to make a reconnaissance.
1. Dr. Charles P. Berkey and Dr. Clarence N. Fenner. Reconnais-
sance report (Berkey, 1915).

(J) Expeditions of 1915.
2. Dr. Chester A. Reeds. Study of Tertiary stratigraphy, and espe-
cially an attempt to establish the subdivisions of the Younger
Series and make paleontologic collections. New base map pre-
pared (published with this paper).
3. Mr. Douglas R. Semmes. Study of the San Juan District (Semmes,
1919).
4. Mr. Edwin T. Hodge. The Coamo-Guayama District.'

(c) Expeditions of 1916.
5. Mr. Bela Hubbard. The Aguadilla-Lares District.4
6. Dr. Charles R. Fettke. The Humacao District.4
7. Mr. Armin K. Lobeck. A general study of the physiographic his-
tory of the island.'

(d) Expedition of 1917.
8. Mr. Graham John Mitchell. The Ponce District.4


3 Report ready for publication.
4 Report interrupted by the war.






BERKEY, INTRODUCTION TO THE GEOLOGY


CLASSES OF STUDIES

The whole list of studies of all kinds fall into three classes:

(a) General studies, covering certain features affecting the whole
island.
(b) Areal or district studies, with accompanying geological maps.
(c) Special investigations on problems suggested by local conditions
or the material collected.

GENERAL STUDIES

Among the general questions which have been considered to belong to
the island as a whole are the following:
New Base Map.-The making of an accurate detailed map, such as
might be represented by topographic sheets of the kind prepared by the
U. S. Geological Survey, is an undertaking much beyond the scope and
facilities of this Survey. Each investigator of a district has undertaken
the correction of the maps available for his own area in enough detail for
the purpose. These districts, however, are not complete to such a point
that a general relief map for the island could yet be constructed from
them. As a step, therefore, preliminary to that result, the data of all
previously published maps, together with such notes as were available
from field observation, have been compiled, under the direction of Dr.
Chester A. Reeds, in the form of a new relief map which is published
herewith.
Physiography of Porto Rico.-After the reconnaissance and first dis-
trict studies were made, it became apparent that a general physiographic
study covering the whole island would be particularly desirable. The
opportunity to make a complete study of an isolated land mass, with a
-.1, -ir.llg1., history probably similar to most of the West Indian
islands, seemed to be particularly good. Mr. Armin K. Lobeck under-
took this study and his work is completed.
Tertiary Stratigraphy.-This general problem, which was considered
too broad for a district study, was recognized as a difficult investigation
that deserved to be considered by itself. Dr. Chester A. Reeds entered
upon the investigation by making a, collecting expedition in the season
of 1915. It was the intention to include in this study a determination of
the principal subdivisions and boundaries of the Tertiary strata for the
whole island, and it was hoped that this could be done on simple enough
lines and promptly enough to be of service to all of the men doing district






SCIENTIFIC SURVEY OF PORTO RICO


mapping and making local studies. The problem has been found espe-
cially difficult and final results are not yet reached. In the meantime
the practical matter of mapping has been solved in each district as well
as may be, and probably no full statement of the Tertiary stratigraphy
will now be attempted till all of these separate studies are available.
Mineral Resources.-Considerable money has been spent in Porto Rico
in mineral exploration. It is said that gold was secured in paying quan-
tities by the Spaniards in the early days. One can see gold panning in a
few places now. Many ores of other metals are also formed as well as
other economic products. Almost none, however, have proven attractive
as the basis of an industry, although much interest is attached to such
possibilities. Mineral resources of sufficient value to establish these in-
dustries are much to be desired, and it is one of the purposes of this
Survey to make a final summary of all matters relating to this question
in a general study after all of the districts have been fully investigated
and reported.
DISTRICT STUDIES, WITH OUTLINE MAP
The basis of all detailed investigations is the district or areal study.
It has been found possible to divide the island into areas, each of such
size that it could be studied in sufficient detail for the purpose in a single
season by one man. These districts are not necessarily of the same size
or form. Their boundaries have been chosen, in part, in consideration of
transportation facilities and field accommodation, and, in part, in con-
sideration of the occurrence of special features deserving particular atten-
tion or special study. It has been possible to assign to each investigator
of a district of this kind some important problem of a fundamental sort
in addition to his regular district mapping and description. On this
basis the area of the whole island has been covered by seven districts.
They vary considerably in size, depending upon the nature of the country
and somewhat upon the complexity of the geological problem, and to
some degree also on the amount of previous investigation in adjoining
areas (see outline map).
The first two districts to be studied completed a broad belt entirely
across the island from north to south, extending from San Juan to a
point beyond Vega Baja along the north coast and from Guayama to a
short distance beyond Santa Isabel along the south coast. This belt was
chosen as a guide study and was intended to more firmly establish the
major geological lines for further study of the rest of the island.
The belt across the island was divided by a line from east to west in
the vicinity of Barranquitas into a northerly and a southerly district.








SCIENTIFIC SURVEY OF PORTO RICO AND THE VIRGIN ISLANDS


OUTLINE MAP
OF
PORTO RICO
SHOWING
DISTRIBUTION OF DISTRICT STUDIES


bA RECI


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-..."-- LEGEND
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S --- drainage DOvides
Geological Boundaries
S*a nabo -......... Approximate Boundaries
.rttiary Formnations
SAr I Dsc -oun-FFa drts
Arro District Boundaries


Scale of Kilometers
1 V *


.ao


.NIJRTOJ a


VOLUME 1, PART 1


BEADLIA A


A








BERKEY, INTRODUCTION TO THE GEOLOGY


The northerly one has for convenience been designated the San Juan
District from its principal city. A study of this district was completed
by Mr. Douglas R. Semmes and the report on its geology, with geologic
map, forms a part of this bulletin.
The south end of the belt, south of Barranquitas, was studied by Mr.
E. T. Hodge. His area has been commonly referred to as the Coamo-
Guayama District. A special study of the nature and probable origin
of the hot springs at Coamo has been included by Mr. Hodge in his
studies of this district.
The Aguadilla-Lares District occupies the northwest corner of the
island, extending southwest as far as Mayaguez. This was studied in
1916 by Mr. Bela Hubbard. The district includes the best material for
a detailed investigation of the Tertiary stratigraphy.
The Humacao District of the southeast corner of the island extends
as far west as Guayama and as far north as Ceiba. This district contains
one of the large areas of granite as well as the principal iron occurrence.
The geology of this district was studied by Dr. Fettke in 1916. It in-
volves these special discussions in addition to the district geology.
The Ponce District of the south and southwest quarter of the island
extends from a few miles east of Juana Diaz to the west coast and from
the south margin to Mayaguez. This was studied by Mr. Mitchell in
1917 and was made to include the special problem of elevation and sub-
sidence as marked by terraces, and also the problem of faulting.
Two other districts are still to be studied.
One lies in the northeast corner, occupying the area north of the
Humacao District, and may be called the Fajardo District. The other
lies west of the San Juan District, extending to the vicinity of Camuy on
the west and Adjuntas and the mountain divide to the south, where it
touches the Ponce District. This is known as the Arecibo District.
The accompanying outline map indicates the location of these districts
and gives the names of the men who have been responsible for their
individual study (see outline map).

SPECIAL INVESTIGATIONS

Special scientific studies not already covered by the general investiga-
tions of group (a) or included in the district studies of group (b) are
chiefly concerned with pure science investigations connected with paleon-
tology. Immense collections have been made and seem to be worthy of
elaborate study. It is expected that they may ultimately form a volume
on the paleontology of Porto Rico.
A considerable list of special topics enumerated in the original Recon-












SCIENTIFIC SURVEY OF PORTO RICO


naissance do not deserve so elaborate treatment as that suggested above,
and indeed are quite logically cared for in the different district studies.
Some of them have been mentioned in that connection already. Among
those considered as representatives of the class are the following:

(a) Mapable subdivision of the Younger Series.
(b) Mapable subdivision of the Older Series Complex.
(c) Nature and origin of the San Juan formation.
(d) Reef-building organisms.
(e) Changes in character and composition of the igneous products at
different stages in Porto Rico volcanic history.
(f) Thermal springs.
(y) Studies in magmatic differentiation in an isolated volcanic center.
(h) Origin of the Iron Ores, etc., etc.

An occasional special topic has been the basis of a separate paper
(Fettke and Hubbard, 1918), and some of these have been or will be
published separately in an effort to furnish useful information as
promptly as possible.
In spite of the complexity and extent of the original undertaking, most
of it has already been completed so far as relates to the general studies,
district descriptions and special problems. One season's field-work on
the scale of that of 1915 or 1916 would complete such work, and it would
then be possible to make a final geological map of the island and to write
the summary of its areal, structural and economic geology and its geo-
logical history.
A completion of it to that consummation may be regarded as the
immediate object of these geological field studies. The paleontologic
studies, in so far as they are necessary to the historical treatment and
stratigraphy, are to be included in the geologic investigations. Subse-
quent studies of all kinds and to any extent may be based on the collec-
tions, and additional investigations along paleontologic lines will then be
included in a separate volume under paleontology.















BERKEY, INTRODUCTION TO THE GEOLOGY


BIBLIOGRAPHY

BERKEY, CHABLES P.
1915. Geological reconnaissance of Porto Rico. Ann. N. Y. Acad. Sci.,
XXVI, pp. 1-70.

DINWIDDIE, W.
1899. Physical features of the island (Porto Rico). Harper's Weekly,
XLIII, p. 248.

FETTKE, C. R., and HUBBARD, B.
1918. The limonite deposits of Mayaguez Mesa, Porto Rico. Bull. Am.
Inst. Min. Engin., No. 135.

HILL, R. T.
1898. Cuba and Porto Rico, with other islands of the West Indies. New
York.
1899. Porto Rico. Nat. Geog. Mag., X, pp..43-112.
1899a. The forest conditions of Porto Rico. Bull. No. 25, Div. Forestry,
U. S. Dept. Agric.
1899b. The value of Porto Rico. Forum, XXVII, pp. 414-419.
1899c. The geology and physical geography of Jamaica. Bull. Mus. Comp.
Zool., XXXIV.

SEMMES, DOUGLAS R.
1919. The geology of the San Juan district, Porto Rico. Scient. Survey
Porto Rico and Virgin Islands. N. Y. A. S., I, pt. 1, pp. 33-110.

WILSON, H. M.
1899. Water resources of Porto Rico. Water Supply Paper No. 32. U. S.
Geol. Survey.
1899a. The engineering development of Porto Rico. Engin. Mag., XVII,
pp. 602-621.
1900. Porto Rico; its topography and aspects. Jour. Am. Geog. Soc.,
XXXII, pp. 220-238.







SCIENTIFIC SURVEY OF PORTO RICO


NEW BASE MAP OF PORTO RICO

BY CHESTER A. REEDS

Following a short fossil-collecting season in Porto Rico in June and
July. 1915, for the New York Academy of Sciences and the Porto Rican
Government, the American Museum of Natural History cooperating, it
seemed desirable for plotting results that a small outline map of Porto
Rico should be prepared. During October, 1915, such a map was drawn
at The American Museum of Natural History by Mr. A. Briesemeister
under my direction. Most of the data for the base sheet was obtained
from a large map which had been prepared by the engineers of the
Interior Department of the Porto Rican Government. The four-sheet
edition of the U. S. Coast and Geodetic Survey and the U. S. Post Route
map of Porto Rico were also used. Before the end of October a solio
print edition of one hundred copies of this new base map of Porto Rico
was published by The American Museum of Natural History. Com-
plimentary copies of the preliminary edition were distributed to the
members of the Porto Rican Committee of the New York Academy of
Sciences, to certain officials of the Porto Rican Government, to the
various field men sent out by the New York Academy of Sciences to
Porto Rico and to other interested persons.
During December, 1915, the 100, 500, 1000, 1500, 2000, 2500, 3000,
3500, 4000 and 4500 foot contour lines were drawn on a separate sheet
for superposition on the base map of Porto Rico which had been prepared
in October. Data for this topographic map of the island were taken from
various sources, chiefly the U. S. Coast and Geodetic Survey maps, the
profiles and sketches of the engineers of the Porto Rican Highways Com-
mission, the contour map of a portion of the south side of the island
prepared by the Irrigation Service, the U. S. Geological Survey's topo-
graphic sheet of the Luquillo Forest Reservation, together with H. M.
Wilson's topographic map of Porto Rico, prepared for the U. S. Water
Supply and Irrigation Paper No. 32, 1899. Numerous photographs by
Professors C. P. Berkey and H. E. Crampton of the Porto Rican Com-
mittee of the New York Academy of Sciences as well as a large number
by myself were of considerable assistance in the preparation of the topo-
graphic sheet. Fifty solio print copies of this contour map were issued





























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REEDS, NEW BASE MAP OF PORTO RICO 31

by the Museum and complimentary copies were distributed as had been
done with the base map.
Early in 1916 the 10, 100, 300, 500 and 1000 fathom lines were added
to the base sheet of October, 1915. Areas less than three fathoms in
depth were dotted. This information was compiled from the various
Porto Rican charts of the U. S. Coast and Geodetic Survey.
This map should be of interest not only to the scientist and tourist but
also to the million inhabitants of Porto Rico, for it is legible, convenient
in size and contains a vast amount of information.










THE GEOLOGY OF THE SAN JUAN DISTRICT,

PORTO RICO


BY DOUGLAS R. SEMMES

CONTENTS
Page
Synopsis.............................. .............. ... ......... 35
Introduction......... ...... ..................... ................... 36
Geographic Location.............................. ............... 37
Natural History Survey of I'orto Rico .................. .......... 37
San Juan District ................................................ 37
Physiography of the Island ............... ......................... 38
Complex Mountainous Oldland............. ......... ............... 38
Peneplanation .................................................. 40
The Coastal Plain.................................... ........... 40
Mature Dissection .................. ................... ......... 41
Recent Submergence and Uplift................................... 41
Physiography of the San Juan District ................................ 41
The Dune Area................... ......................... ..... 42
The Playas...................................................... 43
The Tertiary Limestone Belt................. ..................... 44
Terraces ..................................... .............. 46
The Inner Lowland ...................................... ........ 47
The Interior Mountainous Region................................. 48
Incised Meanders............................................. 49
Stream Capture ...................................... ........ 50
Peneplane. ............................. ...... .......... 50
Rock Formations ...................................... ............. 51
Younger Series....................... .............. .......... 52
Recent Deposits ............................ .. ..... ........... 52
Unconsolidated Lime Sand............................... 52
Playa Plains ............................................ 52
Pleistocene (?) Deposits............. ............... ......... 53
San Juan Formation............... .................... 53
Age............................................. ........ 55
Tertiary Limestone Series.................................... 55
Arecibo Formation........................................ 55
Age......................................... .. ........... 57
Older Series ..................................................... 60
Residual Soil and Depth of Decay............................. 61
Corozal Limestone......................................... 62
La Muda Limestone.......................................... 64
Aguas Buenas Limestones..................................... 64
Limestone at K 29 .......................................... 65
(33)








34 SCIENTIFIC SURVEY OF I'ORTO RICO

Page
Juan Ascencio Chert Beds................................... 65
Corozal Jasper Bed.......................................... 66
Shales.............................................. ......... 67
Sandstones ........................ ..... ..................... 69
Conglom erate .................. ............................. 70
Tuffs ................... ............... ..................... 71
Age....................................................... 72
Summary ...................................... ..... 75
Igneous Rocks ....................................................... 76
Modes of Occurrence .................. ........................ 76
Lavas........................ .............................. 76
Sills ........................................................ 76
Dikes ............................ ............ .............. 77
Stocks and Bosses ........................................... 79
Texture......................... .............................. 79
Composition ..................................................... 80
Vitrophyrs .................................................. 81
Rhyolites .................................................... 82
Granites .................................................... 82
Quartz Latites and Rhyo-andesites ............................ 83
Quartz Monzonites ........................ .......... ........ 84
Granodiorites...................... .. ... ............ ........ 85
Dacites............................ ........ ... ......... 85
Quartz Diorites ............................................... 85
Trachytes ................................................... 86
Syenites................................................... 86
Latites (Trachy-andesites) .................................... 87
Monzonites .................................................. 88
Andesites ..................................................... 89
Diorites ...................................................... 91
Basalts........................ ................... ........ 92
Olivine Basalts.......................... : ................... 93
Summary ..................................... .. ...... ....... .93
Structural Features.................................... .......... ... 95
Unconform cities ................................................... 95
Faulting ....................................................... 96
Folding................................................ ........... 98
Metamorphism................................................... 98
Minor Structures............................. ... ....... ......... 98
Cross-bedding......................................... .. .... 98
Solifluction .................................................. 98
Stratigraphical Summary........................................... 99
Mineral Occurrences.................. .............................. 102
Gold.......................... .. .... ....................... 102
Placers....................................................... 102
Auriferous Quartz Mines................... ................... 103
Platinum...................... .............................. 104
Copper................................................... ...... 104







SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


Page
Lead .................................. ......................... 105
M anganese.............................................. ......... 105
Road Metal............................................. ......... 105
Building Stone........................................... ......... 106
Guano.................................................. ......... 106
Illustrations.......................................................... 106
Cross-sections..................................................... 106
Maps........................................................... 106
Acknowledgments ..................................................... 107
Bibliography............................................. ......... 108



SYNOPSIS

The area described in this report, which has been designated the San
Juan district, lies on the northern side of the island of Porto Rico. It
extends from the city of San Juan on the east to a point about two miles
east of Manati and southward as far as Barranquitas-an area roughly
500 square miles in extent. The San Juan district is a typical north-
south section through the northern half of the island, the geologic and
physiographic history of which may be regarded as essentially the same
as that of the whole island.
The physiographic history of the San Juan district is that of a complex
mountainous oldland, which has been peneplaned, partially submerged,
overlapped by a coastal plain, uplifted, maturely dissected, again slightly
submerged and partially uplifted, erosion continuing in the interior from
the time of first uplift.
The formations of the district may be primarily divided into the
younger series, or those that form the coastal plain, and the older series.
or those that lie below it. The youngest formation consists of a coastal
deposit of indurated lime dune sand of presumably Pleistocene age over-
lying unconformably a limestone series, which forms the coastal plain
proper. This limestone series, termed by Berkey the Arecibo formation,
lies almost horizontal (dip 60 N.) and is now maturely dissected, giving
rise to the typical pepino or haystack topography of the island. The
Arecibo formation is regarded as upper Oligocene in age. In the north-
western portion of the island, but not appearing in the San Juan district,
is a lignitic shale member (the San Sebastian shale) which underlies the
Arecibo formation. The age of this shale member is uncertain, but in all
probability it is lower Oligocene.
The older series consists of marine and volcanic sediments with numer-
ous intrusives. It is highly tilted and locally folded, resting uncon-






SCIENTIFIC SURVEY OF PORTO RICO


formably below the younger series. The sedimentary types represented
are limestone, shales, sandstones, conglomerates, tuffs and ashes. With
hardly an exception, these sediments are either of volcanic origin or were
deposited through volcanic processes. Even the limestones show brec-
ciated structure and are, with few exceptions, merely accumulations of
limestone fragments derived from older beds shattered by volcanic out-
bursts. The foraminiferal content of certain shale and limestone beds
in the upper part of the older series indicates that the beds are upper
Cretaceous and Eocene in age. The lower part of the series is no doubt
Comanchic. The peneplanation of the older series culminated along the
coast before the deposition of the coastal plain, for the peneplane surface
can now be traced underlying the Arecibo formation. In the interior,
however, erosion continued until the uplift, which occurred in late Oligo-
cene time.
The older series is everywhere intruded by igneous rocks of many
varieties. The predominant type is an andesitic porphyry. The tuffs
and other sediments directly derived from volcanic sources are also of
this general composition. The igneous rocks occur as extrusive sheets,
sills, dikes, and small and moderate sized stock-like intrusives.
The mineral resources of the district are not large. About 100 ounces
of gold are panned annually from the rivers in the vicinity of Corozal.
Gold quartz veins have been worked in the hills south of Corozal, but
without success. In the barrio Pasto, southwest of Morovis, several
copper prospects have been slightly explored.

INTRODUCTION

The Antilles and the Central American region have remained until the
close of the nineteenth century almost a terra incognita as regards their
geological structure and history. Unfortunately, little study has since
been devoted to this critical region which connects the two Americas, and
until more accurate information regarding it is available our ideas of the
geologic correlation of the two continents must remain largely conjec-
tural. At the close of the Spanish-American war Porto Rico, being the
closest of our newly acquired insular possessions, attracted a good deal
of attention. During the first years of the American occupation much
was written about the island and its inhabitants. Several very good
papers describing its general configuration and geology were published
during this period,' and a number of articles on particular features or
the general aspects of the country appeared in current periodicals and in
1Dorsey (1902), Hill (1899a), (1899c), Wilson (1899), (1900).






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


proceedings of learned societies. Interest in Porto Rico, however, soon
waned, and from 1902 to 1915 nothing regarding its geology was pub-
lished. In 1915 a paper by C. P. Berkey appeared in the Annals of this
Academy, which gives the first detailed and purely geological description
of the island that has yet been written.

GEOGRAPHIC LOCATION

Porto Rico, the smallest and easternmost of the Greater Antilles, aver-
ages 35 miles in width and 100 miles in length, presenting a regular
rectangular outline three times as long as broad and alined nearly ex-
actly east-west. Its shoreline is nearly straight and quite regular. The
coast is usually low, with a few low promontories. The fringing keys
and deep indentations characteristic of a more youthful stage in coastal
development, and so abundantly developed on the Cuban coast, are en-
tirely wanting. Its area is 3670 square miles, just 500 square miles less
than the area of Jamaica, or about three times that of Rhode Island. Its
population, considered as a whole, is the densest of any country on the
globe.
NATURAL HISTORY SURVEY OF PORTO RICO

In 1914 an appropriation was made by the Insular Government of
Porto Rico for the establishment of a Natural History Survey which was
to be carried on in conjunction with the New York Academy of Sciences.
The geological phase of the survey was begun in 1914 when Dr. Berkey
and Dr. Fenner spent a month on the island, making a geological recon-
naissance, preparatory to a subsequent and more detailed survey. The
following year the writer was assigned to the San Juan district to begin
this more detailed work.

SAN JUAN DISTRICT

The region selected may best be called the San Juan district, as San
Juan, the capital, is situated in the northeastern corner. The district is
that area comprised between longitude 660 05' and 660 27' west, and an
east-west line through the town of Barranquitas about 21 miles south of
the northern coast. The areal extent of the district is roughly 500 square
miles. A corresponding area of similar width to the south was studied
at the same time by Mr. E. T. Hodge. In this way a belt 25 miles wide
was surveyed entirely across a typical portion of the island. Three
months were spent in the field, in which time a topographic map was
constructed, the mineral resources studied, and the geology plotted in as






SCIENTIFIC SURVEY OF PORTO RICO


much detail as possible, considering the shortness of the time and the
size of the area involved.

THE PHYSIOGRAPHY OF THE ISLAND

The general configuration and topography of the island have been well
described by several writers. Dorsey (1902) has written an admirable
description of the several types of topography one meets on crossing the
island, with an especially detailed description of the limestone topography
of the coastal plain. Wilson (1899) also has given a good general de-
scription of the physical aspects, and Hill (1899) described the physiog-
raphy in connection with his geological reconnaissance of the island. The
recent article by Berkey (1915) gives the most modern and detailed de-
scription of the physiography and physiographic history of the island.
The island of Porto Rico may be described as a complex mountainous
oldland, in the nth cycle of development, which has been peneplaned, or
reduced to an old stage of topographic development, submerged, over-
lapped by a coastal plain, uplifted and maturely dissected, partially sub-
merged, then re-uplifted and further dissected.

COMPLEX MOUNTAINOUS OLDLAND

The most characteristic topographic feature of Porto Rico is its com-
plex mountainous aspect. That portion of the island that rises above sea
level is merely the eroded crest of a submerged mountain range of great
complexity and magnitude. This eroded crest rises steeply from the sea
to an average altitude of 2500 feet along the divide. Owing to difference
in rainfall on the northern and southern side, the divide has been pushed
to the south by the encroachment of the larger northern streams until it
now stands two-thirds of the way toward the southern coast. The present
position of the divide has also been accounted for by a theory of block
faulting of the whole island with the front face of the block facing south
(Berkey, 1915, p. 40). In the western portion of the island the divide
is double, where it is parted by westerly flowing streams. Near Adjuntas
the two divides join at the headwaters of the Anasco River to form a
single ridge, which continues to Cayey, where it first swings southward,
and then sharply to the northeastern corner of the island. The Luquillo
range forms another westward extending branch of the divide nearer the
northern coast.
The texture of the mountainous topography is medium to fine, except
for certain lowland areas of coarser texture. The interior topography is






SNEII.1IEA', GEOLOGY OF THE SRIAN JU.IS DIITRIICT


predominantly the ridge or cuchilla type. The slopes are extraordinarily
steep, slopes of 30-40 degrees being often seen. Between these ridges the
streams run in V-shaped valleys. Some of the valley walls are very steep,
but all variations from a canyon-like gorge to mature, open valleys are
to be found. The streams north of the divide are very numerous and of
large volume, due to the abundant precipitation of the large per cent of
run-off. Loisa, the largest river, rises only eight miles north of Arroyo


FIG. 1.-Valley of the Plata, just below the power dant at K 19, Conterio Road
Such steep valley walls and boulder-strewn beds are characteristic of almost all of the
mountain streams.

on the southern coast. The Plata, next in size, rises the same distance
north of Guayama. The larger streams rise high up in the mountains,
and in the first few miles develop steep gradients characterized by falls,
rapids and boulder-strewn beds (Fig. 1); then passing out into the lower
coastal margins they acquire much more mature courses and finally
swing in large meanders over the flat playa plains bordering the coast.
Thus a typical stream such as the Plata on the northern coast may show
every stage in development from youth to late maturity in its compara-
tively short course to the sea. The complexity of this mountainous old-






SCIENTIFIC SURVEY OF PORTO RICO


land is readily recognized from even the most cursory observation of its
geologic structure. An endless variety of sediments and volcanics, steeply
tilted and locally folded, are to be found intruded, permeated, and satu-
rated by igneous rocks of many kinds, assuming an endless variety of
forms.
PENEPLANATION

If one stands upon the summit of one of the central sierras and looks
out over the innumerable ridges that rise in all directions he may be im-
pressed by the even crest line formed by the general accordance of sum-
mits. This crest line apparently marks the present level of a peneplane
the development of which began in Eocene and culminated in late Oligo-
cene time. It is quite possible that the peneplane was never developed in
any degree of perfection over the entire island. In some districts, how-
ever, its present position is clearly defined, and near Mayaguez and Rincon
undissected portions of this old upland surface can be seen. In this
vicinity the level-topped areas are locally known as mesas. In other
parts of the island no trace of an accordant skyline can be found. How-
ever, a discordant skyline might well be developed in local areas through
differential erosion or by subsequent warping. The evidences for and
against peneplanation are discussed in more detail in another part of this
report.
The present elevation of the peneplane ranges from 2000 to 2200 feet
near the divide to sea level along the coast. On the northern side it is
partially covered by the younger coastal plain strata, but in the stream
valleys it can be seen still preserved beneath the coastal plain. Elevations
rising decidedly above its maximum elevation, such as El Yunke (3790
feet), Los Picachos (3700 feet), Ala de la Piedra (3650 feet) and Guilarte
(3600 feet), must be regarded as monadnocks upon the peneplane surface.

THE COASTAL PLAIN

Overlapping the mountainous region, there extends for nearly the
whole length of the northern coast a coastal plain of limestone and shaly
beds, which reaches as far inland as Lares and to an elevation of 1200
feet. A similar coastal plain appears along the central portion of the
southern coast. They are both of early Tertiary age, but whether they
are exactly contemporaneous is uncertain. This fringing coastal plain
varies in width from a mere line of isolated limestone hills, as seen south
of San Juan, to a belt over ten miles broad north of Lares. On the
southern coast it nowhere exceeds two or three miles in width. Overlying






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


the coastal plain on the northern side, and occurring as a string of fring-
ing islands or low coastal promontories, is the San Juan lime sand. It is
essentially an indurated dune sand composed of shell fragments and
quartz grains. This sand extends nearly the whole length of the northern
coast, except for the extreme eastern part, but it has not yet been reported
upon the southern coast.

MATURE DISSECTION

The coastal plain is characteristically mature in its stage of develop-
ment. Some portions of it might be regarded as sub-mature, and towards
the eastern end of the district, south of San Juan, dissection has con-
tinued to late maturity. The larger area comprised between these two
extremes, however, shows the rugged appearance of maximum dissection,
characteristic of maturity.

RECENT SUBMERGENCE AND UPLIFT

Since the deposition and uplift of the Tertiary coastal plain there is
evidence of still later movement. The rock terraces described by Berkey
(1915, p. 48) on the southern side of the island indicate that the sea was
once at that level, while the presence iupon them of recent shells fixes the
time. On the northern coast recent marine shells are found some distance
up the larger valleys at elevations of 100-200 feet. The subsequent uplift
was not as great as the previous subsidence, for large areas of the San
Juan sandstone, which must have originated at or above sea level, are now
submerged, forming islands fringing the coast. The available evidence
seems to indicate, moreover, a series of minor oscillations extending from
Pleistocene time until the present, that is, the submergence which formed
the fringing island was not continuous but consisted of numerous positive
as well as negative movements.

THE PHYSIOGRAPHY OF THE SAN JUAN DISTRICT

The San Juan district, as already stated, extends from San Juan west-
ward to a point two miles east of Manati and southward as far as Barran-
quitas. Lying in the central part of the northern side of the island, it
enjoys a very uniform humid climate. The mean annual rainfall varies
from 50 to 90 inches. The dryest months are December and January,
and the wettest month is May. The mean annual temperature varies
from 74 to 78. Though the climate is strictly tropical, the heat is
rarely oppressive; a breeze is always stirring, and at least one or two
showers may be expected each day.






SCIENTIFIC SURVEY OF IORTO RICO


The district represents a typical section of the northern half of the
island, and its physiographic history has in general been the same as that
given for the island as a whole. The accompanying block diagram (P1. I)
is an attempt to picture graphically the topography of the San Juan dis-
trict. Traversing the district from the coast to the southern boundary,
one would cross successive belts of differing topography, which may be
regarded as the topographic provinces of the district. On the coast is a
narrow belt of modern sand dunes, in some places overlying the older
Pleistocene consolidated sand dunes. Following this, there is an expanse
of flat plains locally known as playas. These extend from the coastal
dune area inland to the Tertiary limestones. In places the flat expanse
of the playas is broken by low, rounded hills of the Tertiary limestones
rising through it. To the south of the playa plains the typical pepino or
haystack topography of the limestone belt begins. This belt extends
inland and adjoins the oldland. It is occasionally broken by transverse
valleys, such as the valleys of the Plata and Cibuco Rivers, which have
completely removed all the limestone so that the alluvial plains of the
river valleys rest directly on the oldland. Separating the coastal plain
and the mountains proper is a low-lying area called by Hill (1899a) the
"parting valley." This is the inner lowland developed between the
coastal plain cuesta and the oldland. It is well developed as it crosses
the San Juan district, broadening out towards the east to form a lowland
area several miles wide between Bayamon and Guaynabo. The moun-
tainous area extends southward from the inner lowland to the southern
boundary of the district.
THE DUNE AREA
The dune area presents topography of two types: first, the low promon-
tories and coastal islands developed in the consolidated dune sand of
Pleistocene (?) age; and, second, the modern dune topography that bor-
ders the coast. The first type is best represented in San Juan City. The
promontory, rising some 75 feet above the sea, on which the city stands,
is entirely made up of the consolidated dune sand. To the west it can be
traced either as a string of low islands, or occasionally as a promontory
jutting out into the sea. Near Vega Baja Playa the island type is well
developed, the islands ranging in size from a few yards to several hundred
yards in length, and occurring out as far as a quarter of a mile from the
coast (Fig. 2).
The second or modern dune type of topography is a narrow strip of
live dunes composed of lime sand similar in composition to that which
forms the consolidated deposits. The dune belt follows the coast for the
entire length of the district, varying in width from a few feet to several









SCIENTIFIC SURVEY OF' PoRTO RICO AND TIH VIRGIN ISLANDS


VOLUME 1, PLATE 1


BLOCK D/AGRAM
Or THE
5AN JUAN DI53T/CT PB.






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


hundred yards. It is broadest in the western part of the district, but its
maximum width is developed farther west than the San Juan district
extends. The origin and source of the Pleistocene, as well as the modern
dune sand, will be discussed under a subsequent heading.

THE PLAYAS

The playa plains, a name given by Hill to the fan-like plains that ex-
tend from the foothills of the dissected limestone cuesta to the coast, are
to be seen along the entire coast of the San Juan district. Separated
from the sea by the dune belt, they extend inland to the Tertiary lime-
stones, and follow up the larger valley until they lie directly on the rocks



















FIG. 2.-San Juan consolidated dune sand forming fringing islands at Vega Baja Playa

at the interior complex. The playa plains rise only a few feet above sea
level, and until recent years large areas of the plains in the San Juan dis-
trict have been covered with water, forming inland swamps and lagoons.
Since the value of the land has been recognized for sugar-cane production,
all the available areas have been drained, and now constitute the most
productive and valuable lands on the island. The low, flat, monotonous
expanse of the playa plains is occasionally broken by low, rounded hills
of the Tertiary limestones rising above the alluvial (Fig. 3). These
limestone hills are isolated remnants of the coastal plain here buried be-
neath the recent deposits. Owing to the slight elevation that they afford
above the blistering heat of the surrounding plains, they are selected as
town and dwelling sites. Both Dorado and Toa Baja, the only two towns






SCIENTIFIC SURVEY OF PORTO RICO


situated in the center of the playas, are built upon these limestone rem-
nants. Others are chosen as sites for the individual dwellings of the
natives. The playa plains follow the three principal valleys of the Cibuco,
Plata, and Bayamon Rivers for some distance inland, and in the case of
the latter two extend well beyond the limestone belt.
The geological history of the formation of the playa plains has, no
doubt, been complex. It is evident that they are in part of fluvial origin,
for they grade directly into the true alluvial plains of the river valley.


FIG. 3.-The playa plains near Arecibo
Showing a limestone remnant rising above the floodplain deposits and the limestone
hills of the Arecibo formation in the background. Photographed from report of Bureau
of Soils, U. S. Department of Agriculture, 1902.

Just how important a role the sea has played in their formation is not
definitely known. It is the writer's opinion, however, that both factors
have been influential. This point will be further discussed below.

THE TERTIARY LIMESTONE BELT

As the northern coast of Porto Rico is approached, one readily distin-
guishes two kinds of topography; the rugged mountainous skyline, and
the lower, angular, maturely dissected limestone coastal plain. On closer
inspection it is seen that the topography of the coastal plain consists of
numerous isolated or closely grouped angular hills not exceeding 100 feet







SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


in altitude, but of such angular and rugged outline, and so densely cov-
ered with tropical vegetation that they form the most inaccessible and
impassable regions of the district. Slopes of 400 are quite common and
occasionally vertical faces of limestone rising 10 or 20 feet are to be seen.
Though from a distance the hills appear well rounded, yet at close range
they are exceedingly rough and jagged, and are, as one of the Americans
living in the vicinity expressed it, "very hard to scramble up." The
unusual appearance of these limestone hills was early noticed by the na-


FIG. 4.-Typical haystack topography as developed in the icestern portion of the San
Juan District
Photographed by H. E. Crampton.

tives, who call them pepinos or cucumber-shaped hills. Berkey has used
the term "haystack hills," a term peculiarly fitted to picture their appear-
ance, rising abruptly as they do from comparatively level plains (Fig. 4).
Both Dorsey (1902, p. 800) and Berkey (1915, p. 52) have independ-
ently come to the same conclusion as to the origin of the haystack topog-
raphy. Berkey summarized the steps in their development as follows:
The reef limestones are not uniform in composition or structure. They
have more or less intermixture of earthy matters which are distributed irregu-







SCIENTIFIC SURVEY OF PORTO RICO


larly but chiefly at certain horizons, as more earthy or shaly beds of no very
great lateral or vertical extent. Such conditions are repeated at occasional
intervals in successive horizons. As such a series is lifted above sea level and
subjected to ordinary erosion and weathering, the tendency is, (a) for the
purer and more massive reef limestones to be attacked by the solvent action
of percolating water with a development of underground channels, porous rock
condition and actual caverns, (b) for the more earthy layers to resist and
limit such action at the levels where this matter is present in sufficient abun-
dance, with a development of residuary material. As this action progresses
toward maturity, many of the larger caves collapse and sink-holes are thus
formed. With still further development, the sink-holes merge into each other
in local areas where solution has been most active, the earthy debris forms a
soil in the bottom corresponding in level with the first important earthy layer,
and adjacent remnants of the limestone reef stand out as sharp, rugged hills
separated by irregular notches that represent other smaller, collapsed caves.
The result of such action and conditions, finally, is the numerous 'haystack'
hills standing on flat soil-covered areas or surrounding such areas as if they
were just set down as bunches on the surface.

This is no doubt the best explanation for the origin of the typical hay-
stack hills. Towards the south boundary of the limestone belt, however,
the haystack hills are not typically developed. The topography of this
region is more rounded, with better drainage pattern. Occasional sink-
holes are developed, but the topography as a whole resembles more that
developed by normal stream erosion. This difference is due to the change
in character of the limestone. The haystack type is developed on pure
limestone, the latter on shaly limestone and shales.

TERRACES

The haystack hills rise abruptly from almost level plains, just as hay-
stacks on a grass field. The plains are local developments and do not
continue over extended areas. On passing from one of these plains to
another, one is conscious of the fact that they rarely are of the same alti-
tude, that is, they are not separated portions of a continuous base-level.
On the contrary, they assume a step-like succession, forming a series of
seaward-facing terraces. The level of an upper terrace corresponds
roughly with the summits of the haystack hills on the next succeeding
lower terrace. The terraces are best developed west of Vega Baja. Two
can be recognized on following the road from Vega Baja to Morovis at
K2 42-43 and K 3-4 (Morovis road) respectively (see topographic map).
Others less prominent can be detected by means of the aneroid.
There are two possible explanations for the origin of the terraces:

K indicates kilometer posts on the public roads.






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


(a) The presence of a shaly layer in the purer reef limestones may be
the controlling factor in the development of a plain at a particular level
through difference in solubility. The pure limestone would soon develop
large caves, extending down to the level of the shaly layer, which, col-
lapsing, would leave the haystack hills on a fairly level base. This plain
would slope gently with the limestone strata towards the sea. After a
time erosion would finally break through the shaly layer and a new ter-
race would be formed with the next shaly layer as its base (Fig. 5). In
this way successive terraces would be formed facing the sea. The differ-
ence in appearance of the pure limestone and the shaly layers might well
be so slight as to escape notice, so the apparent absence of shaly beds
throughout the upper limestones would not disprove the theory. The
writer located several shaly beds in the upper limestone that might easily
have caused the terraces, but no exact correlation could be made from the
barometric readings.
(b) It is also possible that these may be marine terraces. Evidence
for recent subsidence has already been mentioned, and marine terraces of



FIG. 5.-Section through the Arecibo formation
Illustrating the possible origin of terraces from differential solutioh caused by shaly
layers at A, B and C.

about the same elevation have been described from the southern coast
(Berkey, 1915, p. 48). Both causes may have been influential, but the
writer is inclined to adopt the first, since no marine shells have yet been
found on the terraces. Moreover, the haystacks on the plains are more
numerous and more promiscuously scattered than marine stacks should
be, unless one considers the possibility of marine terraces later dissected
to form the haystack hills.
The limestone belt extends across the whole district. It is wedge-like
in general outline, being about five miles wide on the western border of
the district, and thinning to a mere string of isolated hills on the east.
Its inner margin reaches the elevation of 700-800 feet north of Corozal.
In this locality and to the west its southern boundary is a well-defined
cuesta which is locally known as "The Walls" (Fig. 6). To the east of
Corozal the cuesta form is destroyed by more advanced erosion.

TIHE INNER LOWLAND

This topographic province, described by Hill as the "parting valley"
(Hill, 1899a, p. 105), extends entirely across the San Juan District. It






SCIENTIFIC SURVEY OF PORTO RICO


lies between the infacing cuesta of the coastal plain on the northern and
the mountainous region to the south. It is best defined at Corozal (Fig.
i) and to the west. Eastward it broadens as the limestone cuesta wedges
out, until south of Bayamon it is represented by a broad expanse of rolling
country, extending southward past Guaynabo and merging gradually
into the interior mountain. Its topographic development is more than
that of the interior mountains, that is, its stage of development is late
maturity, with mature streams crossing it in broad and often meandering
courses.
THE INTEI1IOR MOUNTAINOUS I:EGIOx

South of the inner lowlands the topography of the San Juan District
is typically mountainous. On approaching the island the rugged serrate


















Fi<. i u.-Iufc of vnuesti and inner lou'lanu near Coro.:ul
Showing relation of the C(orozal limestone in the older series.

character of the skyline can be distinguished soon after its outline rises
above the horizon. Passing through the mountainous area one is im-
pressed with the extreme degree of the dissection. Almost no level
ground is to be seen. Throughout most of the mountainous region of
the San Juan District the country has attained the maximum ruggedness
characteristic of a fully mature stage in topographic development. Other
portions show more or less advanced stages, but the topography of the
region as a whole is mature. Between the numerous streams the divides
form a network of sharp ridges, so steep that the natives have appropri-
ately named them cuchillas or knives. These are likewise closely and
deeply corrugated bv ravines and gullies, the whole region assuming very
much the appearance of a wrinkled handkerchief, a simile said to have






SE.IMMES, GEOLOGY OF THE SAN JUAN DISTRICT


been used by Columbus in describing the topography of the Antilles to
Queen Isabella.
The drainage of the mountainous area as a whole is dendritic in pat-
tern. The adjustment of the stream courses to underground structure is
very rare. A few cases were noted where some adjustment had taken
place, but nowhere was it conspicuous. Although the mountainous region
is underlain by a complex of formations of much variety, yet there is little
or no evidence of this in the topography. Occasionally a dike could be
traced for a short distance as a poorly discernible ridge, and it was no-


FIG. 7.-The inner lowland and the inface of the cuesta at Corozal
The almost vertical inface is locally known as "The Walls." Photographed by
H. E. Crampton.

ticed that the highest points were usually underlain by highly indurated
siliceous tuffs, as, for example, Torrecillo, northwest of Barranquitas, and
Helechal, some distance'south of Bayamon.

INCISED MEANDERS

Some of the streams have developed deeply intrenched meanders as
they pass out from the mountainous oldland. The best example is that






SCIENTIFIC SURVEY OF PORTO RICO


of the Orocovis River south of Morovis. In this region the stream shows
larger swinging meanders entrenched to the depth of 800-1000 feet (see
topographic map). The origin of these incised meanders and their re-
striction to the inner lowland and the adjoining mountainous region is
comparatively simple. At the time peneplanation reached its maximum
the streams passing out over the flat oldland surface acquired meandering
courses on nearing the sea, which then reached inland as far as the present
mountainous region. Subsequent gradual uplift has slowly entrenched
these meanders as the coastal plain emerged and assumed the present
position.
STREAM CAPTURE

Although there are undoubted cases of stream capture in other portions
of the island, no striking examples are to be found in the San Juan dis-
trict. Near Comerio, however, there is a wind gap that was evidently at
one time occupied by a tributary of the Plata River. This tributary, now
represented by Mora Creek, formerly flowed from the southeast through
the wind gap and occupied the upper valley of the Arroyato River (see
topographic map). Subsequently another tributary of the Plata, work-
ing down from the north, intercepted the larger stream, forming the
present Arroyato River, which joins the Plata at K 23 on the Comerio
Road. Both streams are now completely readjusted, but it is still evident
that Mora Creek occupies a much larger valley than would be expected,
considering its volume.
PENEPLANE
It has already been stated that the interior mountainous region has
been partially truncated by peneplanation. In the San Juan district
practically nothing of the upland surface remains. In some portions,
however, its present elevation is well marked by the even character of the
crest line. In the southern part of the district it can be traced at an
elevation of 2000-2200 feet or less towards the eastern corner. From this
elevation it slopes to the edge of the mountainous region, where it is
approximately 1200 feet. At this point the mountainous region breaks
sharply down to the inner lowland, forming a well-defined escarpment.
The escarpment can readily be traced on the topographic map, especially
in the central portion of the district just north of Naranjito. The
peneplane level, occasionally discernible among the crests south of the
escarpment, is not again recognizable until it plunges below the lime-
stone strata of the coastal plain. At Corozal its elevation as it passes
under the coastal plain is about 300 feet. From this it would appear that
the peneplane surface was slightly upwarped along the southern edge of






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


the coastal plain; but these irregularities in the present surface of the
peneplane may well have been original. The peneplanation was by no
means perfect. In some portions of the district no indications of its *
former existence are to be found. Between Barranquitas and Barros
much of the country rises far above the peneplane level. Such an area
can be regarded as a monadnock group. On the other hand, near Cidra,
in the southeastern corner of the district, the general elevation of the
country lies below the peneplane level. In this area the peneplane either
was less uplifted originally or its original elevation has been entirely
destroyed by subsequent erosion or warping. Indeed the present irregu-
larities of the skyline and the variations in the general elevation of the
upland surface are of such magnitude that the writer is forced to admit
that the area may never have been peneplaned, and that the apparently
even crestline that can be seen in parts of the district may be partly the
result of an optical delusion on the part of the observer, and in part a
real evenness, not necessarily caused by peneplanation, but due to weather-
ing in situ and the formation of a thick soil mantle, which would tend to
destroy the minor irregularities in the skyline.

ROCK FORMATIONS

The formational units of Porto Rico may be primarily divided into
those that form the coastal plain and those that lie beneath it. These
have been named the Younger and Older Series by Berkey. They are
everywhere separated by a marked unconformity, so that they are entirely
distinct structurally as well as lithologically and faunally. In the San
Juan district the younger series includes all the formations encountered
in passing from the coast to the inner lowland. These are, in the order
of their respective ages, as follows:
1. The modern sand dunes of the coast and the adjoining playa plains.
2. The Pleistocene (?) San Juan lime sand.
3. The Oligocene limestones and shaly limestones (Arecibo formation).
4. The San Sebastian shales, developed near Lares, but not in the San
Juan district. Probably Upper Eocene or lower Oligocene in age.
A detailed subdivision of the younger series will, no doubt, be possible
when its fauna has been studied with care. The Arecibo formation is
essentially a unit, though somewhat complex, and showing considerable
variation laterally. The lower shale member is evidently a separate for-
mational unit, merely representing a local development, which is not
found in the San Juan district. A subdivision of the underlying older
series, however, is quite another matter. The complexity of the forma-







SCIENTIFIC SURVEY OF PORTO RICO


tions and their lateral variations are so great that nothing more than a
general subdivision will probably ever be possible. The local development
" of many of the formations, together with their disturbance and alteration
by igneous agencies, make even the most generalized correlations largely
conjectural.
YOUNGER SERIES

RECENT DEPOSITS

Unconsolidfated Lime Sand.-The modern dune sand, the occurrence
of which as a narrow belt along the coast has been mentioned, consists of
shell and coral fragments with occasional grains of quartz. Its constit-
uents are almost identical with those of the associated consolidated de-
posits-a fact which leads one to infer that this sand is but a modern
representative of the older consolidated San Juan sand. The belt of
modern sand dunes is usually narrow, its greatest elevation of 8-10 feet
being reached within 75 yards of the coast; from this point it slopes
gradually inland and merges into the adjoining playa plains.
Playa Plains.-The playa plains, which occur as an east-west belt
across the district, lying immediately south of the dune district, are
partly fluvial and partly marine in origin. They are composed of ma-
terial derived from the interior, of the island through the ordinary proc-
esses of stream erosion and deposited by the streams passing out of the
mountainous interior upon the comparatively level coastal plain.
These streams are now slightly entrenched, giving occasional exposures
of a few feet in thickness. Such exposures show typical torrential bed-
ding and deposits of fluvial character; however, marine shells are occa-
sionally found upon the playas. These can be accounted for through
temporary inundations. During the hurricane of August, 1898, practi-
cally the entire expanse of the playa plains of the district was flooded.
At this time the simultaneous occurrence of heavy rains and strong north-
east gale caused the inundation. It is evident, moreover, that the sea has
not had direct access to the plains since the consolidation of the San Juan
sand. Indeed the formation of an offshore sand bar in Pleistocene time
may have been the controlling factor in the formation of the playa plains.
In the sheltered lagoons formed behind the bar the alluvial material may
have been deposited and subsequently uplifted with the San Juan for-
mation. This would account for the typical fluvial appearance of the
deposits, as well as their occasional evidences of the action of marine
agencies.






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


PLEISTOCENE (?) DEPOSITS

San Juan Formalioin.-This name has been given'by Berkey to desig-
nate the formation upon which the city of San Juan is built. It here
forms a striking promontory that extends as a spit several miles out from
the adjoining mainland and at its extremity attains an altitude of almost
a hundred feet. This maximum elevation is crowned by the picturesque
El Morro, the massive walls of the castle being built upon the very edge
of the steep seaward face of the promontory (Fig. 8).














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

.^. :i : -2 ','; --
~~~~;;:':,~~,x5w -


FIG. 8.-Morro castle and a portion of the city of San Juan, built upon a promontory
composed of the San Juan consolidated lime sand
Photographed by H. E. Crampton.

The San Juan formation as seen at this point is a very porous, medium
to coarse grained sandstone composed of minute fragments of shells and
well-rounded quartz grains, bound together with calcareous cement (Fig.
9). The calcareous grains predominate and in some localities form prac-
tically the whole of the rock, so that the formation is essentially a lime
sand. In other localities the quartz content is higher, but in no ease did






SCIENTIFIC SURVEY OF PORTO RICO


it constitute as much as fifty per cent of the rock. In the vicinity of
San Juan two distinct types of the formation were found. First, that
which occurs well above sea level near Morro Castle. At this point the
lime sand is exceedingly porous. The cementing material does not fill all
the interstices, the grains being cemented only at the actual points of
contact. This mode of cementation has created a rock in which the vol-
ume of the pore spaces is about 20-30 per cent of the entire rock. The
second variety of lime sand was taken from a quarry near Borinquen
Park, about three miles east of the first locality. The material from this


FIG. 9.-San Juan consolidated lime sand


quarry, coming from below sea level, is dense and closely cemented, being
locally used as a building stone. It is evident, therefore, that the degree
of cementation is dependent upon the length of time the formation has
been exposed to the free circulation of calcareous solutions. The upper
portion of the formation, being re-elevated after only a short period of
submergence, now shows only the initial stages of consolidation, while
the lower portion, still submerged, has consolidated into a dense and very
durable rock. The question then arises whether the porosity of the upper
portion of the formation is not due to subsequent leaching rather than to






SEMMES, GEOLOGY OF THE SAN JUAS DISTRICT


initial poor cementation. A microscopic examination of the lime sand,
however, shows no evidence of any appreciable amount of subsequent
leaching, for a subsequent dissolution would have affected the readily
dissolved calcareous fragments as well as the calcareous binding material.
West of the extremity of the San Juan spit, the continuation of the
San Juan formation can be traced by a series of islands, the largest of
which, Cabras, rises just in the center of the harbor entrance. Small
fringing islands of this type, wholly composed of the consolidated lime
sand, can be traced along the coast far beyond the western boundary of
the district. All the headlands and points that rise above the general
level of the coast are likewise composed of this material. Throughout
this entire area one finds that the lime sand is characterized by very pro-
nounced cross-bedding of an eolian type. This cross-bedding character
is well shown in the face of the promontory below Morro Castle, as well
as in numerous other localities to the west. The eolian type of cross-
bedding, together with the fact that the San Juan lime sand is almost
everywhere associated with modern dune sands, indicates that it was for-
merly of similar character to the modern dune sands, merely having been
subsequently consolidated.
Age.-The exact age of the San Juan formation is uncertain. None
of the modern sand dunes shows even the initial stages of consolidation.
Indeed, much of the material composing them appears to have been de-
rived from the destruction of the San Juan formation. Moreover, evi-
dence of the final oscillations of the coast is clearly seen in the cliffs of
this formation. Large portions of the consolidated sand still remain be-
neath the sea, so it is evident that the formation was formed before these
coastal movements occurred. The formation contains no conclusive
faunal evidence of its age. The shell fragments are so minute that none
could be determined. Occasional foraminifera occur, but these proved
valueless as an index of the age. The formation has been regarded by
Berkey as being of Pleistocene age. This conclusion is derived from its
general appearance, situation, and close association with modern dune
sands of similar composition. Indeed, the upper portion of the formation
might possibly be post-Pleistocene in age, though as yet no direct evidence
has been found to support this view.

TERTIARY LIMESTONE SERIES

Arecibo Formation.-Underlying the San Juan formation, but sepa-
rated from it by an extensive erosion interval, is the Arecibo formation,
so called by Berkey from its type locality, the town of Arecibo. The






SCIENTIFIC SURVEY OF PORTO RICO


formation consists of a series of reef and shell limestones alternating
with layers of calcareous shale. Towards the base of the series the shales
become predominant. In the vicinity of San Sebastian, in the north-
western part of the island, there is locally developed beneath the Arecibo
formation a series of dark, lignitic shales, quite different from the yellow-
ish, calcareous shales that occur in the base of the Arecibo in the San
Juan district. No equivalent of these shales could be found in the San
Juan district. It is the writer's opinion that the San Sebastian shale is
merely a local development, which is not equivalent to the Arecibo, prob-
ably being much younger in age (see discussion of age of younger series).
At Juana Diaz, on the southern coast, a similar shale member is to be
seen. At this point, however, the beds dip as steeply as 36, according to
Berkey (1915, p. 13), which would seem to indicate either that they
underlie the Arecibo unconformably or that they have been tilted by later
movements. In the San Juan district the Arecibo formation dips 60 to
the north. Its maximum thickness is not developed, but towards the west-
ern boundary it is at least 500 feet thick. It extends entirely across the
district, forming a belt several miles broad on the west, but wedging to a
mere line of isolated hills to the east. Much of its former extent has, no
doubt, been destroyed by erosion, but it still covers an area many times
larger than that occupied by any other single formation.
Along the valley of the Plata, where a good section of the formation
can be seen, it consists, in the upper part, of a series of white reef and
shell limestones of medium density, with occasional layers of yellow, im-
pure, shaly limestone. The shaly layers increase in number and become
essentially light-colored calcareous shales towards the base. The basal
shales are of this character in this locality, and are entirely unlike the
dark lignitic shales of the San Sebastian region. East of the Plata Val-
ley the base of the formation is not exposed, but there is no evidence that
its basal shales differ in composition from those seen in the Plata Valley.
The Arecibo formation as a whole appears to be a structural unit. Al-
though it is not uniform in character and composition throughout its
vertical extent, yet there are no unconformities or erosional breaks, nor
are the variations in general appearance distinct enough to warrant a
closer subdivision. Since the formation varies laterally as well as verti-
cally, no specified section would hold true for the entire district, yet as
one passes over the formation certain general changes are observable,
which are continuous throughout the district. The upper part of the
series, as has been noted, is characterized by the predominance of a white
or pinkish, chalky, poorly bedded limestone, partly composed of shells.
Locally this limestone assumes a hard, dense, and massive character, being






LSEMMlES. GEOLOGY OF THE S. I JU,/[. DISTRICT


quite brittle and ringing when struck by a hammer. It is in this phase
of the limestone that the caves, which are so numerous through the lime-
stone belt, are developed. The absence of shaly material makes it readily
soluble by underground waters. Locally the upper purer limestones show
the characteristics and content of a coral reef. Near Morovis the lime-
stone has such an appearance. Close examination shows that it is com-
posed almost entirely of corals and is even more irregular and massive in
structure than the white, chalky limestone. On passing downward
through the series, one notes that the bedding becomes better defined and
occasional layers of shaly limestone are to be seen. Thin, shaly layers
occur even in the highest beds, but they do not become noticeable until
better developed towards the base. In the lower portion of the formation
the shaly layers become predominant and the massive, purer limestone is
entirely lacking. The base of the series is entirely composed of calcareous
shales, which are porous, coarse-grained, and well bedded. Such a suc-
cession would naturally be expected under the conditions which seem to
have prevailed. After submergence the coarser, shaly material would be
laid down first near shore, followed by finer shales and shaly limestones,
with occasional beds of purer limestone marking temporary periods of
clearer waters. Finally, the upper limestone beds and coral reefs would
be deposited when clear-water conditions had been established.
Age.-The fauna of the Arecibo formation has never been carefully
studied. The fossils are as a rule poorly preserved, nothing remaining
but the internal molds which are rarely determinable. The older series
was formerly thought to be Paleozoic, but the limestones of the coastal
plain were early recognized to be of Tertiary age.
In 1868 Cleve (1883, p. 189) first approximately determined the true-
age of the older and younger series. He stated "that the island of Porto
Rico consists largely of very thick, almost undisturbed limestone beds of
Miocene age; but in the interior of the country, around Utuado, rocks
similar to the Cretaceous of the Virgin Islands are met with."
In 1898, after a reconnaissance of the island, Hill concluded that the
ages of the Tertiary formation were as follows (Hill, 1899a, p. 109) :
The upper part of the Pepino hills is made up at their surface of a rather
hard lime marl full of coralheads, with occasional indurated strata of firm
white porous limestones. These rocks (the Pepino formation) are of Miocene
age, as determined by Mr. T. Wayland Vaughan from the corals collected by
me, similar to certain rocks of Antigua hitherto not known in the geologic
sequence of the Greater Antilles. Their tilted position, standing at 1200 feet
where they meet the older volcanic mass, testifies to the great geologic move-
ments which have taken place in the West Indies in late geologic time.
Below this limestone, which is at least 100 feet thick, are fossiliferous






SCIENTIFIC SURVEY OF PORTO RICO


greensand marls of undetermined age (Eocene or Oligocene), which in turn
rest upon a great thickness of thinly stratified reddish lignitic clays and sands
of Eocene age (the Richmond formation) which outcrop near San Sebastian,
Guatemala and Mocha on the western end of the island, and near Carolina on
the northeast coast.

The pepino formation is in part equivalent to the Arecibo. No green-
sand marls were found in the San Juan district, but these probably cor-
respond to the lower shaly Arecibo. The lignitic clays and sands are
what has since been termed the San Sebastian shales. In the older lime-
stone beds of the interior, Hill mentions the occurrence of a Cretaceous
fossil (Rudistes), thus confirming Cleve's earlier supposition that the
older series was of Cretaceous age.
Through the aid of Dr. William H. Dall, the material which the writer
collected in the San Juan district, as well as some of that collected by
Dr. Berkey the summer previous, was roughly determined. The material
consisted almost entirely of internal molds, some of them being so poorly
preserved that even the genus could not be determined. In only one case
was a species positively identified, but the accumulation of evidence from
quite a number of genera, as well as the general appearance of the forms,
made the determination of the ages of the respective members of this
series fairly reliable. The forms studied should be divided, according to
the locality from which they were collected, into three groups:
1. Collected by the writer from the base of the Arecibo, near Toa Alta
and from the upper part of the same formation near Vega Baja. The
forms from these two localities are so much alike that no attempt was
made to distinguish between them.
2. Collected by Berkey from the Ponce formation just northwest of
Ponce, on the southern coast.
3. Collected by Berkey from the San Sebastian shales (upper portion)
near Lares.
The fauna from the Arecibo formation consisted of pelecypods, gastro-
pods, and corals, of which the following genera were identified:

Metris trinitaria (Dall), an index of the Bowden formation, upper
Oligocene of Jamaica, found throughout the Arecibo formation
in the San Juan district (P1. II, la, lb).
Pecten sp. (P1. II, 5).
Mittha sp. (P1. II, 2a, 2b).
Haminea sp. Very abundant (P1. II, 3a, 3b).
Cyprea sp. Abundant (P1. II, 4).
Amauropsis sp.
Conus sp. (P1. II, 6).
Ampulina sp.






SCIENTIFIC SURVEY OF PORTO RICO AND THE VIRGIN ISLANDS


FAUNA OF THE ARECIBO FORMATION


VOLUME I, PLATE II








SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


Strombus sp. (P1. II, 7).
Orbicella aff. cavernosa (Linn.).
Fungid coral, not determinable, but similar to those found in the
Oligocene of Antigua, Anguilla and Cuba (P1. II, 8).
Venus sp. (P1. II, 9).
Although only the first of these forms may be regarded as even an ap-
proximate index, yet the fauna as a whole from its general appearance is
regarded by Dall as Upper Oligocene. The occurrence of Metris trini-
taria at the base as well as in the upper portion of the formation would
indicate that the entire Arecibo formation as developed in the San Juan
district is Upper Oligocene.
The second group of fossils was collected from the Ponce formation
on the southern coast. This material was also so poorly preserved that
the genus could not always be determined. This group includes:
Lucina sp.
Lucina sp. small specimens much like forms from the Chatta-
hoochee (Middle Oligocene of the Gulf Coast). (P1. II, 11.)
Pecten sp. like P. thetidis (Sowerby). (P1. II, 10.)
Fasciolaria (P1. II, 12).
Amauropsis ocalana (?) similar forms in Ocala limestone, Vicks-
burg formation, lower Oligocene of the Gulf Coast (P1. II, 13).
As the relative position of the specimens in the Ponce formation was
unknown, the geologic range could not be determined. It is probable
that the specimens were collected from the lower portions of the forma-
tion. These forms at least indicate that the formation is lower to middle
Oligocene and that it is in part practically equivalent to the Arecibo
formation of the northern coast.
The third group was collected near Lares from the first shell bed of the
San Sebastian shales. This shell bed is located in the upper portion of
the shales. It contained:
Venericardia sp.
Ostrea sp.
Conus sp.
None of the forms of this group could be used as an index of its age,
but from their general appearance Dall concluded that they were lower
Oligocene or possibly upper Eocene. Therefore, since these forms were
collected from the upper part of the San Sebastian shales, it is probable
that these shales are, in greater part, of lower Oligocene age.






SCIENTIFICC SURIHEY OF' POIRTO hICO


OLDER SERIES

Underlying the Arecibo formation is a complex series of sediments of
both aqueous and volcanic origin, intimately associated with numerous
igneous rocks. It is this complex that Berkey has termed the older series,
since it is unconformably separated from all the formations that have
been described and is in appearance much older than these strata. The
older series outcrops in all parts of the San Juan district south of the
inner lowland, an areal extent of roughly two-thirds that of the whole
district. The contrast in appearance of the two series is strikingly evi-
dent as one passes abruptly from the nearly horizontal beds of the Arecibo
formation, showing comparatively simple structure and only minor varia-
tions in lateral and vertical extent, to the steeply tilted beds of the older
series, where sediments and volcanics of all varieties are dissected by an
infinite number of igneous rocks, both intrusive and extrusive, ranging
in size from small stringers to major intrusives of many square miles in
extent. The subdivision of this intricate series is an exceedingly diffi-
cult task. Many of the formations are of such insignificant thickness
that they would necessarily have to be grouped to appear on a geologic
map of moderate scale. Such grouping would be on an arbitrary basis,
for the formations involved do not naturally fall into larger groups. Con-
sequently, only generalized subdivisions have been made on the geologic
map. the detail in structure and composition being shown on the accom-
panying cross-sections (P1. IV). The rocks of the older series include:
Sediments essentially marine.
Sediments essentially volcanic.
Igneous rocks.
Marine sediments which have been wholly derived from the land
through the ordinary processes of erosion and stream transportation are
almost unknown in the older series of the district. Practically all of the
marine sediments contain volcanic material. Some are composed wholly
of volcanic material cemented by carbonates. These sediments have been
well assorted and bedded, and often contain marine organisms. The
marine sediments-that is, those that show more strongly the character-
istics of marine rather than those of volcanic sediments-include lime-
stones, shales, sandstones, and conglomerates. These form only a small
percentage of the total thickness of the older series. Occasionally shale
and conglomerate beds of 100 yards or more in thickness are to be seen,
but the limestone beds are usually less than 100 feet in thickness. With
the marine sediments, the extended series of volcanic ashes and tuffs is






SElHUIES, GEOLOGY OF THE SAN JUAX DISTRICT


intimately associated. The ash beds are usually well bedded and a cer-
tain degree of assortment can be seen in the material that has been either
ejected directly into the sea or subsequently reworked. The tuffs are in
greater part massive; others are well bedded and show evidence of marine
assortment. The ejectmental material is of all sizes from fine ashes com-
posed of grains less than .01 mm. in diameter to coarse tuffs containing
fragments a foot or more in diameter. A continuous series is formed
between these two extremes, so that the distinction between ashes and
tuffs is merely an arbitrary one based on size of grain. The complexity
of the older series is primarily due to large number of igneous rocks that
occur closely associated with the sediments. But for the fact that the
older series has been so thoroughly dissected by igneous intrusions, its
correlation and geologic mapping would be a comparatively simple matter.
As it is, however, correct mapping can only be done after detailed study
of areas where outcrops are numerous.

RESIDUAL SOIL AND DEPTH OF DECAY

None of the rock formations of the district is free from surface altera-
tion and decay. The older series, longer exposed to the weathering agen-
cies, has been altered to considerable depths and is now almost everywhere
covered by a residual soil locally reaching many feet in thickness. Its
greatest development is in the inner lowland, where the relief is moderate
and stream transportation slower. In this region it is often possible to
make excavations of 15 feet in depth without encountering bed-rock.
Consequently, in this lowland region outcrops are uncommon and the
few that occur are often so highly decomposed that the exact nature of
the original rock remains uncertain. Even in the mountainous region
where outcrops are plentiful many of them are badly decomposed. The
stream beds and recent road cuts afford the best opportunities for the
study of extended surfaces of fresh rock. Even though the temperature
changes are small, exfoliation is common, producing peculiar concentric
arrangements that are strikingly prominent in many of the shales, vol-
canics, and igneous rocks (Fig. 10).
One of the most striking characteristics of the topography of the old-
land is the remarkable stability of the soil mantle. As has already been
noted, the soil mantle, remaining practically in situ, might well be re-
garded as an important factor in determining the regularity of the crest
line, in many places giving rise to well rounded hill summits that have
been regarded as good evidences of peneplanation. Hillsides that slope
as steeply as 400 can be cultivated with little or no loss of soil. Knife-
edge divides with 40 slopes on either side retain their soil to the very






SCIENTIFIC SURVEY OF PORTO RICO


summit and can be ploughed without fear of washing, even after the
heaviest rain. Berkey ascribes the causes of the stability of the soil
mantle to three factors (1915, p. 35); first, the dense cover of vegeta-
tion; second, the small range in daily and annual temperature; and,
third, the low content of inert or refractory material, such as quartz, in
the rocks whose destruction has furnished the soil. Neither of the first
two of these factors would explain the stability of the soil when unpro-
tected, subsequent to cultivation. The third, however, is of vital signifi-
cance. The igneous rocks as well as the volcanic material are predomi-
nantly of andesitic composition, contain large amounts of feldspars and
ferromagnesian minerals, and little or no quartz. This on weathering
would give rise to an argillaceous soil, essentially a clay, which would be
very impervious. A soil of this character would not permit the free cir-












FIG. 10.-Exfoliation producing concentric arrangements in ash beds south of Bayamon

culation of ground water and would not readily be removed and trans-
ported by the surface run-off.
The presence of a widespread mantle of such impervious soil might
well explain the peculiar absence of meteoric springs throughout the
island. True springs are almost unknown in the San Juan district. The
so-called springs are merely rivulets of surface water, hidden in their
upper courses by dense vegetation. On the southern coast several thermal
springs have been found, but it is now believed that these derive their
water from magmatic rather than meteoric sources. Whether the imper-
viousness of the soil is the sole cause of the absence of springs is not
yet determined, but it is at least a possible explanation of an otherwise
unexplained phenomenon.
COROZAL LIMESTONE

Just southwest of the town of Corozal, as one crosses the river at the
ford on the road to Barros, can be seen an outcrop of limestone. Close






SEMMES, GEOLOGY OF THE SAX JUAX DISTRICT


inspection shows that it is a dense, grayish, brittle rock, comparatively
easily dissolved, as is indicated by the few outcrops that rise above the
residual soil. Moreover, the limestone is strongly fragmental. It con-
sists of angular limestone fragments, usually less than 2 inches in diame-
ter, and occasional particles of feldspars or ferromagnesian minerals,
thoroughly bound together by a calcareous cement. The strike of the
bed is roughly N. W.-S. E. Its dip could not be accurately measured, as
none of its structural relations could be determined from the few limited
outcrops. The dip, however, was judged to be -about 15-'0 X. E.,
since it was apparently conformable with the neighboring types which
show approximately that dip. The thickness of the bed is also uncer-
tain, but it is not believed to exceed 100 feet. This limestone can be
followed from a mile southeast of Corozal until it plunges below the
Tertiary strata to the northwest.
The origin of the fragmental structure, which is characteristic of this
limestone, is quite puzzling. An examination of the rock under the micro-
scope shows that the fragments are angular, indicating that they have
undergone little or no transportation. Moreover, particles of feldspars
and ferromagnesian minerals, which are perfectly fresh, occur with the
limestone fragments, so one must also infer that consolidation took
place very soon after deposition. It is the writer's belief that the frag-
mental structure in the limestone is directly due to volcanic agencies,
that is, brecciation was caused by volcanic eruptions which probably took
place as submarine eruptions through a limestone bed. The shattered
limestone was then deposited with intermingled feldspars and ferro-
magnesian minerals, and the whole cemented by further precipitation
of CaCo, from the sea. The presence of marine organisms in the inter-
stitial CaCo, makes it evident that the breccia was at least reworked by
the sea, and the angularity of the fragments would indicate that the
ejectmental material was thrown directly into the sea, or that the erup-
tion itself was submarine. Such an explanation for the origin of the
Corozal limestone breccia is not without its defects. The breccia is of
considerable extent, yet no traces of the older limestone beds, which
were shattered to form the breccia, could be found. It is possible that
the locality just south of Corozal, where the brecciated structure is best
developed, marked the locus of the eruption, and that in other unex-
posed portions of the limestone bed the fragmental structure might be
lacking. In this case the fragmental material would be practically in situ,
forming only a brecciated phase in the original limestone bed. If such
were the correct interpretation, however, one would expect the breccia
to contain a large amount of volcanic material. In reality it is comn-






SCIENTIFIC SURVEY OF PORTO RICO


posed almost wholly of fragments of pure limestone, but it occurs closely
associated with an extended series of bedded tuffs, indicative that the
deposition of the limestone fragments occurred first, followed by that of
the tuffaceous material. The Corozal limestone breccia contains abun-
dant foraminifera, which will be described with the other foraminiferal
beds under the discussion of the age of the older series.

LA AMUDA LIMESTONE

Berkey (1915, p. 22) has described the occurrence of this limestone
just east of the San Juan district near La Muda. The same limestone
was found by the writer just south of the town of Guaynabo, where
it is exposed in a small quarry as one leaves the town by the trail to
La Muda. Its strike is N. 48 W. and dip 300 S. W. The thickness
of the bed was roughly determined as about 40 feet at this point. To
the east, according to Berkey, its thickness materially increases. The
formation could be traced along its strike for a distance of about 5 miles.
Northwest of Guaynabo it finally disappeared under the heavy residual
soil of the inner lowland. To the southeast no outcrops could be found
beyond K 26 on the military road. It possibly continues farther, as no
careful search was made for it east of the San Juan district.
The Muda limestone shows a fragmental structure similar to that
seen in the limestone near Corozal. It is not so striking, however, as
that seen in the Corozal limestone, because of closer cementation of the
fragmental material which gives rise to a dense, grayish-white limestone,
in which the fragmental structure is only betrayed by its somewhat
mottled appearance. Microscopic examination of the limestone shows
the fragmental structure well defined, as well as small amounts of
tuffaceous material and foraminifera scattered throughout the cementing
material. The limestone, on the whole, is very similar to the one at
Corozal and both, no doubt, have originated in the same manner. The
Muda limestone is likewise closely associated with tuffaceous formations.

AGUAS BUENAS LIMESTONE

Two miles south of the town of Aguas Buenas are the great caves of
Porto Rico (Dinwiddie, 1899). These are developed along the strike of
a limestone bed about 100 feet in thickness. The strike of this bed is
roughly N. 500 E. and dip 200 N. W. In the valley of the Bayamon
River, to the west of the caves, two limestone beds are to be seen, which
are apparently conformable. To the southwest the same beds may again
be seen in the valley of the Arroyato River, so the strike was closely de-






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


termined by plotting the outcrops. The strike of these limestone beds is
practically normal to the general strike of the formations throughout
most of the district. This pronounced variance in strike indicates that
the Aguas Buenas limestone beds are unconformably related to the other
beds of the older series. Above the thickest of these two limestones, in
which the caves occur, is a coarse conglomeratic tuff which contains
fragments of the older limestone. Beneath the limestone are other tuffs,
or rather tuffaceous limestones, which resemble Muda limestone, except
that they contain larger amounts of tuffaceous material. No forami-
nifera or other organisms by which the age of these formations could be
determined were found, but from the structural relations it would be
inferred that the Aguas Buenas limestones lie below and are uncon-
formably separated from the limestones previously described. The struc-
ture of this area, however, was not satisfactorily worked out.. If the
area to the east of the San Juan district is eventually mapped, the rela-
tionship of the Aguas Buenas to the Guaynabo limestone may be made
clear, for the former should be truncated by the latter in the valley of
the Loisa River north of Caguas, provided each continues that distance
along its line of strike.
LIMESTONE AT K 29

On the Comerio road, at K 29, a thin bed of impure limestone approxi-
mately 5 feet thick occurs interbedded with tuffs and ashes. The lime-
stone contains a large amount of tuffaceous material, being in reality
merely a tuff that has been highly carbonated. Although insignificant in
size, this limestone bed serves as a typical illustration of the frequent
occurrence of limestones partly of volcanic origin, occurring intimately
associated with volcanic sediments. Impure limestones were also noted
on the trail from Barros to Corozal and in the lower valley of the Bar-
rancas River.
JUAN ASCENCIO CHERT BEDS
Northwest of the Aguas Buenas limestones in the barrio of Juan Ascen-
cio there is a chert bed of considerable thickness, the strike of which is
approximately parallel to the limestone beds (N. 50-60 E.). The dip
and thickness of the bed could not be determined from the few scattered
outcrops that were found. This chert is a hard, dense, grayish white
rock, which consists wholly of an exceedingly fine-grained aggregate of
silica, showing an absence of any bedding. Just north of the ridge on
which this chert outcrops a siliceous rock showing fine crumpling was
discovered. A microscopical examination of this rock proved that it was






SCIENTIFIC S'UREY O'F PORTO RICO


essentially the same composition as the chert to the south, except that it
was quite highly colored in the weathered outcrop by iron oxides. The
first of these chert beds probably originated through the silicification of
some sedimentary bed, either a limestone or a calcareous shale. The
second chert is either a phase of the same formation or a different one,
which slumped before solidification. Subsequently the formation was
silicified, the intense crumpling due to solifluction being preserved in the
secondarily formed chert. Still later, the iron content of the chert has


Fli. 11.- ('mnplini dliec to soliflutiom in the luain Ax'-euio chert


been oxidized to linionite and hematite which occur in bands following
the crumples, thus serving to emphasize the intensity of the crumpled
structure (Fig. 11).
COIiOZAL .l.\SI'ER BED)

A bed 10 feet thick of Jasper was found in the Corozal River two miles
above the town. Only one outcrop was located, from which it appeared
that the bed was conformable with the surrounding formations. From
the amount of jasper float found in the streams it was inferred that the
bed was of considerable extent.






SEMMES. GEOLOGY OF THE SAN JUAIN DISTRICT


SALES

One might conclude from a hurried examination of the island that
shales are commonly encountered in great extent, but in reality typical
shales are only occasionally found. Many of the formations that have
been recorded as shales are well-bedded ashes. Indeed, such a gradation
exists between the shales and ashes that in some cases one cannot be sure
in just which class a given formation belongs. All the shales found in
the San Juan district contain some volcanic material. Many of them are
composed almost wholly of ashy material. The intimate association of
shales and ashes would lead one to infer that either might have been de-
posited under essentially the same physical conditions, that is, the change
from shale to ash accumulation need not mark any noticeable change in
the climatic or physical conditions under which they were deposited.
Many of the shales are calcareous. Some contain such large quantities
of calcareous material that they have formerly been regarded as lime-
stones. Foraminiferal shells occur in some of the shales and occasionally
a shale is almost entirely composed of these minute organisms.
The largest accumulation of shale in the district is near the town of
Barranquitas, where the aggregate thickness of the shale series must be
several thousand feet. These sales are very calcareous, so much so that
they were originally regarded as limestones by Hill (1899, p. 108).
They represent the calcareous extreme in the shale series, but strictly
speaking they should not be termed limestone. The shales extend from
about K 40 on the Comerio-Barranquitas road to a point a good distance
south of Barranquitas on the road to Aibonito. They form the crest of
the island in this vicinity, extending down some distance on either side
of the principal divide. The shales contain a good deal of ashy and
tuffaceous material and are locally interbedded with ashes and tuffs. The
strike of this series is roughly N. W.-S. E., which indicates that it is now
simply a phase in the extended series of tuffs and ashes that form the
basis of the island.
On the Bayamon-Comerio road, at K 5, there is an outcrop of well
stratified shales dipping 450 to the S. W. and apparently striking parallel
to the general N. W.-S. E. strike of the region. Fresh specimens can be
obtained in the road cut, which when examined under the microscope
show an exceedingly fine-grained texture. The material is in such a fine
state of subdivision that nothing can be recognized except minute grains
of quartz. The rock is very dense, with little or no pore space. Faint
banding can be discerned in the hand specimen, but no orientation or
bedding is visible-under the microscope. Berkey has suggested that these






SCIENTIFIC SURVEY OF PORTO RICO


shales may be equivalent to those exposed at Mayaguez and Fajardo. The
material from Fajardo, however, is quite different in appearance. It is a
light, porous shale, the porosity of which is due to numerous minute cavi-
ties which apparently were previously occupied by foraminiferal shells.
The leaching of these calcareous organisms has produced a porous struc-
ture which is not to be found in the dense shales exposed at K 5. It is
possible that this shale may be equivalent to those exposed at Fajardo and
Mayaguez, but the writer does not believe that correlation of such widely
separated formations can yet be made with any degree of certainty.


FIG. 12.-Typical foraniniferal shale


In a road metal quarry just off the Comerio road, at K 8, there is an
exposure of sandstone with included fragments of shale. The shale is
dark, dense, and very fine-textured. Its occurrence as angular fragments
in the sandstone leads one to infer that it has undergone much the same
process of formation as the Corozal limestone breccia. This shale occur-
rence is insignificant in size, but it is of great interest in that it contains
numerous foraminifera, which not only indicate the conditions under
which it was deposited, but also give an idea of its age (Fig. 12). The
foraminifera will be described in the discussion of the age of the older
series.






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


A somewhat extensive series of shales was discovered in barrio Pifias,
southeast of Toa Alta. These shales are well stratified. Their strike is
about N. W.-S. E. and dip 200 S.W. On the weathered outcrop the
shale appears as a light, porous rock, which is composed of an aggregate
of very fine, shaly material with numerous minute cavities, some of which
are now filled with quartz and zeolites. These cavities were in part at
least originally occupied by foraminiferal shells, which have been leached
out and the cavities in part refilled with quartz and zeolites. A large
percentage of them, however, is still empty and as a result the shale is
exceptionally porous. This shale is very similar to a specimen collected
by Berkey from Fajardo, but as yet too little is known about either of
the formations to attempt any correlation.
Another shale of similar character occurs in barrio Cuchilla, where the
Morovis trail crosses the Unibon River. The shale strikes N. W.-S. E.
and dips 300 to the N. E. It contains a great deal of ashy material, to-
gether with numerous foraminifera. The formation might equally well
be described as a volcanic ash series that has been thoroughly reworked
and assorted by the sea.
SANDSTONES

Pure quartz sandstones are almost unknown in Porto Rico. This is
due to the fact that so few of the rocks contain quartz in any appreciable
quantity. Only the granites and rhyolites, which are comparatively rare
in the San Juan district, could afford a source of quartz sand, so it is not
surprising that quartz occurs only as an unessential constituent of the
sandstone formations. The few sandstones that are found in the district
are composed chiefly of tuffaceous material that has been well sorted and
somewhat rounded by stream and wave action.
The most interesting occurrence of sandstone that was found is to be
seen in the quarry at K 8 on the Comerio road. It is a dense, medium-
grained, dark greenish rock showing good bedding and containing numer-
ous inclusions of shales. The formation dips 420 to the S. E. and appar-
ently strikes N. 650 E. This is nearly normal to the general strike of
the district, and agrees more closely to the limestones south of Aguas
Buenas than with the conglomerates and tuffs just to the south. It is the
writer's opinion that this sandstone is of limited extent, representing an
ancient delta or fan. Such a theory of its origin would explain its ab-
normal strike. The sandstone is composed of a granular aggregate of
ferromagnesian minerals, feldspars, and occasional grains of quartz. This
material is undoubtedly of volcanic origin. In other words, this sandstone
is merely a fine-grained tuff. the constituents of which have been some-







SCIENTIFIC SURVEY OF PORTO RICO


what rounded and assorted in being transported from the point of erup-
tion to the point of accumulation. The angularity of many of the grains,
and especially of the shaly fragments, leads one to infer that the material
has not undergone extended transportation.
In barrio Padilla, where the Corozal-Morovis trail crosses Piedras
Muellas Creek, there is an interesting occurrence of arkose sandstone.
The sandstone is a medium to coarse-grained, greenish rock, consisting
of an aggregate of feldspar and quartz grains with interstitial chlorite.
The feldspar grains, which are predominantly plagioclase, are somewhat
murky, but no actual decomposition has yet set in. This sandstone also
is essentially a volcanic tuff. The presence of such a large percentage of
unaltered and angular feldspar grains indicates that the material was
deposited almost at its source.
A fine-grained, reddish quartzite was discovered in barrio Santa Olaya,
just south of Helechal Peak. This was the only sedimentary rock found
in the district that consists chiefly of quartz. It appears to have been first
deposited as a fairly pure quartz sand. Later this sandstone was further
silicified, probably under some degree of pressure, as a faint orientation
of the grains can now be seen. The quartzite apparently does not repre-
sent the product of regional or contact metamorphism, but is merely a
highly silicified sandstone.

CONGLOM RATES

Conglomerate beds are frequently found in the older series. In aggre-
gate they form a large percentage of its total thickness. They are, how-
ever, much more abundant on the southern than on the northern side of
the island. The conglomerates occur in belts trending N. W.-S. E. or
parallel to the major tuff series, which apparently form the backbone of
the island. It has been suggested that the conglomerate belts represent
breaks in the older series, separating it into disconformable or possibly
unconformable groups.
The only important conglomerate that was found in the San Juan dis-
trict can be best seen in the cuts along the Comerio road at K 12-13.
This is a boulder conglomerate, which must be over 500 feet thick, though
there are not enough outcrops to determine its exact thickness. Its strike
is about N. W.-S. E. The material of which this conglomerate is com-
posed is almost entirely volcanic in origin. The boulders are chiefly
igneous rocks, especially andesite porphyries. Occasional boulders of
older tuffs were also found. It is possible that this conglomerate was
formed by the erosion of some older tuff formation, but it is more prob-







SEIMMES, GEOLOGY OF THE SA'. JU.IN DISTRICT


able that it represents an accumulation of tuffaceous material directly
derived from volcanic sources, and rounded by stream action during
transportation to its point of deposition. This view is supported by the
fact that the material is exceedingly poorly assorted. Large boulders and'
very fine tuffaceous material are intermingled in all proportions. If this
conglomerate were produced through the ordinary agencies of stream
erosion, some assortment of the material would be expected.
The conglomerates are in many places closely associated with igneous
activity. At K 12.5 on the Comerio road a felsitic dike, 10 feet thick,
intrudes the conglomerate (Fig. 11). At other points it is intruded by
larger igneous bodies. Northwest of Aguas Buenas, in the bed of the
Bayamon River, there is a conglomerate which appears to have been satu-
rated by the still molten magma, for boulders can now be seen in a crys-
talline matrix. The phenomenon has been reported by Berkey as occur-
ring in some of the conglomerates on the southern side of the island. The
extensive series of Muda tuffs also shows conglomeratic phases, but as this
formation is essentially a tuff, it will be described below under that head.

TUFFS

The conglomerate formations in many places grade imperceptibly into
bedded and massive tuffs, which constitute the most abundant rock type
that is found on the island. They occur in an extensive series of great
variety, forming the most dominant structural feature of the older series.
The tuffs for the most part are massive in structural habit. Bedded tuffs.
however, are frequently encountered which have been worked over and
somewhat assorted during deposition. The finer material shows better
bedded structure, and the finest ashy material is often perfectly bedded.
The tuffaceous material shows a good deal of variety in composition.
For the most part, however, they are composed of fragments of igneous
rocks, predominantly the porphyritic andesitic type, so that this variety
may be termed andesitic tuffs; yet a close examination of a large amount
of the volcanic material reveals the fact that practically all the rock
types of the older series are represented. Fragments of all the important
igneous rock types were found, as well as occasional fragments of older
tuffs. At K 7.5, on the Comerio road, a tuff containing numerous frag-
ments of limestone was found. The limestone fragments are dissolved
out on the weathered surface, giving the tuff a pitted aspect. Near this
point is the tuffaceous sandstone which contains large fragments of shale.
It is evident, therefore, that the tuffaceous material, though of general
andesitic composition, contains a large variety of rock types of different






SCIENTIFIC SURVEY OF PORTO RICO


composition, some of which locally constitute large portions of the entire
tuff formation.
The composition of the tuffs is often locally modified by the intrusion
of igneous rocks. The cutting of tuffs by igneous dikes and larger intru-
sions usually has little effect upon them, but it often happens that the
magma, while still molten, has permeated the tuff and there crystallized,
forming a tuff with a crystalline matrix. In consequence, it is often
exceedingly difficult to distinguish between a tuff that has been invaded
by an igneous magma and a true intrusive igneous body that contains
numerous inclusions.
The tuffaceous rocks are especially liable to alteration and modifica-
tion. As they are usually poorly consolidated when first deposited, they
form mediums readily permeated by aqueous solutions of all kinds. Con-
sequently, many of the tuffs have been so highly altered that their original
structure is almost wholly obscured. The commonest process that has
modified the tuffaceous rocks is a thorough silicification, which changes
the formerly non-resistant tuff to an indurated, exceedingly resistant
rock. Silicified tuffs of this character underlie the most pronounced
topographic features of the island. In the San Juan district it was
noted that they formed both Torrecilla and Helechal peaks, the two most
prominent features in the topography of the whole district. The latter
peak rises abruptly from a rolling surface of about 500-800 feet in alti-
tude to the height of 1800 feet.
It would be impractical to describe individually the numerous tuffa-
ceous formations of the district. They occur in N. W.-S. E. belts and are
especially well developed in the southwest portion of the district. In
this region they form practically all the country rock, with the exception
of the tuffaceous shales and igneous intrusives. An excellent develop-
ment of tuffs can be seen along the entire stretch of road from Barran-
quitas to Barros. In the vicinity of Comerio they are also well developed.
The Muda tuffs represent the most extensive series found in the north-
eastern portion of the district. These also strike N. W.-S. E. and their
aggregate thickness is probably several thousand feet. They are well
exposed in the bed of the Guaynabo River, and also along the Guaynabo-
Muda-Aguas Buenas trail.
AGE

The distinctly older appearance of the older series, as well as its highly
tilted position, leads one to infer that an erosion interval of some magni-
tude separated it from the overlying coastal plain. Moreover, the evi-
dence of extraordinary igneous activity throughout the deposition of the






SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


older series and its complete absence in all the subsequent formations
indicate either an extended time interval or a rather sudden change in
physical conditions. It was the consideration of these points that led
the first observers to regard the older series as probably Paleozoic in age.
This idea, however, was soon abandoned; for, as above noted, Cleve, in
1868, suggested that the formations of the older series much resembled
the Cretaceous of the neighboring islands. Hill later confirmed this view


FIG. 13.-Chart showing the relative positions of the principal foraminiferal beds
1. Unibon shale.
2. Shale inclusions in the sandstone at K 8, Comerio Road.
3. Corozal limestone.
4. Muda limestone.

by the discovery of the Cretaceous fossil Rudistes in a grayish limestone
bed that strikes from Cabo Rojo eastward to about 15 kilometers north
of Ponce on the Adjuntas road.
The formations of the older series of the San Juan district are, with
the exception of certain foraminifera, almost universally unfossiliferous.
The few other forms that have been found come without exception from
the southern side of the island. In the upper part of the series, in the
San Juan district, the beds of foraminiferal shales and limestones which
have been mentioned serve to throw some light on the age of the series.






SCIENTIFIC SURVEY OF PORTO RICO


On the accompanying chart (Fig. 13) the four most important of these
formations have been indicated numerically:
1. Unibon shale, at juncture of the Morovis trail and Unibon River.
Strike, N. W.-S. E. Dip, 300 N. E.
2. Shale fragments in sandstone at K S, Comerio road. Strike, N.
65 E. Dip, 42 S. E.
3. Corozal limestone breccia. Strike, N. W.-S. E. Dip, 15-20 N.
E. (?).
4. Muda limestone. Strike, N. W.-S. E. Dip, 300 S. W.
The Unibon shales show typical chalk foraminifera (Upper Creta-
ceous). Since only sections of the forms are obtainable, it was often im-
possible definitely to determine the species. Through the assistance of
Dr. R. M. Bagg, of Appleton, Wisconsin, the writer was able to recognize
a sufficient number to leave little doubt as to the age of the formation.
The following forms were identified:

Globigerina cretacea, d'Orbigny3 (P1. III, 1).
Globigerina bulloides, d'Orbigny.4
Truncatulina lobatula, Walker and Jacob5 (P1. III, 2).
Nodosaria communis, d'Orbigny6 (P1. III, 4).
Nodosaria farcimen, Soldani.7
Textularia sp.

While none of these forms is a conclusive index of the chalk horizon,
yet considering their abundance and general aspect, they could hardly
be regarded older than Upper Cretaceous.
A foraminiferal fauna very similar to this was found in the shale at
K 8. Its forms are even more like those characteristic of the English
chalks. This fauna comprises:

Globigerina cretacea
Globigerina bulloides
Orbulina universe
Truncatulina lobatula
Textularia globulosa (P1. III, 3)
and a few Rotaline forms.

The foraminifera of the two limestone beds are essentially the same.
They are characterized by numerous Miliolide, as well as Rotaline and
Textularia-like forms. In these limestones were found:

8 Report of Challenger Expedition. IX, p. 396. Pl. LXXXII, 11a-c.
4 Op. cit., p. 593, PI. LXXIX, 1-7.
5 Op. cit., p. 660, Pls. XCII, 10; XCIII, 1. 4. 5.
Op. cit., p. 504, P1. LXII. 9-20.
SOp. cit., p. 498, P1. LXII, 17-18.




SCIENTIFIC SURVEY OF PORTO RICO AND THE VIRGIN ISLANDS VOLUME I, PLATE III


FORAMINIFERA


41p

r^r_








SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


Miliolina seminulum, Linn6 (Pl. III, 5a, 6a, Gb).
Nonionina sp.
Pulvinulina sp.
Textularia sp.
Truncatulina lobatula (?).
Though few species could be definitely determined, owing to the altered
condition of the limestones, yet the general appearance of the material
under the microscope was that of a typical Miliolina limestone, which
suggests lower Tertiary age. These limestones, however, lie unconform-
ably below the Arecibo formation, which is not later than Oligocenc.
From these species one may be forced to conclude that the upper part of
the older series of the San Juan district is of Eocene age.
The lower members of the older series are wholly unfossiliferous. Tt
is the writer's opinion, however, that the older series as expressed in this
district is entirely post-Jurassic and probably post-Comanchic in age.

SUMMARY

The material composing the limestones, shales, and conglomerates, as
well as the tuffs and ashes of the older series, is in great part derived
from volcanic sources or through volcanic agencies.
The discordance in strike of some of the limestone beds, especially
those near Aguas Buenas, indicates the presence of an unconformity in
the older series, the limestones being older than the rest of the series.
The conglomerate belt that extends across part of the district may also
be taken as an indication of another break in the series.
The low content of inert refractory material in the rocks of the older
series gives rise to an impervious soil that clings tenaciously to excep-
tionally steep grades.
Many of the formations show the effects of subsequent alteration. The
tuffs are especially liable to the attack of modifying processes which have
resulted in many cases in their being altered to extremely indurated and
resistant rocks.
The formations of the older series are everywhere associated with igne-
ous activity, expressed either as intrusions or by the invasion of liquid
magma forming fragmental rocks with crystalline matrices.
The age of the older series is regarded as Cretaceous in its greater part.
The uppermost portion is doubtless Eocene, and it is probable that the
lower part is Comanchic in age.






SCIENTIFIC SURVEY OF PORTO RICO


IGNEOUS ROCKS

MODES OF OCCURRENCE

In all parts of the island the older series is characterized by an abun-
dance of igneous rocks, showing a variety of forms and complex struc-
tural relations. After the deposition of the coastal plain strata, igneous
activity apparently ceased, for in these formations no igneous rocks of
any kind are found. It is possible, but improbable, that post-Oligocene
intrusion has occurred in the older series of the interior, which has not
affected the limestone strata of the coast. The modes of occurrence of
the ignous bodies of the district are somewhat variable. Extrusive and
intrusive rocks occur as lava flows, sills, dikes, and irregular intrusive
masses, varying in complexity from a simple flow or single dike to an
interrelated series of complex intrusives so intimately associated that
accurate and complete differentiation is often wholly impossible.

LAVAS

The occurrence of lava flows in the San Juan district is comparatively
rare. Several examples of such flows are to be found on the Comerio-
Barranquitas road near K 35. At this point the lava flows occur associ-
ated with bedded and massive tuffs, dipping at steep angles. The lavas
often show a vesicular habit which distinguishes them from the sill-like
intrusives. In this locality and near the Bairoa River just north of
Aguas Buenas strongly vesicular lavas are found. The vesicules are often
filled with quartz, calcite, and zeolites, forming a fairly well developed
amygdaloidal structure. The liquid lava occasionally seems either to
have permeated the underlying ash and tuff beds, or to have been satu-
rated with ashy and tuffaceous ejectments, forming a rock of tuffaceous
appearance but of crystalline groundmass, which often proves very con-
fusing to the geologist. The lavas found were almost invariably ande-
sitic in composition.
SILLS

The most abundant of the simpler forms assumed by the igneous rocks
are the intrusive sheets or sills. These are conformably intruded in the
ashes and tuffs which are now tilted at steep angles or locally folded.
The individual sills are usually thin, being often less than a foot thick
and rarely more than a few feet in thickness, yet they are so numerous
that they add materially to the total thickness of the formation in which
they occur (Fig. 14). Except where modified by subsequent disturbance,







SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


the sills are almost perfect in form, and often show so indistinctly their
intrusive nature that on weathering they are hardly distinguishable from
the neighboring strata. Incorporation of tuffaceous material by the in-
truding magma frequently occurs, giving the sill a decidedly fragmental
structure. Should such a sill occur in a series of tuffs composed of simi-
lar fragmental material, it might readily be entirely overlooked.

DIKES

Intrusives assuming the form of dikes are not uncommon in the district.
Many of the existing likes were probably overlooked or mistaken for


FIG. 14.-Diorite porphyry sills intruded between layers of shales and ashes at K 20,
Comerio Road
The light-streaked and banded layers are shales and ashes; the massive portions, best
seen on the left side of the print, are sills. Photographed by C. P. Berkey.

more massive intrusives, where the outcrop showed none of the structural
relations. It sometimes lhippvrl that a dike cutting some non-resistant
formations such as the conglomerate at K 12.5 Comerio road, now stands
out as a plainly visible topographic feature (Fig. 15). Such cases, how-
ever, are rare, for a dike intruding an igneous body of similar composi-
tion and resistance would only be revealed by a careful, detailed study of
suitable exposures. Good exposures are to be had along the carreteras
and principal stream beds; elsewhere the residual soil is so widespread









SCIENTIFIC SURVEY OF I'ORTO RICO


FIG. 15.-Faulted trachyte dike crossing the La Plata River
This dike is intruded in the conglomerate that crosses the Comerio Road at K 12-13.






SEJMMES, GEOLOGY OF THE SAN JUAN DISTRICT


tlmt it is usually impossible to determine the underlying structure. The
likes vary in size from narrow stringers and apophyses from neighbor-
ing intrusives to massive structures many feet in width. The dike rocks
are usually of felsitic texture. No relation between the coarseness of
texture and the size of the dike could be discovered. The smallest dikes
sometimes showed microgranitoid or even granitoid texture. The texture
seemed rather to depend upon the conditions of intrusion than upon the
size of the intrusives.
STOCKS AND BOSSES

The majority of the intrusive masses of the district should be classi-
fied as stocks and bosses, the term stock including all small batholithic
intrusions, and boss being restricted to those stocks roughly circular in
ground plan. It is often difficult to make this distinction unless the
boundaries of the intrusive are accurately known. The stocks of the
San Juan district are as a rule small. In the bed of the Bayamon River
a perfectly formed stock not measuring over 20 feet in diameter was
found. On the other hand, southwest of Morovis a large intrusive of
granite covers an area of several square miles, and in other portions of
the island intrusives 10-12 miles in diameter have been found. Large
intrusives, however, are rare in the San Juan district. Only two, cover-
ing an area of over a square mile, were located. The majority were less
than 100-200 yards in diameter. Fragmental inclusions are character-
istic of all the stocks of the district. In the larger ones these inclusions
may be of large dimensions. In the granite southwest of Morovis inclu-
sions of the adjacent tuffs measuring many feet in diameter were found.
It frequently happens that the invading magma incorporates quantities
of tuffaceous material. Such a rock, after consolidation, will so resemble
a massive tuff that it will often be confused with one. Several speci-
mens collected from supposedly massive tuffs proved after microscopic
examination to have a crystalline structure. The variation from the
massive crystalline to the tuffaceous phase of an intrusive may be so
gradual that no line of demarcation can be made, or so sudden that it
appears as if the boundary of the intrusive has been passed and that the
tuffaceous phase of the intrusive is a separate formation of massive tuff.

TEXTURE

Rocks of felsitic texture proved the most abundant in the district.
Almost all of these showed porphyritic habit, often not discernible in the
hand specimen but evident from microscopic examination. An evidence
of the almost universal tendency toward the porphyritic texture in the







80 SCIENTIFIC SURVEY OF PORTO RICO

felsitic rocks is that many of them show three generations of crystals more
or less perfectly developed. The larger phenocrysts occur in a matrix of
smaller phenocrysts and a felsitic groundmass (Fig. 23). Though the
rocks are predominantly porphyries of felsitic texture, yet quite a variety
of textures and structures are encountered in a study of much material.
No true glasses were found, but a rock of vitrophyric texture, in which
the glassy groundmass made .up most of its bulk, was seen (Fig. 16).


FIG. 16.-Photomicrograph of ritrophyr occurring north of Cidma
E. Feldspar phenocrysts altered to epidote. C. Chloritized glassy groundmass.

Another specimen shows a peculiar radial, felt-like structure much re-
sembling that of a natural slag (Fig. 22). Microgranitoid texture is
common among the larger intrusives. Rocks of granitoid texture are
abundant and widespread, but none of the coarser granitoid rocks showed
pronounced porphyritic texture.

COMPOSITION

The igneous rocks of the district are fairly uniform in composition.
The predominant type is an andesitic, or the coarser-grained dioritic







SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT


equivalent. The widespread occurrence of andesitic types is noticeable in
the fragmental material constituting the ashes and tuff as well as in the
igneous rocks in situ. Many of the tuffs are wholly made up of andesitic
material. However, when one is afforded the opportunity of examining
a variety of material, such as is to be seen in the boulder beds of any of
the streams that drain a large area of the older series, one soon realizes
that considerable variety in composition is represented. In examining
petrographically the material brought from the district, the writer found
almost every variety of the igneous rocks from acid to medium composi-
tion. Basic rocks, however, were uncommon; basalts and olivine basalts
being rare, and typical gabbros or more basic rocks almost unknown.
As almost every square foot of the older series is intruded or modified
by igneous rocks, it would obviously be impossible to attempt to describe
each individual intrusive or to locate them geographically. Accordingly,
they have been grouped on a basis of composition, and the more important
members of each group will be briefly discussed. The gradations from
one group into the next usually proceeds so uniformly and gradually that
the introduction of several intermediate groups is unnecessary. A com-
prehensive classification of the igneous rocks of the district should include
all of the following varieties:

Vitrophyrs Trachytes
Rhyolites Syenites
Granites Latites or Trachy-andesitcs
Quartz latites and Rhyo-andesites Monzonites
Quartz monzonites Andesites
Granodiorites Diorites
Dacites Basalts
Quartz diorites Olivine basalts

VITROPHYRS
These rocks are of rare occurrence in the San Juan district. On the
trail between Aguas Buenas and Cidra, about a mile north of Cidra,
there is a prominent outcrop of a rock which proved on microscopic ex-
amination to be a vesicular vitrophyr. Phenocrysts have for the most part
been altered or completely weathered out, and the cavities since filled with
epidote, chlorite, and quartz. The glassy groundmass is still fairly fresh,
but there seems to be some indications of incipient devitrification. The
rock shows a slightly fragmental structure and occasionally the outlines
of former large crystals are to be seen, but these are believed to have been
inclusions rather than original constituents of the rock (Fig. 16).






SCIENTIFIC SURVEY OF PORTO RICO


RIIYOLITES

Rocks of rhyolitic composition are also quite rare. The best example
was seen northeast of La Torrecilla in barrio Barrancas. This is a
porphyritic rhyolite with phenocrysts of quartz, orthoclase, and mica in
a fine-grained acidic groundmass. The whole rock has subsequently been
highly sericitized (Fig. 16a). In the Bayamon Valley just east of Hele-
chal Peak a number of acidic intrusives were found, among which were
several rhyolite porphyries.


























FiG. 16a.-Rhyolite porphyry
Large phenocryst of quartz in center, crossed nicols.

GRANITES

Granitic intrusives are widespread and fairly abundant. All of those
studied microscopically proved to be rich in soda feldspar, or that variety
known as soda granites. The large intrusive mass which covers an area
of several square miles southwest of Morovis is of this composition. The
rock is light-colored, with scattered patches of the darker ferromagnesian
minerals, and coarsely crystalline in texture. It is composed chiefly of
quartz, plagioclase, orthoclase, and hornblende, with scattered titanite,




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