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About Google Book Search Google’s mission is to organize the world’s information and to make it universally accessible and useful. Google Book Search helps readers discover the world’s books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web at http://books.google.com/ fy fy t COPTER ty 4 PO} v Ls 115 NT hy ee = Te SS ea} ) Paares lf i 27 oF THE 1 q c) Le ¢ at a en ———e | ee lal Le at conan Hosted by Google Hosted by Google ari ie wf ve was a Ps ~ ~ By vs is fs tee ran yi a Ee 3 ot eg ran eo a o ee he Hosted by Google Hosted by Google NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands eee VOLUME I—Part 3 Geology of the Ponce District—G. J. Mitchell OF NEW YORK: FP) BLISHED BY THE ACADEMY 1922 Hosted by Google GEOLOGY OF THE PONCE DISTRICT, PORTO RICO By GrRaHAM JOHN MITCHELL CONTENTS Page AGG os 0X6 06 C61 5 C6) 8 Sm 231 Nature of investigation... .. 0... cc cece cece wee ce cette eee e eee eace 231 Geological and topographical map........-.. ccc cece cece e eee ee eee 232 Routes of travel... .. ccc cece cee cee ee tec e eee c ee eee tetas eeceeeses 232 Climate and vegetation.......... 0 cece cee cece eee e rere eees wc eeee 232 Acknowledgments. ...... cece cece cece cence erence cece eee eeeeeee 232 PHYSIOZTAPNY.. 1... cece ee wee cece rete eee ee tte eee weet ee eee nes 233 Introductory statement... ..... cece cece cece eee eee eee ee ences 233 Complex mountains....... ccc cece cece eee e eevee ceees eee e cece ee eee 234 EXtent.... ccc ccc cece eee cet eee eee eee eee eee et tee eeeeees 234 Relief... ccc ce ccc cee cee cere tee e tee e eect eee eee e eee eeeee 234 Relation of topography to rocks and rock structure............. 235 Drainage........eeeceesees peewee eee ree teen ee eee gen cccscesees 236 TerraceS......ccee eens eee cece eee eee eee e reece eee tenance 237 Tertiary coastal plain. ...... ccc cece eect cece eee eet tent e eevee eee 237 Bb. 5) 0 237 Relief... ccc ccc cc cece ee eee eee ee ee ee tenes we eee ee eeeees 238 Relation of topography to rocks and rock structure...... .......... 238 Drainage... . ccc ccc cece cece eect e eee teens weet eee eens 238 TOLTACES. cc ccc ccc cee ee ee eee eee eee eee eee tee Hee e ee eeee 239 TLOW]andS.... ccc ccc ce cee ce eee ee ee eee eee tenet teeter eee eee eeee 242 Valleys... cc cece c cect e cece eet e eee t eens Sees 242 Guanajibo Valley... ... cece cece eee cece tence beeen eeeeeees 242 Yauco-Boqueron Valley.... ccc cees cece e eee reeee eeccereees 243 Ponce-Juana Diaz Valley....... 0. cc cere eect tween cence eees 243 Yauco, Guayanilla, Tallaboa valleyS..........6++ esses eeeee 244 PIAYVAS. .. cece cece eee eee twee eeeees ee 244 Minor physiographic features.........:eee cece eee eect tet eer eceeees 245 Slumping. ...... cece cece cee eee eee teen neon Mee eee cece ence cece 245 Sink-holes........ Ce cece cece ce eee ce eee erect eect ets cenenseees 246 Detailed description of formations..............++- eee e cee eeee 246 A 246 Shale... cc ccc cece cee cee cee eee eee e eee et tenet ee eeeeeee 248 General statement........ esc c cee cere cere et weneeeenee 248 Rio Yauco shale... ... cece sce e cee cece es ete re ceceece 249 Pefiuelas shale... ..... cece cee eee ences cee ee eens 251 Ensenada shale.........ceceee cee eeceeee ‘eee e cece eweee 252 TAMeStONE. . cc cee eee eee cece eee e teen eee eee eeens 253 San German limestone......-.. ee cere eee tee e ence eee 253 230 SCIENTIFIC SURVEY OF PORTO RICO . Page Coama tuff-limestone.............. sete eee cee cece eee 255 yuayabal limestone... ... cc cece cece eee ee tee e ee eet tees 256 Tertiary sediments...... eee e ee enee eee ee eee eee eee et eee e ee eees 258 Introductory statement..................00. se eeee cece eee seeee 258 Ponce formation............. cece eee e cee eees ean eaee eee e ee eee 258 Quaternary deposits... ... cece cece eee eee eee eee ra 260 San Juan formation....... see e eee e eee ee eee scene wee cence 260 ATIUVIUM. 6. ccc ce eee cece cece eens see e eee nee 261 Igneous rocksS...... 0. cee cee eee ee eee eee vee e cece eee we eee eae 261 Metamorphic rocks..... cece eee ee eeee Cece e cece eee ete cere eect wees 22 Anamorphic rockKsS......... 00. c cece cece e eee eee eeee es wee e eee 262 Katamorphic rocks.......... 00. ee eee cece cee e eee sec c eee ee eeee 262 Structure......... coat eee ete cence See e eect eee eee ee tees seeeee 262 Introductory statement......... 02. cece cece cee eee e nee ete e neces wee 262 Folding............ eee eeee sce c eee e cere cette eee e cence « se eeeeeee 263 Late Cretaceous folding.........cccceeeceeceeeeee se eeweseecees 263 Tertiary folding............ eee cece e eee eee eee e ree ceee seeeee 264 Faulting......... ee eee eee eee eee ee tenet tence ese ee eee ee ees 260 In Cretaceous rocks.......... se eeee pce rete eee eet cence ees 265 Late Tertiary faulting. ........ 0c ccc cece eee ce eee eens 266 b G22) a6) C0) 2 a 269 Introductory statement......... cee cece ee ee ee eee eee eee eeees 269 Igneous intrusive rocks................ se eee eee renee eee eceeses 269 Quartz diorite.... 0.0... cc cece cece cece e een ees secon see eeee 269 Diorite...... 0... 0. cee eee ewer eees see e cece eee eee see e eens 270 Trachy-andesite..... ccc cece ee cer cece cece eee e eee elseeee eens 271 Hornblende andesite. ...... ccc ec eee e eee eee tenes see cence eeeee QTL Augite andesite... 2.0... cece ee cece ce eee eee see e cere eee 272 Diabase.......... se eee enee seer e ee eee eee cette ee eae oF eee eeee . 273 Augite porphyrite............. eee ce eee eee cece tee eee tees 275 Pyroclasties....... wee e cece . see e eee ee eee cece e ee wenee weees 215 Tuff... ccc eee cee eee eee sce c wee eee eens . se eeeee ween. 25 Sedimentary rocksS.........ce cece eee eeeee eee eee ee ete eee wee eee eeee 277 Shale............+. see e eens cece cece ec ee eee estan ee eeene wee 217 Chert..... ccc cece reece ener cces ee eeee cece ee eee eeee eee ceeeeee 279 San Juan formation......... dace eee cece ee eee se ceecceceees 209 Metamorphic rocks..........5+5 ran rn ee eee eee ee 250 Contact metamorphics............ see e eee eeeeeee ee cece ee eeeee 280 Garnet rock..... cece eee eee see eee ee tee eee eae we eeeeee ee 2H) Garnetiferous limestone....... see e cece wees cece ee eeee ee 290 Epidote rock............ cece ee eeee se ee eee ee eee cee se eeee 281 Katamorphic rock... .. cece eee cece e eect e eee leeeeceeevecees 281 Serpentine... 0. ccc ccc cece c cece rece eee ee essence seeeeeees 28h Paleontology. ....--e.eeeeereee eee reece eee teen eee ee eres eee cece wee. 282 Introductory statement.......... eee ee ee eee eee eects ene seeeeeeee 282 Post-Tertiary fossils......... secs ee cene ee eeee eee ee cece cece eee es 28 Tertiary f0SSi1S......c ec cee cece cece e erect rete e erences eceees 203 285 Cretaceous fossils.......... eee cece ee eee tenet ee cee seer eres MITCHELL, GEOLOGY OF THE PONCE DISTRICT 231 Vage Historical geology... . cc ee cece eee cece e tee ccceeeeucteeeeueee 287 Introductory statement....... cc ce cece cece c ese e cee seecccarecesaees 287 Cretaceous rock deposition................ccccceceees eee eee e eee eee 288 Deformation and intrusion...........0.... ccc ce eee eee ees eee eeee 288 Erosion of Cretaceous... ..... ccc ec cece cece cece c enc teceeceneceacs 288 Tertiary sedimentation............ 0... ccc cece cece ec cee t eeeseneees 289 Deformation and uplift of Tertiary................... occ ee cece eee 289 Erosion Of Tertiary... 0... ccc ccc ccc ccc ccc cece eee c cscs neeetceees 289 Submergence with terrace-cutting and formation of San Juan forma- 1 C0) | 290 EMeCYVgence..... cece cece eee e ec eee ewe eee e tee eetnees eee ee eee 290 ECONOMIC ZEOLOLY .. 1... cece ewe cece ee eect cee cent tee ee ees weeeteeseeees 290 General statement... ..... cece cece ccc cece cette tee cne eeeeecenes 290 Manganese... ccc ccc ee ec cece eee tee ec neteees eee eee eceeee 290 Magnetite. 0... cc cece ee eee cece eee eee eaes cece eee eee 291 TIMONICe. 2. eee cee nee eee eee ee te eeeeeeeneeaes 291 C50 9) 9) se 294 S21 0a 295 Building stone... .... cc cece cece ce cece eee cece teens eee cee eee 296 Road-metal..... cece ccc cece reece cece eter n tent ences cute eeeecees 297 Cement material... ... ccc ccc cc cece eee eee tee neee eee eee eee 297 Petroleum... 0... Lee cece cee eet e eee ete rete ne wee eeeeetenes 297 INTRODUCTION NaTurRE OF INVESTIGATION A geological study of the Ponce District, an area of about 850 square miles, in the southwest corner of the island, was undertaken with the expectation that in this less humid section some points in the geology— for example, the structure and age of the Older Series and the question of recent changes of level—might be determined. The relation of this area to other parts of Porto Rico may be seen by reference to the Outline Map.1 The field-work occupied the period from June 1 to September 8, 191%, and the report was completed in the Geological Laboratory of Columbia University during the succeeding winter. Field-work was carried on from the principal cities and towns as headquarters, saddle-horses being the chief means of transportation. The formations along all roads, nu- merous trails, and many stream courses were studied, and it was along such routes that the best exposures and decipherable structures were found, especially in the Cretaceous rocks. Seven main traverses were 1 Scientific Survey of Porto Rico and the Virgin Islands, I, pt. 1, p. 26. 232 SCIENTIFIC SURVEY OF PORTO RICO made from north to south, with many detours into adjoining territory, thus making it possible to examine all formations of any consequence. GEOLOGICAL AND TOPOGRAPHICAL Map The base map for the region was compiled from maps of the United States Coast and Geodetic Survey and of the Insular Government, sup- plemented by the writer’s field-notes. The scale used, 1: 50,000, or ap- proximately one and a quarter inches to one mile, makes it possible to show areally small intrusive bodies which could not have been represented on the available smaller-scale maps. The topography is generalized, but care has been taken to depict the surface features in as much detail as time would allow. Routes or TRAVEL On the Geological Map an attempt has been made to differentiate the first-class macadam roads from those of second grade. All important trails are also shown on this map, but the innumerable local paths of the natives have been omitted. CLIMATE AND VEGETATION The climate is tropical, but modified by the trade winds and by the land and sea breezes. The district as a whole has less rainfall than other parts of Porto Rico. In fact, the extreme southwest corner borders on aridity. The mountains along the northern boundary of the district receive a copious supply of moisture from the sudden downpours, which were observed to be a daily event during the period spent in the district. Tropical vegetation is rank in most of the higher mountains. It forms a tangled mass, through which it is often necessary to chop one’s way unless roadways or streams are followed. However, all the mountain slopes are not so heavily coated with plant growth, some being compara- tively barren; for example, the serpentine ridges north of Sabana Grande and some of the hills of Tertiary limestone along the south coast. ACKNOWLEDGMENTS The many courtesies extended by residents throughout the island and the material assistance given by not a few of its people not only added pleasure and comfort throughout the work, but were in a measure respon- sible for the satisfactory completion of the field investigation. Among those who rendered special service were Col. George R. Shanton, Chief of Insular Police. Governor Yager’s suggestions of a general na- MITCHELL, GEOLOGY OF THE PONCE DISTRICT 233 ture and his assistance in getting the field-work started were highly appreciated. Dr. Lippitt, of the Bureau of Sanitation, gave information concerning sanitary conditions and also furnished letters to residents in the district. To Sefior Manuel Gonzales, of Salinas, the writer is in- debted for the loan of saddle-horses, the securing of which made it possi- ble to cover the area in much greater detail than could otherwise have been done. Deputy United States Marshal George Trautman, of Ponce, spent one day in the field with the writer, pointing out features of inter- est in the vicinity of the manganese deposit north of Juana Diaz. The efficient service of Sefior Fernando Oliver, Jr., as interpreter was a con- siderable aid in the field-work. The field-work and preparation of the report have been under the gen- eral direction of Dr. Charles P. Berkey, whose interest has been a source of inspiration. Dr. Amadeus W. Grabau has made valuable suggestions in stratigraphical and paleontological matters, as has also Dr. J. J. Galloway. Dr. T. W. Stanton, of the United States Geological Survey, determined the Radiolites sp. and Actaconella sp., and thus made it possible to place the age of the oldest rocks as Upper Cretaceous. Dr. Robert Tracy Jackson identified the Clypeaster rosaceus and the new species Hemiaster berkeyi. He has also kindly furnished a description for the latter. Lastly, the writer wishes to express his acknowledgments to the New York Academy of Sciences, which institution made it possible to carry on the investigation. PHYSIOGRAPHY INTRODUCTORY STATEMENT The broader relief features of the Ponce District may be grouped as follows : (1) A complex mountainous area, the westward continuation of the central Cordillera, which is characterized by angular ridges and deep, narrow valleys cut several hundred feet below the general level of the mountain surface, the altitude of which rises to over 3600 feet in the highest peaks, in the region to the southwest of Adjuntas. These moun- tains stand on the north side of the district, and their southerly and westerly slopes stretch toward the coast with more or less uniformity and pavs beneath the remnants of Tertiary coastal plain sediments which border the south coast. The southwestward continuation of this “old lan? surface is broken by two conspicuous, broad valleys, one extending from Yauco on the east to Boqueron, on the west, and the other occupy- 234. SCIENTIFIC SURVEY OF PORTO RICO ing the triangular lowland from San German to Cabo Rojo. The most rugged section extends along the northern side of the region from the eastern boundary, east of Villalba, to the vicinity of Maricao, the hills becoming lower and the valleys broader as one approaches the south and west coasts. (2) A much-dissected coastal plain of Tertiary sediments, which at one time lapped over the eroded surface of these complex mountains, but is now faulted down against the older rocks and forms a border along the south coast. These coastal-plain deposits are cut into isolated remnants by streams flowing through to the Caribbean Sea. (3) Lowland areas, some quite extensive, occupy present and former river valleys and stretch along the coast in broad, gently seaward sloping plains, called playas. The most striking of such lowlands are the valleys between Yauco and Boqueron and the valley of the Guanajibo River. The most extensive playas are Ponce, Guayanilla, Cape Rojo and Mayaguez. In the following pages the physiographic details of these broader relief features will be presented, together with a discussion of minor surface configurations. CompLex Mountains EXTENT The complex mountains occupy over three-fourths of the region. They extend beyond the district boundaries on the east and north and reach the sea on the west. The southern limit is marked by the fault separat- ing the Tertiary and Upper Cretaceous rocks. RELIEF The relief is rugged along the northern border, whereas the ridges hecome more rounded and the valleys broader as the southern and west- ern boundaries are approached. When viewed from the hills south of Yauco, the upland surface has a gradual slope southwestward from the main drainage divide north of Yauco to the low hills of the west coast, extending from Mayaguez on the north to Cape Rojo Playa (Fig. 1). On closer inspection the intensive dissection which progresses daily under the tropical showers greatly impresses the observer. The steep slopes of the higher mountains turn the waters into narrow gorges, where, afte? heavy storms, raging torrents are ever deepening and widening the!" valleys. The heavy coat of tropical vegetation checks the rapid run-ol’. but the impervious character of the surface soil facilitates it. Suc: MITCHELL, GHOLOGY OF THE PONOE DISTRICT yalleys as the upper Rio Yauco, Rosario, Guayanilla, Tallaboa, Canas Bucana, Guanajibo, Guaba and Preito are typical of this mountain region. The evidence of penepla- nation of the older rocks be- fore the deposition of the Tertiary is not so conclusive as on the north side of the island. There the old ero- sion surface, developed at the close of the Cretaceous and early Eocene, is plainly visible (Berkey, 1915, p. Ht). In the Ponce Dis- trict the only locality where this old erosion surface ap- pears to be preserved — is on the “Mesas” near Maya- guez. RELATION OF TOPOGRAPHY TO ROCKS AND ROCK STRUCTURE The influence which dif- ferent rocks and rock strac- tures have exerted in. the development of surface fea- tures is shown in many instances. The formations most resistant to weathering are the San German and Cuayabal limestones, the igneous intrusives, and the lighly indurated tuff. One need only see the white ridges of the San German ind Guayabal formations, irrounded by lower-lying vreas of tuff and shale, to 4 8 ° S a cd EI & & & BIMOTT) OONB| ptmoasosoy ayy uy AoqRA ¢ ponpx fo yynos siti Mavy4ay aut w 236 SCIENTIFIC SURVEY OF PORTO RICO realize the importance of these highly calcareous beds in the physiography of Porto Rico. The tuff and ashy shale weather readily, although it must be remem- bered that the more strongly indurated tuffs are among the rocks most resistant to erosion. The ashy shales are easily disrupted and cut away, and it is along belts of such material that the head-waters of some of the streams are forming deep valleys with sharp ridges between. Shale higher in lime content, as, for example, that at Ensenada, is a good ridge- maker. Reference to the geological and topographical map brings out the fact that the largest valleys, as, for example, Yauco-Boqueron and Guanajibo, are formed principally on tuff. A factor which has acceler- ated the erosion of tuff in the southwestern part of the district is ex- foliation. By this process numerous tuff exposures are undergoing dis- integration. The structural attitude of such rocks as the San German and Guayabal limestone is in part responsible for some of the most striking surface features in the region. The tilted position of these strata, with more easily eroded material both below and above, has given rise to ridges like the one to the east of Guayabal reservoir and those forming the south face of Yauco-Boqueron and Guanajibo valleys. The development of subsequent valleys by headward erosion along weaker rock-belts can be seen north of Yauco, where the head-waters of the Guayanilla are working back along belts of tuff and shale. Other smaller, subsequent valleys are found along the south coast, as, for ex- ample, the one at Ensenada and the extensive lowland between Yauco and Boqueron and the valley along the Guanajibo. DRAINAGE The present drainage is divided among fifteen main streams and their tributaries. The area east of a north-south line drawn approximately through Yauco is drained by eight principal rivers, among which are the Jacaguas, Tallaboa, Guayanilla, Bucana, Inabon, Portugues, Canas, Yauco and Susua. These streams, whose head-waters are in the moun- tains to the north, cross the geological structure approximately at right angles in the lower three-fourths of their course to the Caribbean Sea. Each has cut a deep valley along its upper course, but on reaching the coastal plain the breadth of this valley is considerably increased, and in instances, like the Ponce Playa, the confines of individual streams are lost in the flat coastal lowland. The course of the drainage across the structure supports the idea of superposition from a former more exten- MITCHELL, GEOLOGY OF THE PONCE DISTRICT 237 sive Tertiary coastal plain, and, as will be shown later, this has probably been the case. The principal westward-flowing streams are the Guanajibo, with its tributaries, the Viejo, Rosario, Hoconuco, Cain, Flores and Mayaguez. The history of the Guanajibo is of special interest, due to its capture of streams to the east of San German. These streams formerly flowed south through the Yauco-Boqueron Valley to Guanica Bay. The original fuanajibo, which drained westward from San German, had a shorter course to the sea than the streams of the Yauco-Boqueron Valley. As a consequence, it pushed back its head-waters, which were then west of San German, to a point east of the city. The Cain and Flores, with their branches, were then following a longer southerly route and developing the Yauco-Boqueron lowland. With the eastward advance of the Guana- jibo, this southward drainage was captured and diverted to the west. The water gaps in the hills southeast of San German strongly support this conclusion. Stream gravels are present in some of these gaps. The north-flowing streams are Mayaguecillo, Guaba, Bucarabones, Prieto, and Blanco, all of which pass into the adjoining area, studied in detail by Mr. Bela Hubbard. These streams are working mostly in shale and tuff and are rapidly cutting back their valley heads along the main drainage divide. TERRACES The only terraces found along streams are those formed by the en- trenching of formerly deposited river alluvium. Such terraces are more strongly developed along the lower stream courses, but a few examples are found bordering the wider sections of the upper stream valleys. It is mv belief that these terraces are connected with the recent changes of level which have affected Porto Rico. A wave-cut terrace has been formed on the Cretaceous rocks making up the point just south of the Reform School, southwest of Mayaguez. The details of the terraces due to wave action will be found in the section describing the Historical Geology. Tertiary CoasTaL PLAIN EXTENT What remains of the Tertiary coastal plain along the south coast of Porto Rico extends from a point a short distance east of Juana Diaz to Cape Rojo and Point Aguila, in the southwest corner of the island. This physiographic province is not continuous, but is cut into isolated areas by the rivers traversing southward. The largest portion is confined to 238 SCIENTIFIC SURVEY OF PORTO RICO the vicinity of Ponce and continues west to the region of Ensenada. The exposures at Cape Rojo and Point Aguila are small and underlie the San Juan formation at the former locality. The northern boundary is deter- mined by the fault, which is discussed later under structure. RELIEF In contrast with the complex mountains, the coastal plain has a much smoother aspect. Viewed from a distance, it is seen to approach, with a gentle south slope, the “old land” on the north. The boundary between these two physiographic units is in most cases characterized by an erosion scarp forming the south wall of the discontinuous fault-line valley devel- oped along the fault zone between the Tertiary and Cretaceous areas. The surface is diversified by stream valleys both across and parallel to the strike. The greatest altitude reached is a little over 900 feet, in the area southeast of Pefiuelas, and the slope is southward and southwestward from this point. RELATION OF TOPOGRAPHY TO ROCKS AND ROCK STRUCTURE The soft, chalky character of much of the strata making up the coastal plain offers little resistance to erosion. Streams have slight difficulty in developing their valleys, and would cut them much faster were it not for the fact that the rainfall in this part of the island is much less than in other sections of Porto Rico where the Tertiary is found. Harder strata, sometimes a foot or more in thickness, have protected the underlying material, thus producing low cliffs. This, however, is a minor feature. On the north coast shaly and harder strata have been in part responsible for the haystack topography (Berkey, 1915, p. 51). None of this type -f topography was seen in the Ponce District. The ease with which water enters and dissolves the limestone is respon- sible for the pitting of the surface with depressions having no apparent outlet. Such sinks are to be seen in the region of kilometer 67.0 along the Ponce-Pefiuelas road. These depressions are very inferior to similar occurrences on the north side of-Porto Rico. DRAINAGE The principal drainage consists of the main streams, which rise in the mountains to the north and traverse the coastal plain through open val- leys bordered by low hills. All of these streams lie to the east of Ense- nada, the Tertiary area to the west being void of any but intermitten' streamlets. Among even the larger rivers, few maintain a constant flow MITCHELL, GEOLOGY OF THE PONCE DISTRICT 239 to the sea. Observations during the three months spent in the district proved most of them to be of torrential habit, at times filled to overflow- ing, then receding again to dry channels. The amount of rock waste which such torrents transport is large. Boulders several feet in diameter can be found in stream channels during low water. Gaps in which water-worn gravel is found occur east of Ponce, in the Tertiary limestone ridge along the south face of the lowland between Ponce and Juana Diaz. The largest of these gaps is just southeast of kilometer 127.0, on the road from Ponce to Juana Diaz. Formerly the Bucana River occupied this depression, but has since taken a westward course to the Ponce Playa. Other gaps notch the’ Tertiary ridges, but conclusive evidence of stream capture in such instances was not found. TERRACES Both stream- and wave-cut terraces are present. The former are sim- ilar to those described under the complex mountain area, but are more extensive ; the latter are represented by old marine levels along the south coast. he stream terraces are best seen along the Bucana River north of the military road from Ponce to Juana Diaz. The successive levels at this locality have cut through alluvium to underlying rock. Wave-eut benches on the Tertiary along the south coast of the district are well developed on the headlands at the mouth of Guanica Bay and a short distance both east and west of this point. The elevation of these levels ranges from 10 to 200 feet and bevels the south-dipping Tertiary limestone beds. In some instances cliffs at the inner margin are partially preserved, but in most cases the soft character of the chalky limestone has prevented their preservation. There has been some difference of opinion, among those who have studied the district, concerning the question of recent changes of sea- level in Porto Rico. In the following pages the evidence secured by the writer in the survey of the Ponce District will be presented. The locali- ties listed range from east to west and include terraces cut on the Cre- taceous, as well as on the Tertiary rocks. Berkey (1915, p. 48) describes terraces in the region near Guayama, on the south coast, which he at- tributes to wave action. Their levels range from 100 to 200 feet. The evidences of recent changes of level, together with conclusions on the (question, are as follows: (1) One-half mile southwest of Juana Diaz, on the north bank of the Jacaguas River, the folded Tertiary beds are beveled and a deposit of silt, sand and gravel 2 to 12 feet thick covers the surface. In this surface 240 SCIENTIFIC SURVEY OF PORTO RICO covering, at an elevation of 130 feet, are found numerous Strombus pugilis. (2) At kilometer 72.5, on the Ponce-Pefuelas road, recent marine fossils are found in finely stratified material of estuarine character. In this deposit a layer of black mud averaging one foot in thickness occurs at a depth of from 2 to 5 feet below the surface. In this black mud are found Strombus pugilis, Lucina jamaicensis, Lucina tigrina, Arca ‘tuber- culosa, and Byssoarca ziebra. These fossils are also found in other parts of this deposit, the elevation of which is 180 feet. (8) Across the west branch of the Cafias River, just east of the above locality, the same species of fossils as occur at (2) are found in the stratified sands and gravels at a depth of 314 feet below the surface and an elevation of 160 feet. (4) Southeast of Yauco, 114 miles, in the Rio Yauco Valley, abundant fossils are found in the surface covering of the river valley at an elevation of 150 feet. The fossils include Murex elongatus, Arca rhombea, Lucina tigrina, Arca tuberculosa, Turritella imbricata, Pecten nucleus, Venus cancellata, Ostrea virginica, Pterna sp. (5) East of Yauco, one-eighth of a mile, the pre-Tertiary rocks are truncated, and in the gravel and sand which cap the beveled strata are found Arca tuberculosa and Lucina tigrina, occurring at depths of 1 to 2 feet below the surface. The elevation at this point is 200 feet. (6) On the coast southeast of Yauco a terrace surface bevels the Ter- tiary limestone at an elevation of 60 to 160 feet, the inner margin being marked in places by a cliff. The following fossils are found on this sur- face: Strombus acciptrinua, Fissurella nodosa, Arca rhombea, and Turbo pica. (7) Just north of the lighthouse at Guanica the Tertiary limestone is beveled by terraces at levels of 10, 50, and 150 feet, and in the surface soil on the two upper terraces are found Arca tuberculosa, Lucina tigrina and Turbo pica. At the ten-foot level large numbers of these fossils are found in the lime sand and silt which coat this terrace. (8) East of Guanica one-eighth of a mile, on the east side of the Susua Valley, a terrace at an elevation of 50 feet contains in the surface layers the forms Lucina jamaicensis, Arca tuberculosa, and Turbo pica. (9) At the town of Ensenada (Central Guanica) the shale is trun- cated and a deposit of shells, mud, silt and sand covers the surface to a maximum depth of 5 feet. The fossils occur at an elevation of 45 feet and include the following: Murex elongatus, Manicina sp., Venus can- cellata, Operculum of Turbo, Arca rhombea, Cerrethtum litteratunc. Ostrea virginica, Arca tuberculosa, Byssoarca ziebra. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 241 (10) On the south side of Pardas Bay, south of Ensenada, the Ter- tiary limestone is again terraced at an elevation of 65 to 100 feet, and the fossils Arca rhombea, Arca tuberculosa and Lucina jamaicensis are found buried in the soil of the surface. (11) On Cape Rojo, in the southwest corner of Porto Rico, the San Juan formation, which has been interpreted by Berkey as a lime sand of dune origin, is found at an elevation of 75 feet, overlain by 3 feet of conglomerate consisting of well-rounded pebbles. In the San Juan for- mation occurs a Conus sp. very close to the recent form Conus porto- ricanus. (12) On Aguilla Point, the extreme southwestern portion of the island, recent gastropod shells are found in consolidated gravels at an elevation of 11 feet. At an elevation of 25 feet they occur on the beveled surfaces of the rocks which make up this point. (13) Three and three-quarters miles southwest of Mayaguez, on the coast near the Reform School, a terrace is cut on the Cretaceous rocks at an elevation of 50 feet. The inner margin is marked by a cliff, and the following fossils are found in the surface soil: Arca tuberculosa, Venus cancellata, Lucina jamaicensis. The argument has been advanced by Lobeck that where recent fossils have been found in Porto Rico they are associated with Indian mounds. Such an interpretation, however, could not explain the existence of shells buried in stratified material of estuarine character at depths of from 2 to 5 feet. Furthermore, although in each of the thirteen localities cited above the writer made careful search for artifacts, in no instance was evidence found to substantiate the Indian-mound theory. Based upon the evidence presented in the thirteen above-mentioned cases, the writer draws the following conclusions: With the recent changes of level the old river valleys were embayed, allowing the sea to enter with its marine fauna and to lay down deposits of sand, silt and mud. That these deposits—for example, at localities Nos. 1, 2, 3, 4, 5, 8 and 9—were laid down in Quaternary time is evidenced by the fact that over 95 per cent of the fossils are of the same species as those living at the present time in the adjacent sea. In the remaining instances, Nos. 6. 7, 10, 11, 12 and 18, the truncation of the underlying beds of limestone and other formations along the south and west coasts and the presence of cliffs at the inner margins of some of these terraces, together with the recent fossils found on the surface, are facts hard to explain if they are hot connected with the work of the sea. In considering the question as to which has been the shifting element, the land or the sea, the evidence indicates a change in the elevation of the 242 SCIENTIFIC SURVEY OF PORTO RICO land. If the sea-level had varied and the land had remained stationary, one should find some uniformity in the terrace levels at particular stages. Such uniformity does not exist. In summing up the conclusions the writer feels justified in stating that there has been differential uplift of the land in Porto Rico in recent time, with a maximum change of at least 200 feet. Low1Lanpbs VALLEYS Two of the most interesting valleys from a physiographic standpoint are the lowlands occupied by the Guanajibo River, especially the trian- gular portion between San German, Cabo Rojo and Hormigueros, and the broad stretch of nearly level area between Yauco and Boqueron. The development of the former has been at the expense of the latter. The Guanajibo River occupies a valley, the head of which lies just east of Sabana Grande. The lowland extends westward along the river, and narrows north of San German to less than one-quarter of a mile. A short distance westward it again widens into the triangular area already de- scribed. In the northwest corner of this triangle the valley is once more reduced in width and finally opens out into the Mayaguez Plava. The history of the development of this valley is intimately associated with the rocks, rock structure, and drainage of the region. The underlying for- mation is chiefly tuff, with some shale. The strike is in general parallel to the length of the valley. Berkey has suggested that the triangular portion already mentioned is related to an old voleanic vent, where the extremely broken and easily disintegrated rock material would allow rapid removal by streams and at the same time account for the shape of the depression. The evidence in the case is not clear, because a deposit of alluvium covers the valley floor to such an extent that only scattered ex- posures of the underlying formations could be examined. Where such exposures were seen they were found to be tuff like that exposed in the hills to the east and west. The narrow portions of the valley, for example, north of San German are bordered on the south by peridotite, largely changed to serpentine. In the case of the depression now occupied by the Guayabal reservoir, north of Juana Diaz, the evidence of a former center of voleanic eruption is clear (Berkey, 1915, p. 37). The extremely broken and heterogeneous mixture of voleanic fragmental material has offered comparatively little resistance to erosion, with the result that © similar but much smaller lowland than the one along the Guanajibo ha- been formed. Whether or not the lowland along the Guanajibo has * similar history to the one marking the site of the Guayabal reservoir, | MITCHELL, GEOLOGY OF THE PONCE DISTRICT 243 was not able to determine from the evidence presented. However, it was found that the more easily eroded rocks underlie this area, the chief eroding agents being the Guanajibo and its western branch, the Viejo River. Another factor in controlling the shape and extent of this low- land is the San German limestone, which forms the south wall and has developed an erosion scarp with east-west trend along the southern bound- ary. .The relation of the history of the Guanajibo Valley to that of the Yauco-Boqueron lowland is closely associated with drainage modifications of the Guanajibo. By working eastward, the head-waters of this stream captured those which formerly flowed south through the Yauco-Boqueron depression to Guanica Bay. Further details of this capture will be found in the discussion of this latter valley. The Yauco-Boqueron Valley, a name suggested by Lobeck, extends from Yauco on the east to Boqueron Bay on the west. It is the most pronounced physiographic feature of its type in the district. Its south wall is formed by the San German limestone and tuff. The northern boundary is less strongly set off from the low hills of the complex moun- tain province. Its greatest width is approximately three miles, with a length of over 21 miles. Southwest of Lajas the valley narrows; then widens again westward of that point. The elevation ranges from sea- level at Guanica and Boqueron bays to an altitude of 150 feet just south of Lajas. The underlying rock is chiefly tuff, with small patches of shale and two remnants of San German limestone in the hills east of Guanica Lake. Tuff remnants can be found protruding through the alluvium filling. The geologic structure which has influenced the development of this lowland is that associated with the southward-dipping shales and limestones. Reference to geologic cross-section (B.B*), Plate V, will illustrate the structural habit of the rocks involved. It will be noted that the formations dip south at angles near 45°, and the main depression is cut on the tuff, with the limestone forming the greater part of the ridge to the south. The history of the development of this valley dates back to a period before the capture of its principal drainage by the Guanajibo. At that time the waters of the Cain, Flores and what is now the upper (uanajibo flowed south to Guanica Bay. It was during this period that the valley was carved. Later diversion of the drainage by the Guanajibo left only a few intermittent streams in the eastern portion, which have continued to furnish water for Lake Guanica. The western portion of the valley was formed by the Boqueron River and its branches. The depression between Ponce and Juana Diaz differs from the two just described, in that it has been formed on the Tertiary marl and lime- stone which has an anticlinal structure. Structure (Section H. H’, 244 SCIENTIFIC SURVEY OF PORTO RICO Plate VI) is across the eastern part of this valley and shows the relation between structure and topography. The fault along the north side cuts diagonally southwest across the anticline, and in the vicinity of the Ponce-Adjuntas road brings the southern limb, which is composed of chalky limestone, into contact with the Cretaceous rocks. This feature can be seen in Section G. G*, Plate VI. The south wall is determined by the erosion scarp developed on the south-dipping Tertiary limestone. ‘The surface of this valley does not present the same smoothness which characterizes the depressions at the west end of the district. Low hills are present and stream terraces are cut below the general level of the valley floor. Other lowlands along the south coast are illustrated by the valleys cut by the Guayanilla, Yauco and Tallaboa rivers. All of these depressions have essentially the same history, having been formed by the streams which now occupy them. Their location and extent are shown on the Geologic Map. Their most conspicuous portions are near the mouths of the streams and are cut in the Tertiary Coastal Plain sediments. In the case of the Rio Yauco and Guayanilla, the valley floors developed on the Tertiary rocks are much wider than along the Tallaboa, but in each case the mouth of the stream is characterized by a coastal flat. To the east are similar valleys, as, for example, Canas, Inabon, Portu- gues and Jacaguas, all of which merge seaward into the Ponce Playa. PLAYAS Playa is the local name given to gently sloping areas bordering the sea. In other countries similar areas have sometimes been designated as nar- row coas‘al plains, to distinguish them from more extensive tracts of the same character. In Porto Rico, playas are prominent features, especially _ along the north and part of the south coast. The four most extensive ones in the Ponce District are Ponce, Guayanilla, Mayaguez and Cape Rojo playas. Ponce Playa, the largest of the four, is over 15 miles long and 314 miles wide at its broadest point. The gentle seaward slope is traversed by the Canas, Portugues, Bucana, Inabon and Jacaguas rivers, all of which contribute to the gravel, sand and silt which coat the surface of the underlying limestone to depths of 25 feet and over. The topmost layer is generally a black soil, made blacker by fertilization with cane waste from the sugar “centrals.” The formation of Ponce Playa and others to the westward is due to combination of river flood-plain and delta processes. The detritus froi the land has been carried to these lower areas, where it has been spreat! MITCHELL, GEOLOGY OF THE PONCE DISTRICT Q45 out by successive floodings. The occurrence of recent marine fossils in the playa material shows the presence, at one time, of the sea over these areas, and its influence in their formation is a factor to be considered. Guayanilla and Mayaguez playas are smaller than the one at Ponce, but have essentially the same physiographic history. At Mayaguez the underlying rocks are of Cretaceous age, while in the case of Guayanilla the basement formation is of the same geologic age as that at Ponce. Cape Rojo Playa is of special interest, due to the presence of a thin coating of reddish, siliceous sand which covers the surface. The rock upon which this sand rests is not extensively exposed, but where seen it is tuff of the same general character as that at Ensenada. The origin of the siliceous sand is probably in the numerous quartz veinlets which occur in the shale and tuff; also from chert masses in these rocks. The red color is due to iron oxide coating the grains and preserved in the arid climate which now characterizes the extreme southwest corner of the dis- trict. The concentration of sand at this point is most likely the combined work of streams and sea. Conclusive evidence of former streams entering this area from the north was not found, but the gap to the north.of the playa suggests the presence at one time of a stream of considerable size. The level character of the surface is in large part the work of the sea when it stood at a higher level. Minor PHyYSIOGRAPHIC FEATURES SLUMPING Slumping has produced minor physiographic changes, especially evi- dent along the sides of river valleys which traverse the Tertiary limestone, and in places the material has been reworked by the estuary waters, which at one time filled the lower portion of these river valleys. The erosion scarp along the north-facing Tertiary has also been modified by the same process. Large quantities of broken Tertiary rock were found in slumped areas, especially along the Ponce-Mayaguez road between Yauco and Ponce. The slumping has covered much of the fault which passes through this section. In the Cretaceous rocks, slumping is best seen in the serpentine northwest of Yauco, near the trail to Maricao. At this locality a large mass of rock has moved a considerable distance down the steep side of the valley. This slide is so recent that the surface along which the movement took place is plainly visible for some distance. Slumping of small areas, especially in road cuts, were encountered on the Mayaguez-Consumo-Maricao road near Maricao.. What appeared to be tulf and ashy shale al % x 7 ‘ Ne kK base and foraminifera NN xs & S u ~ shaleand /imesine nearer 3 N S94 g s\ u N the top » s . nN n v This series has be So So? > svaded by intrush Vos NS ~ 2 aS RN invaded DY Mnirusve 22 R N ° Diorite, Andesite, Trae 8 N s s 8 5 Andesite, Diabase, Augil Ss ae * ty porphyrife and peridoltle eo 2s 8 gs ERR 6 & g § e & Hosted by Google MITCHELL, GEOLOGY OF THE PONCE DISTRICT 269 cannot be determined and is probably much greater than the figure given. The age of this disturbance was at least late Tertiary, since the upper Oligocene beds are involved in the movement. The total displacement is the sum of a number of slippings at intervals throughout the later Ter- tiary. That the region may still be subject to further structural defor- mation is indicated by the rather strong earthquakes which were felt during the month of August. Besides the structural evidence already presented, the physiographic habit of the island, as pointed out by Berkey, supports the view of faulting with uplift of the island as a whole, along the southern side, and tilting of the block to the northeast. The presence of the fault along the southerly margin of the island has been questioned by Dr. A. K. Lobeck. After a study of the physiography of Porto Rico, he has concluded that the evidence of faulting can be attributed to sink- hole formation along or near the contact of the Tertiary and Cretaceous rocks. The abnormal dips of the beds near the fault he attributes to drag, produced by caving action accompanying the sink-hole formation. Such a “sink-hole hypothesis” seems incapable of explaining the struc- tural features involved. PETROLOGY INTRODUCTORY STATEMENT The purpose of this chapter is to summarize the petrographic details of a selected set of typical rocks from the area investigated. The material chosen includes sedimentary and igneous representatives, most of which are of sufficient field extent to be shown on the geologic map of the dis- trict. Some contact metamorphics are also described, but are not differ- entiated from formations of which they are only an altered portion. Ianreous IntTRUsIVE Rocks QUARTZ DIORITE Rocks of this type are well exposed at kilometer 23.4, on the Ponce- Adjuntas-Aricebo road, and where seen occur as an intrusive in the form of dikes or small bosses. Hand specimens of the quartz diorite are char- acterized by medium-grained texture and the presence of hornblende and feldspar, giving a greenish gray color on fresh fracture. Upon weather- ing, a whitish, sandy soil is produced, in which a few quartz grains can be identified. The leading primary essential mineral in this rock is plagioclase, with hornblende next in prominence. The feldspar ranges from oligoclase to 270 | SCIENTIFIC SURVEY OF PORTO RICO labradorite, andesine being the most abundant. The hornblende is of the pale-green pleochroic variety, occurring in rectangular plates and irreg- ular broken crystals. Quartz is interstitial and contains many dust-like inclusions and gas bubbles. The texture is that of a medium-grained, erystalline rock. The primary accessory minerals, in the order of their abundance, are: titanite, orthoclase, magnetite, ilmenite and apatite, Alteration has taken place in the minerals to a considerable degree, the feldspars and hornblende having been most affected. The former have given rise to a sericitic aggregate which marks the central portions as well as the borders of the crystals. A saussuritic complex, in which epidote is the prominent constituent, occupies portions of the labradorite crystals. The change in hornblende has resulted in the formation of chlorite, which in some instances is penninite. Titanite has remained unchanged, while the ilmenite has in places passed to leucoxene. Magnetite has given rise to limonitic iron, which in places stains the sections yellowish. The effect on orthoclase has been similar to that on the more acid plagioclase. Quartz is practically unaffected and contains many stout little apatites in perfect condition. DIORITE Typical diorite is found at several points in the district, the best ex- posures being at kilometer 23.9, Ponce-Adjuntas-Arecibo road; four miles up the Portugues River from Ponce, and at kilometer 29.5, Ponce- Adjuntas-Arecibo road, just north of the Ponce District. The forms in which these rocks occur are the same as those of the quartz diorite. The diorites, however, are much more extensive and represent a type closely related to the rock just described. In the field, diorite is marked by a medium to coarse texture, in which feldspar and hornblende are the chief minerals. The greenish black hornblende is more strongly developed in some localities—for example, on the Portugues River—while at kilometer 23.9, Ponce-Arecibo road, feldspar with porphyritic habit is the promi- nent mineral, In thin sections the rocks of this group prove to be normal diorites, consisting of plagioclase (andesine and labradorite) and hornblende as primary essential minerals, with a few crystals of oligoclase present. The accessory constituents are ilmenite, titanite, pyrite, pyroxene, magnetite. apatite and biotite. The texture ranges from medium to coarse and in places tends toward ophitic. In a specimen from kilometer 23.9, Ponce- Arecibo road, large crystals of hornblende not only act as “host” for the metallics, but inclose numerous large plagioclase crystals, producing 4 beautiful ophitic structure. In many instances the plagioclase shows JUTCHELL, GEOLOGY OF THE PONCE DISTRICT Q71 zonal banding, with the successive zones becoming more acid from the center outward. Among the most prominent products of alteration are chlorite, sericite and leucoxene, the chlorite developing frofn the horn- hlende and pyroxene and the sericite from the feldspars. Leucoxene is secn filling zones of decomposition along the crystallographic directions of the ilmenite—a feature characteristic of these two mineral associa- tions. Epidote is present as an alteration of hornblende, and pyrite has been changed along the crystal borders to limonite. A small amount of lime carbonate was noted as coming from the hornblende. TRACHY-ANDESITE A specimen collected from kilometer 22.2, Ponce-Adjuntas road, proved upon microscopic examination to be a trachy-andesite. Although the areal extent of this type will not permit its differentiation from andesite proper on the geologic map, it is thought best, for the sake of completeness of the petrographic series, to describe it. At the above locality this rock occurs as a small dike and is probably related to the larger dioritic mass which occurs in close proximity. In the field, this rock was classed as an andesite, for its true character is only seen in thin sections. What is true of the andesites as to field characteristics holds also for this rock. The interesting feature of this specimen is the prominence of ortho- clase in a rock which otherwise satisfies the requirements of a hornblende andesite. Oligoclase and andesine are both present, the former being the more abundant. Hornblende of both the light brown and the uralitic varieties is plentiful, the former appearing as basal sections in which the cleavage is well shown, and uralite in more rectangular plates, closely associated with secondary epidote. Orthoclase occurs interstitially as well as in the groundmass, while the plagioclases are arranged in such a manner as to simulate diabasic structure. Magnetite is the chief primary accessory mineral, with apatite and pyrite the only other representatives present. Alteration of hornblende has given rise to epidote and a few patches of lime carbonate closely associated with the epidote. An earthy substance having the appearance of kaolin occupies portions of the feld- spars. Magnetite shows practically no alteration. HORNBLENDE ANDESITE A typical exposure of this rock is found at the north end of Guayabal reservoir, on the road to Villalba. At this locality the andesite is in the form of a small dike cutting the tuff. The megascopic characteristics of 272 SCIENTIFIC SURVEY OF PORTO RICO this rock are its greenish white color and the very fine-grained ground- mass, in which are set a few feldspar and hornblende phenocrysts. In the field the material looked exceedingly fresh, but thin sections show that alteration of the minerals has progressed to a marked degree. Although the rock is badly altered, there is still enough left of the primary minerals to determine its original character. Andesine and labradorite, together with hornblende phenocrysts, are set in a dense, fine-grained groundmass made up chiefly of small plagioclase crystals. As accessories, a few oligoclase, pyroxene, magnetite and apatite crystals are seen ; the oligoclase and pyroxene being interstitial, while the magne- tite occurs chiefly as inclusions in the hornblende. Some grains of mag- netite, together with apatite, are distributed throughout the groundmass. The alteration of the feldspars has produced an aggregate of sericite which in places incloses carbonate. Hornblende has altered chiefly to chlorite and carbonate, with some epidote. Apatite is unaffected. AUGITE ANDESITE The following localities will serve as representatives of this rock type: Just north of the limestone quarry at Ensenada, “Guanica Central”; one mile west of “Guanica Central”; on the east line of the district, two miles north of the Ponce-Santa Isabel road; kilometer 64.2, Ponce- Pefiuelas road; the southeast end of Guayabal reservoir; at kilometer 8.1, Mayaguez-Las Vegas road; and on the west branch of the Canas River, north of the Ponce-Mayaguez road. Augite andesite generally occurs as sills or sheets in the shales and tuffs. In some instances, as, for example, in the tuffs, it is hard to determine the relation, as bedding in this rock is not clearly shown and jointing is so perfect that it might easily be confused with bedding. As seen in the field, rocks of this class vary in color and in their tendency toward porphyritic texture. Specimens one mile west of Ensenada and at the east end of Guayabal reservoir are porphyritic and have a reddish hue, due to the presence of more iron oxide (readily seen in thin sections), while the specimen at the east end of the district, north of kilometer 17.0, Ponce-Santa Isabel road, is dark greenish and shows a stronger porphyritic habit. In all occurrences. pyroxene crystals are visible as phenocrysts, but feldspar cannot always be recognized. The mineralogy of this rock is that of a normal pyroxene andesite. consisting of plagioclase (andesine, labradorite) and augite as primary essential minerals. The feldspars occur both as phenocrysts and in the groundmass and are marked in many cases by zonal structure. Inte'- growths of one feldspar in another are also seen in parts of the thin se~ MITCHELL, GEOLOGY OF THE PONCE DISTRICT 273 tions. Augite is developed mainly as idiomorphic crystals with basal and longitudinal sections visible. The minor primary minerals are mag- netite, ilmenite and hornblende, the latter being very sparingly repre- sented. The texture varies in some localities, as, for example, just north of the limestone quarry at Ensenada, where there is not the porphyritic habit that is seen in the rock just west of Ensenada. In the former the feldspars have a stubby habit and the contrast between phenocrysts and groundmass is not at all pronounced, while in the latter case the typical porphyritic habit of the andesites is well shown, with feldspars and augite as phenocrysts. The common alteration products are chlorite and iron hydroxide. An example of the former is very well seen in a specimen procured at kilometer 64.2, Ponce-Pefiuelas road, where the chlorite is developed from augite, which occurs both in the groundmass and in phenocrysts. Jron hydroxide is developed chiefly from magnetite. Seri- cite is present as a result of the decomposition of feldspar. Carbonate has been developed both in feldspar and augite. Some of the lime car- bonate, however, clearly shows an introduction origin. A small amount of epidote is closely associated with chlorite developed from the pyroxene. Leucoxene is seen to come from ilmenite, which in some slides is rather generously distributed. Native copper, associated with amygdules in a pyroxene andesite, was found on the west branch of the Canas River, just north of the fault between the Tertiary and Cretaceous. DIABASE Good exposures of diabase are found at the following points: Kilo- meter 22.5, Ponce-Adjuntas-Arecibo road; kilometer 3.2, Yauco-Lares road; and kilometer 2.8, Mayaguez-Las Vegas road. Dikes and sills of diabase are not an uncommon feature in the tuff and shale of the Older Series. The above-mentioned localities are a few of the occurrences noted in the field. In size these intrusives vary from small stringers to masses 50 feet or more across, as, for example, the exposure at kilometer 3.2, Yauco-Lares road. The most prominent feature of the hand speci- mens of diabase is the texture, which ranges from rather coarse diabasic at kilometer 3.2, Yauco-Lares road, to very fine in the rock at kilometer 22.5, Ponce-Adjuntas road. The coarser varieties have a grayish green color, while the finer-grained ones are a dark greenish shade. Outcrops weather to a muddy yellow color, and the soil derived from such material is of a darker hue. The essential primary minerals of this rock are plagioclase feldspar, of the labradorite and andesine varieties, and augite. The feldspars are arranged in characteristic diabasic fashion, with the augite filling the Qt4 SCIENTIFIC SURVEY OF PORTO RICO spaces between. This structure is well seen in the rock from kilometer 2.3, Mayaguez-Las Vegas road. In other instances the diabasic fabric. although present, is not so prominent a feature. Of the primary access. ories, magnetite and ilmenite deserve first mention. The proportion of these minerals vari In the sample from kilometer 2.3, Mayaguez-Las Vegas road, ilmenite is much in excess of magnetite, while in the other specimens magnetite appears in greater quantities. Apatite in needle- like crystals and stubby cross-sections is also present. Alteration has S. Kia, 6.—Augite porphyrite showing zoned augite crystal set in fine-grained groundinass Taken in plain light, magnified about 30 times. K. 8.6, Mayaguez-Consumo-Maricao road. resulted in the production of chlorite from the pyroxene, a feature well shown by the rock from kilometer 22.5, Ponce-Adjuntas road. Sericiti- zation of the feldspar has advanced in some cases to such a degree as to completely replace the original crystal. Leucoxene developed from ilmenite and iron oxide from magnetite complete the list of alteration products, In the material from the Mayaguez-Las Vegas road, lime cat bonate is present in veinlets cutting the other minerals, showing |t* introduction from outside sources. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 275 AUGITE PORPHYRITE Although porphyritic habit in rocks of the Ponce District is not un- common, two instances of that type of texture are such striking examples of porphyrite that it is thought best to give them more prominent men- tion than they might otherwise deserve. ‘The two occurrences are at kilometer 8.6, Mayaguez-Consumo road, and two miles north of kilometer 15.0, on the proposed extension of the Yauco-Lares road. In both locali- ties the augite porphyrite occurs as dikes in the pre-Tertiary shale and tuff. On the Yauco-Lares road the intrusion is much larger than the one on the Mayaguez-Consumo caraterra, being over 75 feet across. This rock is readily identified in the field by the presence of the large, well- formed pyroxene crystals set in the dark greenish, fine-grained ground- mass. The specimen from the Mayaguez-Consumo road is of a lighter greenish color and the augite crystals are not so conspicuous. The most striking petrographic feature of this rock is the porphyritic habit of augite. These large pyroxenes, showing marked zoning, are set in a fine groundmass composed of feldspars and partially decomposed ferro-magnesians. Oligoclase and andesine are present, both in the vroundmass and as phenocrysts, with the former more strongly developed in the groundmass. Some glassy matter is also present. Magnetite is prominent as inclusions in the pyroxene and feldspar and as small parti- cles scattered through the finer portion of the thin section. Alteration has progressed farther than would be suspected by an examination of hand specimens. Chlorite is the chief result of this alteration, and has developed from feldspars and ferro-magnesians in the groundmass and from the phenocrysts of these minerals. Cavities which were originally filled with other mineral matter are now occupied by chlorite. Magnetite is partially changed to iron oxide, which has stained portions of the groundmass. PYROCLASTICS TUFFS From many specimens of tuff a set has been selected for microscopic study, which covers the lithologic range of this class of rock. The dis- tribution of the samples chosen is such as to permit comparison in differ- ent sections of the district. The thin sections include specimens from the following localities: Kilometer 3.9, Mayaguez-San German road; kilo- ncier 19.8, Ponce-Adjuntas road; at the intersection of Ponce-Adjuntas and Jayuya roads; kilometer 13.7, Ponce-Adjuntas road ; kilometer 14.9, Ponce-Adjuntas road; kilometer 12.8, Ponce-Adjuntas road ; Guaniquilla 276 SCIENTIFIC SURVEY OF PORTO RICO Point ; one-quarter of a mile southwest of Lajas; on Jacaguas River where the road to Villaba makes first crossing; kilometer 31.1, Yauco-Mayaguez road ; kilometer 11.3, Ponce-Adjuntas road; just west of Ensenada; north three-quarters of a mile from kilometer 72.3, Ponce-Pefiuelas road; 2 miles northeast of Maricao; and on the divide ten miles east of Maricao. In all the above localities the tuff occurs as massive, faintly bedded ma- terial varying greatly in thickness and attitude. On the whole, the tuff shows evidence of fragmental habit, the included rock fragments being plainly visible. However, in some exposures the rock is thoroughly in- durated and does not show evidence of its pyroclastic origin. The color varies from greenish to reddish tints, with shades of yellow and white. It is found on microscopic examination that the colors are due chiefly to the prominence of different secondary mineral products, the reddish varieties being richer in iron oxide, the green in epidote, and the whitish in decomposed feldspar. The petrographic features of the tuff may be summarized as follows: (1) The composition is predominantly andesitic, with trachytic, basaltic and glassy varieties present. (2) Fragmental habit characterizes all the slides, although this feature is more strongly developed in some instances than in others. The frag- ments show wide range in size, varying from pieces plainly visible in hand specimens to those of microscopic dimensions. Jagged outlines of crystal and rock fragments are characteristic features. (8) The matrix in which the fragments are set generally contains a quantity of ashy matter, which packs in around the fragments, forming a compact mass. (4) Many of the individual crystals, as, for example, feldspars and ferro-magnesians, occur as broken fragments. (5) Included fragments showing spherulitic habit are sometimes present. (6) Secondary mineral products, including calcite, chlorite and epi- dote, are strongly developed; the calcite and epidote favoring the feld- spars and the chlorite favoring the ferro-magnesians. Where calcite is strongly developed, epidote shows a decrease. All of these minerals also show introduction, especially the calcite and epidote (7) Magnetite is abundant in some slides and shows alteration tu hematite and limonite. (8) The geologic history of this group of rocks, as far as can be asce'- tained from the study of thin sections, suggests the following steps: (a) Accumulations of volcanic ejectamenta on land surfaces, wi'l) marginal portions of these deposits reworked by water. This margi'! MITCHELL, GEOLOGY OF THE PONCE DISTRICT Q77 reworking accounts for the rudely stratified portions in which the frag- ments show more or less rounding. (b) Induration involving carbonitization, chloritization and epidoti- zation, which binds some of the rock into a hard, resistive mass. (c) Fracturing and introduction of carbonate, epidote an‘d chlorite. (d) Weathering, producing iron oxides and hydroxides. SEDIMENTARY Rocks SHALE Specimens from the following localities have been chosen as typical representatives of the range of rock included here under the general head of shale: (1) Kilometer 5.6, Mayaguez-Consumo-Maricao road. (2) Kilometer 3.7, Mayaguez-Consumo-Maricao road. (3) Lapena Point, north side of Mayaguez Bay. (4) Kilometer 6.4+-, Mayaguez-San German road. (5) Kilometer 14.6, Yauco-Lares road. (6) Kilometer 20.5, Ponce-Adjuntas road. (7) Kilometer 22.7, Ponce-Adjuntas road. (8) Kilometer 1.1, Jayuya road. (9) Kilometer 1.1, Jayuya road. (10) Kilometer 21.0, Ponce-Adjuntas road. (11) Intersection of Ponce-Adjuntas and Jayuya road. (12) Kilometer 16.6, Ponce-Adjuntas road. (13) East bank Jacaguas River, two miles south of Villalba. (14) One and three-fourths miles south of San German, on road to Lajas. (15) One-fourth mile south of San German, on road to Lajas. (16) Kilometer 7.2, Yauco-Lares road. (17) Kilometer 111.9, Ponce-Juana Diaz-San Juan road. (18) One mile north of kilometer 114.0, Ponce-Juana Diaz-San Juan road. (19) One mile northeast of manganese deposit northeast of Juana Diaz. (20) Kilometer 112.1, Ponce-Juana Diaz-San Juan road. (21) One mile northeast of manganese deposit northeast of Juana Diaz. The most evident field characters of the shale are the bedded and laminated structure and the color. The former is by far the more con- 278 SCIENTIFIC SURVEY OF PORTO RICO stant and has an important bearing on the solution of structural relations of the pre-Tertiary formations. The colors exhibited by this class of material have a wide range, including red, bluish gray, black, yellowish and green, with gradation between these colors. Although the color is the first feature which attracts one’s attention in the field, especially in the case of the reddish varieties, it must be borne in mind that the colors are the result of alteration and, as will be shown in the petrographic discussion, are dependent upon the decomposition of certain minerals. The petrography of the shales in the Ponce District may be sum- marized as follows: (1) On the basis of composition a general division can be made into those high in carbonate of lime and those high in ashy matter. The lime content is present in the remains of foraminifera shells and as finely dis- ‘seminated carbonate. When the ashy content is predominant, forami- nifera are scarce or entirely absent, and the rock in thin section has the appearance of fine volcanic dust with angular crystals distributed through it. Numbers 1, 2, 3, 4, 6, 9, 10, 11, 12, 15, 17, 19, 20 and 21 are typical ashy shales. Numbers 5, 8, 13, 14 and 16 represent characteristic limy shale. There are gradations between these two extremes, as shown by numbers 7 and 18. (2) When crystal fragments are large enough to be identified, they are found to be feldspar. (3) Glass fragments are present in the more ashy shales. (4) Pyrite, crystalline and massive, is plentiful in numbers 1, 2, 4, 5, 6, 9, 10, 11, 12, 13, 16, 17, 19 and 21. (5) Magnetite occurs in some slides, but not so extensively as pyrite. (6) In number 2 an inclusion of coarser-grained material shows lithic fragments of andesite. (7) The foraminiferal content varies in amount, but is greatest in specimens of the dark bluish gray, strongly bedded rock. A list of the identifiable foraminifera in the different specimens of shale will be given in the paleontological section. (8) Finely laminated structure is present in numbers 2, 8, 11, 12, 13, 14,17 and 18. This feature is plainly visible in hand specimens. (9) The chief alteration has been the oxidation of the iron-bearing minerals, chiefly pyrite. The color of the red and yellowish shale is duc to finely distributed earthy hematite and limonite respectively. Leu- coxene is present as finely disseminated specks. (10) The greenish rock, well exposed at kilometers 19.6 and 15.%. Ponce-Adjuntas road, derives its color from the presence of abundant chlorite. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 279 (11) Introduced veinlets of carbonates, chiefly calcite, are prominent in numbers 5 and 14. Many slides show micro faults and fractures. (12) Cavities left by the removal of mineral matter characterize num- bers 1, 83, 6 and 9. These specimens represent the more highly altered and porous rock. CHERT The two samples of chert chosen for thin sections are from Lepena Point, on the north side of Mayaguez Bay, and kilometer 6.+, Mayaguez- San German road. At both localities the chert occurs as small, irregular masses in shale and tuff. Hand specimens are grayish to red, with weath- ered surfaces colored whitish. Some dark red and green chert was found in the vicinity of Villalba. The rock is massive and is traversed by numerous veinlets of quartz. In thin sections, the rock presents the exceedingly fine-grained quartzose make-up characteristic of that class of material. In some cases the quartz is large enough to be determined optically. Many veinlets of quartz traverse the sections. Pyrite is prominent and hematite in a few grains is identified. Oxidation has produced limonitic substances from the pyrite. This yellowish material is distributed through the rock. Crushing has developed weaknesses which. have been filled with quartz veins. A search for radiolaria was made, but none was found in the sections. SAN JUAN FORMATION As seen in thin section, the rock is made up chiefly of lime carbonate in more or less rounded grains and irregular masses, through which are distributed angular quartz grains. Feldspar crystals are identified, but are not a prominent constituent-of the rock. One crystal of tourmaline was noted. The angularity of the quartz and feldspar crystals is a notice- able feature and shows that the material has not been subject to much wearing action of stream or wind. The quartz and feldspar, together with the tourmaline, which were in all probability derived from the near- by Cretaceous rocks, have been subjected to very little wear before their incorporation in the lime sand. The carbonate of lime is of two types: (1) Rounded grains. (2) Crystalline and massive carbonate, filling in around the other minerals. This carbonate of the latter type is responsible for the binding together of the mass. 280 SCIENTIFIC SURVEY OF PORTO RICO METAMORPHIC Rocks CONTACT METAMORPHICS Garnet Rock—The garnet rock described here is located at the iron prospect of Sr. T. Blasini, about four miles north of Ponce, on the Portugues River. This rock occurs as a contact phase of limestone which has been cut by an intrusive dike. The structural relation of the rocks involved are discussed in another section, under Magnetite. The most noticeable feature of this rock in the field is the knot-like aggregate of reddish brown garnet set in a light greenish, fine-grained mass which is cut by calcite veinlets. Chalcopyrite and pyrite are present in appreciable amounts. Pale yellowish brown garnet, determined to be grossularite, makes up approximately 90 per cent of the thin section. The garnet is cut by numerous fractures filled with introduced mineral matter and secondary alteration products. Among the other constituents, named in the order of their abundance, are lime carbonate, quartz, chlorite, epidote and sulphides. Carbonate, quartz, chlorite and sulphide have been intro- duced, while epidote is derived from garnet. The order in which the introduced minerals have come in overlaps. All have entered the rock at approximately the same time with the sulphides, slightly in advance of the others. Carbonate has continued as the last introduction product, with quartz and chlorite following in the order named. The time rela- tions of these introduced minerals is clearly shown in the veinlets which cut the garnet. The sulphides, chalcopyrite and pyrite fill fractures and veinlets in the garnet and are also included in the carbonate. The chief interest in this rock and the other two, about to be described under the contact metamorphics, is in their relation to the magnetite. A mineral group association which characterizes this rock and which might serve as one example of the result of contact metamorphism of limestone is the following: garnet, calcite, quartz, chlorite, chalcopyrite and pyrite. Garnetiferous Limestone.—This rock is from the same locality as the garnet rock. The garnetiferous limestone is a contact metamorphic phase of the San German limestone, which at this locality occurs interbedded in the Cretaceous tuff and shale. Fresh material of this class is com- posed of closely packed calcite and dolomite crystals, with numerous red- dish brown garnets dotting the mass. Pyrite cubes and massive chalco- pyrite, which shows the iridescent tarnish, are present. Calcite and dolomite crystals, packed. together so closely that cryste! outlines have merged one into the other, are the chief constituents of thi: MITCHELL, GEOLOGY OF THE PONCE DISTRICT 281 rock. Calcite is much in excess of dolomite and has the crossed-twin lamella well developed. Pyrite cubes and massive chalcopyrite are in- cluded in the carbonate and also lie at the junction of several of the cal- cite and dolomite grains. The garnet is the pale yellowish brown variety, grossularite, in which are quartz and chlorite crystals. Minute dust-like inclusions are common in the carbonates. The rock is very fresh; the sulphides show but little alteration. Epidote Rock—The epidote rock is also from the Sefior Blasini pros- pect, on the Portugues River. It occurs as a contact phase of limestone and an igneous intrusion. The nondescript appearance of this rock hardly suggests the beautiful development of epidote which is shown in the thin sections. Veinlets of epidote cutting the dark crystalline mass are the most noticeable feature in hand specimens. Dark brown garnet has a very slight development and a few sulphides can be identified. In thin sections epidote and quartz are seen to be the chief minerals present. The time relation of these minerals is practically the same. Both act as “host” and “guest.” Where “host,” the minerals occupy large, irregular patches; and where “guest,” they are present as small inclusions. Actinolite is identified as inclusions both in quartz and epi- dote. Patches of lime carbonate with chalcopyrite and pyrite inclusions are identified. The sulphides have been slightly attacked by weathering, but otherwise the rock is quite fresh. The mineral association is epidote, quartz, actinolite, calcite, chalcopyrite and pyrite. KATAMORPHIC ROCK Serpentine-——The distribution of serpentine rock can be seen on the Geologic Map. Its most extensive development is in the region north of Sabana Grande and southeast of Mayaguez. Specimens chosen for micro- scopic examination are from one-quarter of a mile south of the Reform School, southwest of Mayaguez; north of Sabana Grande five miles; from kilometer 41.7, Yauco-Mayaguez road. At the first and last locality men- tioned this rock oceurs in the form of dikes, but in the region north of Sabana Grande it takes the shape of a large, irregularly outlined intrusive mass in the Cretaceous formations. The exposures of serpentine are most frequently dark green to nearly black, with light shades of green charac- teristic of the more altered material. Some exposures are coated with a whitish alteration product, while in other instances iron stains the out- crop yellowish or reddish. The only minerals recognizable in hand speci- mens are pyroxenes and chrysotile, the latter forming small veinlets cutting the mass, and the former as phenocrysts, best seen on freshly broken surfaces. More thoroughly serpentinized exposures have a char- 282 SCIENTIFIC SURVEY OF PORTO RICO acteristic oily appearance and soapy feeling. In the specimens south of the Reform School, pyroxene crystals have weathered in relief, giving the surface a rough appearance. Much of the rock is strongly jointed and sheared and slumps where exposed in steep banks. Microscopic examination shows the serpentine to be derived from peri- dotite. Enstatite and olivine are the most abundant primary minerals, the former occurring generally as large, lath-shaped crystals showing varying stages of alteration to serpentine, and the olivine is distributed as individuals among the pyroxene and as inclusions in the enstatite. Residual cores of olivine in serpentine are prominent in portions of the slides. Bronzite and diopside are present as scattered individuals. Oli- vine is more abundant in the rock north of Sabana Grande and south of the Reform School. Magnetite and chromite are plentiful, much of the magnetite being secondary after olivine. Alteration has produced anti- gorite and chrysotile from the olivine. Cores of olivine surrounded by serpentine characterize sections where this mineral is present. At times only the outline of the former olivine crystal remains, the center having been filled with a serpentinous product. Veinlets of chrysotile cut por- tions of the slides. The decomposition of olivine gives rise to magnetite, which in turn has furnished the iron oxide and hydroxide which appear as stainings. Enstatite has been changed to bastite, which replaces entire crystals of the former. In other instances the attack has progressed only co far as to produce a change on the border and along lines of weakness in the pyroxene. The lamellar structure of the enstatite is inherited by the bastite, giving a structure easily distinguished from the core-like pattern of the decomposed olivine. PALEONTOLOGY InTRODUCTORY STATEMENT The paleontological material collected during the survey of the Ponce District lends itself to the following general classification : (1) Post-Tertiary fossils found in elevated gravel, sand and silt of former embayments; shells on the elevated wave-cut terraces along the coast and in the surface layers of playas. (2) Fossils from the Tertiary limestone and marl along the south coast. (3) A few forms of upper Cretaceous age from the limestone and limy shale of the Older Series of Berkey. In this report one new species and one variety are listed. The paleon- tological publications in the bibliography contain descriptions and illus- MITCHELL, GEOLOGY OF THE PONCE DISTRICT 283 trations of the other species here identified. The post-Tertiary fossils, all of living species, are found in perfect state of preservation. Details as to location and occurrence of these forms have been given under the discussion of terraces. The Tertiary formation has yielded a variety of forms with representatives from the lower and upper Oligocene. In the discussion of the lithology of these beds reference has been made to the distribution of the fossils and the lithologic character of the rock in which they are found. Upper Cretaceous Radiolites sp., Acteonella sp. and Hemiaster berkeyi n. sp. have been identified from the formations making up the older rocks in the district. The first two forms have been deter- mined by Dr. T. W. Stanton, of the United States Geological Survey, and the Hemiaster berkeyt by Dr. R. T. Jackson. Details of the forma- tions in which these forms occur have been discussed heretofore. Post-TERTIARY FossIis Post-Tertiary fossils were found in the elevated terrace gravel, sand and silt and in the estuarine deposits on the floodplains of some of the rivers along the south and west coasts. The list of forms identified are: Conus sp. like Conus portoricanus, Strombus pugilis, Lucina jamaicensis, Lucina tigrina, Arca tuberculosa, Byssoarca ztebra, Murex elongatus, Arca rhombea, Turritella imbricata, Pecten nucleus, Venus cancellata, ‘ Ostrea sp., Strombus accipitrinua, Fissurella nodosa, Turbo pica, Manicina sp., Cerrithium litteratum, Pterna sp. TERTIARY Fossius The Tertiary fossils from the Ponce formation have been identified as forms belonging to the Oligocene. The following is a list of those de- termined : Clypeaster rosaceus, Lamark (Fig. 7), Pecten sp., 281 SCLENTIFIC SURVEY OF PORTO RICO Stroimbus spe. Rulla sp. Teredo incrassata, Gabb, Lrea sp. Npondyls Spex Lucina sp., Leda sp. Corbula sp. ee a at ve ie Ae ae o, * oy be ae va be pon} va ot bye med ee ee 4 # i 9 ax aa fy ae 8 at ae oe ihe a ee cS 47 = Ae ee ‘J hy i if é i oe Bh 24 3 a ae in a a ihice es > At 2, ek ‘os ah ¥ e ig) a? igs 7 % a vas S sh ed ee te vite! ay be re. i Hlupeuster POSALEUS, Lamark Orhitolites complanata, Lamark, Ninmitites sp. frea sp. Cardin sy. . Dall. Peclen chipolanis Ensis sp. Peelen sp., Liv plistigina te sxont, VOrbigny, (ry prea Sp., MITCHELL, GEOLOGY OF THE PONCE DISTRICT 285 Natica sp., Amauropsis sp., Astarte sp., ‘ Cardium sp., Turritella halensis, Dall, Turritella halensts alpha, new var., Cythara sp., Fasciolaria sp., Pecten sp., Laganum sp., Orbitoides mantellt, Morton, Leaf fragments. A new variety of Turritella halensis var. alpha has been found, asso- ciated with T. halensis. Its variation from halensis is given below in the description of the form. Turritella halensis var. alpha This form differs from T. halensis in having the secondary spirals less prominent and also by the lesser prominence of the primary spirals on the main surface of the whorl. Only the upper two spirals are strongly developed, the third one being very weak, thus giving a distinction to that portion of the whorl. The lower two spirals are pronounced and become confluent into a nearly uniform ridge on the later whorls. The beading is well marked on the upper two spirals, but obsolete on the others. There may be a slight change in the surface of the shell, due to solution; but the general characters could not have been affected to the extent shown. Locality.—Jacaguas River, northeast of Ponce; occurs in the marly heds above the Orbitoides mantelli. CretTacEous Fossiis The following fossils have been collected from the San German lime- stone Radiolites sp. (Upper Cretaceous) and Actwonella sp. (Upper Cre- taceous). The Radiolites sp. is abundant in large and small fragments. It can be easily identified on weathered surfaces by the mesh-like struc- ture produced by weathering of transverse sections. The Aci@onella sp., together with the Radiolttes sp., serves as an index of these limestones. From the Cretaceous shale and limestone the: following foraminifera have been identified : ‘1) Shale just south of Melones Point. These beds are the westward extension of the shale at Ensenada, which carries Hemiaster berkeyt. 286 SOIBNTIFIO SURVEY OF PORTO RICO Textularia sp., Pulvinulina sp., Globigerina eretacea, d’Orbigny, Orbulina universa, d’Orbigny, Globigerina bulloides, dV’ Orbigny ete Ny, - CS +e, as by As x 8/2 Vig. 8.—Hemiaster berkeyi n. sp., Jackson. (2) Guayabal limestone, one mile east of the manganese deposit north of Juana Diaz. Globigerina sp. Much fragmental foraminifera not identifiable. (3) Ensenada shale. A large amount of fragmental foraminifera. The description of the new species of Hemiaster (Fig. 8) found in this MITCHELL, GEOLOGY OF THE PONCE DISTRICT 287 shale has been furnished by Dr. R. T. Jackson, who states that its affin- ities are with the Cretaccous. His description follows: Test large, cordiform, high, subconical, rounded below, from the highest point of the test dorsally sloping anteriorly, posteriorly and laterally to the ambitus. Ambulacra petaloid, wide, in deep broad furrows. The anterior ambulacrum III is narrower than the others and lies in a shallow furrow which reaches to the ambitus. Ambulacra II and IV, the anterior pair, are widely divergent at an angle of about 95° to each other and are in grooves which extend nearly to the ambitus. They measure 30 millimeters in length. The posterior ambulacra I and V lie in grooves and are narrower and much shorter than the anterior pair, measuring about 18 millimeters in length. The pores in the paired ambulacra are at the ends of narrow slit-like grooves. The interambulacra are narrow and elevated dorsally. The apical disc is very close to median in position, perhaps actually median, but from imperfections posteriorly exact measurement in the antero-posterior axis cannot be made. The apical disc is quite well preserved and shows the four genital plates with large perforations; the madreporite is of medium size, meeting but not sepa- rating the other genitals. Oculars I and V are in contact on account of the absence of genital V, as is characteristic of the genus. From imperfections the peristome and perisproct are not preserved. Small perforate tubules with subtubules are scattered over the test, somewhat distantly, and small granules thickly cover the spaces between the larger tubules. The specimen measures 40 millimeters in height, and this is a close approximation to the real height, though, if not worn ventrally, it would be slightly higher. It measures about 65 millimeters in length and 65 millimeters in width. The specimen is very much larger than any other fossil species found in the West Indies. It differs also in its shape from any known North American species. At the request of the collector, Mr. Graham John Mitchell, this species is named in honor of Prof. Charles P. Berkey, of Columbia University. GEOLOGIC HISTORY InTRopUCTORY STATEMENT In an historical summary of the geology of Porto Rico, Berkey (1915, p. 60) noted the following steps: (1) A long geologic period of volcanic activity, accompanied by marginal attempts at assorting of fragmental and detrital material of organic accumu- Jations, disturbed from time to time by renewed or extended igneous activity. (2) A dying out of volcanic energy, greater stability of the mass with re- spect to elevation and subsidence, and erosional attack continued long enough to result in extended planation and partial baseleveling, with final extensive submergence. (3) The development of an unconformable overlying series of shales, reef linestone and related deposits, chiefly of organic origin, brought to an end by fial re-emergence. : 288 SCIENTIFIC SURVEY OF PORTO RICO (4) The development of present surface features under stream erosion and marine marginal attack, with modifications arising from oscillation of level. These, with a few additions, are essentially the events in geological history of the Ponce District, a summary of which is given below. Cretacrous Rock DEvosITIOoN The oldest geological record in the Ponce District is represented by a series of interbedded tuff, shale and limestone beds which comprise the Cretaceous formation. These rocks represent a period of volcanic activity in the form of explosive eruptions, with periods of lesser activity marked by the development of limestone and foraminiferal shales. The greater part of the tuif was deposited upon land with the sea encroaching upon the borders of this material and reworking areas to form shale. Heavy showers of volcanic dust contributed to the supply of material for the shales found interbedded in the tuff. That there were intervals during this voleanic period when organisms could exist in abundance is shown by the Radiolites sp., which formed reef-like masses, now represented by the interbedded San German and Guayabal limestone. Foraminiferal shales also accumulated in these less-disturbed intervals. In some in- stances previously formed rocks were broken up by the eruptions and redeposited to form such rock as the Coama tuff limestone. The Guayabal limestone represents a period of limestone formation higher than the San German and Coama tuff limestones, but in the same geological epoch, namely, Upper Cretaceous. The latest Cretaceous record, as shown by the rock in the district, is that of renewed volcanic activity with the formation of tuff. DEFORMATION AND INTRUSION Toward the close of the Cretaceous the region was subjected to dynamic disturbances which resulted in a pronounced folding of the formations. Accompanying this folding came the igneous intrusions of diorite, ande- site, diabase, peridotite, etc. It has not been possible, from the date secured, to work out the sequence of these intrusions. Their effect has been to produce baking, which is exhibited by the shale, and to form mineral deposits, as, for example, the magnetite on the Portugues River. Erosion OF CRETACEOUS Toward the close of the Cretaceous there was uplift of the region, fol- lowed by a period of erosion long enough to wear down the exposed rocks to a partial plain, designated as the Eocene peneplain (Berkey, 1915, P 3800 SCIENTIFIC SURVEY OF PORTO RICO PALEONTOLOGY (References in which fossils identified are figured) Crark and TWITCcHELL. 1915. The Mesozoic and Cenozoic Echinodermata of the United States, U. 8. Geol. Surv., Monograph LIV. Conrab, T. A. 1893. Fossils of the medial Tertiary. Trans. Wagner Inst. Sci. Dat, W. H. 1890-1903. Tertiary fauna of Florida. Trans. Wagner Inst. Sci., III, Parts 1, 2, 4, 5 and 6. ‘ 1915. A monograph of the molluscan fauna of the ee Pugnax zone of the Oligocene of Tampa, Florida. Bull. U. S. Natl. Mus., No. 90. 1916. A contribution to the invertebrate fauna of the Oligocene beds of Flint River, Georgia. Proc. U. 8S. Natl. Mus., LI, pp. 487-524, pls. 83-88, December. HEILprRIn, A. 1887. Explorations of the west coast of Florida. Trans. Wagner Inst. Sci., I. ‘Maury, C. J. 1911. Santo Domingo type sections and fossils. Bull. Amer. Paleontology, V, Nos. 29 and 30. 1912.