^01.8 'Mm iiiiiiii ,,j^^i0MS&^^0M0Mk liiliBiPPi© ....pp...:.....:.:..JS^^ W?S1| I'Siiiiigiiiiss Kft; Wi;;!:: ;. . ;*: ;:,: : . . ,:h ew.;- . - ii-;: ' ^' '^iWEMM^my;, p»f|f:f:»m^^^^^^^ ii|iiiiii|i;|S^^ mam JF'- -- --'^ ^^t"^ I,' K NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands VOLUME I^-^^^Part 3 Geology of the Ponce District^^i. J. Mitchell NKW YOKK: GEOLOGY OF THE PONCE DISTRICT, PORTO RICO By Gbaham John Mitchell CONTENTS Page Introduction 231 Nature of investigation 281 Geological and topographical map 232 Routes of travel 232 Climate and vegetation 232 Acknowledgments 232 Physiography 233 Introductory statement 233 Complex mountains 234 Extent 234 Relief 234 Relation of topography to rocks and rock structure 235 Drainage 236 Terraces 237 Tertiary coastal plain 237 Extent 237 Relief 238 Relation of topography to rocks and rock structure 238 Drainage 238 Terraces 239 Lowlands 242 Valleys • 242 Guanajibo Valley 242 Yauco-Boqueron Valley 243 Ponce-Juana Diaz Valley 243 Yauco, Guayanilla, Tallaboa valleys 244 Playas 244 Minor physiographic features 245 Slumping 245 Sink-holes • • • 246 Detailed description of formations 246 Tuff 246 Shale 248 General statement 248 Rio Yauco shale 249 Peiiuelas shale 251 Ensenada shale 252 Limestone 253 San German limestone 253 (229) 230 SCIENTIFIC SURVEY OF PORTO RICO Coama tuff-limestone 255 (ruayabal limestone 256 Tertiary sediments , . 258 Introductory statement 258 Ponce formation 258 Quaternary deposits 260 San Juan formation 260 Alluvium 261 Igneous rocks 261 Metamorphic rocks , . . . 262 Anamorphic rocks 262 Katamorphic rocks 262 Structure 262 Introductory statement 262 Folding 263 Late Cretaceous folding 263 Tertiary folding 264 Faulting 265 In Cretaceous rocks 265 Late Tertiary faulting 266 Petrology 269 Introductory statement 269 Igneous intrusive rocks 269 Quartz diorite 269 Diorite 270 Trachy-andesite 271 Hornblende andesite 271 i^ugite andesite 272 Diabase 273 Augite porphyrite .' 275 Pyroclastics 275 Tuff 275 Sedimentary rocks 277 Shale 277 Chert 279 San Juan formation 279 Metamorphic rocks 280 Contact metamorphics 280 Garnet rock 28X) Garnetiferous limestone 280 Epidote rock 281 Katamorphic rock 281 Serpentine 281 Paleontology • 28- Introductory statement 282 Post-Tertiary fossils 28*1 Tertiary fossils 28 > Cretaceous fossils • 28 > MITCHELL, GEOLOGY OF THE PONCE DISTRICT 231 Page Historical geology 287 Introductory statement 287 Cretaceous rock deposition 288 Deformation and intrusion 288 Erosion of Cretaceous 288 Tertiary sedimentation 289 Deformation and uplift of Tertiary 289 Erosion of Tertiary 289 Submergence with terrace-cutting and formation of San Juan forma- tion 290 Emergence 290 I]conomic geology 290 General statement 290 Manganese • 290 Magnetite 291 Limonite 291 Copper 294 Salt 295 Building stone 296 Road-metal 297 Cement material 297 Petroleum 297 INTEODUCTION JSTature of Investigation A geological stndy of the Ponee District, an area of about 850 square miles, in the southwest comer 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 Eico may be seen by reference to the Outline Map.^ The field-work occupied the period from June 1 to September 8, 1917, 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 iViund, especially in the Cretaceous rocks. Seven main traverses were 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. EouTEs OF 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 Eico. 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 E. 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 ilie district. To Sehor 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 Senor 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. Eobert Tracy Jackson identified the Clypeaster rosaceus and the new species Hemiaster herheyi. 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. PHYSIOGEAPHY 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 ]>ass beneath the remnants of Tertiary coastal plain sediments which border the south coast. The southwestward continuation of this "old land'' 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 Kojo. 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 Eiver. The most extensive playas are Ponce, Guayanilla, Cape Kojo 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 IjecQine 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 Eojo Playa (Fig. I)' On closer inspection the intensive dissection which progresses daily under the tropical showers greatly impresses the observer. The steep slopes ('f the higher mountains turn the waters into narrow gorges, where, after heavy storms, raging torrents are ever deepening and widening their valleys. The heavy coat of tropical vegetation checks the rapid run-oH but the impervious character of the surface soil facilitates it. Sue! JUTV-HI-lLIu (flvOLOGY OF THE PONOH Ifl.^Th'liJT vsilli'vs as Iho TipiKM' Rio I i II n J! a j i h o, (liijiba ami rroito are t^'yiical of t'hi>i >!i«iiiiil!iin rc'u'ioii. nntion uf i\w o].1 ^ I .n.1 carlv |i:oiH.iie, in plriiuly 5: | isililf (llcrlvcy, inir», p, I s II 1. ill Ihr PoiH:e Dis^^^ J I net Iho ..Illy l.Hiality whcro | | M! l!H> "Mosas- iM-ar Mava- ^ - TO Run\> .\NP la.u'K sTirrt Ti-iii: v> hav.^ <;.verl:!Hl in th. .luiara-nl ..f Mirfacr r,,v fanfos. Ill,' (onnation- -t r.->Hi>|:aiii. in wcnihrnni:. tli.i 8:111 lUaiiioii nw-: iiynha.l lii)H/>tuni/s. lia i.i.ii> iiiiriir^ivc.. and th. dilv iiuliiratv.! iulT, Dn. •-< of iJic 8rtu Conn: (aiayabal (oriijatiui mrukd l.y lowordyii ' "T tiLli: aiid shale. 2e36 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- MfTCHELL, 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, Eosario, 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 Guanajibo, 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 thes6 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 my belief that these terraces are connected with the recent changes of level which have affected Porto Eico. A wave-cut terrace has been formed on the Cretaceous rocks making up the point just south of the Eeform 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 r*orto Eico extends from a point a short distance east of Juana Diaz to Cape Eojo and Point Aguila, in the southwest corner of the island. This pliysiographic province is not continuous, but is cut into isolated areas ^>} the rivers traversing southward. The largest portion is conflned to 238 SCIENTIFIC SURVEY OF PORTO RICO the vicinity of Ponce and continues west to the region of Ensenada. The exposures at Cape Eojo and Point Aguila are small and underlie the San Juan formation at the former locality. The northern boundary is deter- mined by the faulty 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 m 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 Peiiuelas, 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 Eico where the Tertiary is found. Harder strata, sometimes a foot or more in thickness, have protected the underlying material, thus producing low cliifs. 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-Penuelas road. These depressions are very inferior to similar occurrences on the north side of Porto Eico. 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 intermitteiir streamlets. Among even the larger rivers, few maintain a constant flo^ 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 Eiver 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. TERKACES 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. The stream terraces are best seen along the Bucana Eiver 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 Eico. 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 been formed. Whether or not the lowland along the Guanajibo has i similar history to the one marking the site of the Guayabal reservoir, 1 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 Gnanajibo and its western branch, the Yiejo Kiver. 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 iilling. The geologic structure which has influenced the development of this lowland is that associated with the southward-dipping shales and limestones. Keference 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 Guanajibo 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 tlie valley was formed by the Boqueron Eiver 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 8GIBNTIFW 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 YI. 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 Eio 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 31/2 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 ji combination of river flood-plain and delta processes. The detritus froin the land has been carried to these lower areas, where it has been spreao MITCHELL, GEOLOGY OF THE PONCE DISTRICT ^45 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 Eojo 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 Physiographic 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 jality a large mass of rock has moved a considerable distance down the teep side of the valley. This slide is so recent that the surface along which the movement took place is plainly visible for some distance. Shimping of small areas, especially in road cuts, were encountered on the Mayaguez-Consumo-Maricao road near Maricao. What appeared to be ^hmiping of more ancient date was seen along the steep mountain valleys ii\ the areas of tuff and red shale. loc fiOIENTIFIC HUHM'JY OF I'OliTO RICO !INK-I10LKS ImkhI examples xr lis^ e,xt< tliiin elwnvhc Furro IJieu. One placo where undonbte*! sirik^ioles iire ruimd h .iii the viciiiit.v of kildiueier lyl.O, on tlie Voiiee- Peiraolas royil. Tlie liold evj- deuce hIiows that this process lias heeii less iuiporiaut in shivpiiig th. |H'rs(Mit tupo»TaphT of the Ponce I)istri(3t tbiin in the case of the Tertiar) uii th(; iinrth side of Porto Bieo. DHTAILI'-I) DE.SCIUI'TKJNS OF FOHMATHms Tuff A real iJislrihulion. -Tiilf is the most ahmidiiot rock in the distrii-i. Any north-south traverse ol the areti will disclose a strong- dovt'lopnu'id vt this class ol' material Typical outcrops may be seen at EristMJinla rtr.e iu»rth (d; .Pefmelas. Yaoco is niuJerlain l)y this rock. Olher good fx-^ pusures may l)e seen along tlie irauco-LaniS road north^ of YancHj. tin the Poiice-Ad juntas road, at intervals between kilometer IM.O and nerlln ward, good examples of tliis rock are to \m I'onTuP In the broad east^we^i vallev between Yaiico and Boqueroii occasional remnants of tu(T jn-otriuli' tiir(Higli tlie alluvial covering of the valley floor. ThirkncHH. -^^Due to the pinching and swelling of these beds, whosi^ structure is also very obscure, it is diflficult to ilfposited linder (lifl'etvnt conditions. (luaijaJinl Lhiiest.one.^—Tlic type locality for ttiis formation is tfic \"ici,iiitv ut* (jiiayabal iiesiervoir north of Jiiaini Diaz, where the rock makes U]> (he eoiis|>ieuons wliite hiJIs to tlie east and west of this point. Berkev tentatively pkuu-d this formation with the (Joama tuff linn^toiie, but tlie section exposed on the upper east fork of daeaguas lliver shows th,c tjpieal Coanni tulT f>eds overhiin by limy shale^, upon whieh follows tlie (inayahal linuistonA!. As stated in the discussion of the (Joama .tidf limestone, its stratijjfrapliii- fiositioji !)c]ow shale identieal with the Penuelas shale and the pres(>nee of Eadioliie.s in th,e liniestoni' indieate a close relaticaiship between the (,'oama tulT limestone and the San Gerjnaii formation, hnt. noeessitates different eonditions of deposition. Since fiehf relations show the foanui tiilT limestone and the limestone at tht; fiuayalial lleservoir not to be equivalent, it is pro]Josed here to use the term tiuayabal linn:^^' stone for tin.' whJte, nnissive roek (hn'elopetj in tlie vieinity of Gnayahal Reservoir. Tins fnruiation^ eojitiniie.s southeastward in promineid; deveh opment into th,e adjoining district studied by Hodo'e (U>20). To tb- west, beds of this eliarat-ter were traced northwestward im three to si>. nub^s, "^vhen tJiey wei'c found to pinch out. Wliether or not they are dr^ veloped in the district to the nortli cannot be said. It is possilde fh.-o' their de\'elo]anent is hn-al and they will not be foand in the area nort- of the Ponifc District. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 257 The section on the upper east branch of Jacaguas Kiver proved the best for determining the thickness^ which measured 950 feet. Hand specimens of this limestone are a mottled cream color, with den- dritic manganese marking some of the rock. The material is massive and brittle, breaking easily under the hammer. In some exposures jointing is prominently developed, but this is not true of all the material. Bed- cling is rather obscure, but the true attitude of the formation can easily be determined from the structure of the underlying shale. A considerable quantity of fragmental organic remains are present, including Radiolites and foraminifera. The beds strike north 50° to 65° west and dip south 25° to 35° on the north limb of the syncline, and stand nearly vertical on the south side of the fold. It is in this rock that the manganese north of Juana Diaz is found. Differential weathering has produced a characteristic fluting in por- tions of the outcrop. The narrow channels stand vertical and are some- times two inches wide and over three feet in length. In figure 3 a good illustration of this fluting in the white limestone is shown. Diller (1897) describes fluted limestone from Baird, Shasta County, California, of which he says : This limestone is exposed in large masses, making a mountain ridge along the McCloud River. At many points its surface bears peculiar small ridges and intervening depressions. In the field it is evident that the little valleys are lines of drainage, and it may readily be concluded that they are a form of rain erovSion due to the solvent action of the water running over the surface. An examination of many ridges and valleys discloses no difference in composi- tion or structure to which the variation in surface form may be due. From the fact that the flutings are drainage lines, it appears that they may be re- ferred to the rilling of the water on the surface of the limestone. Where the water flows the solvent action is most intense, and a minute valley is formed ))y solution. This form of differential weathering is common upon limestone exposures, but is rarely so well developed as in the example from which speci- men 154 was collected. The fluted character of the Guayabal limestone is a very striking fea- ture where strongly developed and probably has the same origin as that described by Diller. The noticeable absence of fragmental volcanic material would indicate a period of decreased vulcanicity. The presence of Radiolites places the limestone in the Upper Cretaceous. Its structural relation to the Coama tuff limestone, which is probably equivalent to part of the San German formation, would place the Guayabal limestone in a higher position in t lie Upper Cretaceous. 258 SCIENTIFIC SURVEY OF PORTO RICO Tertiary Sediments introductory statement The Tertiary sediments along the south coast of Porto Eico extend from a short distance beyond the southeastern section of the Ponce Dis- trict westward to Point Aguila^ in the southwest corner of the island. They represent a former more extensive coastal plain series deposited upon and partially around an eroded central core of complex rocks. AThether or not these later sediments formerly extended across to the north coast cannot be determined, but the evidence is clear that they once had a greater extent northward. Their stratigraphic relation to the Cre- taceous formations has been complicated by the fault which determines the northern limit of these rocks. However, other structural features are such as to show without doubt that the conditions were similar to those on the north coast, where the Tertiary rests with a marked unconformity upon the older formation. In general, the Tertiary formation is made up at the base of fossil- iferous, yellowish, marly material with occasional sandy layers. This basal portion grades into the yellowish and white, chalky limestone char- acterizing the upper portion. The highest strata contain fragmental masses of coral heads bedded in soft, chalky material. Some harder strata of pinkish semi-crystalline limestone, at times a foot or more in thickness, are found in the higher, chalky portions. From its typical development in the region around Ponce, it is proposed to use the name of that city to designate this formation. PONCE FORMATION The Ponce formation is confined to remnants of variable extent along the southern border of the district. The largest area is northwest and west of Ponce, where the chalky limestone is best developed. To the east of Ponce, and underlying the Ponce-Juana Diaz Valley, is found the best exposure of the lower marly and shaly portion. The upper part of these lower beds is found at the fault contact just east of Yauco and at other points along the military road from Yauco to Ponce; also in the face of the hills just south of Yauco. The lowest beds are made up of yellowish, marly and shaly material in which sandy and more compact strata are developed. These harder layers are sometimes two feet or more thick and contain abundant foraminifera (Orhitoides mantelU), which fossil also characterizes the more shaly and softer portion of the rock. Other fossils identified in the collection from MITCHELL, OEOLOGY OF THE PONCE DISTRICT 259 the lower beds include Laganum sp.^ Feci en sp., Fasciolar^ia sp.^ Cythara sp.^ Turritella halensis, TurriteUa halensis var. alpha, Cardium sp., Astarte sp., Cypra'a sp., Am/phistegina lessoni (abundant ), Ensis sp., leaf fi^agments and fish teeth. Pieces of petrified wood are also present and coal is reported to occur in these beds, but noue was found. Besides the sandy portions, there are tliin la,yers of fine gravel in which shell frag- ments are plentiful. Some of the sandy layers are filled with broken shells and foraminifera, especially the forms Amphisiegina lessoni and Orbitoides mantelli. The lower beds pass disconformably into the upper yellowish, chalky limestone so well exposed in the hills around Ponce. This material is characterized by its yellowish white color and chalky nature. It contains numerous casts of Lucina sp., Area sp., Bulla sp., and other forms. In structure it is generally massive, but hard, thin, yellowish strata are prominent in some localities and are valuable in de- termining the structural habit. The chalky material continues to the top of the formation, which, if anything, becomes more chalky. The upper- most beds are well exposed along the coast from Culebrinas Point to Guanica Bay. At Culebrinas Point the rock contains abundant specimens of Clypeaster rosaceus, together with other forms of Oligocene age. The basal portion of the Ponce formation was deposited in compara- tively shallow water near shore, as shown by the sandy character of the strata, together with the abundance of broken fossil fragments and re- mains of plants. The distribution suggests a broad embayment whose eastern limit was located approximately a short distance east of Juana Diaz and whose western extent was in the vicinity of Yauco, though faulting has made it impossible to determine the exact extent of these k)wer beds. How far north this embayment reached is not certain, but, judging from the lithologic character of the rock and structural features, it was certainly more extensive than at present. The present width of the Island on a north-south line through Juana Diaz is approximately 34 miles. The distance between the Tertiary of the north and south coast along this line is about 18 miles and the eleva- tion of the highest point nearly 3600 feet. Considering the fact that the central mountain mass was much lower at the beginning of the Tertiary period, as shown by the peneplane developed on the north side, it would appear that the lower beds of the Tertiary must have been laid down much to the north and south of their present boundaries, on the two sides (»f Porto Kico at about its central portion. The conditions influencing tlie deposition of these lower beds changed to those under which the ( lialky material was formed. In this later period more quiet and deeper water prevailed, with a continuance of these conditions through the Oligo- 260 SCIENTIFIC SURVEY OF PORTO RICO cene at least, as no record of further deposition is found until the San Juan formation (Pleistocene) is reached. The question as to how ex- tensive the upper beds were is also unsolved. They surely reached farther north, but whether or not they covered the island cannot be determined from any evidence thus far secured. The fact that there is a difference in the Tertiary fauna of the north and south coast, as far as can be told from the fossils already identified, is a fact to be considered in this con- nection. Further detailed studies of the fauna from the north and south coasts may prove or disprove a former connection. The lowest beds, as exposed along the Jacaguas Eiver northwest of Juana Diaz, are characterized by abundant Orhitoides mantelli. They^ are forms which establish the Oligocene age of the Ponce formation, with the basal portion of Lower Oligocene (Vicksburg age) and the higher part representing Tipper Oligocene (Chipola). Quaternary Deposits SAlSr JUAN FORMATION The San Juan formation, first identified and named by Berkey (1915, p. 11), is found at Cape Eojo Lighthouse, in the southwest corner of the district. At this locality it caps the eminence upon which the lighthouse stands. The rock at Cape Bojo has the same general characters as that of ma- terial in San Juan Point, ^olian cross-bedding is present and the rounded grains of carbonate of lime which make up the bulk of the for- mation are plainly visible in hand specimens. A conglomerate bed three feet thick, made up chiefly of pebbles of Cretaceous rocks, is found rest- ing on the San Juan at the edge of the cliff south of the lighthouse. This is a feature not seen at San Juan. The relation of the San Juan to the Ponce formation is unconformable. Details of the petrographic character of this rock, which is one of the most interesting types found in the dis- trict, will be discussed under Petrology. The unconformable relation of the San Juan to the underlying Ter- tiary limestone, as shown in the cliff at Cape Rojo, necessitates a period of erosion at the close of the Tertiary sufficient to bevel the underlying beds. Just liow much erosion has taken place is not known, since the latest Tertiary record in the district is Upper Oligocene, and there is no evidence that Miocene or Pliocene strata were ever deposited. The presence of aeolian cross-bedding indicates a dune origin. The capping of conglomerate made up of well-rounded pebbles and the pres- MITCHELL, GEOLOGY OF THE PONCE DISTRICT 261 ence of Conus sp.^, which resembles Conus portoricanus, show that the San Juan was at one time below sea-level. The age of this formation has been placed as Pleistocene (Berkey, 1915, p. 12). The fossil content and situation of the beds at Cape Kojo support this conclusion. ALLUVIUM Alluvial material can be found along all streams and mantles the greater part of the broad lowlands near the coast. The low coastal regions known as playas are coated with deposits of this nature to a depth of 25 feet or more. Deposits of this type are composed of material ranging from boulders two feet and over in diameter to the finest silt. An examination of the gravel shows representatives of the range of rocks in the Cretaceous for- mations, and in some deposits nodules of Tertiary limestone are present. That some of this material was deposited when the land stood at a lower level is shown by the presence of interstratified, recent marine fossils in the surface layers of the playas and river iloodplains and in elevated estuarine sediments. It may be noted that the alluvial material can be grouped under two general heads : (1) Elevated estuarine gravels, mud and silt, occurring along the present river valleys. (2) Deposits of the same character, covering the present river flood- plains and playas and being augmented by each successive flooding. The first might better be correlated with the San Juan formation, while the latter would represent the more recent deposits. Igneous Eocks For purposes of mapping, the igneous rocks have been divided into diorite, andesite, diabase, and augite porphyrite. The occurrences of quartz diorite are too small to be shown areally on the geologic map. This is also true of the trachy-andesite. However, due to the petro- graphic interest of these rocks, they have been discussed in detail under Petrology, where will also be found a more extended discussion of the igneous rocks. All of the igneous rocks in the area have an intrusive relation to the Cretaceous sediments and occur as dikes, sills and irregular masses. The largest intrusive is represented by peridotite altered to serpentine in the region north of Sabana Grande. The dikes and sills range from thin ?heet-like bodies to masses 50 feet or more in thickness and are all con- fined to the Cretaceous formations. In no instance were intrusives found 262 SCIENTIFIC SURVEY OF PORTO RICO cutting the Tertiary rocks. The general elongation of the intrusive bodies along the strike of the pre-Tertiary beds is a noticeable feature. Metamorphic Eocks anamorphics The anamorphics, including garnetiferous limestone^ epidote rock^ and garnet rock^ are typical contact metamorphic effects produced by igneous intrusions in limestone. The extent of these rocks is limited to small zones. They are of particular interest in connection with the associated iron deposit on the upper Portugues River. Further details of these rocks will be found in the section on Petrology. KATAMORPHIC ROCK The original peridotite from which the serpentine has been derived was intruded into the tuff and shale of the pre-Tertiary series. This intru- sive relation is clearly shown in the area north of Sabana Grande^ where small beds of tuff are found inclosed in the serpentine. At other points^ as, for example, southeast of Mayaguez, the serpentine cuts across and lies along the bedding in the ashy shale, thus placing the age of this rock as later than the deposition of the Upper Cretaceous and before the for- mation of the Tertiary, namely, in late Cretaceous time. In all proba- bility the intrusion of the original peridotite was an accompaniment of the folding which deformed the pre-Tertiary beds. STRUCTUEE Introductory Statement On the basis of structure, the Ponce District is readily divided into two sections—a northern, more extensive area, made up of strongly folded Cretaceous rocks, and a southern border of Tertiary and later formations. The structural habits of these two sections are so strikingly different that one cannot help being impressed by the fact. The marked degree of folding which has affected the Cretaceous rocks, in places throwing them into closely folded, steeply dipping beds, is not to be found in the Tertiary formations. In an attempt to unravel the structure of the different beds, seven main traverses were made across the area from north to south, along lines which gave promise of the best results. One shorter, north-south, section was studied in detail, and the formations between the main traverses were MITCHELL, irtlOLOGY OF Tlltl I'OXCf] lllt id: the dill'orent beds. The iniikijig ot sneh traverses m this reu-ion is ns, easy task. The rank growth oi' ti'opieal wu-cintion .ind ihe rng-Mvdiics^ of thu country, coiid»ine.l with ex(-'i'ssivi- werttherin«i- of ruek exposures, make p'<.lo-ieal work exeretlingly ardiioos. One ractur. howwiT. which iacilitated the work was tlie jnvstiHcc (d' nnniernu< trails and roads iiloiiu^ whieh it was possddo to secure data which prohahly emihl nut laive he..n iir(|oir<.d utherwise. ^lany streams alh^rded 'se.-iinn's wha-h ludped m do- h'rniining si rurtiiral relations. Ida- data eollec-ted alonn^ tlu-' traver>e>, aiiieh in all easo< were neeessurilv ^-ncuous. have Imn) prnjeen'd le. sirai.uiu VoLim >rr..j|.r rolding uf tli.. Cri'laeeons rai h'ature of tic- area ..eniiacd ^^^^l,'il> )'Hlii-<, Mhll, rneU st.rata, wJiicJi ni souu' nistanees t: . l)ii)s of !HP were recoi'ded, l)ut the i nsualhv lM»tween ^h')" and T-'dh In th part ol 264 SCIENTIFIC SURVEY OF PORTO RICO this northern area the beds are generally overturned to the northeast, but on reaching the summit of the main range, along the northern border of the district, the steeper slopes are more often on the southwest side. This would indicate thrusting from the southwest and northeast. The broader features of these foldings are shown on the cross-sections, Plates V and VI. The general strike of the beds is northwest-southeast in the northern half of the area, with a change to east-west along the southern border, as seen in the region around and west of Ensenada. Variations between these directions are due to local flexures or mark the ends of pitching folds, as is well shown by the shales along the Yauco-Lares road north of Yauco. The section exposed along this road affords the best illustration of folding in the Cretaceous rocks. Here is found an almost continuous section, involving shale, tuff, limestone and igneous intrusives whose structural habit is shown in figure 4. Other localities where the structure of the Cretaceous rocks is clearly shown are : (1) On the road from San German to Lajas. (2) Along the Ponce-Adjuntas road. (3) Peiiuelas-Ponce road, especially in the vicinity of kilometer 67.0. (4) In the region around and west of Ensenada. (5) South and west of Cabo Eojo. (6) Forth of Penuelas. (7) North of kilometer 114.0, on the Ponce-Juana Diaz-San Juan military road. (8) On the upper Bucana River, northeast of Ponce. In addition to the larger foldings, there are innumerable smaller crumplings, which are not shown in the cross-sections. Examples of local distortion can best be seen in the shale along the Jayuya road, at kilo- meter 3.0 and vicinity; also at kilometer 22.7, on the Ponce-Adjuntas road. The presence of such marked folding necessitates the action of moun- tain-building forces comparable in part to those which have affected re- gions like the Appalachian Mountains. While the forces which deformed the Cretaceous of Porto Rico have not been active over so long a period as is attributed to those which folded the Appalachians, still the two regions, from a structural standpoint, have features in common. It does not seem possible to attribute the folding of the Cretaceous formations of Porto Rico to igneous intrusions, although such an hypothesis has been advanced. FOLDING OF TERTIARY The Tertiary limestone and marl comprising the Ponce formation have been folded to the extent of producing low anticlinal deformation, which - jjj^TlFlC SUBVET OF POErfO RiCO AND THE ViRGIN ISLANDS VOLUMB I, PAET 3 >* s ^ 4i# \ p^tl'''! ////' ^^^^^^4 ^ * * > x^** * ..^-^f^^iFx^":^''-" " :•';•-* wS^y^i>il\\>- ''':: -'1' y/M///r ^ 4 ^ ^ (h A ^ A * ^a4^.aa^ MEOEND Sedimentary Rocks PleisUeene Oliyfnm iiltj s»itd •ltd Siwel in yaltey htttama SmitTu* Upper Cretaeeeu* L»t« Cr€t0e«*ut Pirm*tJ«n - C»nso//J*ted^ Crassheddtd lime SMd cf dune •rifin P»nc€ Ferm*titfi - Idoatire t» btdded ektiky /imesUne st t*p, p»uiiy inU mtrly mnd saitdy strmlk #/ h^t* Oua/alml Limeahnt - IVh'ih ^ massire ,er^ste/fme /lme*t»/ie Cotm*. Tuff Llmest0m - M»ssire >m6 #/ wftif* •<•• tUisA-m^jr //mestM f/^tfmt^fs miffi /fm—m fnfmmi I Sen Germen Limettone - BUith' frey mettire it bedded eryatef/ine /imestefe. I Jhe/e - Sttenffy bedUmJ meAy *ed fimy tkeki miff heel afe^l^mefita ef fy^ and eyfkmfreT* I Tuff" Meuire, remile tu^ min U*»/ *lnU m.m^ 0ffhm*f»fk ous and Katamorphic RocAs Dierite fi^'V^ Andeeite A»fite Per^hyrite MExl Serpentme Htm 0. 'm.^^^^^^^^'tMwiM% e^^^^^^^^' * "^v^ i///JIjIIi*\v\\\^ fijtBJjI .s.^$^^a^^^'' 1- * V. '//M'/r^ \^ si-^nW-^ tW^—22_—-''^^^SS^^S^f *+*." n < PLATE VI GENERALIZED GEOLOGICAL CROSS -SECTION OF PONCE DISTRICT MITCHELL, GEOLOGY OF THE PONCE DISTRICT 265 in places pitches with low angles to east and west. How extensive this folding has been cannot be determined, because the fault which marks the boundary between the Tertiary and Cretaceous rocks cuts out the lower part of the Tertiary beds, and thus conceals their structural habit. That folding has taken place is clearly shown in the section (H. H^, Plate VI) along the Jacaguas River, north and south of the Ponce-Juana Diaz road crossing. At this point anticlinal structure of the formation is present, with the crest of the fold just north of the Jacaguas River bridge. The north limb of the fold dips with an angle of 15° to 10° into the fault bounded on the north by the tuff. On the south side of this anticline the beds are exposed in the prominent erosion cliff plainly vis- ible to the southwest from the Jacaguas River crossing. Here the strata dip from 35° to 10° southward; the steeper dips are in the face of the cliff, and become more gentle north and south of this point. This is the only locality where such pronounced dips in the Tertiary rocks occur. In general, the inclination is from 10° to 20° southward, the steeper angles being read on strata near the fault zone, and are due in part to dragging up of the beds by faulting. Besides this folding, with axes trending ap- proximately east and west, there are broad, shallow deformations which have nearly north-south axes. This latter folding was determined by tracing the marl and limestone beds from east to west. The Tertiary formation shows none of the minor crumpling so char- acteristic of the pre-Tertiary. The absence of structural contortions comparable to those in the Cretaceous beds clearly indicates a period of diastrophic movement much inferior to that which affected the older rocks. Faulting in cretaceous rocks By far the greater number of faults in the Cretaceous rocks are of the normal class, although, in some instances, as along the Jayuya road, at kilometer 3.0, the shales have been overturned, accompanied by thrust faulting. Other evidence of faulting in the form of crushed zones was seen in many localities (Fig. 5) ; for example, at kilometer 9.0, Yauco- Lares road, where limestone, tuff and igneous rocks are involved in the crushing, and on the Ponce-Adjuntas road, at kilometer 17.0 and 16.8+, where the limestone is extremely crushed and broken, due to faulting. Other localities are in the region north of Penuelas; along the main cirainage divide west of Adjuntas; northwest of Villalba; southwest of San German; north of Hormigueros, and at many other places too nu- nierous to list here. In the majority of cases examined no great displace- 2m HVIEXTIFIV HUR.Vtir OF r(HlTi) RICO iin'iil lirtd liiki'ii [)1rtco, wlii<:li would jiidieate tliat the iuleih^^itv o\' mx- i|<'IV)riTiiit:io!iEil forces wjis riCit so greaJ: as has Ix'eii tlie e«se iii otlier ron-icu of iiioiii)tniii-niiiki]ig-. Tlio jiiost (/onspiciioiiH f!Hilt,< ill till' C^rotaot'ou^ formiiticm arc pho^wii i the Htniotiiro m-ihm (B. 1'.' uiid (X ('\ Tlato V). In the first iiistaiK' i\u- rn|»iiiri' has otrciirreil in the soj-peiitiiic anil is iiiarked hj tho iiih>iis(>l rii>h='(l mid slu-konsidcd roiidili.!]i nf th.; rock, us wtdl ',v< liv phys!^ miihn- l\-nhin's. In tin; hh-Uoii C.C'\ tin- hreak hu> taks-ii piarr alr.n he i-i.niaet (.!' s(M-p./niiiH- and shalr. The Hiiiuuni uf iiH,\i.im'ot in eilh. iis.< ruidd, iH.t he (IrffTiiiiiUH!. Ii! iH.ih casc-^ (he phv^-ioo^nipliM- rxon-s.in J^ATiv I'lOltTlAUV !'Ari;riX(i 'Vlw iHo>l pr..iioiiiH/.-d faiih III IJh' di^tnrd is fhc ..oo inarkiiig ih*- r laci of rhc Tcrliarv limesfoiie and marl on the south, with tJii- Cretacr furiiiations on iho north. Berkcy (11H,\ p. :!!»> was iho lirsl to .-all ait fiuu to rids siriN'tiiral h-atiin\ and lio says: llio most iiroiiiineiit faiilr. in irs ofToc-t ninai Mrcseut iValnros. is ilic mw i markiiitr tlio innor in.-iru'in of llio younger sorii's of oli:illelt ak.ng the south shh' of tho IsL-hkI Ii Jtiiina I>hiz iiast Poin-o a slmrl (listiuu-c to rhe north, eroxsin- tho i'oi Ark-ebo road at Jv^J,S, and IhoiXHi westward, orosshiji tlio I'oiH'c-Ponuolas i- At 1\-:10. 'IMs is the only hnrgo fault actoilly obsoryod I hut is nocossaril) MITCHELL, GEOLOGY OF THE PONCE DISTRICT 267 recent age, although a few others are inferred. It must be of very hite Ter- tiary age, because the chalky Ponce beds are abruptly cut off by it. The older rocks of the pre-Tertiary are lifted with respect to the younger series, forming the present coastal margin wherever this fault has been seen. It has been traced by us from Juana Diaz to the vicinity of Penuelas, a distance of about 12 miles. What becomes of it at either end is not yet determined, but it is believed to extend much farther in both directions. Further evidence secured by the writer during field studies in this dis- trict substantiates the conclusion reached by Berkey and has also made possible the tracing of tliis structural feature to a greater distance east and west. The additional evidence relating to this question has been secured at a number of localities^ extending from the vicinity of Juana Diaz to a point five miles east of Point Melones, on the west coast. Tlie most eastern locality where faulting is clearly shown is on the Jacaguas Eiver, northwest of Juana Diaz. The structural relations of the Tertiary marly limestone to the Cretaceous tuff are shown in cross-section H. H\, Plate VI. It will be noted that the folded Tertiary beds have been dropped down against the upraised tuff. If these Tertiary beds were raised sufficiently to allow them to lap over the eroded surface of the Cretaceous formations, which they must have done before their displace- ment, then we would find that the movement down the steeply inclined dip slope of the fault plane was at least 900 feet. However, the base of the Tertiary is not exposed ; so the exact amount of movement whicli lias taken place cannot be measured. The contact between the marly beds and the tuff is represented by a crushed zone over 50 feet wide, in whicli tuff and marly material are mixed with surface w^ash. The evidence of faulting to the east of this point is concealed by the alluvial cover ; but, judging from tlie position of the Tertiary and Cretaceous beds and the presence of crushed zones, the fault is believed to cross tlie Ponce-Juan a Diaz-San Juan military road in the vicinity of kilometer 112.0 and con- tiime southeastward beyond the limits of the district. Going westward from the Jacaguas Eiver, the fault can be traced to the Ponce-Ad Juntas road where it crossed at kilometer 4.8. Here the Tertiary chalky lime- stone is separated from the tuff by a crushed zone over 100 feet wide. The Tertiary beds are dragged up so that they dip 20° south near the fault and flatten out to 12° and 10° a short distance southward. Con- tinuing westward to the west fork of Canas Eiver, the same relations as seen on the Ponce-Adjuntas road are found. The next locality to be considered is kilometer 67.0 (10.0 by old num- bers), on the Ponce-Pehuelas road. The structural relations of the Cre- taceous shales and Tertiary limestone are shown in cross-section F. F^. The folded shales have been raised so as to cut out the lower Tertiary 268 SCIENTIFIC SURVEY OF PORTO RICO beds and bring the chalky limestone in contact with the Cretaceous rocks. This contact is also marked by crushing, and the Tertiary beds are sharply turned up against the older rocks. From this point the fault passes south of the military road, and the next locality where the evidence is clear is Just south of kilometer 48.0, on the Yauco-Peiiuelas road, east of Yauco. At this locality the marly beds just below the chalky limestone are turned up against the Cretaceous rocks, with a narrow zone of crushed material lying between. Passing farther west, the line of faulting runs north of the base of the Tertiary limestone and marly material forming the hills just south of Yauco. Here structural relations are concealed by the alluvium of the valley ; but farther west, in the region of the San German limestone out- crop, at the east end of the railroad bridge over the Susua Eiver, the faulted relations are again conclusive. At this point, which is three- fourths of a mile southeast of the railroad bridge, the white, chalky Ter- tiary formation is sharply dragged up against the San German beds. The crushed zone, which is approximately 20 feet wide, is filled with powdered Tertiary limestone and fragments of the San German. South of the contact, chalky Tertiary limestone assumes a normal dip of 10°, which is the average inclination maintained to the south coast. Three miles farther southwest the same relations can be seen. Here the San German limestone and the Tertiary chalky beds are again involved in the displacement. From this point the line passes southwest to Ensenada, where the Tertiary and Cretaceous are brought in contact. The relations here are best seen on the south side of the ridge of Cretaceous shales south of Ensenada. By following westward along the south face of the ridge, the clialky limestone is found to approach the older shales where the former is upturned against the steeply dipping Cretaceous. The dip of the Tertiary south of the contact is 8° to 12° south. The narrow crushed zone betAveen the formations is filled with powdered, chalky limestone, fragments of shale, and surface wash. In structural section B. B^ the same relations as seen west of Ensenada are found to be present. The sharply upturned Tertiary limestone abuts against the Cretaceous shale, with a narrow zone of crushing between. Westward, beyond this point, the break has been traced to the playa of Cape liojo, where it passes beneath the red sand deposit which covers that low coastal area. In conclusion, the evidence as shown by the structure sections clearly indicates faulting between the Tertiary and Cretaceous rocks. The Jacaguas Eiver section affords the only opportunity for measurements of the displacement ; but, as already noted, the exact amount of movement iKXTiFic Survey of Porto Rico and tub Virgin Islands Volume I, Part 3 PLATE VII GENERALIZED COLUMNAR SECTION OF PONCE DISTRICT 1 Scale linch ^^ 1800 ft. System Series Formaton SyM Columnar Section Thfekmn Provisional Correlation 7kb/e Lithoiop'c Character f^ysio^raphic Express /on ^ ^ f^rto Pico. SantcBomin^o Jamaica GulfCmsm 5 ft&ctnt .i4l , PL^^'*^ Alluvium -^^^"-^i^&f^hifi-^km^ Pleistbcertc S«tl J-UOm f^mOm s.j: ^^SS^^S^^^ '7'^26 Son tTutm FermaTm Cr^i-bidjJW^^ Chiefly e»/eim>f earS^m ^ § ^ % Pc. r 1 1 taso^ 1 Zone G.HI. Maury) Bowo/en marl Chipola^ Fl. White massive to bedded chalky limestone passing into yellowish marly material at base. Low rounded hills. 6ufi- hoies in local areas. 1 ' 1 ' 1 1 ' 1 * 1 f 1 1 / 1 t 1 ' ) ' 1 1 ' 1 1 di^i -1 P-^—[— Olifocene Tuana Diaz formation UrfC 7S0' tnfefmit^ San Oehastian ShalB Montpelier white limestone Vlcksburg^Fl Marly and shaly beds with th/n sandy strata . Orb/toldes ma^Telii ai>i4n4s/a»t Loi^'land i.e. Fh/?ce - Ji/ana Diaz Valley -=-i-i—~-r^' J L^_-J:^-T 1 -, ' 4t ^ ^ ^ 1 1 xU ^ ^"^ 1 1 1 ^^ 1 1 /ariahle ^ massive tuff of prevailing Andesitic composition with local de- velopments of^shale and 1-^ 1 1 1 agglomerate^ also small irregular masses of e/rert. Ashy and foratniniferal shale with local oleveli^meiA Deep narrow i/afleys separated hy sharp neffes in more youthfully d-ssectai % 0) «3 1 1 of tuff and Sandy strata. Jrreqalar masses ofgr»ylth portions L ess relief and broader Valleys in early ?1 ^ ^ ^ ^ chert. maturely allssecteed areas .0 5 II 5> N 8/S0± 5 1 s5 I. 1 1 Massive to hedJed l^luish <^ray limestone^ massive tuff fimestonCj massive T^rominerjt r/ayes formed by limestones. Broai/er valleys cut 1 white limestone The cibove rochmake up an /ntert>edded Con- in tuff err/if sha/e. V :t ^ 1 ^ 1 5i formable ^s&ties with tuff and ashy shale attk 6(7se a/7ct foraminiferal shafea/iif/imestone /7earer the top. ^ 1 ^ ^ > "i ^ 1 1 1^ -^ 1 1^ V ^ ^ This series has icren m^aded by mtruslyc -^. 1 ^1 D/orltCj AfJ^es/te^ Traehy ^ t V ,^ -^ Andesite^ JDiahase^ Aufite- parphyrite fffrd per/tdotfte 1 1 f^^ 1 ^ -§ ^ .< 1 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. PETEOLOGY 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. Igneous Intrusive 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, crystalline 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 Eiver from Ponce, and at kilometer 29.5, Pouce- Adjuntas-Arecibo road, just north of the Ponce District. The forms in which tliese 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 Eiver—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 metallies, but inclose numerous large plagioclase crystals, producing a beautiful ophitic structure. In many instances the plagioclase shows MITCHELL, GEOLOGY OF THE PONCE DISTRICT 271 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 froiii the horn- l)lende and pyroxene and the sericite from the feldspars. Leucoxene is seen 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 hornhlende, and pyrite has been changed along the crystal borders to limonite. A small amount of ]ime 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 Guayahal reservoir, on the road to Villalba. At this locality the andesite is in the i'<'rm 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 CentraP ; on the east line of the district, two miles north of the Ponce-Santa Isabel road ; kilometer 64.2, Ponce- Pehuelas road; the southeast end of Guayabal reservoir; at kilometer 8,1, Mayaguez-Las Vegas road; and on the west branch of the Canas Eiver, 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 tlie groundmass and are marked in many cases by zonal structure. Inter- 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 [Hid 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-Peiiuelas road, where the chlorite is developed from augite, which occurs both in the groundmass and in phenocrysts. Iron 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 Eiver, 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 2.3, 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 held. 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- niens of diabase is the texture, which ranges from rather coarse diabasic at kilometer 3.2, Yauco-Lares road, to very fine ih 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 arrans^ed in characteristic diabasic fashion, with the augite filling the iJTI 'Si'lEXTIFIC HVRVF/Y OF PORTO KiOO spaces lietwet'ii. Thin structure \> well ."coii in tlie rock t\\m\ kilumrMt ; 2.:!, ^\^,^\ih^\w7AA\^ Yi^'^iu roa, Ifayagiiez^.H. \'e.i.':as ruaih ilnieiiitt^ is uiiieh iu exeosK ol' inagu(>tite. while j]i the elisi-! spce-iirieiis iua,Toundmass 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 ii:roundmass. Pyroclastics TUFFS From many specimens of tuff a set has been selected for microscopic ^tudy, 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- ^nviev 19.8, Ponce-Adjuntas road; at the intersection of Ponce-Adjuntas 'iii'l Jayuya roads; kilometer 13.7, Ponce-Adjuntas road; kilometer 14.9, 5*f'nce-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 Eiver 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-Peiiuelas 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 tui! 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. (3) 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 fehl- 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 to hematite and limonite. (8) The geologic history of this group of rocks, as far as can be ascei- tained from the study of thin sections, suggests the following steps : (a) Accumulations of volcanic ejectamenta on land surfaces, wi li marginal portions of these deposits reworked by water. This margi? -il MITCHELL, OEOLOGY OF THE PONCE DISTRICT 277 reworking accounts for the rudely stratified portions in which the frag- ments show more or less rounding. (h) 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 Eocks 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.+^ 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 D az. I'he 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 SGIENTIFIG 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, yellowisli 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-bearin,<]j minerals, chiefly pyrite. The color of the red and yellowish shale is dne to finely distributed earthy hematite and limonite respectively. Len- 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 abundar.t 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, 3, 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.-f-? 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) Eounded grains. (2) Crystalline and massive carbonate, filling in around the other minerals. This carbonate of the latter type is responsible for the binding toi>:ether of the mass. 280 SCIENTIFIC SURVEY OF PORTO RICO Metamorphic Eocks 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 Eiver. 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. k mineral group association which characterizes this rock and which might serve as one example of the result of contact metamorphisni 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 crystal 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 lamellae 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, o-rossularite, 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 Seiior Blasini pros- pect, on tke Portugues Kiver. 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 Keform 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 occurs 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- niens are pyroxenes and chrysotile, the latter forming small veinlets cntting 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 Eeform 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. OK- vine is more abundant in the rock north of Sabana Grande and south of the Eeform 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. Yeinlets 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 ?o 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 illns 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., Actceonella sp. and Hemiaster herheyi 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 herkeyi by Dr. K. T. Jackson. Details of the forma- tions in which these forms occur have been discussed heretofore. Post-Tertiary Fossils 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 jammcensis, Lucina tigrina. Area tuberculosa, Byssoarca ziebra, Murex elongatus, Area rhomb ea, Turritella imbricata, Peeten nucleus^ Venus caneellata, , Ostrea sp., Strombus acei'pitrinua, Fissurella nodosa. Turbo piea, Manieina sp., Cerrithium Utteratum, Pterna sp. Tertiary Fossils 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), Peeten sp., .1 i I ir >i la I ) i->F I'-f'.irrn in MITCHELL, GEOLOGY OF THE PONCE DISTRICT 285 Natica sp., Amauropsis sp., A starte sp., * Cardium sp., Turrit ella halensis, Dall, Turritella halensis alpha, new var., Cythara sp., Fasciolaria sp., Pecten sp., Laganum sp., Orlntoides mantelli, 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 beds above the OrMtoides mantelli. Cketaceous Fossils The following fossils have been collected from the San German lime- stone Radiolites sp. (Upper Cretaceous) and Actceonella 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 Actceonella sp., together with the Radiolites sp., serves as an index of these limestones. From the Cretaceous shale and limestone the- following foraminifera have been identified : ''I) Shale just south of Melones Point. These beds are the westward extension of the shale at Ensenada, which carries Hemiaster herkeyi. SGIENllFIO HVJIVEY i)¥ PORTO RJC'O TcxluJarvi i^p., .['iilvinulina sp., GlohiijeritiAi creiiicea, il/'Orbigny, Orhuliria universa, t] 'Orbigiiy, Olobvjerina buUoide^R, d'Orbigiiy. (2) Ctiinyabiil liniCHt:«iie, one mile eai*t of the manganese deposit nort c Juana Diaz. (Jlohigerina sp. Maieli frngjiiental foraiiiiidfera not identifiable. {?)) Ensenada shale. A large amount of fragmen tal foramiiufera. Tlie description, of tlie new species of Hemimter (Fig. 8) found in tn MITCHELL, GEOLOGY OF THE PONCE DISTRICT 287 shale has been furnished by Dr. E. T. Jackson, who states that its affin- ities are with the Cretaceous. His description follows : Test large, cordiform, high, subconical, rounded below, from the highest j.oint of the test dorsally slopmg anteriorly, i)osteriorIv 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 lY, 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 reqnest of the collector, Mr. Graham John Mitchell, this species is named in honor of Prof. Charles P. Berkey, of Columbia University. GEOLOGIC HISTOKY Introductory Statement In an historical summary of the geology of Porto Eico, 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- lations, 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 t<» result in extended planation and partial baseleveling, with final extensive submergence. (8) The development of an unconformable overlying series of shales, reef li^iiestone and related deposits, chiefly of organic origin, brought to an end by fii al 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. Cretaceous Rock Deposition 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 tuff 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 volcanic period when organisms could exist in abundance is shown by the RadioUtes 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. DEPORMATIOISr 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 rock? to a partial plain, designated as the Eocene peneplain (Berkey, 1915, p MITCHELL, GEOLOGY OF THE PONCE DISTRICT 289 41). This old surface is probably represented in the Ponce District by the mesas southeast of Mayaguez. Tertiary Sedimentation The next event of which there is a record is found in the deposition of lower Oligocene shaly and marly limestones. The base of the Tertiary in the Ponce District has been obliterated by faulting, which has also de- stroyed the normal relations of the Tertiary and Cretaceous beds. How- ever, as stated elsewhere in this report, the marked unconformity existing on the north side of the island between the formations of these two geo- logic epochs was also undoubtedly present on the south side. The lowest Tertiary is represented by the rocks exposed along tlie Jacaguas Eiver northwest of Juana Diaz. The sandy character of the marly and shaly limestone and the presence of many fragmental fossils and plant remains indicate shallow-water deposition. The change from this near-shore type of deposition to somewhat deeper water is indicated by the passage of these lower beds up into the chalky limestone of the upper Ponce formation. The waters in which these higher beds were laid down were much quieter and freer from sand and mud. Toward the top of the Oligocene (Upper Oligocene) the conditions became suitable for coral growth, as is indicated by the masses of coral heads imbedded in the chalky limestone. Whether or not Tertiary beds higher than the Upper Oligocene were deposited in the district cannot be determined, as the next record of deposition above the Oligocene is in the San Juan formation of Pleistocene age. Deformation and Uplift of Tertiary Toward the close of Tertiary deposition the region was again uplifted, accompanied by dynamic disturbances which produced gentle folds whose axes trend north-south and east-west. The fault between these rocks and the Cretaceous probably started its first movement at that time and reached its present amomit of displacement before the close of the period of erosion which followed. Erosion of Tertiary The next event was the dissection of the Tertiary Coastal Plain and stripping of these deposits from much of the area formerly covered. The i omplex Mountain Province thus uncovered or still exposed continued to undergo erosion and to contribute to the supply of waste which is now 290 ^SCIENTIFIC SURVEY OF PORTO RICO partially represented by the playa deposits. The larger valleys —for ex- ample, Guanajibo and Yauco-Boqueron—were cut during this period. Submergence with Marine Terrace-cutting and Formation of San Juan Formation The submergence which followed the main Tertiary erosion allowed the sea to extend up the river valleys, where it laid doAvn gravel, sand and silt and formed the present elevated estuarine deposits. It was during this period that the highest marginal terraces were cut. The highest recorded terrace is approximately 200 feet, but the amount of submer- gence may have been somewhat greater than that. During this submer- gence the fossiliferous San Juan dune sand was consolidated and a con- glomerate consisting chiefly of pebbles derived from the Cretaceous rocks was deposited upon it. Emergence From that time the movement appears to have been one of periodic uplift, finally reaching the present land-level. These uplifts resulted in the terracing both of the estuarine and inland floodplain deposits and in the cutting of the lower terrace levels on the formations along the south and west coasts. ECONOMIC GEOLOGY General Statement The metallic mineral deposits in the Ponce District are confined to the Cretaceous rocks. The most extensive deposit is the limonitic iron, which is associated with serpentine rock. Manganese is the next in extent and is confined to the Guayabal limestone north of Juana Diaz. Magnetite is found along the upper Portugues Eiver north of Ponce, where it has been formed along the contact between limestone and an igneous intru- sive. No deposit of copper was found, although greenish stained andesite in which small particles of native copper are present outcrops north of kilometer 72. -f-, Ponce-Peiiuelas road. Among the material classed as non-metallics are salt, building stone, road-metal, cement material and petroleum. In the following pages all of the above resources are treated in as much detail as the individual cases seem to warrant. Manganese The only locality in the district where a deposit of manganese was found is north of Juana Diaz, where it occurs in the Guayabal limestone. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 291 The property is owned by a company of which Deputy United States Marshal George Trautman, of Ponce^ is the local representative. Several pits have been opened and a quantity of ore extracted. The method of working is as follows: The good ore is removed from the pits by means of pick and shovel. It is then sorted by hand, sacked and carried by pack- train to the military road, a distance of about three miles, where ox-carts transport it to the Ponce Playa for shipment. The ore occurs as irregular-shaped masses along crushed and jointed zones in the Guayabal limestone. The size of the pits from which ore has been removed averages about 20 feet wide and 30 feet deep. The ore minerals are psilomelane and pyrolusite. The former is the more abun- dant and has the characteristic botryoidal form. It also occurs in layers with the pyrolusite, which shows short, indistinct crystals. Much of the manganese is massive black psilomelane, some of which is carried down the hillside by surface water, leaving a black trail behind. Calcite, crys- talline and in banded form, is intermixed with the manganese. The manganese owes its origin to secondary processes. The mode of occurrence of the oxides eliminates the possibility of a primary origin of these minerals in the limestone. The presence of the ore in pocket-like bodies of no great extent laterally or in depth and the location of such bodies in crushed and jointed zones indicate a concentration of the oxides by surface waters moving through these channels in the limestone. Such fracture zones would offer an easy course to manganese-bearing solutions. Tlie original source of the manganese is not entirely evident. A micro- scopic study of the limestone country rock disclosed no primary manga- nese minerals or other minerals from which the oxides might be derived. A possible source of the manganese would be the jasper, which occurs as irregular masses in the limestone and which is found coated with manga- nese oxides. These jasper masses are not always found in direct associa- tion with the ore, but in most cases are only a short distance away. In one instance reddish jasper was found with manganese oxides coating the jasper and replacing it. From the field evidence, it appears that the jasper was the original source of the ore where the manganese minerals were probably present as rhodonite and rhodochrosite. Subsequent leach- ing, in the process of weathering concentrated the psilomelane and pyro- lusite in the crushed zones in the limestone. Fragments of limestone are 'Ound imbedded in the manganese oxides which have partially replaced the country rock. In thin sections the oxides can be seen replacing the frairniental RadioUtes and foraminiferal remains and cutting the wall roek in a network of veinlets. Harder (1910) discusses the manganese ores in the Franciscan jasper 292 SCIENTIFIC SURVEY OF PORTO RICO of California and states that ^^the original source of the ore is the jasper itself/' Further search for manganese in the Ponce District should be confined to the limestones, especially the Guayabal formation, and those portions closely associated with the jasper should receive first attention. Magnetite A prospect of magnetite along the Portugues Eiver north of Ponce was visited in company with the owner, Seiior T. Blasini, of Ponce. The property is known as Tibes and is about four miles north of Ponce. No development work had been done up to the time of the writer's visit, with the exception of a few shallow pits, which were not sufficient to determine the extent of mineralization. From an examination of the few openings made, it can be seen that the ore owes its origin to contact metamorphic action of a felsitic dike cut- ting the San German limestone. The intrusive is so strongly modified that its original character cannot be determined. However, it has the appearance of an andesite. The magnetite, with a little pyrite and chalcopyrite, is intimately asso- ciated with garnet (grossularite). The ore, as far as could be determined from the openings, occurs in irregular replacements along the limestone igneous contact. The limestone at this point dips with a steep angle and is cut by the felsite dike, which has a nearly horizontal attitude. The property is worthy of further prospecting to determine the full extent of the ore. LiMONITE The most important limonite deposit in the district, on the Mayaguez mesa, has been discussed in a very comprehensive paper by Fettke and Hubbard (1918). Since this paper may not reach all those interested in this iron deposit, it is proposed to summarize here the results of Fettke and Hubbard's study and to add additional localities for this tvT)e of ore. In regard to the distribution and mode of occurrence of the limonite, Fettke and Hubbard state : The limonite occurs as a mantle of brown to reddish brown soil overlying: the top of the mesa and extending part way down its sides. Over most of the steep northern and southern flanks, however, it is absent, the underlying ser- t>entine rock coming to the surface here. The material does not furnish a very fertile soil, so that vegetation on the mesa is scanty, as compared with tlie luxuriant growth of tropical plants covering the adjoining hills. Over most of the mesa the total thickness of the limonite is not exposed, and, as this can- not be determined without drilling, a few measurements only were obtained during the hasty reconnaissance made by the writers. MITCHELL, GEOLOGY OF THE PONCE DISTRICT 293 The gradation from weathered serpentine ^nto limonite, wlierever the con- Uu'f is exposed, is a shaip one. As a rule, the serpentine just beneath the iron ore has been altered from the dark green, dense rock, already described, to a light yellowish green, soft, porous mass, abundantly stained by brown hydrated oxide of iron. The iron ore itself usually has a reddish brown color, but inter- vening between it and the altered serpentine there are, in most cases at least ;i few inches of brown material. The character of the iron ore is summarized by them as follows : Most of the limonite deposit consists of a loose, porous, earthy mass varying ill color from light yellowish to a dark reddish brown. Usually the red por- tions occur nearest the surface, while the brown rest upon the serpentine. The relative percentage of iron in the ore cannot, however, be determined from the color, as freqaently the brown varieties contain a higher percentage of ferric oxide than the darker red. The shade of color is undoubtedly due to the degree of dehydration of the ferric oxide. Toward the surface the limonite tends to lose its combined water and gradually passes into less hydrated foi^ms of ferric oxide and finally to hematite. In addition to the loose earthy material, numerous' boulders and masses of hard ore occur scattered over the limonite area. These boulders retain the porous structure of the soft ore, but contain little veinlets running in all direc- tions and filled with the botryoidal form of limonite with varnish-like luster. These boulders range in diameter from a few inches up to several feet ; appar- ently they have been washed out of the upper layers of loose material. A thin section from such a boulder, under the microscope, showed that the hydrated iron oxide is present in two forms, cryptocrystalline and amorphous. The former variety retains the structure of the serpentine, while the amor- phous has been deposited afterward in little veinlets running in every direction throughout the former ; these show a banded structure. Small grains of mag- netite and chromite appear here and^ there, just as in the serpentine rock. The conclusions of these authors upon the origin of the limonite is <] noted below : From the preceding description, it is seen that the limonite deposits of the ^layaguez mesa are almost exactly similar to those in northeastern Cuba, which have been studied by a number of geologists, all of whom have agreed that they are residual in origin, being derived from the underlying serpentine hy the weathering of this rock. It will, therefore, be sufficient to summarize briefly here the main lines of « vidence that point to the residual origin of the limonite. These have already '>een ably discussed by the geologists who have described the Cuban deposits. The close association of the limonite and the serpentine points strongly to ^iich mode of origin ; where other rocks lower in iron underlie the surface, the ^'>ils are relatively poorer in iron oxides. That the original structure and ^('xture of the serpentine can still be detected in some of the hard ore furnishes ^dinost indisputable evidence that the limonite has been derived from it in this niaiiner. The irregular contact of the iron ore with the underlying serpentine ^^ Hso characteristic of residual deposits derived from underlying rocks by 294 SCIENTIFIC SURVEY OF PORTO RICO the process of weathering. Likewise, the extremely porous structure of the limonite, where slumping has not occurred, shows that it has been formed by a leaching process in which the more soluble constituents have been dissolved by circulating waters, leaving the less soluble constituents behind. The rela- tively high percentage of chromite in the ore also indicates the same origin. Ohromite is a relatively insoluble mineral, and therefore remains behind in the residual soils. Finally, a comparison of the analyses of the serpentine and the limonite furnishes further evidence for supposing that the latter was derived from the former (Table 6). Table 6. Comparative Composition of Serpentine and Limonite Serpentine, per cent. Limonite, per cent. SiOa 38.41 2.44 AI2O3 4.96 20.21 Fe^Oa 6.32 57.69 FeO 1.27 0.85 MgO 33.32 0.61 OaO 0.04 trace NajO 0.27 notdet. K2O 0.10 notdet. H2O at 110° O 0.83 1.09 H2O + ignition 13.40 14.96 TiOa 0.08 0.26 NiO 0.72 100 Cr^Oa 0.42 1.57 100.14 100.68 The only further information which. might be added is to note other, less extensive occurrences of the same type of ore as that found in the Mayaguez mesa. The writer has gone over the area examined by Fettke and Hubbard, and has also studied the serpentine to the east of the mesa and at other points in the Ponce District. The limonitic iron was found at a number of localities in the mass of serpentine, the western end of which forms the Mayaguez mesa. The deposits are very irregularly dis- tributed, much of the serpentine showing only iron stains. The other locality worthy of mention is at the Eeform School southwest of Maya- guez, where shallow pits have been sunk. The limonitic soil at this point is filled with shiny nodules of brown iron ore. Copper An unsuccessful attempt was made to locate a reported copper deposit north of San German. The rock in this region is chiefly serpentine, and, although places showing copper staining were found, no deposit was en- MITCHELL, GEOLOGY OF THE PONCE DISTRICT 295 countered. It is possible that ore may occur here similar to that found on the Pacific coast (Butler and Mitchell, 1916). Copper in the form of small particles was found in andesite north of kilometer 72.+, Ponce- Penuelas road, near the fault contact. The mineral is very sparsely dis- tributed in the andesite, being associated with the filling of amygdules. Weathering has produced greenish copper stains coating the surface of the rock. Salt Salt is derived by solar evaporation of sea water in artificial salt pans along the south and southwest coasts, at four different places. The largest of these is the Salinas de Cabo Rojo, in the extreme southwest corner of the district, near Cape Rojo. A memorandum of the Salinas de Cabo Rojo, furnished by Seiior Arturo Bravo, of San Juan, is included here : The so-called Salinas de Cabo Rojo are situated on the southwest point of this island, about where Morrillos Lighthouse is located. The salt production is divided into three sections, viz : "Fraternidad" on the south, between Morrillos and Punta Aguila ; "Candelaria" on the west, between Punta Aguila and Punta de Hicacos; and the "Corozo," between Punta de Hicacos and Punta de Penones. The salt is obtained by solar evaporation. There are two large ponds, one natural and the other artificial, which receive the sea water through ditches. From these ponds, by means of wooden canals, the water is pumped into the crystallizers, wind power with five mills being used. The salt is taken with wheelbarrows from the crystallizers and stored near by in pyramid piles, and when dried is carried to the shore in small open wagons, driven by oxen, through rails in some instances, and in others with common ox-carts, which unload into lighters at a small wharf, and the lighters take the salt to the vessels. The number of crystallizers amounts to 41, and their capacity of salt varies from 25 to 100 tons each, averaging in all about 3500 tons. In ordinary weather conditions through the year, these crystallizers condense four times, showing a total production of 14,000 tons. There is room for building more crystal- lizers at a very small cost, to duplicate the production, and with some further improvements it might yet considerably increase. Exceptional unfavorable weather is usually experienced every ten or twelve years, and we had to encounter with such contingency in 1913, when the yield amounted only to 2870 tons. Wages are paid at the rate of 50 cents per day for any ordinary work, but the labor on shore for shipping is paid by the weight, at the rate of one cent per hundred pounds. The labor for gathering the salt in the crystallizers, carting it out and piling is paid at 4 cents fanega. Shipping by lighters is paid at $2.00 per hundred fanegas. Each fanega is equivalent to about 300 pounds or over, according to the weight of the salt. Our salt is well known in different American markets and particularly so in l^>oston. During the years 1910, 1911 and 1912 eighteen cargoes were shipped 2iH; t^ViKNTlFlC lr yenr, <»r which w(^ rnrnish about oWM), the other snuiller salt places liere about .jOCio, and the remainder is imported, from Curacao. These im|H»rtnlioiis from Cnr.-e cao have been c(a.islderabl,v increased since ft>rei^'n salt Is coming lu free i.f dnf.v, but we Iwirdly think the iioiiipetitlon with home production may be luurh lonsjTer maiytaliiotl in spite of the exceedingly elu^p prodiK'tioii In the ueii^dr^ boriiii,' island. dm> to small wages and more reduced exinaisi's prevailing there. Our property consists of about 1853 ciierdas (each euerda approximately one a<-rei of lainl. about omvtliird of whicli covers the t^xtonsion recpiired for tin- salt tnauipulation. I'~H!. a. "Moiiiali-n'' SidiiHU ircal t,f aiiuniiM, on xuiitl) aniHt Other jilaeoH prodiiejiig salt are tlic so-called Boqiieroii, Jiear the Siilin «lc Cabo Ji'djo, whieli yields netirly 1000 tons irniinally: (kianiea, on t soiit-li coast, producing 3000 tons, and M^mtalva (Fig. I*)? wear (hiiiiiii loUil tons. The joothods m&l in these smaller saliiias is practicnlly r sanK' as tliat ejnployed at the SaJiims de I'aho Kujo. BpiLniN'o 8'roNK Only three iurmntions in the (lls^triet h>n(l themselves to bnihling pi ]Hises. Tliesi- are the Coama tuft limestone, (Inayabnl limestoni', a ])ortions of tlic Fmiee