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MyM.'^' mm -M Q ip... iF^ii "- '11 ^:-^mm- 111 f.: :':•!: <: Mz^-iij;; ;" : i.^; » :;,• ': ;.;.:'« :'.i .-. :.••.. :.• -j-- . ; :, ^ ^ : :., '• " ; ; •.: : :.iV . :. ....:: ; : flr^tfiilP^ in I,' K N!:VV YOi^K ACADEMY Ol' SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands \C)I UME i CONTENTS OF VOLUME I rage Title-page i Contents iii Dates of Publication of Parts iil 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 38 Geology of the Coamo-Guayama District. By Edwin T. Hodge Ill Geology of the Ponce District. By Graham J. Mitchell 22J) The I*hysiography of Porto Rico. By Armin K. Lobeck 301 Index 3S1 Dates of Publication of Parts l»art 1, September 26, 1919. Part 2, May 25, 1920. Part 3, March 1, 1922. I*nrt 4, December 20, 1922. (iii) iJsT OF njj'STiiArioxs Pr.ATKS I.*:ige \. lU(K-k I Hn.irraiti (>{" tlu' Saii Juan I>istrir{. T. it 42 H. Fnnna of the AriM-iho Format }<»u .">s I [ r. Foninninf<M'a • ''^-t \\\ (;(M»i()ixic Cross-sootiuii \KW^ \. (}(MH^raiiz(Ml (Zoological Cross-soctiou ot; roiico District............. '-iVJ W. (iciicralizcd (Jcolo^ncal ('russ-sfH-tioii of Poncc District.... 2(»4 Nil. <;«MicraliztMl Coluiimai' Section of Ponce District '. . . . 2i)S Fua'HKs Dioritc porphyry sills 2iJ Detail of the double structure in tlie San Juan I'orniatiou at Arecilx) 21 Ha\'slack (pt^pino) liilis 22 N'aliey of tin* Plata, just below th(^ power dam at K lil. Comeiio Road.. . . Hi) San Juan constdidated <iune sand forndnir fi-inirinLi: islands at Vega Bajji IMaya • 4:» The playa, plains near Ai'ecibo 44 'l\vt>icai haystaek topography as develoi>ed in the western portion of the San Juan District 45 SiH'tion through tlse Arecibo foi'niation 47 inface of cuesta and inner lowlan<l near Corozal 48 The iiuier lowland and the inface of the cuesta at (Vu'ozal 49 Mori'o castle and a portioii of the eity of San Juan, built upon a promon- tory c<uni>osed of tho San Juan consornhated lime san<l 5:> San Juan ecnsolidate<l limi^ sand •'>4 i^xfoliation producing coTiceiitric arrangenuMUs in ash beds south (*f Ua\ amon <>2 ("runijding due to solitlu<Mion in the Juan Ascenci() cluM't <*>^> Tyfdcal foraminif<ra] shale <>^ Chart showing tlie ndative positions; of the pri!icii)jfi foraminifera I beds.. T*» Di<»rite porphyi*y sills intruded betW(M^n layers of sliales and ashes at K 20. ( 'omerio Road 77 Faulted] tracliyte dike crossing the f.a I'lata Ulver 7s l*hotomicrograi)h of \-itro})hyr (>ecurring north of (Mdra so lihyoHte porphyry sii Soda granite, slntwing poikiiitic struct vu*e S:i Soda granite oc<'urring south of Naranjito S4 Quartz <liorite sr, 'l'ra<'hyte, showing p<»orly deveh;)]>ed traehytic structur(\ (a*oss(Ml ni<'ols... S7 Latite. showitig veins tilled with epidot(^ and iron oxides ss \n andesite pjorphyry with lai'ge jdienocrysts of plagioclase sun'oumh'd by a ixroundmass. showing the structure of a natural slag s'J Andesite porphyry impregnated with ehrysoi'(»lla. P»arrio Pasto, ....... . UO VOK TENTS OF VOLUME J x IMi»rite i)or[)liyi'y with pyroxene phenocrysts in a ici'ounduiass of iatli- sliaped feldspars < i>la.i?io{rlase) iM Olivine hasait •'- (;ross-i)e(l(le<l strueture in the 8an Joan foi'niation at AreeiiM) Ul lUoek (liairrani of Coastal Plain 117 <;uayani;i upon the upper (M>astal terraee lis (\-iyey in tlu^ liarraiupiitas-Cayey r.owland liM Tutr iHHls r.)\ JJfhoviifnix' s]». in slialt^s of tlie P>arran<piitas-rayey series. \:\:\ Siiells worn, oxi(liz<Mi, and eeuieuted to.i^ether to form one of tlie liuie- ston(^s of tile P.arran<piitas-('ayey series !:;."> 'roi)o,i?raphy of the I»arran(piitas-Cayey Lowland i:'>r> Character of tlu* inieonformity hetween the P.arranuuitas-Cay(\v ami the Sierra dc* Cayey series 1.*J^ Con,u:louierate with an iiiiieous matrix 140 Ash shah's <'ontaininir Orbitolitcs 1 14 (Miert with d(»sieeation fraetures 14.", (^rystal tutf Un hithi<' tuir 1^1 Coamo SjjriuLrs iimi^stone ex[)osed in the I>esoalahra<los (iap l-"*! IJih<tih<niun(ni thalius in eross-seetion in the Coamo Si)rin,irs limestone... l.V> Lithitthdnmiou thalius in lon.u'itudinal section in ( •oam<» Si>rin^^s limestone I.Ki Coamo Sprin.us lian.!4:e loT lAthathinnniou in Coamo Sprin.irs limestone IoN AmphixivijiiHi in Coamo Sprin.i^s limc^stone Itio !*(}r()((if<(Us (<))}<< iifriius in shale KJi* Shakes of the Rio Uescalahrados s(>ries Ki.*', N'alley of the (iuamani t(;s ('(M'ro (Javias. Coastal Terraee, Coamo liivei" !(»!> Felsj)athlc andesite 17- Auixitc^ and(*site 174 Auixite and(^site showimc feldspars comi>letely altered: au.L::ites unaltered., 17.1 Classy-au.Jxite andesite 17(> Olivine andesite 177 Olivine free hasalt... 17s Olivine hasalt 17t) La>a tlow containinii" pyroclasts of I'alcite which are surrounded hy irarnets 1S1 Flow hr<M'(Ma.. , IS'J Iirneous structure of tlu^ Sierra de Cayey coui^lomerate matrix 1^:; iiclation of isotluM'mal surfa<*es to andesitic colunnis ls-i Hornldende i^rranite., is<; Pdotite icranodiorite ls7 HoiiihhMide diorite Iss ICornhlende-auirite diorite 1 st> Hornhiende andesite )*M\ HornhkMHle-auirite andesite P.M /'nrfnijscus concrHtricuy in tuffaceous shale Pjr> ^^l<ih'uji'r\i\4i an<l alii:* in the Ooamo Springs limestone. P.k"; ^1 HVIESTIFIV HURVEY OF VOHTO KWO Page (Miloi-ite (levtOopiiiK fi'oin foldspars which are assoeiattnl with fresh aiigites 2(K^ Epi<l<)te replacing feldspars ^04 Minerals named in order from the center outwards: stained antigorite, stilbite, laiimontite, and ealcite 205 Intimate mixture of quartz, epidote, and actinolite 206 Quartz and epidote replacing feldspar 20T Spherulitic growths of specular hematite and chalcedony. , 208 ( 'oamo Sijrings • • 210 Karrio del Carmen Prospect -21 Portion of Complex mountain province as seen from the Tertiary hills south of Yauco 235 Cuspated ht^ach, west coast. Porto Rico, just south of Melones Point 246 Fluted (iuayahal limestone of Juana Diaz 256 Uio Yauco shah*, as seen from point one mile north of kilometer 15.0, y auco-I.a res road • 263 Faulted shale at kilometer 14.3, Yauco- Lares road 266 Augite porphvrite showing zoned augite crystal set in tine-grained ground- mass ,,./, 274 VtiipcuMrr rofiuccus, Lamark 284 JivndiiHter hcrkcin n. sp., Jackson, x 3/2 286 ''Montalva" Salina, west of (Juanica, on south coast 296 IVIap showing the average annual rainfall in Porto Rico 303 Sketch map of Porto Ri(*o 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 P»lock diagram of the southwest corner of Porto Rico 311 Appalachian ridge structure in the oldland rocks between Boqueron and Parguera 312 A'alley of the Manati River above Clales 313 The valley and flood i>lain of the Rio Grande de I.oiza near Carolina. .... 317 North-south protile through Porto Rico 318 I )iagram of the (4uanica district 318 The inner lowland at ( Juanica 321 The iinier lowland at Guanica 322 P>asal Tertiary south of Yauco, along the (Tuanica 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 3.36 Th<' inner lowland of the south coast, looking eastward from cuesta face ba{*k of Ponce 338 The Tertiary coastal plain lapping upon southern flank of Mariquita Hill. 339 The surface of the coastal plain between l*once-and Penuelas. 340 Reds of the coastal plain dipping slightly toward the oldland 341 Road cut at K9 H9 on the Ponce-l*enuelas road, showing relation of Ter- tiary to older series 342 Se<!tion of hill along Jacaguas River one mile scmthwest of Juana Diaz.. 343 The drowned coast of eastern Porto Rico 345 Lower cours<» of the D<»scalabrado River during the dry season. 350 CONTENTS OF VOLUME I Yll Page Tlie liigli terrace of the Plata River at K 13 on the Comerio road — 351 Alluvial terraces of the Pena Pobre Creek, west of Rio Blaoco 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 clifC 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- dlllas and Isabela 36(> River mouths at the north coast '^^- The coast east of Quebradlllas 3(i5 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. 374 Muertos Island from the north 375 Maps Outline Map of Porto Rico showing distribution of district studies 26 New Base Map of Porto Rico, Complied from the latest Government Sources 36 Geologic Map of the San Juan District, Porto Rleo. Geologic Map of the Coamo-Guayama District, Porto Rleo. Geologic Map of the Ponce District, Porto Rico. Physiographic Map of Porto Rico. ERRATA I*age 58, line 33, and page 59, line 8, read Metis for Mctris NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Forto Mico and the Virgin Islaeds ¥OLUME I^Part 1 History of the Survey -i¥. £.. Britton Geological Introduction^ C F. Berkey Geology of the San Juan District-- 1), R. Semmes NP:W YORK: HISTORY OF THE SURVEY By JST. L. Britton Chairman of the 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 for a scientific survey of Porto Eico was approved and referred to a committee consisting of Professors Boas, Britton, Crampton, Kemp and Poor, at the meeting of the Council in Kovember of that year, at which meeting the Council appropriated $500 annually for five years for the purposes of this investigation. Br. E. 0. Hovey and Dr. E. W. Tower, Eecording 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 oifered to assist the work, and has contributed $3,000. The Porto Eico 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 Kew York Botanical Garden, scientific departments of Columbia IJniversity 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 Eico. The two agricultural ex- periment stations of Porto Eico, one at Mayaguez and one at Eio 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, >^oology and anthropology. Professor Crampton proceeded to Porto Eico 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 Eico.^ 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., XVI, pp. 59-70. 2 SVIENTIFIC SURVEY OF PORTO RICO politicall}' a part of Porto Rico, and made the first represe^tati^ e col- lection of plants obtained on that island.^ During February and part of March, 1914^ Dr. Britton, accompanied by Mrs. Britton, Mr. John F. Cowell and, Dr. Frank B. Lntz, 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 Botarucal Garden a paper on The Vegetation of Mona Island.^ In the summer and autumn of 1914, zoological collections were made over considerable portions of the island by Messrs. Roy W. Miner in marine invertebrates and myriopods, John T. N^ichols in fishes, and Frank E. Watson and H. G. Barber in insects and arachnoids. Many thousand specimens were obtained and important additious 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. Lntz in his List of Greater Antillean Spiders, with Kotes on Their Distribu- tion.** Richols has published some of the results of his work under the title Fishes I^ew to Porto Rico."^ The geological investigation was commenced by Dr. Charles P, Berkey and Dr. Clarence N. Fenner, wlio 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.^ In July and August, 1914, Dr. J. A. Shafer devoted about six weeks to the study of the forests of the Sierra de ISTaguabo, and added a num- ber of species of plants to the known flora of Porto Rico."^ Under the auspices of the 'New York Botanical Garden, Professor IST. 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 alg;p over considerable areas of the island. He obtained many thousand speci- mens of these organisms, which proved to be more abundant there than 2 Joiu'. N. Y. Bot. Gard., XIV, pp. 103-105. 8x\mi. Mo, Bot Gard., II, pp. 33-58, Pis. I, II. (Contributions from N. Y. Bot. Gard., No. 175.) *Ann. N. Y. Acad. Sci., XXVI, pp. 71-148. 5 Bun, Am. Mils. Nat. Hist, XXXIV, pp. 141-146. « Ann, N. Y. Acad. Sci., XXVI, pp. 1-70. (Contributions from Dept. of (.oolosy. (^)- lumbla Univ., Vol. XXV. No. 20.) " .Tour. N. Y. Bot Gard., XVI, pp. 33-35. BIUTTOX, HISTORY OF THE SURVEY 3 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 algae within the American tropics. He has published a narrative report of his field- work.^ The greater portions of the months of February and March, 1915, w^ere spent by Dr. Britton, Mrs. Britton, Mr. John P. 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 soutli to Morrillos de Cabo Rojo and westward to Ponce find 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.^ 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 ISTational 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. ^^ 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.^^ » Jour. N. Y. Bot. Gard., XVI, pp. 132-146. » Jour. N. Y. Bot. Gard., XVI, pp. 103-112. ^« Bull. U. S. Dept. Agric. 1916, No. 354, p. 99. " Ann. N. Y. Acad. Sci., XXVII, pp. 23-29. 4 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 Eico. 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 Baiios de Coamo. Mr. D. E. 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 N"ew York Botanical Garden.^- Professor Stevens described and illustrated the species of the large genus Meliola as repre- sented in Porto Eico, under the title The Genus Meliola m Porto Eico, which includes descriptions of sixty-two new species and varieties. ^'^ 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 obtainino- 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 paiis of the island, adding some 15,000 specimens of insects and large num- bers of spiders to collections previously made. Mr. Eoy 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 Eaymond 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 archaeological investigations at several points during May and June, 1915, aided bv Dr. 52 Jour. N. Y. Bot. Gard., XVII, pp. 82-85. IS lUinois Biological Monographs II, No. 4, pp. 1-86, Fls. I-V. BRITTON, HISTORY OF THE SURVEY 5 II. K. Haebeiiiii, Dr. J. Alden Mason and Mr. Eobert 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 Eican children have been published by Mr. Leslie Spier.^* 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 Riean 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 IJtuado, 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.^^ 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.^^ Dr. Haeberlin has published a paper entitled Some ArcliJBo- logical Work in Porto Rico.^^ An ancient village site at Capa, located by Professor Boas and subse- quently investigated by Messrs. Mason and Aitken, proved to be the most important of all archaBological 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. ^^ " Am. Anthropologist, XX, pp. 37-48. '' Jour. Am. Folk-Lore, XXIX, pp. 423-504. i<5 XIX Congress of Americanists, pp. 224-228. " Ann. N. Y. Acad. Sci.. XXVII, pp. 17-22, Pis. I-V. ^8 Am. Anthropologist, XIX, pp. 214-238. ^» XIX Congress of Americanists, pp. 220-223. G 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 alga^, principall}' along the southwestern coast from Ponce and Cayo Muertos to Cabo Eojo. During this trip he obtained about 3^000 specimens, including some species addi- tional to the known flora of the island.^^ Professor Bruce Fink^ of Miami University, proceeded to Porto Eico 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 and 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 petrogiyphs or rock- carvings. Mr. Bela Hubbard carried out a geological survey of the nortliAvestern 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, whicli 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 ISTew 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. xinthony 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.^^ Preliminary Diagnosis of an Apparently iS^ew Family of Insectivores.^^ ^ Jour. N. Y. Bot. Gard., XVI, pp. 210-225. ^Ann. N. Y. Acad. Sci.. XXVII, pp. 193-203. 22 BuH. Am. Mils. Nat. Hist., XXXV, pp. 725-728. BRITTON, HISTORY OF THE SURVEY 7 ±sew Fossil Kodents from Porto Eico, with Additional Kotes on Elas- modontomys ohliquus Anthony and Ileteropsomys insulans Anthony."^ 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 oiir knowledge of the flora ; he is now preparing a list of the fungi of Porto Rico. Dr. Basil Hicks Dutcher, TJ. 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.^* Another symjjosium was given at the meeting of December 14, 1914.^" 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.^^ At the meeting of the Section of Biology Novem- ber 8, 1915, communications were presented by Messrs. Reeds, J. A. Allen and Matthew.^^ The Committee submitted a report of progress at the annual meeting of December 20, 1915.^^ At the meeting of the Academy held January 24, 1916, Dr. Boas, Mr. Aitken and Dr. Haeberlin^^ presented communications on ethnology and archaeology, and at the meeting of May 15, 1916, geological commimica- tions were presented by Messrs. Hodge, Semmes, Reeds, Fettke, Hub- bard,^^ Lobeck and Professor Berkey.^^ 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.^2 The final geological reports are being written, for the most part, by the ^ BiiH. Am. Mus. Nat. Hist., XXXVII, pp. 183-189. 2* Ann. N. Y. Acad. Sci., XXIV, pp. S69. -• Ann. N. Y. Acad. Sci., XXIV, pp. 398, 399. 2« Ann. N. Y. Acad. Sci., XXVI, pp. 433-436. -7 Ann. N. Y. Acad. Sci., XXVI, pp. 436-439. 2«Ann. N. Y. Acad. Sci., XXVI, pp. 456-462. "« Ann. N. Y. Acad. Sci., XXVII, pp. 250, 251. ^•^ BuU. Am. Inst. Mining: Engineers, No. 135. «i Ann. N. Y. Acad. Sci., XXVII, pp. 277-282. •"- Bull. Am. Inst. Mining Engineers, No. 135, pp. 661-676. 8 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. Yanghan, 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. JST. 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. E. S. Williams. Ilepatics.—Professor A. W. Evans. Lichens,—Professor Lincoln W. Eiddle. Fungi.—Dr. W. A. Murrill, Dr. F. J. Seaver, Professor J. C. x\rthur, Professor P. L. Stevens, Professor H. H. Whetzel, Dr. E. W. Olive, Dr. John A. Stevenson. Algm (marine).—Dr. Marshall A. Howe. Algce (fresh-water and aerial).—Professor 'N. Wille. iSTo arrangement has yet been made for the study of Porto Eican diatoms. The zoological collections are being studied and final reports prepared as follows : Mammals.—Mr. H. E. Anthony.^" Birds.—Mr. Alex. Wetmore has published an exhaustive report on the birds which may be taken as a basis for our document.^''^ Reptiles.—Preliminary work by Miss M. C. Dickerson. Amphibians.—-VTeliminsiTJ work by Miss M. C. Dickerson. Fishes.-—Mr. J. T. ISTichols and Professor C. F. Silvester. Insects: Orthoptera.—Mr. J. A. G. Eehn 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. Eoy W. Miner. ss Mem. Am. Mus. Nat. Hist., N. S., II, Part 2. s^ Bun. IT. S. Dept Agric, 1916, No. 326, p. 140. BKITTOX, HISTORY OF THE SURVEY 9 MoUusliS.—Preliminary work by Dr. L. P. Gratacaj). Ascidians.—Dr. Wiliard Van Name. Crustaceans.—Mr. Eoy W. Miner and Dr. Wiliard Van Name. Annelides.—Professor A. E. Treadwell. Eckinoderms.—Preliminary work by Mr. D. M. Fisk. Bryozoaiis.—Dr. E. C. Osburn. Coelenierates.—Mr. Eoy W. Miner. Eepresentative collections of Porto Eican protozoans have not yet been made. The anthropological and archgeological documents are being written l)y 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 Eico, by Dr. Mason on the archaeological investigation of the very important site at Capa, by Dr. Aitken on the cave sites near Utuado, and hy Dr. Miison and Professor Espinosa on Porto Eican 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 wouhl 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 Eico to form the basis of a Nat- ural History Museum, and other portions will be forwarded to Porto Eico 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 Eico, to the east and southeast. It is highly desirable, before our survey is brought to a close, that these should be studied geologically, zoologically and archseologically. 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 Eico 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. 3- Mem. Brooklyn Bot. Gard., I, pp. 19-118. (Contributions from N. Y. Bot. Gard., No. 203.) INTRODUCTION TO THE GEOLOGY OP PORTO RICO By Chakles P. Berkey CONTENTS Page Introductory statement 11 Geological reconnaissance 14 Major geological features • 15 Rock formations 15 The Younger Series 16 Tlie Older Series IT PetrograpMc 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 au INTRODUCTORY STATEMENT The Island of Porto Kico lies between latitude 170° 54' and 18° 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 Eico 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) 12 SCIENTJFW SURVEY OF PORTO RICO top of El Yujiqiie 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 Eico 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 Eico may have to a former continental mass extending toward and possibly connecting North and South America, how^ever 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 Eico is very mountainous. Ji. central Cordillera extends nearly east and west, starting abruptly at the sea near Eincon 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 many 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 Eico. 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 Eico has been made the subject of a special study by Mr. Armin K. Lobeck and will soon be BERKEY, INTRODUGTION TO THE GEOLOGY 13 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. Kumerous 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 Eican land mass toward the east. All are included as a part of the studies of Porto Eico. 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. Eainfall is very unevenly distributed. At the east end, near El Yunque, there is an annual ]-ainfall of 123 inches. On the other hand, at Cabo Eojo, 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 w^est 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,^ 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, folh^wing 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 1 For discussion of this feature in much more detail, see the foHowinj? nrticlo by Douglas R. Semmes on The Geology of the San Juan District, Porto Rico. 1 ! ^^(JIE^TIFW SUR\I:Y OF PORTO RICO ()\er most r)j' tlie inland havc^ a direct infiLienee on the character of rock decay and disintegration and also on the (jiialitv of the soil. The a\'erage (hiily temperature is 80 degrees, and it rarely rises above iK) or drops kvlovv 'lO, while the maximum is said to be 99 degrees. In area Porto Eico covers ?u(u() square miles, and is therefore about tJiree-fourths the siz(^ of the State of Connecticut. It is roughly rectan- gular in outh'ne, with the longer axis east and west and in actual dimen- sions i.< a])])roxijnately 35 miles in average width and 105 miles long. Ir is tlie fourth in size of tlie AYest India Islajids. Porto Eico is very productive an<l is said to h(* th(^ most (Umsely ])o]nilated country in the world. GKOLOOICAL EE(X)NNAISSAIv[CE The Ishmd of Porto Eico has never had a systematic survey, and com- j)arati\el}' ktv; articles lune been writtcui on its geology. A finv papers, ho\v<'VtM', liave iK'scribed certain spc^cial features and general jihysical con- ditii^ns in a very instructive way. Tlie writings of E. T. Hill (1898, 18iH)), especially, contain a comprehensive outline of the geologic history of the Island of [?orto Eico, and those of IT. M. Wilson (1899, 1900) and \\. Dinwiddiii (1899) give as good general desc.'ription of its physical f(*atur(\< as is anywhere to be found. In the sunjmer of 1911 th(^ Avriter, in conipany with Dr. (Inreiice N. Fenner of the (hM)]Vhysical Laboratory. A\'ashington, made a geological leconiiaissance of the island, with the object of determining the chief liru'S of its geologic history and cmtlining the major problems to receive further study. This reconnaissance was undertaken as a part of th<^ Katural ITistory Survc^y planned by the New York Academy of Sciences and was intended to lay the foimdations of a complete geological survey whidi it was exjXH-ted would become the work of several subsequent expeditions an<l 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 Eecon- miissance of Porto Eico (}>erkey, 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 ])e made, form a part of the plan for this Survey.^ Seven such districts - A dPtaiUnl list of (hose topics and studies is given at the end of this article. BERKEY, INTRODUCTION TO THE GEOLOGY ]5 and areal studies and four general mvestigations were originally planned. Five of the district or areal studies have been made and one general investigation has been completed. MAJOR GEOLOGIC FEATIJEES ' 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 l)riet' outline of the more im])ortant geologic features characterizing the wliole island follows. Rock Foiimations Tliere is ao (jx(;(HMlingly large number of formational rock units in this field and a very great number also of rock types or varieties, and the con fusion as a c{)jise((uenoo is so great, at first sight, that it all appears a hopeless nunldle. This is <>sj)eeially true of the interior of the Island ratiuM- tban tlu^ coastal margins. In the higher interior regions igneous I'ocks and elastics derived mon^ or less directly from igneous action such as tulfs aiid aslies prevail, although formations of simpler sedimentary (\vat(n' laid) babit arc; 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 tlie north coast ^vest 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 w^ould expect them to be separated by a more or less profound break or imconformity. To this extent at least tlie 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 shales, marls, chalks and limestones, little disturbed. As should be expected, h; scji::xTiFJO suuvEy of pojrto rico there is a \erv (li<ti)iet iineoiiformitv between these two major series i-epreseiited hy an erosion plane which bevels tlie upturned members of tlie older complex and seems to have all of the earmarks of a peneplain. The complexity of the older of these two scries su,2rgests again the possi- bility of a subdivision of it^ but all studies so far have failed to show a ]uajor line of separation. The formational groupings are therefore in appj'oximately the order suggested in their original reconnaissance, essen- tially as follows: THE YOUNGER SERIES This includes shale and limestone beds, wel limestones and occasional sandstone and. peb!.)le accumulations. These are readily recognized as btdonging to the Tertiary period, but the greater detail of age relation is a matter necessarily left for the more elaborate studies of ihose districts where these formations are ])rominent or to the results of geiieral strati- graphic aiid paleontologic study. The Younger Series as used in these studies inchules at least two very n^adily differentiated but \ov\ unevenly developed mcMubers which were r(d'erred to in the reconnaissance under names taken from their inost typical occurrence. (a) The San Jnan Fonnailon is the most recent consolidated repre- sentative in the island. It is confined to the immcKliate coast margins, especially the north coast, and the adjacent small islands, the most strik- ing occurrence forming tbe promontory on which the city of San Juan is built. (/;) The Areciho Fonnalion is a thick series of limeston(>s, marls and shales. This is a much more extensive and geologically important mem- ber than the San Juan. Subdivision is dou])tless 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 w^ide appli- cability. They have been, however, of distinct usefulness in all studies thus far made. The most commonly used are the following: 1. The Areciho llmesloiw upper limestone nu'mber on the north sid<^ of the island. 2. The ISan Srhaslian shales low(»r, shale member on the north side of the island, rl. ''Jlu^ Jvan a Diaz marls south side shales and marls near Juana Diaz. 1. The Ponre Chalk beds V(M*v white chalks nrar Ponce on south side of the island. BMRKEY, INTRODUCTION TO TBE GEOhOtlY 17 The greater part of all these beds are satisfactorily (leteriniiied to be of Oligoeene age, but the latest studies indicate the presence of representa- tives of both Eocene and Miocene age as well. THE OLD Ell SERIES This iiichi(his a C()ni);)k\\' association of tuifs^ ashes, sliale, (conglomerate, limestones and a very great variety of intrusives and occasional hiva flows. Tli(\^e nre readily seen to b(^ (Miongh obk^r than the other series to have sii{fcr(Ml nuK'b additional modification of petrographic- charactcM^ and deformation, and it is comparatively sim])le also to determitie that the jncMubers of the series as far as they bear fossil evi(kMice seem to be con- tiiied to late Mesozoie ag(S ])erhaps wholly to the (Veta(!e(nis period. Th(^ {)\(\qv Sei'ies thus far has failed to show evcMi as (h'stinct lines of sub(h' vision as the Yonnger. IVrha])s there are breaks as great as that L>et\veen the Han Juan and tlie Arecibo formations of the Yonnger Sto- ries, but the structures are nuu^h more obscure, by n^ason of (hdormation an<l other changes, and are less fully exposed. There is a suggestion of a possible structural l)reak marked by one belt of limestones found bv Dr. Scmimes in the San Juan district (see San Juan District geologic map) ajid perhaps iji the La Muda Limestone of tlie original rei^mnais- sance acuMUint, but no other supporting evidence is yet at hand, even after a study of more than half of the ishunl in detail. The Older Series is apparently a fairly continuous accumulation of no gn^ater e(miplexity and stru(?tural diflieuJty than shouhl be ex])e('te<l from a lojig and ahuost (H)ntinuous volcanic history. Althcmgh still regarded as a single series of formations, it has been found very conv(>nient in this case also to designate certain well marked units by h)cality names. Xo assum]>tion is made as to their extent or the possibility deserving recognition in attempts at correlation. The Kecon- naissance expedition made free use of such tei'ms to much advantage in \\w Held, and latcn" still others have been used l)y the investigators of special districts. As a matter of fact, the great difficulty found in the begiiniing in following any single tiehi unit very far or identifying it at oth(u- places made the use of many locality and arbitrary names particu- larly serviceable at that time. Subsequent studies have not successfully ehminated them, and it may well be that tlie accumulating proof of the great variability of all these formati(ms as well as other units will com])el the retention of such lo(»aI names. Among the terms that belong to the recomun'ssance stage aiid that bid fair to be retained are the following: IS xcn:\TiFia suu) i-jy of pouto rico 1. The Fajardo shales, a very widespread type of blocky shale. 2. The IMayaguez sliaU^s, similar to Fajardo shales in general stnic- tural habit, n. The ('oaino Tuff-Limestone, a hea\y limestone carrying abundant tuff fi'agments. 4. The Trujillo Alta Limestone, a very limited occurrence of dense limestone. 5. Tlh' ''Shred" Tiimestone, a name suggested ])y the appearance of the elder fossil form. (>. The Tia ]\luda Limestone, a formation that seems to mark a break of mor(^ than average consequeiu^e. i. The Mountain Tjimestone, of Hill—thin-l)e(lded, slialy limestones and limy shales of very extellsi^e deveh^pment along some of the mountain ridges. S. Tlu' (^orozal Limestone, a peculiar local occurrence of limestone ])reccia at Coi'ozal. PETROGKAPHTC RA\(;K The pet rographic variety of this older series is very great, but th(^ ty]>es of large C()nse(pience are not numerous. They may nearly all be included within such group terms as limestone, marls, shales, sandstones, conglom- (n'at<'s, tufTs, ashy tulTs, ashes^, and ash shales among the organic sedimen- tary, and clastic rocks; andesites, andesitic porphyries, diorite, syenite and granite as the chief crystalline igneous rocks; and serpentines and gi'aywackes and rarely some contact products as the chicd' badly modified typ(s. Although tlie range of different ty|)es when stated in this foim 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 ai'eal 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 nimiber of small units which alternate or change so many times in short distances that d(^tailed mapping of them, or even an attempt to represent this detail bv the most carefully constructed geologic cross-sections, seems hopeless. All ]7racticable representations of the geologic structure are necessarily i>eiH^ralizations, BERKEY, JXTEODUCTION TO THE GEOLOOY 19 ^taps of any kind eoyenng portions of 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 limestones, 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 qiiality 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 th(^ soutli 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-])edded 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- positien, are also very common and there are a few large bosses several //:a I'l i'ic >/ h'\ i:y <>i' f(urn) i:i< ' nHV ! iilr irlu.lmi: UTrii? nHloUIlt- of llliT I'f n< kiiHl i> II. .u 111. MiiM.r slrii.iiiriil (v.iiuv,: mi<4i a,^^ Xi^nolilhi.' iiirhiMnii^ in \\w ml ni-.n <•>-. nic .xl,.|isi\..l\ ,!..u.l,.|.c,!. Snin.-liHi.- ;i sill ,.r .lik.- is lihTallv .m-..«.|.m! with tiiir rniu^i)ifii1> -^H rlusfly j.m-kcfi L.-viInt that t!i.^ wlml,' lia^ i.-|..n- ih.. ovnrnil uiihviir.l i!i.|N.:. nui.r of a liitV tliuii a hona ti«l.' nitriiH'vc. l,*.^^ [icaird iiisuc.-ii.Hi t.r tin- r-inirturf. Iihwcmt. Iia- rt'imncu fxcry .i<.iil.i a- i,. tlir ,->','iilial .•..nv.-liM-^ (.1 till- .,h>Mnan<.n. A ratii.M' ivnia rkal.l. HiHHir -tni.-tiiiv III r!i.^ San Jir.ui iuniuiti.m. ultirh i> hrliru-,! i,, l,,- a r(,,iM,|Hlal.-.l .liiii.^ ^aiui. 1^ niiKtratr.! ni ih.- a.v..,,,i,an\ in- ll-air.-. It iti:i>'h f:) ^ I \ I i:i>ni r-fins to nn: fH'jti.oi.) 1'TT'^^^m^mmm^m. Ih.iJKi^ Vi Til,' |.)rs(;iir ri'li.-r Iratiiivs ..f l',,rt,. lii.-u an: all iIm.m; ,,r HHi|.|r .T.Ku.n i( tluf inferior <h.- <1nu-iuiv is x. complex nn.l viiniihl,- lluti Mirhur Inn,,; .•-•111 r.. slxnv liMli^ slrurliinil li.mn.L 'I'li,-^ iiiutnitains nr.- ni.u;uv.L sr<'.-| ihI inv-iil:ir !ii .M^tlm.^ M..|liit;iii, i..ps hiv ali.iosi ..vrrywhr,^.. kintV d.UV ,llM.|..S. ,M, Hinff.T ulM'tll.T Ill..\ ,T<.K^ or fullnW ||,.. rn.^v -^t rucl II IV Thr- r.'Htuivs of ihr .-..H-Uil mjirjiin:-.. h.nu'v.T. aiv ,,,ii.^li ,,mmv iiillu ur.'.l l.\ iumiatioiiji! atfitu.h-. 'Fl..- yoiuip-r series ..f l.ril< li,' on ria rtthcr .•v..ii|\ vrotU-i\ f |H-iH-|>la iiii'd ) <m'\';u-<' ..!' th.- .•oiiiplcx: <.lt!cr M-rics •.•usiuii hir- ^^triupr.! Lack iiiii.'ii cf llii^ yuur^vr r<,v.-r from ih.- uhl Ian.! BERKEY, JXTRODUGTION TO THE GEOLOGY 23 tlius 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. Loheck, 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 remnauts form great num- bers of small conical hills, referred to by Hill as the pepino hills and by Borkey 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 Eico is simple. A com- plete or even a reasonably full account of the geologic history of Porto Eico 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 clastic matters, partly in 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 Eeconnaissance 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. 24 sciEXT/Fic ^(jfn'i:y of poirro rico (2) A dying out of \<)l('aiiic energy, greater stability of tiie niar^s with respect to elevation and subsidence, and erosional attack continued long (Miougli to result in extended ])ianation and i)artia] baseleveling witli final e X ten s i ve s ul ) nie vgvu ce. ()>) Tbc development of an nnc^onrorniable overlyijig sei'ies of shales, nvf h'niestones and relatiMi deposits cliietly of organic origin, bi'ouglit to at! end by final ]'e~e mergence. (I } Tbe development of present surface features uiukn* stream erosion and marine marginal attack, witli modifications arising from oscillation of level. PROBLEMS OP TITK SIRYEY Hie prnblems that were snggeste<l in the R(H;(jinuiissan(re report have engagetj the en(h'a\'ors of several (^xpe<h"tions. Altogether seven have been made and f(mr studies liave been comph^tecL Otbers are yet to ])e made. Tbe wlioh^ ])!an witli its accom]ilishments may be lisicMl as folh^vs: Expi:i)n'iONs (a) Ex])e<lition of 1914 to make a reconnaissajice. 1. Dr. (-liarlcs 1*. lierkey and Dr. Clarence N. Fcnncr. Kecoiuiais- san<v report (Rerkey, 1915). (//) Ex])editions of 1915. 2. Dr. ('liester A. Kc(h1s. Study of Tertiary stratigraphy, and ('S]h*- cially an attcni]>t to establish the subdivisions of the Youngci* S(M*ies and make paleontologic collections. New base map pre- pared (priblished with this ])a])er). .*>. IVIr. Douglas K, Hcmmes. Stmiy of the San Juan Disti'ict (Seiiniu^s. 1919). 4. ^Ir. Edwin T. Hodge. The Coamo-duayama Distri<-t.' (/•) Expeditions of 19irK 5. Mr. Bela Hubbard. Tlie Aguadilla-Lares District. 4 G. Di\ Cliarles II. Fettke. The Humaeao District.^ 7. IVIr. Annin K. Lobeck. A general stn<ty of the physiographic his- tory of tlie ishmd.' (J) Expedition of 1917. a ivir. Graham John Mitchell. The Ponce r>istnct.+ •• Reporr ready for ptibllcatioii. * Report interrupted l»y the war. BERKEY, INTRODUCTION TO THE GEOLOGY 25 Classes of Studies Tlie wliole list of studies of all kinds fall into three classes : (a) (leiieral studies^ covering certain features affecting the whole island. (h) Area! ov districf studies, with acxionipanying geological maps. {(•) SptH'ial investigations on prohlenis suggested by local conditions or tlic material collec^ted. GKN-ERAL STUDIES Among tlie genci'al (juestions which hav<^ been considered to })elong to the island as a whoh^ are the folh)wing: Xrtr llnse Map.—The making of an accurate detailed map, such as nn'glit )>c re])resented l)v topographic sheets of the kind pi"e])ared by the r. Si. (Jeoiogical Survey, is an undertaking much l)eyond the sc()])e and facilities of this Survey. Each investigator of a district has uniUnlaken th(^ correction of the maps available for his own area in enough detail for the ])ur])ose. These districts, however, aiv not complete to snch a point that a general relied' niap for the island could yet hv constructed from them. As a stej), therefor(\ preliminary to that result, the data of all ])reviously ]nd)lished maps, together with such, notes as wei'(» available from fitdd obser\ation, have Ikhmi compiled, under the direction of Dr. Chester A. Reods, in the form of a new relief map which is ])uhlished luMMnvith. I'hydoijraph.y of PovU) Biro.—After the reconnaissance and first dis- trict studies were made, it became itpparent that a general physiographic study covering the whole island would be y)articularly d(ssirahle. Tlie opf)ortunity to make a complete study of an isolated land mass, with a |)livsiographic history ])robal)Iy similar to most of th(» W(*st Indian islands, seemed to he particularly good. Mr. Armin 1\. T.ol)eck under- took this study and his work is completed. Trrfinry Hf rail f/rap]ly.—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. I)r, Chester A. Eeeds entered upon the investigation by making a collecting expedition in the season of 101.";. Tt was the intenti(ui to iiudude in this study a determination of the principal subdivisions and boundaries of the Tertiary strata for tlie whole island, and it was hoped that this could be done on simple enough lines and ])romptly enough to be of service to all of the men doing district 26 SVIEXTIFW SURVIJY OF PORTO RICO luappiiig aiifl making local studies. The problem has been found espe- eiallj' difficult aud final results are not yet readied. Iji the meantime the practical matter of mapping has been solved in each district as well as }nay be, and probably no full statement of the Tertiary stratigraphy ^v'ill now be attempted till all of these separate studies are available. Mineral Eesomxes.—Considerable money has been spent in Porto Eico 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 I'ew plac(\s now. Many ores of other metals are also formed as well as other economic pi'oducts. Almost none, however, have proven attractive as the basis of an industry, although much interest is attached to such possibilities. Mineral resources of sutTicient 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 g(^n(^ral study after all of ihe districts have been fidly investigated and reported. DiRTHK^T sM^in)n<:s, ^xrni outline map The basis of ail detailiMl iinestigations is the district or areal study, it has been found possible to divide tlie 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 cbosen, 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 l)asis the area of the wliole 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 ad|oining 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 ]K)int 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 Barraiiquitas into a northerly and a southerly district Scientific Survey of Pobto Rico and the Virgin Islands Volume 1, Part BERKEY, INTRODUCTION TO THE CEOLOOY o; The northerly one has for convenience been desigiuited 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. TIio geology of this disti-ict 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 corjier, occupying \he area north of the Humacao District, and may be caUed 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 {h) are chiefly concerned with pure science investigations connected with paleon- tology. Immense collections have been made and seem to be Avorthy of elaborate study. It is expected that they may ultimately form a volume on the paleontology of Porto Eico. A considerable list of special topics enumerated in the original Eecon- 2S i<('li:\TIFI(J i<l]R\ I:Y of PORTO RICO naissaiice do not deserve so elaborate treatment as that suggested above, and indeed ai-e (|uite logieally eare(t for in tlie different distriet studies. Somo of tbeni ha\e been jnentioned in that eonneetion already. Among tliose considered as repi'esentatives of tlie ebiss are the following: ('() Mapable sub(li\ision of tlie Younger Series. (b) 31ai)al)le subdivision of the Older S(M'i(\s Complex. (c) Xature and origin of the 8an duan foi'mation. (//) I?e(d'-buildiiig organisms. {(>) Changes in eharaeter and composition of the igneous ])roduets al dinVrent stages in i^)rto Jiieo Aoleanie history. {/) I'hei-jnal s|)rings. ( // ) Stn<lies in magnndie differentiation in an isolated voleani(* eenter. (//) Origin of t1ie iron Ores, et(^, etc. An occasional special topic luis been ilie basis of a sc|)ara(e paper ( h'ettkc and Hubbard, HUS), and some (d' these lia\'e been or will be pul)lished separately in an effoi't to furnish useful information as pi'omptly as possible. In s[)ite of the complexity and extent of tlie oi'iginal undertaking, most of it lias already hecMi completed so far as relates to the general studies, district descriptions and s))ecial pr()l)lems. One* season's tield-woi'k on the scale of that of 11)15 or 191 (> would com])lete such work, and it Avould then be ])ossibl(» to make a final geological map of tlu^ island and to write file summai-y of its areal, stnu^tural and economic geology and its geo- logical history. A completion of it to that consummafion may be regarded as the immetliate ohjecd of these geological f\v](\ studi(^s. The |)aleontologic studies, in so fai" as they are ]iec(\ssary to the liistorical treatment and stratigraphy, are to he included in the ge(dogic iuvestigations. Siihse- quent studies of all kinds and to any extent may he based on the collec- tions, and additional investigations along |)aleontologic lines will then Ix^ includccl in a separate ^otume under paleontology. lil]HKE\\ INTRODUCTION TO TJIIJ (HJOLOGY 99 BIHLIOCniAPllY I>ERKEY, Ci1AHIJ:S P. 1915. Geological reeoniiMissaiu'e of I'orto Rico. Aim. N. Y. Acn<l. Sci., XX\ I, pp. 1-70. DlXWlDDlK, W. 1899. PhysicMl features of the island ( Toi-to JUco). Harper's Weekly, XLTTI, p. 248. Fp]TTKE, C. K., and Hubbard, B. 1918. Tlie limonite deposits of Mayagnez ^Nlesa. Porto Kico. P»nll. Am. Inst. Mill. Engin., No. 135. Hill, K. T. 1898. Cuba and l\)rto Kico, will) otlier islands of the West Indi<'s. Xew York. 1899. Porto Rico. Nat. (Jeog. IMag., X. pp. l.'MIl!. 1899a. The forest conditions of Porto Kico. P>ull. No. LM, lHv. F<M-(>stiy. IT. S. Dept. Agric. 1899?>. The value of l»orto Kico. Forum. XXVII. pi). 414-4P.). 1899c. The geology and i)hysical geography of Jamaica. P.ull. Mus. Com)). ZooL, XXX lY. Skmmes, Douglas K. 1919. The geology of the San .luan district, Porto Kico. Scient. Survey I'orto Kico and Mrgin Islands. N. Y. A. S., 1, pt. 1. ]»p. MrMK). Wilson, H. M. 1899. Water resoui'ces of Porto Kico. Water Supi>ly Papei- \o. .'^2. W S. Geol. Survey. 1899a. The engineering develoi>ment of I^orto Kico. Engin. ]\Iag.. XA'fl, pp. 602-621. 1900. Porto Kico; its topography and aspects. .T(mr. Anu Geog. S<»c.. XXXIT. pp. 220-2.^S. ><riKXTJFic ^I'ln i:y or porto jiico NEW BASE MAP OF POKTO IU(() P)Y Cin:sTi'it A. Ekkhs r\)]l<)wiii,o- a sliort fossil-eollectiiig season in Porto Eico in June and Jti1>'. lOlT), for the I^ew York iVcadomy of Sciences and the Porto Kican (lovcrnnioiit, flic American Museum of Natural Ilistor}^ cooperating, it s(M^m(Ml desirable for plotting results that a small outline map of Porto Pic<^ should he prepared. During October, 1915, such a map was drawn at The Amcriciin Mus(uun of Natural History by Mr. A. Briesemeister under my direction. Most of the data for the base sheet was obtained from i\ large map which had been prepared by the engineers of the Interior Department of the Porto Rican Government. The four-sheet editio?) of th(^ TL S. Coast and Geodetic Survey and the XL S. Post Poute ma]) of Porto Pico were also used. P)efore the end of October a solio pi'int edition of one hundred copies of this new base map of Porto Pico was publislied l)y Tbe American Museum of Natural History. Com- ])]imentary copies of the preliminary edition were distributed to the membei's of tlie Porto Rican Committee of the New York Academy of Scienc(\s, to certain officials of the Porto Rican Governraeut, to the various held 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, 8000, 3500, (000 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 vai-ious sources, chii^fly the TJ. 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 pri^pared by the Irrigation Service, the IJ. S. Geological Survey's topo- graphic sheet of the Luquillo Forest Reservation, together with H. M. Wilsoiv's topographic map of Porto Rico, prepared for the IT. S. Water Supply and Irrigation Paper No. 32, 1899. Numerous photographs by Professors C. P. P>erkey and H. E. Crampton of the Porto Rican Com- mittee of the New Y^ork Academy of Sciences as well as a large number bv mvself were of considerable assistance in the preparation of tlie topo- urapbic sbeet. Fifty solio print copies of tliis contour map were issued V OUT ME I, Part _ 65^30' 20' 10' ^}i » ;__ I ^ I I » NEW BASE MAP PORTGRIGQ' -Xompiied frorn the latest Government Sources ^"^Ey-ckESTER A.REEDS aTidA.BRIESEMEISTER October 1915 18° 30' i);iind8 ^&. i^\ \ no. kf' CrossCay I E Y^ Q o- js <! o ^/ iv R ^ - 10' l2^ 65°30' LEGEND Elevations in Feet. Depths in Fathoms. Three Fathoms and iess, dotted Area t.'<""*.L if .u^ Roads completed ^.tf^*»wt<ei>' Roads under Construction .,,/u-v---*x...-j..«ai. Roads to be constructed ._-._—— Native Roads 20' J,./, 1^ ^ .^. v^:. a REEDS, NEW BASE MAP OF FORTO 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 Kican 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 (U)NTENTS Synopsis ;•».") I iiti"c)(lii('tioii '{(> (i!eo,sj:rai)liic Location :{7 Nntnral History Survey of Porto Kico .'{7 San Juan District :;T ri)ysio.i;rai)liy of tlie Island :5S ( 'oniplex Mountainous ( )](llan(l , :;s INnicplanation 40 The Coastal Plain 40 Mature 1 )issection 41 Kecent Submergence and U])lift 41 PliysiojL^rapliy of tlie San Juan District 41 'I'he I )une Aiea 41* Tlie Playas 4:i Tlu^ '^rcM'tiai-y Limestone P>elt 44 Ten-aces 4() Tlie Ijnier ] ^owland 47 Tli(^ hiterior Mountainous Keju'icui 4S Incised Meanders 40 Stream Capture 50 l*en(^plane ."iO Hock Formations .">1 YounjL^er Series ry'2 liecent Deposits .)2 Unconsolidated Lime Sand 52 I'laya I*lains 52 I*leistocene ( ?) Deposits 5;j San Juan Formation 5:5 Age 55 Tertiary Limestone Series 55 Arecibo Formation 55 A^^' '. 57 Older Series (;() Kesidual Soil and Depth of 1 )ecay CJl Corozjil Limestone 02 Ia\ Muda Limestone 64 A j^uas Huenas Limestones 04 Limcv^t one at K 29 (j.-^ (•>3) 31 XCIFjSTIFIC *S7 7iM7;y" OF PORTO h'KJO I'age Juan Ascencio Chert Beds (>5 Corozal Jasper Bed C>^» Shales (>T Sandstones ^>-^ Conglomeratej^ ^^^ Tuffs Tl Age T2 Summary 75 Igneous Rocks TO Modes of Occurrence T(» Lavas 7(> Sills 70 I )ikes 77 Stocks and Bosses 70 Texture 7i) Composition 80 Vitrophyrs 81 Rhyolites 82 (Iranites 82 Quartz Latites and Khyo {nnlesites 8.*] Quartz Monzonites 84 (Jranodioritea 85 Dacites 85 Quartz Diorites , 85 Trachytes 8(5 Syenites 80 Latites (Trachy-andesites ) 87 Monzonites SS Andesites , 81) Diorites 1)1 Basalts 1)2 Olivine Basalts iV.] Summary 9:* Structural Features 1)5 Unconformities 95 Faulting DG Folding 1)S Metamori)hism 1)S Minor Structures • 98 Cross-bedding 9,S Solifluction 9S Stratigraphical Summary 99 Mineral Occurrences 102 (lOld 102 I *lacei*s 102 Auriferous Quartz Mines lOr, Platinum j04 <>>PIH'i' 104 >^/';j/ ]//;>s, a/joLOGY of the ^an juan district 35 Page Lead 105 Manganese 105 Road Metal 105 Building Stone 106 Guano 106 I llustrations 106 Cross-sections 106 Maps 106 Acknowledgments 107 bibliography 108 SYNOPSIS The area (leseri[)ed in this report, which lias 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 ])hysiographic history of which may be regarded as essentially the same ns 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 vslightly sul)merged and partially uplifted, erosion continuing in the interior from tlie time of first uplift. The formations of the district may be primarily divided into the yomiger 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 <i('lH)sit of indurated lime dune sand of presumably Pleistocene age over- 1\ ing unconformably a limestone series, which forms the coastal plain proper. This limestone series, termed by Berkey the Arecibo formation, lios almost horizontal (dip 6° IST.) and is now maturely dissected, giving I'ise to the typical pepino or haystack topography of the island. The Arecibo formation is regarded as upper Oligocene in age. In the north- ^\'estern portion of the island, but not appearing in the San Juan district, 1^ a lignitic shale member (the San Sebastian shale) which underlies the Arecil)o formation. The age of this shale meml)er is uncertain, but in all P'o!)a])ility it is lower Oligocene. The older series consists of marine and volcanic sediments with numer- '^^ intrusives. It is highly tilted and locallv folded, resting uncon- 'Ml ^VIESTIFKJ ^URVI-JY OF PORTO RICO roriiial)])- Ik'Iow the yoini^^er series. Tlic sedimentary types rei)reseiited are limestone, shales, sandstones, eongloiiuH-ates, tnfTs and aslies. ^^ itJi hardly an (weeption, these sediments are either of voleanie origin or were (h^posited tliroiio-h voleanie ])r()eesses. Even the limestxxnes show ])rec- eiated structure and ar(\ with few exceptions, merely accumulations of limestone rra^i^iiients derived from older beds sJiattered by volcanic ont- hursts. 'I'he roraminifei'al content of certain shale aiul limestone heds in the upper part of the older series indicates that the l)eds are -ujvpei' Cretaceous and Eocene in age. The lower part of the series is no douht Comanchic. The peneplanation oi' the older series culminated along the coast hcdore the deposition of the coastal |)laiM, for the peneplane surface can now he traced underlying the Areciho formation. In the interior. h()W(^ver, erosion continu(Ml until the u])li ft, whicyii occurred in late Oligo- cene time. The older series is everywhere intruded l)y igneous rocks of many varieties. The predominant type is an andesitie porphyry. The tuffs and other sediments directly derived from volcanic sources are also of tin's gxMUTal com])osition. The igneous rocks oc(nir as extrusive sheets, sills, dikes, and small and moderate sized stock-like intrusives. The ]nineral resources of the district are not large. About 100 ounces of gold are panned annually from the rivers in the vicinity of CV)rozal. (h)ld (piartz veins have been worked in the hills south of Corozal, but without success. In the barrio Pasto, southwest of ]\Torovis, se\eral co])])er ])ros])ects have been slightly explored. IWYROUUCTIOK 'J'he Antilles and the Central Americian region have remained until the close of the* nineteenth century almost a lerra incof/tti/a as regards tluur geological structure and history. Unfortunat(dy, little study has sijice l)een devoted to this (jritical region which connects the two Americas, and until more accui'ate infornuitiou regarding it is available our id(n\s of the geologic correlation of tlie two contineJits must remain largely conjec- tural. At the (dose of the Spairish-American war Porto llwo, ])eing the closest of our ncnvly ac<pnred insular possessions, attracte(l a good deal of attention. During the tirst years of the Ameri(^an occaipation nnich was written about the island and its inhabitants. ScNcral \ery good papiu's describing its general configuration ami geology were j)ublished during this period,^ and a ]rumber of articdes on ])arti('ular features <>r the general aspects of the country appeared in currc^it periodicals and in 1 Dorsey (1002), HiU (ISOOff). n899r), Wilson (1809), (1900). procrtMlings of lejinied societies. Interest in Porto liico, however, soon waned, and from 1902 to 11)15 nothing regarding its geology was pub- Ijslied. In 1015 a paper by C. P. P>erkey appeared in tlie Annals of this Academy, whi(i]i gives the first detailed and ])urely geological description of the island that lias yet ])een written. (}i;()(;ijAPiiK* Location Porto Pico, the smallest and easternmost of the Greater Antilles, aver- ages .')5 miles in width and 100 miles in lengtli, presenting a regular rectangular outline three times as long as broad and alined nearly ex- actlv east-west. Its shoreline is nearly straight and quite regular. The coast is usually low, Avith a few low ])romontories. The fringing keys and deep indentations characteristic of a more youthful stage in coastal de\'elopm(^nt, aiul so abundantly developed on the Cu1)an coast, are en- tirely wajiting. Its area is ruriO square miles, just 500 square miles less than the area of Jamaica, or about three times that of Rhode Island. Its |M»|)ulation, considered as a whole, is the densest of any country on the globe. Natuilvl ITisrouY S(jkvi':y of Poirro Pico In 11)11 an a])pr()[)riation was ]nade bv tlu^ Insulai* Government of INtrto Pi(M) 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 ])hase of the survey Avas begun in P) 1 1 when Dr. Berkey an(! Dr. Feinu^r s])ent a moidh on the island, making a geological recon- naissance, preparatory to a snbsequent and more detailed survey. The following year the writer was assigned to Wm^ San duan district to begin 1ln's more detailed work. Sax Juan DisTincr The region selected may best be called the San Juan district, as San dtian. the ca])ital, is situated in the northeastern corncM*. The district is that area comprised between longitude ()()° 05' and ()r)° 27' west, and an •ast-west liiu^ through the town of Pjarraiupiitas about 21 miles south of die northern coast. The areal extent of the district is roughly 500 square nnles. A correspcmding area of similar width to the south was studied It the sanu> time by Mr. E. T. Hodge. In this way a belt 25 miles wide was sur\eyed entirely across a ty]u'cal ])ortion of the island. Three "»'»nihs were spent in the liidd, in which time a topographic map was "it>tructed, the mineral resoui'ces stndicMl, and the geology plotted in as .*)S srjtJXTJFic SI R\ i:y OF roRTo lavo iinich detail as possible. consideriD^- the sliorlness of tlie lime and tlu size of tlie area involved. 'vwv: iMivsi()(MiAniY OF Twi: island ^V\\v i^cMieral eondiiuration and io])oii,ra})liy of the island liaxc been well deserilx'd by sexeral writers. Doi'sey (UK)'^) bas written an ad]nii'al)Ie description of the several types of topoiira|)by one meets on erossin:^- tJie island, witli an es])eeially detailed deseri])tion ol' the limestone top(\u;raphy ot tlie coastal ])lain. Wilson (IS!)!)) also has a'iven a. i^ood <;-eneral de- seription ot the physical asjjects. and Hill (1<S!)!)) descrilxMl the ))hysio,ir- raphy in conmndion Avith bis u'colooical reconnoissance ol* the island. Thi^ rec(Mit articde by Jk^rkey (11)15) i^ives the most modern and detailed de- sci'iption of the ])hysio^iiraphy and ))bysiographic history of the island. The island of I?orto Kico may be deseril)ed as a (H)m|)lex mountainous oldland, in the /d'' cycle of de\cdopment, which has been pene|)laned, or rediK^ed to an old sta^i;(' of toj)Oiira])lnc; development, snbmerged, over- lap})ed by a coastal plain, npliftcil and matui-ely dissected, partially siib- merg(Ml, then re-npli Fted and fnrther dissected. ('()MPLi':x .MorxrAixors OnnnAxn The inost chai'acteristie topoi^ra])hic feat n re of Porto Ivico is its com- plex monntainons asjxH't. Idiat ])ortion of the island that rises al)ove sea le\el is merely the eroded ('rest ot a submerged monntain i-ang'e of i^wwi complexity and ma^^nitnde. This eroded (a'est rises steeply from tlie sea to an average altitude of 2500 feet along tbc^ divide. Owing to dilTerence in rainfall on the northern and southern side, the divide^ has been ])ushe(l to the south by the encroachment of the larger northern streams until it now stands two-thirds of the Avay toward tlu^ sontbei'n coast. The ])resent ]K)8ition of the divide has also been aceounted foi- l)y a theoj-v (d' block faulting of the wliole island with the front face of the block facing south (Berkey, 1915, ]>. 40). In the western ])orti()n of th(^ island the divide is double, where it is ])arted l)y westerly tlowing sti'eams. Near Adjunta- the two divides join at the tiead waters of the Anasco Jliver to form a single ridge, whicli continues to (^ayey, where it iirst swings southward. and then sharply to the uortheastern corner of the island. The Lnquillo range forms another westward ext(vnding brancli of the divide nearer th'' nortliern coast. The texture of the mountainous topogra])hy is uuMlium to fine, exc(^]» for certain lowland areas of coarser texture. Idie intei'ior topogra])hv r •v'/-;!/. !//•>'. <:FJ>iJH;y of the s\\ ./i\\.\ i>i>;Th'if"i' ;->,;i prciktininiiiitly tlic ridge or vurlilUn \\\m^. The slupes rtre cxtrrturdii'iarily sUTp. slojM'H of .KM4I demves liuin,ff oftc.ii seen. ln,>lwfeii thivc ridu-en thi; >tmnns run in V~<lmf.fd vnlleyj^. S«hii<> of the valley wall^ are very r^teep, lo h.' luiiiul. T.lic i^irvmm iiurth uf tin- divide nrv very miiiu-n.u.^ and of lai'.i!!' '.^olimif. diu' to tJie ahundiint |ii'('fipitrtti(Hi of Hit- large per rciu id' ruii^uir. IjoisiL til*' hirgcst river, rises milv eiyid luileH north of Arrovo ,el'i,r Ihf innrvr >lai,i irt K . "n llie *<Hiilierii coiiHt. The I'bita, next in nze, rises the .-anio distance ii'.rih of (iiuiviiniii. The hirger streainn rise high up in the nionntain?, ••nid ni the first few luihi's develop Ktee]) ,u^nhent^5 ehiiraj-teri/.ed hy fall^:. nipids and he.ulder^strewn heds (Fig. 1 ) : then pa-sing out intn the Inw.n- >w!ng in hirge meanders over the Hat playu plains br-rikn-ing tlie enast. Ttiiis a ty])ieal stream su«-li as the IMata nn tlie northin-n enast imu- show vf-rv stage in development from y<aith to hdx; matnrity in its trompara- ::\<'1v sliurt eourse to the sea. The eomplexity of tin's iimmilainoiis ..dd,- 40 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. Peneplaistation 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 Eincon undisseeted 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 8AN JUAN DISTRICT 41 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. Eecent Submergence axd 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 upon them of recent shells fixes the time. On the northern coast recent marine shells are found some distance up tlie 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 JUAX DISTRICT The San Juan district, as already stated, extends from San Juan w^est- 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. 42 SCIENTIFIC SURVEY OF PORTO 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 (PL 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 Elvers, 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. N^ear 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 <o sh-MMt:s. (,'h'(fiji<n OF Till-: six ./r lv fusrinrr ;1 ill tJlr W.-sHTll |>;l|-( ..f till' (lislrirf. hut ,1 fiirlluT \\v>t tli.iii till- S;iii .hiiUi 'li-iri .]>,. lln> nri-iii ami wniixr of {In.' Pl.-isUM-eiic n> well iH thr m-Mlt. <;ui<!. will l-e (liscii..s(;(l iiij.l.T n ^^iib<c(|iiriii hcn.lin-. rh.. pliiyii plains, a iiriiin! -iveii hy Hill i.. ilic lati-likf phnn? iliat .-x^^^ ,1 fn.m ill.; r.H.tlnLI.< of tJu; ilissiH.-U'd Vnm-y.Umn nit'sla lo the fua<i. arc he ^wii alniiu; tlio ciitin; f<nist ..f tlir Sail .ruaa ij!>tri<'l. S,. pa rale. I 11, ihi: sea l.y th.' <Iuih: Ih'Iu i1i<\v v\Uhu\ inland tn ihe T.aiiary Irnir- ii.!s. and jullnw n|) Iho lar.ufr valkw imtil th-') lit' dir.;i:tly mi ilin r(.rk< t the niieriur eoniplex. The phiya phmib rise tnily a fnw feet ahnve sea •\rl. ami unlil recent years large arean of the plains in the San J nan dis^ riet have been eoverod witli water. I'urinin.u- inland swamps and la-<M.ns. dn.-o rhi- valrie nf iJie Jand^ \ms been reeoonized lur sng-ar^caim pr.idnctii.n. II the available areas In-ive lieen drained, and m.w eunsiiiuh! the jnust n-ndni:rive and valriablt^. lands tai the island. Tim lew, llai. inonnrniiuns xr.an>e (.1: the plava -fthiins is i.rcasitmally Inniken by low, rtnnnle.d liihls r'lln. TeriJaTY liinesluni-s rising al.nve the alluvial (Rig. :}). These inie.stcme Inlls'aiv isolate.! rnniiiants of the coastal plain here buried bt- iealh the recent dt^p'«>J>- khving tn the slight eh-vali-.n llnit t1iey atrord, ii-.v^. ilie blister iiig bear of the snrr(mndiiig [.dains. ibey are seleeted as •wn and dwelling sites, llotli D.trado ami T<«t Baja. llie only twe. t..\vns 4 1 ^cih'XTiFK' ,sfini-:y of porto h'fvo situut.'d in thi; center of tlie i}hipv<, iiro lniilt open tlicwc liinestoiu' rcii^ luiiitr-. Otlicrs jirc vh.omi as nitcs for the imljvi.lual <lw('lliii«-> ol' the njitivcH. The plava pUiiii^ follow the tliiTH' ]>n"!ir'ii)a] viillcp of the Ciljueo, I'lHtii, ami BavrtmoH Ihxevi^ for some -lishuicc inlHiul. and in tho case o( \hv IiitJrr two cxtond well bovonil the liiiieHtouc belt. The --eolooieal history of the formajion of th.e ].kTa plains has, no (lutibt. been e<Hn,ph'x. It is evi«lent that thev are in part of fluvirtl ori.am, for they -nHl(> direetlv into the irne alhivinl phiins of the river valley. .Fust flow important a rblu tlif sen hiis played in ilieir formal ion is \i (lelinitely kjiowii. It is the writer's opinion, Iiowever, that both faeto have been intluential. This point wilH.e further disenssed helow. Tiu-; Ti-.ui"iAi!Y Lj,MJ-:s:ro>.i' Hiihr As the northern eoasi of Porto Wwo is approaehed, one readily distin- _mn'sbes two kinds of topti_<:-ra)ihy ; tlie riig'<iH'd moiiniajjions skyline, and the bnver, an^e'idar. maturely disserted linieslone eoasfal plain. On closer inspeetion it is seen that the topon-raphy <d' the tr.asfaJ ]dain consists of niinierons isobitefl or elosely j^i-nnped aniiidar hillh; not exceeding 100 feet .n7v.I/J//-;.v. (JEi)IJHlV (IF Tilt: S\X JVAX l^hSTRICT 4.5 I! iiltiliit'le, Imt of s^iich iiii,!i"uhir mul rrig;!>"t'i| oulliiH'. iiinl .«n tlcu^^oly cov- Tc'l with trnpifiil Yi!«ii'tjitinii t'hrtt they Jorin thi; most iiuu'cessihlo aiitl !ii|niH^rthlc' re<>-ioiiH oi' the district. Slopes f»f }0" aro quite comiiiuii iiinl H-,'!i^ioiinlly \i^rtiortl faces of linu'stoiic risiii^u- !0 or 21) iV-ct jito to he seen, rhuiigh froni a, (listaiu-c th,e hiJls appear well romuli'il, yet at eloso range hey arc exceed i ugly roiigii aiul Jagj^'eih tuHl arc. as one of the .Vmericaiis iviiig ill the vi<'inity expressed it, "very hard to seTHiubk! up."' The 111 usual appearance t»f these limestone hill< was early iiotieeel hv the iia- ih».(l l.y li. 'cs. w'lu) call them pr jiiiui.-^ or ciiciiniher^fihapcd hills. I>crkey has nsct • • term "haystack hiJIs.*" a term pe.'uliarly fitted to piclure their a'ppcar «v, rising ahniptly a> tbe.\ do from comparatix'cly level plains (Fi.u;. I 1 r»oth Dorscy (l<H)-.\p. HUO) and J'.erkey (HMo, p. :»2 ) have indepcnd liy come to the same coneln-^ion as to the oid-in of the luiystar-k to|ioy r'iiv. tlerkev siimmari/,.'.! the steps in^ their devrdopment a.s follows: 5V(. indi'e or less ititeriiiixtii eartliy niatlers whifli are ilistrlhiited 11r 4:6 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, (&) 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 Yega Baja to Morovis at K^ 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. ^EMMES, GEOLOGY OF THE SAN JUAN DISTRICT 4,7 (a) The presence of a shaly layer in the purer reef limestones may be the co]itrollin<^ factor in tlie development of a plain at a particular level through difference in solubility. The pure limestone would soon develop large caves, extending down to tlie 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. o). 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 tlu^ barometric readings. (h) 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 Areciho formation lUiistrating the possible origin of terraces from differential solution 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 i? 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 v^edge-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. THE INNER LOWLAND This topographic province, described by Hill as the "parting valley" (Hill, 1899fl, p. 105), extends entirely across the San Juan District. It 48 SCIENTIFIC 8URVJJY OF PORTO RICO lies between the iiifaciiig cuesta of the coastal plain on the northern and the mountainous region to the south. It is best defuied at Corozal (Fig. T) 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 soutlnvard past Guaynabo and merging gradually into the interior mountain. Its topographic development is more tban tliat 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 INTEIUOR MOUNTAINOUS ]n:Gl()N South of the inner lowlands the topograpby of the San Juan District is typically mountainous. On approaching the island the rugged serrate Fig. (>. }iif(i(c of (i(est<i (trtd inner loirhiiKJ near Coio.tdl Sh()\vin!L>- relation of the Corozal limestone in the older series. character of the skyline can be distinguished soon after its outline rises above the horizon. Passing through the moimtainous area one is im- pressed witli 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 hkewise closely and deeply corrugated l)y ravines and gullies, the wliole region assuming very mucli the appearance of a wrinkled handkerchief, a simile said to have Nf;j/ !//>•, fiHoijjav OF Tin-: klv .hwj niminvT 4!i Ivrii x\<vA bv (^)liinilnis in ,|<-<cril)iiiff the topo^mphv nC tho Aiitillrs to (.UnH-ii Isabe'lUi. Tliu (IniLnap/ of the JuouiitaiiimiH arcsi as a wliolo is fl(?ii(lritie in 'pat- h'lii. TIh' rtdjyshHeiii of the strcaio I'OiirsHs tcj iin(ler<i- round strnetun; \> \a:.v\ fiire. A, few ea>os weiv noted where Home adJiiHtmynt: had taken pliicc, hot nowhere \va> it eoii<i>ieii(vus. Altlmngli the iTunintaiiuju.- region H nnderlain hy n complex of formations of miieli variety, jet tliere is little if'iH'ed fur a short dislanee as a. poorly tliseeTnihle rid.a'e. and it was ne^ / UH'I tllV illf„re .,f leed iluit The higliest points were iisnally underlain hy hJ^'hly indiinited -dieeoos tiillV, aSj for example, Torreeilh', .northwest of Ihirrampiitas. and •i''leelial, some distaiiee soiitli of* 15avamon. oine of the streams have ilevelo|X'd dee-plv intrenched meanders as .' puss out from tlie mumitainoius oldland. 1'he Itest example is tliat 50 SCIENTIFIC SURVEY OF FORTO RICO of the Orocovis Eiver 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. Kear Comerio, however, there is a wind gap that was evidently at one time occupied by a tributary of the Plata Eiver. This tributary, now represented by Mora Creek, formerly flowed from the southeast through the wind gap and occupied the upper valley of the Arroyato Eiver (see topographic map). Subsequently another tributary of the Plata, work- ing down from the north, intercepted the larger stream, forming the present Arroyato Eiver, w^hich joins the Plata at K 23 on the Comerio Eoad. 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 ISTaranjito. The peneplane level, occasicfnally 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 w^ould appear that the peneplane surface was slightly upwarped along the southern edge of SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 51 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 regar"ded 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 v^riter 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. EOCK FOEMATIONS The formational units of Porto Eico 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 f 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 somev\rhat complex, and showing considerable variation laterally. The lower shale member is evidently a separate for- ^national unit, merely representing a local .development, which is not Sound in the San Juan district. A subdivision of the underlying older series, however, is .quite another matter. The complexity of tlic forma- 52 SCIENTIFIC SURVEY OF PORTO RICO tioiis 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 Unconsolidated 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. ilt 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. si-:m'mi-:s. nFjiuxiv (u-' tiik k-i:v ,/c:,i:v ihstuict l'LK|i<T«H'KM^: ( r) DKl'OSlTS naif tlic format iuii ujmhi -which the city oi Saji Jniiu i> huili. It Iiere forms a >rrikiii_y- pnHiioiitMry lluit I'XtiMnls as n spit sevei-al mih^'S out J"r<tiii the adjuiiiiii^ti- iiHjiiihiiHl :rii(l at its cxtremitT attaiji.- a.ii aliiliuh;- of alinu,.^!: a hiiiidrHl loft. This maximimi ch^valiun, is t-rcwiied l»y llu- inetiircsqiu! HI ^Inrn., the mas>iu' walls uf tlu- i-asth- hfiii^ hiiilt upmi the very ^hp- ..r tla (Fie. s). The San Jiiaii formation '<\-> seen at this point is a very porons. medinni I'l coarse graJned faiuistone eomposed of miTnite fragiiien,ts of shells and '.vell-rcroiided. quartz grains, hennd tcjgetdier with eahrareous cement (Fig. •^). The eal<?areous grains prcdomiiuiti! and in scmie localities form prac- tically tlie whole of the rock, so tliat the fcninaiion is essentially a lime •^and. In other hjtalities the quartz content is hii>-her. hut in nn casc> did Hi-ll-:STI FH' t^lini'.X OF I'ftUTi) RICt ii; n,.n>tit.ij{(; as iiu!c-li as liftj per cent of the rock, in the virtiiiiry nf ^Sijii .liiaii two distiiK;!,. types ot: the roiinatinii were IV.uihL Vim. that which oeetirH well above sea Jevel near ^h.rro (•a>tic. At this pniiit tlip lime sand is exeecdingly jioreus. The itciucnrin.u^ material <h.es not (ill all eoiiiaft. This iut..le of cementation has crearcl a roek in whieh the vuh nine of the pi>re spac-es is a.l>mit W-:W [u-r eeiit of the entire rocrk. The >ci'on-(l variety uf lime sand was taken frum a qiia.rrY neiir Borin<pien Park. !ihuur flirec mile^ cast ni the firsr Icealitv. The material from this "^ffW: '''"*'"^'J/P'^ eiiarry. c..iiini.u^ from hcl.nr sea level. i> <len.M. ;ind clo^elv ecaiic-uicl l.-eiilly use.l a> a Imililm.e- sr,o!ie. It is evident, fherehuv. that the of ecmeiitation is (iependent upon rhe hmgth c,r tune Uie h.rmati 'been exposed to the free eiri-iihii ion of calearcMnis selntions. Ttie pceiiuii of the fornialiuii, hein-- rc-eh'\ated jiftcr oe1\ a, short pe ;-nhmer2enee, innv shows only tlie initial >la,ii-es ot .••onx.lidat loje Ihi- lower portime still siihra<a>n.d, ha- lamse.jnlafed iirto a dense m dural.le rock. The t|nestjoi! then arises whether lie- p.rro.:it\ ..f lie, i.i.rtion (W tlie forinHlieaj !.> ne»! due to suhscipienr leaehiiej; j-a.!eer 1 SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 55 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 w^estern 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 56 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 6° 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 Yalley. 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 SEMME^, GEOLOGY OF THE (^AN JUAN DISTRICT 57 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 tlie 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 reconnoissance 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 58 SCIENTIFIC SURVEY OF PORTO RICO greensand marls of undetermined age (Eocene or Oligoeene), 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 Clove's earlier supposition that the older series was of Cretaceous age. Through the aid of Dr. William H. Dall^ the material v^hich 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 Yega 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 : MeAris trinitaria (Dall), an index of the Bowden formation, upper Oligoeene of Jamaica, found throughout the Arecibo formation in the San Juan district (PI. II, la, Ih), Pectens^. (PL II, 5). MiWiasp. (PL II, 2a, 2h). Haminea sp. Yery abundant (PL II, 3a^ Zh). Oyprcea sp. x4bundant (PL II, 4). Atnaiiropsis sp. Comis^, (PL II, 6). .1 m pulina sp. N.-^ i}i' I'm-: AIM SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 59 Strombuss^p. (VIII, 7). Orbicella aff, cavernosa (Linn.). Fungid corah not determinable, but similar to those found in the Oligocene of Antigua, Anguilla and Oaba (PL II, 8). Venus s^, (PL 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). (PL II, 11.) Pecten sp. like P. thetidis (Sowerby). (PL II, 10.) Fasciolaria (PL II, 12). Amauropsis ocalana (?) similar forms in Ocala limestone, Vicks- burg formation, lower Oligocene of the Gulf Coast (PL 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 : Venericardm 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. (JO HCIf'JXTlFIC ^UR] f]Y OF PORTO RICO Older Series IFnderl}dng 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 (PI. IV). The rocks of the older series include: Sediments essentially marine. Sediments essentially volcanic. Igneous rocks. ^larine 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 SEMMEt^, GEOLOGY OF THE SAX JUAN DISTRICT 61 intimately associated. The ash beds are usually well bedded aud 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 tliat 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 ^N'one 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 w^ithout encountering bed-rock. ( ^(msequently, in this lowland region outcrops are uncommon and the few that occur are often so highly decomposed that tlie 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 40° 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 st'il'X'flFll' srm'HY <IF PORTO Riro iiiiiiiiil liticl <-aii he pJouglu'il wJihunr. foar cf WiiHliiii-. oven alh-r tin- u'aut'>,t raij). lierki-y asrribe^; ilif i-anM'^ ul: ilio htahiliiy of tlif <.til iiarill)' f(. thi'i'c fiiclors { liM-"), p. .i:*) : Jir^^t, th.; dciisc .;'!.\(>r nl" icgctH- itiii: scH'und. the small rarigo in daily ani' aiinuiil tcMujK'nitiirc : and. hird. ihi! low (•(.ntent of itiort; ui- rpl'iiu'tury iiuiloiaal. .-ludi as <|i,iailz. jii In- rutd^s wlinse do>frurtinii \\m fjinii^hi-d the mW. Nciilipr ui: the liist w., of tJieso rart«.rs wnuhl oxplaan tht- <1alHlity of tlio noil w\w\\ iini.ru^- ertfd, >tdi«><|iir'jit. t(> indiivalioii. 'rhc third, however, is, id: vjlal siuniii^^ aiH;»\ The io-n,Mai> T<xd<K as wcill as the xiAvnnw matorjal axe i)re.|(,i)ii- laiilly of aiidesitdi^ ('(aii|M(>if ion, roiitaiii large a.moinils d' iVldspars and .rr.MHci.u-nrsiaii ndnerals, and little or iw quaii.z. 'I1ii< on weatla-riri- voidd -ive rise to an aroillaeeous >oil, essi-ntiallT a elay. whieh would, hv ery nii|.ervi,)iB. X veil of this eliaraelTT wnidd not perinit i.he rr«'e «'ii^^. Hth of Bui eiilatioii of ui-oniid waler and woidd tiol readlijy he rcmou'd, and iraiis- jiorted bv {he siirfact! riin^ulT. 'IMie preseiice^ of a^ widespread mantle ed' sueli impervious sod, lui.uiit well explain the peeuliar atwnee of meleorje sprin.u's throudioui the i.hriid. True sprin.H's are atmost uid<nown in the San .1 nan distnet. The so-ealled sprin.n-s ajv merely rivideis of snrtaee water, hidden in their nj)per eoni'ses l»v dense veg-etiitiim. On the souJiiern coast several thernuil spririg-s have heeii /nun<l,, hut it is now helievetl thnf ttiese derivr- their wiWin- from magniatie rather than meteorit' soiuves. Whether the iinper- vionsness of the soil is the soh> i-ause of the ahsenee rd* spriii^-s is not yet deterjnined, hnt it. is at lea.st a pussihle exnhniatior! of an otherwise unexplained pheimnienon. eoia.>z\L i.ni r:sToxi^: ddisi southwest of the tctwn of Corozal. as <aie crosses the river al the fiird oil the rnatl to Barros. ean 1h> seen an ontero|i of limestone, rlnse SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 03 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 l)ed 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 l5"-20° N. E., since it was apparently conformable with the neighbormg 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 CaCog from the sea. The presence of marine organisms in the inter- stitial CaCog 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 file locality just south of Corozal, where the brecciated structure is best developed, marked the locus of the eruption, and that in other unex- ]>osed portions of the limestone bed the fragmental structure might be hu'king. In this case the fragmental material would be practically in situ, '*»i'ming only a brecciated phase in the original limestone bed. If such were the correct interpretation, however, one would expect the breccia h> contain a large amount of volcanic material. In realitv it is com- 64 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 MUDA 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 K. 48° W. and dip 30° S. W. The thickness of the l)ed 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 LIMESTONES Two miles south of the town of Aguas Buenas are the great caves of Porto Eico (Dinwiddle, 1899). These are developed along the strike of a limestone bed about 100 feet in thickness. The strike of this bed is roughly JST, 50° E. and dip 20° IS^. W. In the valley of the Bayamoii Elver, 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 Eiver, so the strike was closely de- SEMMES, GEOLOGY OF THE f^AN JUAN DISTRICT 55 term i lied 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 unconforniably 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 tulfs, or rather tuffaceous limestones, which resemble Muda limestone, except that they contain larger amounts of tuifaceous material. No forami- iiifera 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 x\guas Buenas to the Guaynabo limestone may be made clear, for the former should be truncated by the latter in the valley of the Loisa Eiver north of Caguas, provided each continues that distance along its line of strike. LIMESTONE AT K 29 On the Comerio road, at K 2d, 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 Eiver. jua:n- ascencio chert beds Xorthwest 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 f'sck, which consists w^holly of an exceedingly fine-grained aggregate of silica, showing an absence of any bedding. Just north of the ridge on ^vliich this chert outcrops a siliceous rock showing fine crumpling was ^I'^covered. A microscopical examination of this rock proved that it was t's.seaiially the ssiim- coin pun i lion as the eJiert to tlie south, exi:o|)t that it was quite highly colonsd in t1ie wcathorcMl oidrrop by iron r.xi.h-s. TIjc hirst ol' thcw chert; hoik probably orio-iriatct! through the silicifientioii of suiiie sediiricntory bed, citlier a Jiinestone or a calcareous shah\ The RH'ond elicri is either a j;diasc of the .^amc lorinaliuo or a dilTerdit oho. wliioh Kluiiiped before stdidiiicaiioii. Sid)s<>c|nciitly the fomiatioji wa> sili(-ifi(>d. the intense enioipling due t,o soli tl net iou f)eitig preserved in rh.- sec-ondardx^ fornuHl ehi-rr. Still hiter. tlic iron eont<'nt of the '•iicrt Inis been oxidized to limonite an<l hematite whicli - the erninples. thus ser\-ing to emphasize \\\v ir struct ri re ( I'hg. 11). ba!.d> folhoviu, .»r the rrnurph; A l>e.l !(» feet tluek of Jasper was found in tlie (\mv/A\\ M'wvr t\v<. unlr jiheve the town. Only one oiiferop was toeated. front wliirli it apjieare thai the Ited was eceibyrnnfbh' with the sumninding formations. Froi the omoinit (»f Jiisper H<»at b)und in the streams it was irdVrred that th bed was of eonsiderahh^- extent. 81JMMES, GEOLOGY OF THE SAN JUAN DISTRICT 67 SHALES 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 shala 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 shales 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 45° 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 ot" quartz. The rock is very dense, with little or no pore space. Faint i'anding can be discerned in the hand specimen, but no orientation or 'edding is visible under the microscope. Berkey has suggested that these i;,S SflF.XTI F1P HV'RVt'.V OF I'ORTO HIVO .^liiilcs Jiiiiy l)f! etjiijviilciit to tlKW ('X|h)«m1 at Mayn.miez and Fajiinlu. Tin- iiiateriiil from Fajiirtlu. Jiuwi'vcr. ih: t]u\t(} diUVri'iit in iVpperti'aiiw. Jt h n li.dit. |M)rr.i!s hJuiIc, Ww porosity of wliioh is duo to lllInl0^^il^; iidlillTi; cavi- ties wlii<'h iippiiroiitly wero previously ueeiipuHl by forainiiiiferal shells. The lea.-hino- of these- nikniVAim oru^iurisms lia^ pr<Hhice<l a porou.- ^inn- ture whieh is not 1o he found in ihe d<-l!Se shale.s expr.sed at 1\ ">. It is possihie that tliis shale inav ho equivjilent to those expos(.<l at Faiard.. and 'Mayaeiiez. hut the writer dfH'H not helieve that eorrehitiou of siu-h wiihd) sepiirateil htrnisit ions rnii vet !>< um<\v witli any i]i%riH; of eertiiiiitv. T.iipknl f,>,„,„ ill Ifentl s U\ a road iiu'tal quarry just otT Hm Cotnerio Toad, at 1\ H. there is n\ exp.»Mire of .sandsltuie with ioelud(Ml fragments o( shah^. The shaU> n dark, <lcii«*. and very line-textiired. ft^ oeeiirrenec as angular fragmenl^ in tlic .sandstone leads oni' tx^ infer that it, has undoTgone mtieli tin' sanv process (tf formatton as the (/orozal limos^ti»ne hreeeia. This sliale ot^ajr r<>nee w iihsigiiiheanf in size, hut it is of "Teat interest m that it eonlaiiT iiiimerous foramiiiifera. -whieli not only in<lieate the conditions iindei widch it wa.s deposited, hut also pve an idea ol' its a.e^c (Fi,- 1:>). Tl- forainhiifera. will he (h^'serihe.1 in tlic disenssion of the age of the old. ^ SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 69 A somewhat extensive series of shales was discovered in barrio Piiias, southeast of Toa Alta. These shales are well stratified. Their strike is about N. W.-S. E. and dip 20° S. W. On the weathered outcrop the shale appears as a lights 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 TJnibon Eiver. The shale strikes K. W.-S. E. and dips 30° 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 lias been thoroughly reworked and assorted by the sea. SANDSTONES Pure quartz sandstones are almost unknown in Porto Kico. This is due to the fact that so few of the rocks contain quartz in any appreciable f|uantity. 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 nimier- ous inclusions of shales. The formation dips 42° to the S. E. and appar- ently strikes N^. 65° 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 I'erromagnesian minerals, feldspars, and occasional grains of quartz. This material is undoubtedly of volcanic origin. In other words, this sandstone i"^ merely a fine-grained tuff, the constituents of which have been some- 70 SCIENTIFIC SURVEY OF PORTO RICO ^viiat rounded and assorted in being transported from the j)oint 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 v^as 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. CONGLOMEIIATES 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 IS, 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- 8EMMES, GEOLOGY OF THE SAN JUAN DISTRICT 71 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 tuifs 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, liowever, 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. ISTear 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 72 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 oi'iginal 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 botli 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 Earros. 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 probalbly several thousand feet. They are well exposed in the bed of the Guaynabo Eiver, 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 73 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 .>|0'''" .^^" ^^' ^p—>^^^^^^^-^ ^^' yfVI^JGVERO Sw^,.,v_-vxr-'^ )r ^ ^.i^^^^ ^^^uke" I)r^-HSJ^DOJI ^c: LONETA^^^;^^^BAJAS^^ i I UA 1BOS'^is//y^^'1^^^ ^te^ )l^ANATl /^W^KA L^-N ::;;:^^^^' V ^^ / ''"^ \^ "7!(^^^B^Ah^ N- feow s { s o^jf^AAITA y\ ^ :p& V \ GUAmA^Vk ^ i flv^ MOiRO^ Ax....q^ 5 J^'* HMbda 3 ^~/ I / cf ^sNARANJIji1^^ ^\ V n?" xi \>^^%4 A O ^7-/ 0>AGUASBUENAS_Qx:' /^\ V. '•'T\\ ^ .^ ^ ^ ^Ti 1 y r '-/ r^ ^ ^'^^ ^ u v S\ ».^~v ^'^ / ^ "w** s^^^r8|^^ >-- ?sK IffiiRJfjITA?k./f, ^\c* ^ Idrs^;^ >^I^NL Pig. 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 Eudistes in a grayish limestone bed that strikes from Cabo Eojo 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 whicli have been mentioned serve to throw some light on the age of the series. 74 SCIENTIFIC SURVEY OF PORTO RICO On the accompanying chart (Fig. 13) the four most important of these formations have been indicated numerically: 1. Unib on shale, at Juncture of the Morovis trail and TJnibon Hiver. Strike, ]\^. W.-S. E. Dip, 30" 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, IST. W.-S. E. Dip, 15-20° N. E. (?). 4. Muda limestone. Strike, N. W.-S. E. Dip, 30° S. W. The TJnibon 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. E. M. Bagg, of x^ppleton, 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'Orbigny^ (PI. Ill, 1). Globigerina bulloides, d^Orbigny.* Truncat/ulina lobatula, Walker and Jacob^ (PI. Ill, 2). Nodosaria communis, d^Orbigny^ (PI. Ill, 4). Nodosaria farcimen, Soldani."^ Textidaria 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 Globig erina bulloides Orbidina nniversa Truncatulina lobatula Textularia globulosa (PL III, 3) and a few Eotaline forms. The foraminifera of the two limestone beds are essentially the same. They are characterized by numerous Miliolidae, as well as Eotaline and Textularia-like forms. In these limestones were found: 8 Report of Challeuf/er Expedition. IX, p. 506, PI. LXXXII, lla-r. * Op. cit., p. 593, PL LXXIX, 1-T. -' Op. cit, p. 660, Pis. XCII, 10 ; XCIIT, 1, 4, 5. 6 Op. cit, p. 504, PI. LXIT. 0-20. ^ Op. cit, p. 498, PI. LXII, 17-18. V.:.Kr\fK r. VhXV): III ml\mi.\im^:i:a 8EMME8, GEOLOGY OF THE SAN JUAN DISTRIci" 75 MilioUna seminulum, Linne (PL III, 5a, 6a, 6h). Npnionina sp. Pulvinulina sp. Textularia sp. TruncatuUna 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 unconforni- ably below the Arecibo formation, which is not later than Ohgocene. 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. It 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. %|i| Many of the formations show the ^ects 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 Ci^etaceous in its greater part. The uppermost portion is doubtless Eocene, and it is probable that the lower part is Comanchic in age. ]ii Hvi t:\TiFii: ^riiVEY of I'ttirro rich IGXKors IJOCKS in all ruirts oi: tlu- island tin- (.Idcr s^'rics J.. <'liariu-teriziMl by an alriin- (laiH'c of j\i:ii(KniH riM.-k.s, .<lio\viii<f ji vurititv of forms mid eoiiiplox strm- tunil ri'latJoiis. After the (lo})oj<iti(xu of the coastal plain stnita. jgiicons iietJvity appan'titly i'«-asorl. for in these formations no igneous rot'-ks of aurv kind aro fonjid. It u possible, but iniprobat)le, that post-OlJo-oeene inti'iisioii has oe<;iirred in, tlie older series ot th<' interior, whieh. has nut jiJTectod the Jinnwtone strain of tlie t:oast. 'I'he modes of oe<-nrrenee of the i,a-nons bodies of the distriet are soniewlnit variriblt'. Kxtsnsive and intrnsive roeks oeeur as lava flows, sillSj dikes, and irregidar inlrnsive masses. \ar\'in,ii' in eornplexity from a simple flow or single elike to jin inteiTelaled series of complex iiitriisives so intimately assoeia1<'d that a<-enratc- aJid etnnph'te dilferentiatiun is ufltai wholly impossible. 1'he oeeurreni'e <d' lava Hows in the San Juan <listriet is eonrparatively rare, Se\eral exampk-s of sneb flows' are to lie fonrjd on the Comerio- Barranrpiitas roa<l near K o5. At this point the lava Hows oeeur associ- ided with bedded and massive tiitl's, dippiriu- at Htee|. angles. Tim lavas often ^^llow a vesicitlar habit wlsieh distinguishes tltem from the sdldilce intrusives. In this loeality and near the Bairoa liiver Jost north of Aguii.s tlnt-nas strongly vesicafbir bivas an- fouml. The vesiiailcs are often ^ lilleil with (piaj'tz. ealrite, and zeolites, forming a, fairly well developed aniygilaloi<bal slruetui-c. Hie liipjid/lava oeeasionall}' seems eitlier to have porineated the uiiderlvmg ash ^all^tuff beds, or to have been sattr^^ rated with ashy ami tulfaeeous ejeefnu'iits, bM-ming a rnek of tuiraeeous appearance lait of erystatlinc groiiudnniss, which often proves verj' <-ofn^ fusing to the- geologist. 11u' hivas louml were almost iiivariahlv' aiide- The most abnndaiil of the sim|)ler forms assumed liy the ignc<uis roeks are rho iidrusive sheets or sills. These are eorrt'orjnably infrinled in the ashe> ami lull's whieb are now tilted at steep angles or hieally b)lded. The imiividuai sills are nsually t'bin. hein- often less than a foot tbit-k ami randy more than a few feet in thiekne^s. \et thr-y an- so numerous that they add mnterially to the total tbiekness of the formatitm in wbiefi rfiey oeenr ( Fig. II). Exe(:|4t wdiere modifiofl by snljsequent distnrbanee, !SEli:ilES, (IKOLOaV OF rilK SAS .HAS liimiUi'T 'I] the .<iHs art! iiliuo.^i pertw-t Jii lurtii, and oftvii show su iii<listimnJv their intrusive Jiatiire that mi weathering- iliey are hardlv <lisn"iiguishal)le froiri ihe ueighborjjiu- strata. IncorporatJoii of tiilfad'ons material hv the in- iriidjug magma. .ri;(H|neiitIy (werus. giving the sill a. tlecidedly I'ragmeiital >triieture. .Should <ueh ;i sill oeevir in a series of fsdTs eomjMwed of yinu- iar fi-agmental nniTerial. it might readily he eni.irely overl(M.ke<l. Inlrnsivcs assn.mJng tlie fonn nf ililas mv nui i I any of Hi,, existing dikes wen- |H-(>J,iddv ovc nnmon m the diytrnd^ irnii-e massive intrnsives, where the ou.tei-o|i shewed rioiie of the Htrnety,ral iviatiuns. ' Jt sometimes liat»|H;n> that a dike eu-tting sdjiic- jion-resisbint inrmatioms sueli as the eonglomerate at Iv 12.5 ('omericj road, now stands ^'Ht tis a plainly visible to)x.grapliie feature (Fig. 15). Sueli eases, how- for a tlike inlr us 1h !ir eomp( ion ajid^ resistanre would only he revealed hy a earel'nl, detailed study of nd principal stream l.eds: elsewhere tlH> residual Rul is so wulespread fii-'M'iFiv sr/fvi:)^ r)f I'dirro hioo \< liKinirted lo the Uiiul at K JL»^d.*; SEMMES, QEOLOdY OF THE ,SAN JUAN DISTRICT 79 that it is usually impossible to determine the underlying structure. Tlie dikes 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. Xo relation between the coarseness of texture and the size of the dike could be discovered. The smallest dikes .sometimes sliowed microgranitoid or even granitoid texture. The texture seemed rather to depend upon the conditions of intrnsion 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 fi'equently 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 ,i.n'adual 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 liocks of felsitic texture proved the most abundant in the district. Almost all of these showed porphyritic habit, often not discernible in the l^aiid specimen but evident from microscopic examination. An evidence ^'^ the almost universal tendency toward the porphyritic texture in the ,SU .N'f///-;,V'f/Fif;/ SURVHY O/-' I'ORTO liKJO J'elsif.ic rocks is thnl iniuiy of tlieni sliow three giiii era t ions of crystals moTc or ItsH perfectly developed. The larji-er pheiioervsts occur in a nitttrix of Hioaller plieiiocrvsts and a febitie gronnxhiiass (Fig*. 2o). Though the rocks arc predoniiiuuitly porphyrie.8 of felsiti<' texture, yet quite ii varietv of Ix'Xtures and structures are ene<)uni;ered in a study ot* much matcriaj. ;Xo true whisscH were found, but a rock of vitropliyjie textrrrc, in which tlic .liiassy grounthuass uuide np most of its frulk, was scon (l''ig. Hy). Aniilher spcciuicu shows o peculiar radial, feli-likc structure uiueh n seuibliiig that of a luitural shig (l^hi. 2i}. .Mit;rogrftnit,oid texture ' conjuion aiuuug th,e hirger intrusives. Eoeks of grajiitoid text.ure »i alnnnhint iind widospreml. hut none of ihc eoarser granitoid rocks showe p!-ononnced porphyriti<' textur'c. Compos J Tiox 'I'he igneous rocks of the district iire fairly uniform iji cHnnpositi* The j>redominant type -is an andesitic. or the coarser-grained diori l^EMMES, GEOLOGY OF THE SAN JUAN DISTRICT 81 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 wliolly 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- j)rehensive classification of the igneous rocks of the district should include all of the following varieties : Vitrophyrs Trachytes Rhyolites Syenites Granites Latites or Trachy-andesites Quartz latites and Ehyo-andesites Monzonites Quartz monzonites x\ndesites 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 T'ock shows a slightly fragmental structure and occasionally the outlines of former large crystals are to be seen, but these are believed to have been ir. elusions rather than original constituents of the rock (Fig. 16). <'H-:\TIFI(' SVh'VKV OF I'fU.'TO h'K lf(H-k^ of rJiyoJitic roin|K.<ifioit nvv hIso .(iiii<. vntv. T\\v lu'st fXaiiipl.; was .<(.(.!! iiurilu'jis^t of La, Tonvcilla in harrid Ikrrrtiicii^. This is .,, porpliyrilic rliyoljt.c wilh |)lu'iHHi'ysts ol' qiiiirt/., orrlnn-hisc. jiihI mica in rt Jim-gmiiicd aciilii- ,uToiiiHliiirtss. Tiic wJiolr rock luis subse<ni<'iitly Ihm-h hiMlily siTiciiiwd (Kiu-. 1 <;?,,). hi the F»ayanK.ii Valicy jiisl cast of Ilch-- several rlivolire iH.riihvries. (iraiiitie intru>ives are wiilespread iuhI fairly alnnMlant. All ef i!i< sirulied niieroseopieally proved tn he ru-h in suda feldspar, or that varis kiiou'i) as wala «4a'anites. The hirg'e iJitrosive mass wdiieh rovers an ai of several square inile>s southwesl of ."^lorovis is of tliis romposiiion. T roek is li<.-ht-e<)h,r(.il. with seaifered )»a1ehes of the darker ferroimpmiesi minerals, and eearsely crystalline in Kwtiire. It is efnnposed ehielly qnartz. (da.moelase. ortho«-lase. and liornhlende. with seatfered titam SLWIMKS. i;i:f>IJH;V OF the S \X JI:A.\ IHKfUH'T h;5 nuni'litc, rtiul npatirc i:rystjil.<. This intrusivi' \> i'>|H'{'iHlly interesting- tJuit it .sIhhvs a \vellHlevi.|ope.l poikiliti*- struftuiv. The plagioela.se. .riil.ien.li'. ill). I titanitv crystals mvm in the erthot-hise, whieh .sh«nvs iw v^-tal evitUne. The urtheehise rtpparently erystnlli/.ed last with the lart/,. hoth .now oreiirring as interstitial luiiterial (Fig-. K). .Minute inlets of seeon.hirv horiihlende and aetinolit... iM'rasioiinllv Iniverse the i-k for short distan<^i«s. Minor ('upper irnpri-gnjitiiais wjn'o seen at sev- iil pnints. hilt these oceurrenees were ot no iTonuniie iinpiHlanoe, A ?: .. r \:\\\i'v inlnisive (,.(' similar oeen ^\ south ot Xaranjiti. (Fig. I qiMVV:/. l.ATV I Vt u the- telsirio rortvs where the individual ervstals that eonstitiite th^ inidina>s an- too >uia,ll to ht» aeeuraf.ely (h>teru.iined. the tin(rr distine I ill Miriety eannot l»e .luadc Aoeordinglv. the tiuartz latite and rhyo '.<ite.-. tho felsitie equix-aleirts of the tpiartz uionzoivite and grarn.) 8.1: SCIENTIFIC )><vu\:ey OF POBTO jnco dioi'ites, respecii\'ely. arc groupcil togetlier. Pekitic rocks of tliis oom- position are ©(.'casicvnallv foiind. Th(3y consiBt chiefly ol' quartz iind limo- soda feld.<par with iiotidile amounts of ]H)tasli feldspar and siibordjiiato l'erroM'ia«iiicsiaji mineraL«. Sucli rocks are also known as dollcnitcs.^ A tvpical example of this group is to he torind Jiortli of Aguas Bneiiap. The speciinciis (;oliected consisted of pJieoocrysts of quartz, feldspar, predonii- naid-lv the lime^sodu Tarieiv, and mica., in a finc-graiiied oTouiidmass. p. This rook oeeurs associated witli otlier acidic rocks of almost tiie !?ame composition^ and is (k)uhtless merely a difrert'n,tiated fdiase of .some hirgcr ijdTiisive. QUAirrZ ^^lUKZONlTKS Tliose are granitoid rocks consisting chiefly of (piartz, mediirm phigio- clasc, witli consideraliJc orthoclase. With these may octair l)iotite, Ikji'ii- hleiide, or pyi-oxeiie. The)' are the closest associates of tlie grairites ami SmiMES, GEOLOGY OF THE SAN JUAN DISTRICT 85 are often classed as such. They differ only slightly from the soda-gran- ites already described^ yet they serve to express a somewhat more alkaline step in the continuous series of intergrading varieties that is characteristic of the igneous rocks of the district. A small intrusive of this variety was found in the bed at the Bayamon Eiver in barrio Sonadora. It is a massive granitoid rock of bluish color ^ made up of quartz plagioclase, orthoclase, and hornblende. The rock has subsequently been somewhat sericitized and carbonated. (JliANODIORITES The granodiorites are granitoid rocks of nearly the same composition as the quartz monzonites. They contain less potash feldspar than the latter and more of the lime-soda varieties, that is, they are intermediate in composition betw^een the quartz monzonites and quartz diorites. Soutli- west of Naranjito^ in barrio Cedro Abajo, a fairly large intrusive of this ('omposition was encountered. From a casual examination it would be classed as a granite, but under the microscope it is seen to consist chiefly of quartz and plagioclase (oligoclase) with subordinate orthoclase and hornblende and scattered crystals of magnetite, titanite, and apatite. This intrusive shows striking mineralization, which apparently took place in two stages; first, the introduction of additional magnetite, then sul- ])lude solutions entered and deposited pyrite around the magnetite grains. DACITES Those rocks of andesitic composition Avhich contain appreciable amounts of quartz are here grouped as dacites or quartz andesites. This variety is rare and only one typical example was found among the rocks studied, which occurred on the military road near La Muda. It is a porphyritic dacite, consisting essentially of phenocrysts of plagioclase (oligoclase), biotite, and quartz. All the constituents have been somewhat altered, especially the biotite, which has been replaced by chlorite, carbonates, and bleached mica. The intrusion has taken place through a series of coarse tuffs, and some of this material has been incorporated into the intrusive itself, giving it a fragmental structure. QUARTZ DIORITES The quartz diorites are more abundant than their felsitic equivalents, the dacites. Two miles southwest of Morovis an intrusion of this variety occurs in the large intrusion of soda-granite alreadv described. Another KK l<('ll::XTJFl(' Sl\hni-]Y OF l'i)HTO h'K'O .scrieiir'. cblnriic, ff>itloto, ami caleitc. Kaolin, iron oxitles, ainl leiieoxeno al^o 1're(jiieiill\' occur in a^ppreciable qiiaiitities. Since the li-itiT<'i« are not .liHt!n_!i-ui.<ln;iblt' ivmn tlie andes.ites in the field, no ntteniftt will lie Jiuuie t.n ]m, tlioir oeeurreiiee. Mmw of them wcri- there siniplv deHignated as ainlesiten. }in<1 jim specnmens were collected 1'or inir-ruvcopie >:rndy (Fi,^. 21). \i,oxy.o\'i'ri'!> th'ld eonid |>rol)abJy liave tteou rv.tv.vnH] to thin g.ronp had tliey beiui e:\^^ ainijioil inioro.^PopieallT. ''J'wo iutrusivcB of this variety are to be found on the (Vymerio-BarrampiitaH road at K S**.") and 1\ ?dl.T. 'Tliey ai*»' inicrogTairitoid rooks, eonsisting of lath-shaped orthoelasc and pla<4'io- j'bnse ervf^tals arranj^-ed in a coarse, felt-like iigi^-reo-ate with subonlinate anroiints of pyroxene and hornldende. SEMAII-J.S, Gh'OLOiiY OF TJIK SAX J I 'AX lilSTUli'T The aridcBitcs arc the most alnuularit us well as \\w iiiuHt widrr^prcnd of I he igTicons rocks. Th.cy arc lii^iia.lJy oi ]:>or|iliyrit.ie tt'xt.iu'o aiiil a>>iiiim' all the strur'tiiral roriris"'lliat, are rcprc^cnte.l iii the (lisli-e-i. The esseii^ tial consiilneiit.^ are phio'iocJase of uiediirm cniiipoFitiiHi (oliti-uehibi' ami ami liorahJcTide. Certain varieties of ilie andesite iM.rphvric.s jii which ilu' phejioer3'sts were chiefl)' fehlspar showed a i^potte<l appearanee due to iIh' white fehlspar pheiiocrvsts in a <hirk ^lii'tHirulniaHS. Other varieties in whieli the pheiioerysts were composed ehitdly of the I'erromagues! an iniii- 'rals ha.d a darker and more hasic appearance. Diopsidc is much tlie • Host abiiudant oC the forronjagnesiari inineraJs; hornhlemk; oeenrs hifr«'^ i«entl\', am] biotite quite raTeJy. The t»resent mineral constitnents ol' aianj of the andenites are largely the result of seeoodary alteration, ''•'''•me of them contain sneh large (juantities oi' seeoinlary carl)onates that ^eey effervesce slighlJv in oold dilute acid. Serieite," chlorite, kaolin. iH) ^•r 7 l-;.V 77 ,/••/( .V7 7.'l HV OF I'Ofri'O h'll'O liilc ;i larui> |M'r«-..iitjino ..f tiic ciifirt. iiniss ..r the ruck. A few of tiir muit stnkiii.u- Du-inlHTs i.r this n-rniip hit ln'iTwitli hrii'tly (lfs,-rilM'<L Ar 1\ i: on tlic roinerio road ii purphvritii- hurnldcjHic rtiuli'siU' was louml. Sih- ondiiry altcrntHHi lind .a-n-ativ rlisuiu^pd the nrigiiial (-(Hisiitm'nt^ of the ru.-k. 1'li(. ori.umal uiitliiK's ul' tlif IioriiLleiid.' phcnccrysts cuidd Uv vU'twly si'i'ii. rhoii-li liiu horiihlciidc itself had h.-eii .dmiioxMl info Hn'lviiv. .•iilrir.'. and .-hloritf. Anoth.'r lioriihlrtidi' aiidesiii. was fuund at K II on the Alilitary read, soiiili of Ca.Hiias. thi jlu' P)arn.s4'(»ruznl trail ai vpHloihiHl andcsitc |H)r|thyry was fuiind. Hie rork roiisisfr-d .-hioliy ol v\iuUAv and phi.uKMdasi'. Thn«o distiiM-t riviicrarioiis of cryslals wm shnwii. Another eiddotizcd aiidcsile is loeaiod just north of A^^:na^ r.iu'iias near tho river F»iiir«.n. 1'liis is an amv.uxlaloidal aiide.Hle with e|M<h»re lillin^u^ the ea,vifies. On I he Coinerio road at l\ ."» an aiuh'sit. |H.r|»hyry with a remarkable radiatiii.u-. I'eltdikt> -nnindiiiass was diseov^ ored. Us ^irncliire reseinides verv iriueh that of a natural slan' (fh^. 2^lh st:MMi-:s, aHiHjpdV of tui: nj.v .ivas biKTUirr M Om- r.r tlif two loiwst intrusive- in the disf.ricl i> an aiifloite purpliyry. it <HH'iir> ju>t 8()iitli of the Or,H'<»vis l^iver cii the Murovis^-krros rua.l. extend iii.a- out of the district on iJie west. Large plu-nocrysts ..r light^^ roli.red I'ehlsimry in the dark M-rouiHlmass giM>.< the rook a .<|...ite.l ii|.|..,u- ane... li is in this vm-k tJuit the eo|>|M<r pmspeels v^ harri<. Pasto neeur. l-'roni niierusi-()|He examinatinn it is seen tliat this vuvk nhu slunvs tJiree genorii.tions o^ <iTsial growth. Large pheiiuervsts ueenr iii a |i<)rnhyriii<- (Fig. T'y), A ,. ^- m H, i^:i| fSl#^ Litnisives uf dioritic eoin|iusitioii ai-(> of l're(|iHait uetairrtau-e, iheaigh not so widc^^pread as tho aiidesites. Thev are as a rule dense, massive, dark Idiiish I'oeks oft(>n showing hirge phenoervsts of jnToxcne. 'rJioy s'oHsirt ehielly of idagioehiso frddspar ami pyroxeno, with sidMirdiiuit..- amounts of <.rtho(da.sc and hornhLaHle. Diopside is mmdi more ahniidani tliaii aiigitc. 3Ianv of thi- <liorites show rragmental strnetiire. Some ed' ihem haxQ iiieorjMjriitod !«:<> iiiiioli fragiiicntiil iriattirial that they may he inist.akt'ii for tiifl's. The «Iioritos arc imiform in compositJoii and ,t>"ei!era) {ip|)earaiice, and wlien not inoilified hv inchjsioiis are readily recognized in the field (Fj«;. 31). Many of the aiuh'sitic rocks might be mistaken for basalts by tlie casual oh«'rver. hot tyrneal basalts are deeidediv rare. Basic varieties of ande^- sites whirb havi' l>een called l)a>alts are more t^ommon, but all of these fHoks. fhon.iil! I»asalri(^ in appearanee. show only the intcrn.ie.lirtt<' vrtrietie^ of the plagiochise feldspars. At K It, on the fomnrio road, an ainvgda- loidal p<H-phyrifK' TO<-k of basaltic appearance intrudes the tirlTs. It is e»)rHpos(,Hl id* hitb-shafHMl phenoerysts of plagioelase in a dark, basic gronnilniass of ferromag'nesian minerals. 1die vesiiadar cavities liaw' been filled with f|nartz find zeolite. While this ruek bus a typieol basaltic appearance, the pkgioclaso sh.nvs low extinction angles (less (bun 10'^). SE3IMES, GEOLOGY OF THE SAN JUAN DISTRICT 93 which indicate that only the acidic varieties are present. Hence the rock should correctly be classed as a basic andesite. OLIVINE BASALTS Only one olivine basalt was found in the material collected for micro- scopic study. This was an intrusive body about two miles south of Coro- zal. It is of porphyritic texture, consisting of phenocrysts of olivine, and, to much less extent, plagioclase, in a groundmass composed chiefly of plagioclase and magnetite. The olivine was largely altered to serpen- tine and chlorite. The feldspars have likewise been so chloritized that their exact composition was indeterminable (Fig. 25). isTo rocks of gabbroic or more basic composition were found in the dis- trict. Summary ^o definite conclusions can yet be drawn as to the exact range in composition of the igneous rocks of the district or their relative abun- dance, for by no means all of the igneous bodies have yet been located. In the present survey all of the larger instrusives were located, and many of the smaller ones, where well exposed in stream beds and along the carreteras. There remains undiscovered a large number of minor igneous masses either covered by the residual soil or located between the traverses that were made. Sufficient material has been collected and studied, liow- ever, to give an accurate idea of the general range in composition and form of the igneous bodies. In the preceding pages minor differences and variations in composition have been unduly emphasized in order to give a correct idea of the relative range in composition, and the con- tinuous gradation that exists between the more acid and basic varieties. The usage of many of the varietal terms that have been introduced here would be inadvisable for ordinary field work and general description. The petrographic study of the igneous rocks of the San Juan district can be summarized as follows : (1) The structural forms assumed by the igneous rocks of the district are of various sizes and shapes. The sills and stocks occur most com- monly, while lava flows are rare. (2) The igneous rocks are predominantly felsitic in texture. No relation of texture to size of the intrusive could be obseiTed. (3) The rocks of felsitic texture are almost invariably porphyritic. The porphyritic habit is sometimes poorly developed, but in some cases 94 SCJl^XTJFIC ;SUR\ EY OF rORTO RIVO the teiideiicj towards (levelopmeiit is so strong tliat three distiiiet genera- tions of crystals have heen formed. (4) Fragmeiital inclusions ate the rule ratlier than tlie exception in all the varieties of the igneous rocks. The fragments vary in size from large masses of tlie country rock to tine, tnffaceous and ashy material. IhifPaceous inclusions are often so ahundant that the igneous rock assumes the ap|)earance of a massive tuff. (5) The large majority of the igneous rocks of tlie district are of ande- sitic composition^ a fact which would seem to indicate that the source of the material was from a magma of approximately andesitic composition. 3[ost of tlie fragmentary material composing the tuffs is also of ande- sitic composition. Acidic and hasic variations of the })redominant aiide- site or diorite are widespread. The acidic variations represented l)y the granite, rhyolite, and related rocks are more ahundant tlian the l)asaltic varieties. (()) After plotting the distrihution of the intrusives, it was noticed that the rocks of related composition tended to group themsehes in separate provinces. In the valley of the Bayamori River, southeast of Helechal Peak, there is a large group of acidic intrusives. A numher of granitic intrusives are located in an east-west helt roughly following tlie inner lowland south of the towns of IMorovis, ("Jorozal. and Naranjito. In the southwestern portion of the district the igneous hodies were gen- erally of andesitic composition. (?) The igneons rocks sliowed extensive secondary alteration. Almost ]i()ne was perfectly fresh. In some the alteration products constituted tlie hulk of the rock. Products of deep-seated as well as surface altera- tion were represented. The former included epidote, actinolite, tremo- lite, (puirtz, and some calcite. The latter type was represented hy chlorite, sericite, kaolin, quartz, calcite, iron, oxide, and leucoxene. (8) Evidence of regional metamorphism is wanting. Moreover, there is surprisingly little evidence of contact metamorphism. One rock con- sisting almost entirely of actinolite was fcmnd, which suggested contact effects. Tlie abundance of epidote and diopside is suggestive, hut the diopside is usually primary and the epidote products of ordinary altera- tion. No typical contact zones were found. (9) Many of the intrusives showed more or less mineralization. I^vrite and magnetite w^ere the usual products. Two intrusives were located which showed cupriferous mineralization. SEMMES, GEOLOGY OF THE SAN JUAN DISTRICT 95 STEUCTUEAL FEATUEES Many of the larger structural features have been nientionetl in (\)ii- uection with the formations involved. The most striking structural features are related to the igneous and fragmental rocks. The great series of inter>bedded tuffs and ashes^ with occasional lava sheets and sills, which occupies most of the southwestern portion of the district, may be classified as a structural unit, forming the fundamental basal structure of the whole island. In this great volcanic series the minor igneous structures are numerous and complex. The interbedded sills, the flows, the dikes, the intrusives, all serve to increase the complexity of the series as a whole and incidentally cause a variety of minor structures. Unconformities The most striking unconformity in the district is that between the older and younger series. The almost horizontal limestones of the coastal plain resting upon the upturned beds and intrusives of the older series form a structural feature that will be noted by even the most casual observer. This unconformity represents the erosional interval in which the formations of the older seri-es were uplifted, tilted, and finally reduced to low relief. It also marks the close of all volcanic and igneous activity. This erosional interval seems to have occurred either during late Eocene or early Oligocene time, an interval comprised between the Eocene beds of the older series and the overlying limestones of upper Oligocene age. Above the unconformity there is another erosional interval represented. Though this interval is shown by no structural break, yet it represents a much greater period of time than that which transpired between the deposition of the older and younger series. This disconformity occurs 1»etween the Arecibo and the San Juan formations and represents an interval from upper Oligocene to Pleistocene time, during which the island was above water. During this time much of the erosion that has partially destroyed the Arecibo formation doubtless took place. N^o direct evidence of an unconformity was found in the older series, but indirectly at least one important break was indicated. The discord- ance in strike of the limestone beds south of Aguas Buenas suggests cither an unconformable or faulted relationship with the other forma- tions of the series. The writer believes that these limestone beds lie unconformably below the other formations and have been exposed in this neinity by more advanced erosion. The prominent conglomerate forma- tion that strikes 'N. W.-S. E., crossing the Comerio road at K 12, indi- DO SCIENTIFIC SURVEY OF PORTO RICO eates another break in the older series. To the southwest of tliis con- glomerate the formations strike K. W.-S. E. with hardly an exception. Xortheast of it outcrops are rarer. The shale beds for the most part appear to strike K. W.-S. E., but the sandstone at Tv 8 gives an excep- tional strike of N. (>5° E. Northeast of the conglomeratic ]\Iuda tuffs no strikes were obtainal)k3 from the few scattered outcrops. Berkey, how- e\'er, noted that the shales on tlie ^Military road south of Eio Piedras strike E.-W., and the tuffs farther south strike T^T. 20° E. The variations in strike over the northeastern portion of the district are at first very confusing, but when one considers that the material forming the sediments is of volcanic origin and was not derived from a single source, l)ut prol)ably from many vents, it seems surprising that any uniformity in strike would exist over an extended area. The vol- canic material ejected from a number of vents would be concentrically deposited around each, forming a succession of local formations inter- hngering with the deposits from neighboring vents. Intense igneous activity has accompanied the deposition of all this tuffaceous material, and finally the whole complex thus formed was uplifted and folded by sul)sequent dynamic movements. The end-product of such a series of events w^ould be a volcanic ash heap of great complexity, in which ex- tended series of conformahle beds would be the exception rather tlian the rule. Considering Porto Pico as a ])ortion of sudi a mass, one can better interpret the many complex and erratic structures that are to be found as well as tlie local character of many of the formations wbirh are so confusing to one who attempts a solution of tbe areal geology of a ])or- tion of the island. Eaijlting Indirect evidences of faidting are abundant, but in no case was the disj)lacement itself visil)le nor tlie amount of movement determinable. Abrupt changes along the strike of certain formations indicate faulting on a large scale, but outcrops Avere too scattered to determine positivelv their presence. Evidence of faulting on a small scale is much more abundant. Along the Comerio road occasional shear and crush zones arc visible, which apparently involve faults of some magnitude. ^Fhe best examples occur at K 17 and K 21.5. In both cases, however, the com- plexity of the structure involved was so great tluit no accurate estimate of the amount of movement could be made. sf:\nii:s, (;FjHA>f!V of tiif. n lv ,// i,\ hisiini-r ^^^'..:;r .--%.: 98 SCIENTIFIC SURVEY OF PORTO RICO Folding x4l11 of the sedimentary formations of the district show some folding or tilting. The limestones of the coastal plain have been only slightly tilted. Their maximum inclination is not over 10°, and no indications of folding or warping within the formations themselves could be dis- covered. The beds of the older series are characteristically highly tilted, and in many places well-developed folds are to be seen. The folding is on a small scale and is confined to certain less competent shales and ash beds. The fact that the sediments generally dip towards the sea on both sides of the island would suggest the possibility of a large anticline with the axis along the crest of the island. Minor folds are not uncom- mon in the shale series that forms the divide in the southern part of tlie district. Moreover, this shale series is anticlinal in its larger structure, with its crest just north of Barranquitas—a fact which gives additional weight to the theory of anticlinal structure for the older series as a whole. Metamorphism ISlo evidence of regional metamorphism was found in the rocks of the older series. Evidence of contact metamorphism is also surprisingly scarce. The amphibolite already described is the only rock found that might safely be regarded as due to contact effects. Minor Structures cross-bedding The San Juan sandstone shows well-developed cross-bedding of tJio eolian type. Berkey (1915, p. 43) has described with some detail the double character of the cross-bedding where it is best exposed near Areci])o (Fig. 26). SOLIFLUCTION The crumpling, due to the slumping of certain formations, either from external pressure or from the force of its own gravity, gives rise to what has been termed solifluction. This type of structure was noted in sev- eral of the thinly bedded ash formations, but it was best developed in tlie Juan Ascencio chert bed, which has been previously described (Fig. II). ^"^EMMES, GEOLOGY OF THE SAN JUAN DISTRICT 99 STRATIGRAPHICAL SUMMARY The geological history of the island of Porto Rico has already been comprehensively summarized by Berkey (1915, p. 58). Little can be added to this from the data collected by the writer, except certain changes ill the age relations of the formations. The geological history of the San Juan district or that of the whole island, for the former involves practi- cally every step of the latter, still remains in great part uncertain. An accurate and detailed statement of it can only be hoped for as an end- product of the series of investigations that are now in progress. Each investigator, however, can contribute his share toward the final result. The writer, therefore, re-summarizes the historical statement of Berkey, making those modifications which he thinks are justified by subsequent investigation. As stated above, the formations that compose the so-called ^^older series'' were early recognized to be geologically young. While some of the shales and limestones have the appearance of being older than they really are, yet none of them is at all similar to the so-called ^^ancient rocks'' of other regions. The age of the lowermost member of the older series is still undetermined except in the most general sort of way. The discovery of a Comanchic coral by Mr. Hodge in the shales south of Cidra indicates that these shales, which apparently form the base of the older series, are no older than the Comanchic age. Flanking these shales both to the north and south are the succeeding members of the older series, which are, as a whole, younger. Such a succession of the members of the older series would suggest a large anticline extending across the island along the belt surveyed by the writer and Mr. Hodge. Other portions of the island, liowever, do not support this theory, for a section from Arecibo to Ponce shows no indication of the anticline. The writer merely suggests it for the San Juan district and the district to the south, because the shales which form the crest of the island at Barranquitas are anticlinal in struc- ture and are flanked on either side by younger formations. Whether the Barranquitas shales are regarded as the oldest formation of the older series or not, there still remains little reason for supposing that the base of the older series is older than Comanchic. Indeed, the whole of the older series might well have accumulated in a single geologic age, for all of its members give indications that their accumulation was, for the most pJ' d; a rapid process. The deposition of the various members of the older series was accom- p;nned by igneous activity on a very large scale. Great masses of volcanic 100 SCIENTIFIC SURVEY OF PORTO RICO ejectments were thrown forth to accumulate in fan-like deposits both above and below sea level. This was accompanied by periods of submer- gence during which the shaly and calcareous beds were deposited, and periods of emergence in v^liich the recently formed sediments were up- lifted and sometimes shattered by later volcanic outbursts. During the accumulation of the older series there seems to have been no great change in the nature of processes that took place. Xo change of sufficient magni- tude occurred by which the series might be readily subdivided. It is true that changes in physical condition, such as would cause a shale or ash to accumulate instead of a tuff, frequently occurred—so freciuently that these minor changes cannot be used as a basis for consistent subdivision. With the exception of the large conglomerate that crosses the greater part of the district from southeast to northwest, there is no formation which can be used as a definite division line. Other divisions are not even con- tinuous across the San Juan district, and might never be recognized in the adjoining districts. In early Eocene time this period of volcanic activity finally came to an end. Naturally, from the time of the first volcanic outbursts portions of the island had been dry land. Toward the end of the period of volcanic activity the lower portion of the older series must have been above sea level. This land-mass afforded the material for the late Cretaceous and early Eocene foraminiferal shales and limestones. It is probable, there- fore, that much of the island had been reduced to a submature or mature stage of topographic development before the final cessation of volcanic activity. As such activity completely ceased, erosion continued and the entire island was reduced to a peneplane or to an old stage of topographic development. The development and extent of this peneplane have already been discussed. Until the whole island is examined with this point in mind it will be impossible to say to what extent it was developed. The period in which it culminated, however, was geologically short. Eocene formations are found beveled by the peneplane, and Oligocene strata lie above it. As the erosion of the older series proceeded, sediments were deposited unconformably around the margins of the island as it existed then. In shallow water, and doubtless in the estuaries and bays of the coast of that time, were first deposited a series of shales. This phase of deposit was not well expressed in the San Juan district, but to the west it is re]>- resented by the San Sel)astian and Lares shales. This earlier stage in t1i<' deposition of the younger series probably occurred before the interior ha^l been reduced to an old stage of development. As erosion continue'!, clearer water conditions prevailed and the lower shaly beds of the Arecil"> SEMMEIS, GEOLOaY OF THE ^AN JUAN DISTRICT lOl liinestoiie were deposited. Later, after clear water conditions had become stable, the upper pure limestone and reef limestones were finally de- posited. At the beginning of Miocene time there was a marked emergence, and the early Tertiary limestones and shales were elevated to essentially their present position. This emergence was accompanied by warping, so that now these strata are inclined at an angle of about 6°. From that time until the present, these formations have been subjected to erosion, and in consequence the comparatively easily destroyed limestones and shales have been maturely dissected or almost completely removed. Since this emergence there have been frequent minor movements, ex- tending down to recent time—movements that have been rcorded in the marginal terraces by the occurrence of marine shells at considerable dis- tances up the larger river valleys, and by the partial submergence and induration of the San eJuan dune sand. The latest movement has appar- ently been one of slight emergence. Berkey (1915, p. 60) summarizes the more important steps in the geo- logic history as follows : 1) A long geologic period of volcanic activity, accompanied by marginal attempts at assorting of fragmental and detrital material and organic accumu- lation disturbed from time to time by renewed or extended igneous activity. 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 base-leveling with final extensive sub- mergence. 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. The geologic column of the formations of the San Juan district is the same as that formulated by Berkey (1915, p. 61) for the entire island, with certain minor modifications : liecent alluvial deposits Submergence with marginal (playa) deposits Younger series San Juan dune sand (Pleistocene?) Submergence with terrace cutting Post Arecibo emergence and erosion (Miocene) Arecibo formation (upper Oligocene) Lares shales, etc. (Lower Oligocene or upper Eocene) not expressed in San Juan district Marked u ii cc )i i form i ty 102 SCIENTIFIC SURVEY OF PORTO RICO Older series Corozal and Muda limestones (Eocene) Foraminiferal shales and limestones (Unibon shale, shale at K 8, Comerio road) (Upper Cretaceous) Barranquitas shales and shaly limestones. Probably the base of older series (Comanchic ?) MINERAL OCCURRENCES Throughout Porto Rico there is evidence of widespread but apparently feeble mineralization, which has given rise to a variety of mineral de- posits of small extent and uncertain importance. Mineralization in the San Juan district has produced even less results than in other portions of the island. Prospects occur in considerable variety, but no extensive deposit was found in this district. Up to the present time none of the mining ventures that have been promoted in the district has proved a success. This, however, has not been due wholly to the low quality and limited quantity of the ores encountered, but is due partly to the difficul- ties in transportation and the mining methods employed. Without the exercise of caution and economy in the development of small ore bodies such as are found in the district, success cannot be expected. Much money has already been wasted by attempting more elaborate methods than are warranted by the extent and quality of the ore occurrences. The prospector is soon impressed by the extent of the mineralization ill the igneous rocks of the older series. Numerous examples of miner- alized igneous rocks have already been cited. An abundance of magnetite and pyrite are to be found in almost all the igneous rocks of the district, yet the resultant ore deposits are surprisingly small and unimportant. Native gold from quartzose zones and placers is the only metal that thus far has been produced in paying quantities. Indications of copper are abundant, but in only one locality does it occur in sufficient quantities to attract development. Non-metallic deposits of value are entirely want- ing, except, perhaps, limestones for road metal or similar uses. Gold PLACERS The principal gold placers of the island are located in the vicinity of Corozal. From the streams in this vicinity gold has been washed by the natives since the days of early Spanish occupancy. Tales of the great abundance of gold and the size of the nuggets found during those earlv 8EMMES, GEOLOGY OF THE SAN JUAN DISTRICT 103 days are widespread and form a favorable subject of conversation among the older native panners. No statistics are available, however, either to disprove or verify these statements. It cannot be denied that a large amount of gold has been panned out of the streams of this vicinity in past time. At present the work is slow and tedious and pays poorly even in the most favorable localities. The native panner stands up to his neck in a pool, which he has dug in the stream bed, and assiduously scrapes up the fine sand in a cocoanut shell, separating the larger pebbles by hand. If the pool is deep he dives to the bottom for the sand. If ten hours of this work nets him eighty cents he is fortunate, for the average daily washings do not exceed fifty cents. Indeed, the peon prefers steady farm labor at forty to fifty cents per day to the precarious living he can derive from the stream beds. During recent years about 100 ounces of gold are annually washed from the streams of the island, the larger part coming from the Corozal district. Several years ago an unsuccessful attempt was made at hydraulic min- ing. A large dam was erected in the headwaters of the Corozal River and the water v^as conducted down by a 10-inch conduit to a point where the gravels were especially abundant in the same stream just above the town. Due to the difficulties in transportation, the cost of construction of the dam and conduit was very great, the material being brought some 20 miles from San Juan to Corozal by ox-cart and thence into the hills by pack. Unfortunately, the conduit proved inadequate to withstand the pressure of so large a head of water and soon went to pieces. The project was then abandoned for lack of funds and has never been reattempted. Gold occurs not only in the Corozal and Negros Elvers, but is to be found in minute quantities in almost every stream of the district. It has been washed in paying quantities from the Plata, Mavilla, Naranjito, Imibon, and Orocovis Rivers, but its presence in these streams is so rare tliat no regular panning is practised. AURIFEROUS QUARTZ MINES South of Corozal, in the barrios Negros and Palos Blancos, some ex- ploratory work has been done in an attempt to discover the lode or veins which might have been the source of the placer gold (see topograpliie iriap). In the barrio IS'egros some of the old Spanish workings have been reopened in recent years, and two new shafts of shallow depth have been ^unk in an attempt to discover a large vein. A so-called vein about three feet wide has been exposed. The country rock is highly weathered. 104 SCIENTIFIC SURVEY OF PORTO RICO but appears to be an andesitie tuff. On close examination the vein proven to be a silieified zone composed of many quartz veinlets and stringers, none of which is over a few inches in width. These veinlets carry the gold^ and when bunched in large numbers are workable. Xeither the (quantity nor quality of the ore can be depended upon, for it is no loiiger workable when the quartz veinlets become scattered. In the Palos Blancos mine the conditions are nmch the same, thougli this prospect has been more fully explored. The country rock is an andesitie tuff with associated intrusives. The silieified zone, or vein, is somewhat larger than in the other case, and has been followed by one tunnel some 180 feet. A fair grade ore could be concentrated, and one shipment was made to the States. Neither of the two prospects is now worked. The Palos Blancos mine might prove a success if properly devel- oped and capitalized. Platinum Occasional particles of platinum are panned out witli the gold from tlie placers near Corozal. The metal is called plafa by the natives, who think it is silver. Although it sometimes occurs in quite appreciable quantities, the natives get nothing for it from the local buyers. The writer in pur- cliasing the results of a day^s labor from one of the native panners found that it contained, roughly, 20 per cent platinum : so in this particular ease the platinum content, discarded by the native as valueless, was ap- proximately as valuable as the gold, which locally brings only $12.00 an ounce. Such high percentages of platinum, however, are rare. Copper In the barrio Pasto, on the west central margin of the San Juan dis- trict, several copper prospects were visited and examined. These are the easternmost of a group of similar prospects that occur south of Ciales. The others, extending beyond the boundaries of the district, were not visited by the writer. The country rock is an andesitie porphyry, w^hich has been described and figured on Fig. 22. This porphyry occurs as a large instrusive extending out of the district on the west. In this intru- sive the ore occurs as impregnations along cracks and fissures. Several of the prospects in this vicinity have been partially explored. The prin- cipal one was examined by an American mining company and one ship- ment of several tons was sent to the States for smelting. In this prospect the principal vein, or impregnated zone, is about four feet wide. This had been followed about 30 feet into the hill. It is intersected bv several ^EMMEl!^, GEOLOGY OF THE *Sf.LV JUAX DJSTRJCT 105 other similar veins of smaller size. It is evident tliat tlie ])orpliyrv lias been fractured and subsequently was mineralized. The copper-bearing waters have followed these fractures and deposited in tliem the copper minerals, impregnating the neighboring porphyry for distances varying from a few inches to several feet. iVt the juncture of two of these min- eralized fracture quite a showing of copper is developed. In the speci- mens collected malachite, chrysocolla, and azurite appeared in the great- est abundance. Cuprite and chalcocite are rare at the surface, but may prove more abundant in depth. No idea as to the extent and quality of these occurrences can be had without further exploration. The ore bodies that have been unearthed in the ditches and tunnels of former explora- tory endeavors have been subsequently concealed by infalling detritus and dense vegetation, so not even an approximate estimate could be made as to the quantity of ore readily accessible. The inaccessibility of this region has discouraged all attempts at the development of these prospects. The topography is exceedingly rugged and the distance from all means of transportation is considerable. At present the ore must be packed down to the valley of the Manati, from whence it can be carted to the main road. If the transportation facilities are eventually improved, these copper prospects may be successfully worked. Lead iSio occurrence of lead of economic value was found. A small quartz . veinlet carrying galena was seen in barrio Magueyes, south of Corozal, hut no other indications of the presence of this metal were found in the district. Manganese In the same locality a thin bed of psilomelane was seen. Its extent could not be determined, but it did not appear to be of enough importance to warrant exploration. Egad Metal lioad metal is abundant in all parts of the district. The Tertiary lime- stone is extensively used as such on all the roads that pass through the limestone belt. It is said that the purer beds of the limestone give a .i^^ood quality of lime on burning, but the scarcity of fuel makes this in- dustry impracticable. The tuifaceous sandstone at K 8, Comerio road, is quarried and used as road metal for the carretera for some distance. In the interior the andesites and massive tuffs are used and, where accessible, the interior limestone beds are quarried for this purpose. lOG SCIENTIFIC SURVEY OF PORTO RICO Building Stone Many of the igneous rocks of the district that would make excellent building stones are not, and may never be, quarried, owing to the limited demand for such material and the difficulties of transportation. In San Juan, where the local demand is greater, the San Juan sandstone is used to some extent in the resideatial section, chiefly in building foundations, since it forms a pleasing contrast to the cement. Guano Bat guano is found in many of the caves. Large deposits have accumu- lated in the caves south of Aguas Buenas. These are locally used by the natives as fertilizer. ILLUSTRATIONS Cross-sections Soon after the writer began the geologic study of the district, it became apparent that the structure of the older series was in greater part too complex to be shown in even moderate detail on a geologic map of small scale. It was decided then to supplement the map by a few more detailed cross-sections. Therefore, careful traverses were made across the district along definite lines, so that three cross-sections might be constructed showing to some degree the complexity of structure encountered (see Geologic Cross-sections, Plate IV). The data collected in each case have been projected upon a straight line. In this way the windings of the road or trail have been eliminated and the normal proportions and relative positions of the associated formations more clearly shown. The section through Bayamon and Comerio is the most complete and accurate because of the better and more continuous exposures. The other two sections are based upon occasional exposures and are therefore somewhat more generalized. Maps The difficulty of obtaining a map of sufficient accuracy on which the geology might be plotted was soon realized. Available maps, however well adapted they might be for other uses, proved to be so unsuited to this particular purpose that it became necessary to construct a large scale topographic map before beginning the study of the geology of the district. The accompanying map is the result. It was constructed by means of Scientific Survey of Porto Rico and the Virgin Islands Volume 1, Plate IV HgjdmJL Scale cf Miles F^^ur^ /^ Norj-h-Sott^hSecHonfrom \/<e^ BajaPlaya^ihru \^aBaJa,P'tontY^ arte/ B^f-ro* f7gur» B^ ^oaef Pro/i'iefi-om Son Juan, Utry BafamSn andC^meno. fo Borronqultas -N*tmtrmU atlitd^kHom**»rft9mH0ioo^iht e^rrtUrt) ftfure3-^ North-Sotiih eecfi»n^om San Juan, Hira Guaynabo, la Mt/da'and Agumm Boena*. -hCfJra {/arfical 9caf€ I" wJOOO' GEOLOGIC CROSS-SECTION 8EAIME8,, GEOLOGY OF THE SAN JUAN DISTRICT 107 the plane-table and telescopic alidade, the contours being sketched in from aneroid readings. The main roads and streams are shown with some degree of accuracy, and a reasonabl}^ correct idea of the character of the topography can be had from the contours. When one attempts to map an area of 500 square miles of country as complex geologically and topographically as Porto Eico, in a single sum- mer, any great degree of accuracy is manifestly impossible. The sub- divisions of the older series are necessarily somewhat arbitrary, for the members of the series, as a rule, do not maintain sufficient constancy to make close subdivision possible. Many of the division boundaries shown on the map are generalized or are compromises because there is a con- tinuous gradation between some of the formations. Even the boundaries of the intrusives are obscure in much of the area. Some were located by single or widely separated exposures in districts of almost continuous soil cover. The dominant rock type is indicated, however, in each case and the approximate distribution and general trend and relative impor- tance of the larger formational units are brought out by the different color patterns. ACKNOWLEDGMENTS I am greatly indebted to the professors of the Department of Geology at Columbia University for their constant advice and valuable assistance in the preparation of this report; especially to Dr. C. P. Berkey for his field directions and assistance in making petrographical determinations, to Dr. D. W. Johnson for his advice and assistance in preparation of the manuscript, and to Dr. A. W. Grabau for his aid in the determination of many of the fossils collected. For the identification of other species I am indebted to Dr. Dall and Dr. Bagg, without whose assistance few of the conclusions as to the relative ages of the formations could have been derived. I appreciate, moreover, the advice and aid given me by the officials of the Government of Porto Eico, and desire to thank especially Judge Bonner, the Auditor; Colonel Shant-on, the Chief of the Insular Police, and Mr. Bishop, the Assistant Auditor, for advice, assistance and support in organizing equipment and getting established in the field. Many others throughout the island, by their courtesy in giving infor- mation and their never-failing hospitality, did much to further the com- pletion of the work and make my visit to Porto Eico an enjoyable one, and, I hope, a serviceable one as well. 108 SCIENTIFIC SURVEY OF PORTO RICO BIBLIOGRAPHY Based upon the bibliography published by Berkey (1915). Additional articles have been added and the bibliography revised to date. Alexander, W. A. 1902. Porto Rico—Its climate and resources. Bull. Am. Geog. Soc, XXXIV, p. 401. Brief notes on the natural resources, including mining. Berkey, Charles P. 1915. Geological reconnoissance of Porto Rico. Ann. N. Y. Acad. Sci., XXVI, pp. 1-70. The most modern and detailed description of the geology and physical features of Porto Rico. Cleve, p. T. 1871. Geology of the northern West Indies. Kongl. Sevenska Vetenskaps- Akad. Handlingar, IX, No. 12. With m^ps. 1883. Outline of the geology of the northeastern West India Islands. Ann. N. Y. Acad. Sci., II, pp. 185-192. Cretaceous age of the older series recognized. Crampton, H. E. 1916. Porto Rico. Am. Mus. Jour., XVI, Jan. DiNWIDDIE, W. 1899. Physical features of the island (Puerto Rico). Harper's Weekly, XLIII, March 11, p. 248. 1899rt. The great caves of Puerto Rico. Harper's Weekly, XLIII, March 25, p. 293. DOMENECH, M. K. 1899. Mineral resources of Porto Rico. Mines and Minerals, XIX, pp. 529-532. Dorset, C. W. 11K)2. Soil survey from Arecibo to Ponce, Porto Rico. Report of Bureau of Soils, pp. 793-839. An accurate and comprehensive description of the topography and gen- eral configuration of the island. Falconer, J. D. 1902. Evolution of the Antilles. Scot. Geog. Mag., XVIII, pp. 369-376. A brief geologic history of Central America and the West Indies. Gannett, Henry 1901. Gazetteer of Porto Rico. Bull. U. S. Geol. Survey, No. 183. GUPPY, R. J. L. 1909. Geological connections of the Caribbean region. Trans. Canadian Inst., Jan.. pp. 373-391. 8EMME8, GEOLOGY OF THE SAN JUAN DISTRICT 109 FisKE, Amos K. 1899. The West Indies. G. P. I»utnani's Son, N. Y. Social and economic conditions. Hamilton, S. Harbert 1909. Note on some of the ore deposits of Porto Rico. Eng. and Min. Jour., LXXXVIII, Sept., p. 518. Hill, Robert T. 1898. Cuba and I*orto Rico, with the other Islands of tlie West Indies. New York. An Illustrated book describing the islands and their inhabitants. 1899. The value of Porto Rico. Forum, XXVII, June, pp. 414-419. 1S99«. Porto Rico.' Nat'l Geog. Mag., X, pp. 93-112. An excellent description of the topography and a brief r^sum^ of the geology of the island. 1899&. The geology and physical geography of Jamaica. Bull. Mus. Comp. ZooL, XXXIV. A detailed description of the geology of .lamaica. 1899c. The forest conditions of Porto Rico. P>ull. U. S. Dept. of Agric, No. 25. A discussion of the forest conditions as well as the topography of the island. 1899f/. Mineral resources of Porto Rico. Ann. Rept. U. S. Geol. Surv., XX, part VI. A resume of the mineral resources and a short description of the most important occurrences. Newberry, J. S. 1882. Geology of the W^est Indies. Trans. N. Y. Acad. Sci., I, pp. 2;L{-24. NiTZE, H. C. B. 1899. Investigations of some mineral resources of Porto Rico. Ann. Rept. U. S. Geol. Surv., XX, part VI, pp. 779-878. Report U. S. Weather Bureau, 1914. Porto Rico. SlEVERS, W. 1895. Zur Kentniss Puerto Ricos. Mittheil. Geogr. Ges. Hamhurg, Heft. II, pp. 217-236, pi. 4. Spencer, J. W. 1904. The Windward Islands of the West Indies. Trans. Canadian Inst.. VII, pp. 351-371. 1903. On the geological relationship of the volcanoes of the West Indies. Victoria Inst. Jour. Trans., XXXV, pp. 189-207, 1 fig. General discussion of the relationship of the West Indies and the Wind ward Islands to the Central and South American continents. Vaughan, T. W. 1902. Earliest Tertiary coral reefs in Antilles and United States. (Ab- stract.) Science, XV, March 28. 110 SCIENTIFIC SURVEY OF PORTO RICO Wharton, W. J. L. 1894. The physical condition of the ocean. Geog. Jour., IV, Sept., p. 255. Wilson, Hebbert M. 1899. Water resources In Porto Rico. Water Supply Paper No. 32, U. S. Geol. Survey. Well illustrated. The physiography in connection with the water re- sources is briefly described. 1899a. The engineering development of Porto Rico. Eng. Mag., XVII, July, pp. 602-621. 1900. Porto Rico, Its topography and aspects. Bull. Am. Geog. Soc, pp. 220-238. With maps. A good physiographic description. Anonymous. 1778. North American and West India Gazetteer. London, XXIV. 1898. Porto Rico, its natural history and products. Sclen. Am. Suppl., XLVI, July 30. Brief notes on geology. 1916. Overcrowded Porto Rico. Geog. Review, March. if THE UNIVERSITY OF MICHIGAN DATE DUE >* : -' ,t "JISITY OF MICHIGAN BOUND OCTxi. iy43 3 9015 03458 5607 DO NOT REMOVE OR MUTILATE CARD N