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^Bwc^i:r--:' m: llili niSl-P W^BSmW^m^^»M ||i||Slii:-i i;;aim;l-.. . . ...-mm ll^iiiilSiS^* liMiiiSlii: l.iiiwIiiiilSS: I B 52801 8 mwmmmmmmmnm Rff. K;i ^ ;::'*::; J-. ... . : ..ri.: t %;< ''f- ' ::v.;.::V:.7;^^.< :;:. :: :.>".. : : : -/, ; : ... . ?... •..:.. WW?^'i:SsMx^£-'io...^ :.;;;?. JWS i; K:;!;r ;;::|j:; .y^?" ;>;; ^j; ;;i:^' ;;: ; rf^@... iiiliiiSlftiS^^^ mmsimmmmssmm:^ik iifiiii^l- I,' K NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Yirgie Islands VOLUME I^Part 2 Geologylof the Coamo-Guayama IMstnct—E.lT.iHodge XKW YORK: .^r THE GEOLOGY OF THE COAMO-GUAYAMA DISTRICT, PORTO RICO By Edwin T, Hodge CONTENTS Page [ntroduction 114 Location and area 114 Purpose of the study 114 Acknowledgments and previous workers 115 Geography 115 Physiography , 116 General statement « 116 Coastal plain 116 Distribution and general character 116 Hills ,• 118 Terraces and sea cliffs 119 Coast line , 119 Mountains , 119 Distribution and general character , 119 Height, relief, dissection 120 Concordant elevations 121 Interior valleys 122 Drainage. , 122 Relation of streams to trend of valleys and mountains 122 Entrenched stream terraces 123 Coastal streams 123 Capture ^ 124 General geology 124 Difficulties in obtaining data 124 General physical character of the rocks ^ 124 General structure , 125 Sedimentary types of rock • 126 Tuffs 126 Volcanic breccias 127 Agglomerates 127 Conglomerates 127 Shales 128 Limestones 128 Types of igneous rocks present ;. 128 Hydrothermally altered and mineralized rocks 129 Basis for an attempted division of the rocks. 129 Rio de la Plata series 130 Tuffs. 131 Limestones • 132 Agglomerates 132 (111) 112 SCIENTIFIC SURVEY OF PORTO RICO Page Barranquitas-Cayey series 132 Shales 133 Limestones • 134 Tuflfs 135 Physiographic effect * 137 Genesis 137 Sierra de Cayey series 13T Distribution. • 137 Structure • 138 Composition and correlation 139 Ck)nglomerate 139 Physiographic effect 141 Genesis- • • • 142 Guayama series — — • • • • • 142 . . General .statement • • • • 142 .... .Shales. 143 . „ . Cherts. 1^4 .... .Tuffs. • 14^ Limestones 147 Conglomerates 148 Agglomerates ......,..,-.. 148 Physiographic effect • • 148 Genesis 1^^ Rio Jueyes series l'^^ General character 1'*^ Structure • • 1^^ Limestones ( 1^^ Shales. . , 1^1 Tuffs. 1^1 Conglomerates and agglomerates 152 Physiographic effect 152 Genesis. - 1^^ Coamo Springs limestone series 153 Structure 153 Limestone— 155 Tuffs. 156 Physiographic effect 156 Genesis.' • 157 Origin of the dolomites 159 Rio Descalabrados series • 161 General character 161 Structure 1^1 Limestones • 1^1 Shales and cherts 162 Physiographic effect 163 Genesis • • •' 1^ Areclbo formation ^^^ Physiographic effect. 166 HODGE, GEOLOGY OF THE OOAMO-GUAYAMA DISTRICT 113 Page Santa Isabel series 166 Occurrence 166 General character 166 Physiograpliic effect ., 167 Genesis 170 Early intrusives, flows and pyroclastics 170 Occurrence 170 Composition types 170 Rhyolites 170 Felspathic andesites 171 Augite andesites 173 Olivine andesites 176 Basic augite andesites ^ 178 Olivine-free basalts 179 Olivine basalts 179 Special structural types 180 Genesis 184 Batholithic rocks 184 Occurrence 184 General character i 184 Hornblende granite 185 Genesis 185 Later intrusive rocks 186 Occurrence 186 Biotite granodiorite 186 Hornblende diorite 187 Hornblende-augite dioriie. 189 Hornblende andesite 189 Hornblende-augite andesite 191 Genesis 192 Conclusions as to the age of the various series 192 Sedimentary rocks 192 Igneous rocks 197 Summary of age relations 198 Historical summary 198 Early period to end of Comanchic time 19S Cretaceous and Eocene 199 Oligocene 200 Miocene to Pleistocene 200 Pleistocene ( ?) and Recent 201 Vulcanism 202 Hydrothermal effects 202 Mineralization 200 The Ooamo thermal springs 209 Location and purpose of study 200 Occurrence of springs and wells 210 Location near a fault 211 Source of water 211 114 SCIENTIFIC SURVEY OF PORTO RICO Page Possibility of a meteoric source 212 Seasonal rains and variability 212 Floods and underground circulation 214 Temperature 214 Location near volcanic center 214 Constancy of composition » 215 Possibility of connate, entrapped and strayed magmatic waters 210 Possibility of secondary and primary magmatic waters 217 Assimilation 217 Diffusion 217 Assimilation or diffusion in the katamorphic zone 218 Mineral resources 219 General considerations 219 Metallic minerals 220 Coal 222 Oil shales and petroleum 222 Road metal 222 Salt 22;{ Clays 22n Lime 224 Natural cement 224 Structural limestone 224 Thermal springs 224 IKTEODUCTIOISr LOCATIOK AND ArEA This report covers an arbitrarily bounded area, which occupies the south central part of Porto Eieo/ and will be called the Coamo-Guayama district. This district occupies about one-eighth of the 3668 square miles belonging to Porto Eico and measures 23 by 20 miles. Adjacent on the north is another area, the side lines of which are a continuation of those of the Coamo-Guayama district. It is called the San Juan district and has been described by D. E. Semmes.^ Purpose of Study The purpose of the investigations made by Mr. Semmes and myself is to give a geological cross-section of a typical part of Porto Eico. In addition, I will describe in some detail the complex geological structure and the peculiar rock types of exceptional petrogenetic history; I will 1 See Outline Map of Porto Rico, Volume I, Part I, p. 26. 2 The Geology of the San Juan District, Volume I, Part I, pp. 33-110. HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 115 discuss the origin of the Coamo thermal springs^ and attempt to present the data establishing the main events in the geological history of Porto llieo. Acknowledgments and Previous Workers The author is grateful for much assistance in the preparation of this report. The investigation was made possible by the generous support of the Porto Eican Government in co-operation with the Few York Academy of Sciences. The rigors of tropical field-work were more than compensated by the uniform courtesy and exceptional hospitality of the citizens of Porto Kico. The author wishes to express his appreciation of the counsel and guidance of the professors of the Department of Geology, Columbia University, especially that of Dr. Charles P. Berkey. The laboratory investigation of this problem was conducted in the Geo- logical Department of Columbia University. N'umerous travelers and scientific men have published observations on Porto Eico, among whom are the following : P. T. Cleve, Brig. Gen. G. W. Davis, W. Dinwiddle, M. K. Domensch, Amos K. Fiske, S. H. Hamilton, E. T. Hill, J. S. Newberry, H. C. B. Nitze, T. W. Vaughan, W. J. L. AVharton, H. M. AVilson and C. P. Berkey. With the exception of Berkey (1915, p. 69), none of the above more than briefly mention the Coamo-Guayama district. The reconnaissance study by Berkey cleared up the fundamental problems in this district, and the results of this work have been extensively used in prosecuting the present survey. GEOGEAPHY The distribution of the essential geographic features is shown on the Outline Map. The area includes the City of Guayama in its southeastern corner and the villages of Cidra, Aibonito, Cayey, Coamo and Salinas. A magnificent road enters at the northeastern corner and leads south- westward through Cayey, Aibonito and Coamo toward Ponce. At Cayey a branch road leads southeastward to Guayama. A narrow-gauge rail- road and a wagon road traverse the coastal plain from west eastward to Guayama. The area is divided iiito a northern mountainous part, which occupies about two-thirds, and a southern low coastal plain. The mountains are ^levoted to grazing and to coffee and tobacco culture. Large crops of ^^ugar-cane are grown upon the coastal plain. The character of the ^li'ainage pattern is shown by the accompanying topographic map. Because the island lies across the path of the prevailing trade winds, ^^^ere is a great variation in rainfall in the various parts. The northern 116 SCIENTIFIC SURVEY OF PORTO RICO slope of the mountain ranges, where the moist trade winds are forced to rise, receives more than an abundance of rainfall. In the northwestern corner it is said to rain continuously week after week and the mountains at that season of the year are heavily clothed in mist. In contrast to this the southern coast, which is swept by winds deprived of their moisture, is extremely arid. Months frequently pass without a drop of rain falling, and during these periods all the vegetable life in areas not irrigated withers up and cattle must be driven to the interior. Because of the aridity an elaborate system of irrigation has been developed on the south coast, and to this all streams of the district contribute all or a part of their waters. The harbors are poor and exposed. Aguirre is the best, but the ap- proach to it is through a circuitous channel left by rapidly spreading mangrove swamps. PHYSIOGRAPHY General Statement Forming a southern border variable in width is a coastal plain which slopes gently upward from the shores of the Caribbean Sea to an eleva- tion of about 300 feet. North of the plain is an old-land composed of a rugged mountainous interior with a main belt of mountains, known as the Sierra de Cayey, extending from the northwest corner of the district southeastward almost to the southeast corner. Because the old-land is dissected to maturity, broad sprawling spurs stretch southward and north- ward from the main range, and the mountains are not sharply separated from the plain, embayments of the latter extending northward between the spurs for many miles. Two major interior subsequent lowlands occur, one in the northeastern corner and the other forming a linear belt reach- ing northwest and southeast through the south central part of this dis- trict. Coastal Plain distribution and general character The distribution of the coastal plain, which will first be described, is shown by figure 1. It has an average width of four miles. Guayama, Coqui and Salinas are located upon its inner margin. An embayment of the coastal plain into the mountains reaches a point where the Guamani River crosses the road, another reaches up the Salinas River to La Lapa, a third almost to Coamo and near the western border one extends almost to the Military Road. HODQE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT II7 Ylg HVIENTIFIC l^UKVHY OF PORTO Ml CO llio (•t>jis:!tal plain slopeB gradiuilly toward the sm. At Ckiayaina llu? elevation is 2(H) Ivet and at Coamo Springs it is 240 feet This gentle iucdinalioji is broken by nicndip bilk wliich rise like islands above its general level by wave-eut and rivcr-byjlt terraces, and by meandering atui Hligbtly eiitrenelied streams^. The slope, tbe fine alluvial cover, an,d tbc generallv nndnlating t'baractcr of the plain nudvo it admirably atbifited for sngar culture. Uiiiii/uma upon the upper i-oam .-cly to the CD lowfr and (2) ri by I'rof.'ssar Cluiiies 1'. Berk«\^ iriLLH The few mendip hills I'orm prominent landmarks, siieli as the two large bills, 700 feet bigli, vvbieb lie jnst east oC Onayama and arc nearly surrounded by the eoastel plain (Fig. 3). A group of low hills, the highest of whicli is ^100 feet, ocH;nrs adjacent to tlic coast and wi!st of AgniiTC Harbor and are called Aguirre Hills. Nortli of Aguirre Hills and west of (\)Cpii is a low linear mound 800 feet higli, known as Monte SabatcT. There are two large bills «)ecarring as detached portions of the inner mountains, one of v?hieh, north of Salinas, is 600 feet above sea- lev(4 and the othtn% Just east of (joamo Reservoir, is 300 feet high. All ol these arc portions of tfie old-laud protru<ling through the alluvial coastal plain. HODGE, GEOLOGY OF THE COAMO-GVAYAMA DISTRICT HQ Discontinuous linear swells of the coastal plain, 400 feet high, lie south of the mountainous tract and extend from Salinas Eiver westward to the boundary of the district. TERRACES AND SEA CLIFFS Other features interrupting the general slope of the coastal plain are terraces and sea cliffs, of which two are well shown near Guayama, which is situated upon the highest of these terraces, 200 feet above the sea-level. The photograph (Fig. 2) shows very well the abrupt termina- tion of the terrace against the Guayama Hills in a sea cliff. Ninety feet below, as indicated by the arrows, is the largest terrace, which descends by a gentle inclination to the sea. Along the Eio Jueyes three sea cliffs are to be found. One occurs very near the coast, another a mile from the coast and the third, formed by the Cerro Easpado, occurs two and a half miles from the coast ; the outer margin of the lower terrace is 35 feet high. The outer margin of the upper terrace is 100 feet higher than that of the lower. Traced eastward, both disappear before Salinas is reached, but both can be traced westward, though much dissected, to positions north of Santa Isabel. The upper terrace is well developed just south of Coamo Eeservoir dam where its outer margin is about 160 feet above sea-level and rises to over 200 feet at Coamo Springs. The lower two terraces are much dissected. The southernmost part of the Aguirre Hills has a poorly developed sea cliff, which forms the nearest approximation to a clifted promontory found in this district. COAST LINE Aside from local irregularities the south shore of the Coamo-Guayama district has an angular pattern. Thus a wedge extends seaward to Pta. Ecjuilarte; another to Pta. de Cayures; a third between Aguirre and Sahnas ; and a fourth south of Santa Isabel. Between each of these angular projections of land is an equally angular extension of the sea. I'his irregularity is due to drowning of the subsequent valleys which lie 5it angles to the coast line. Mountains distribution and character rhe dominating range in Porto Eico is the Sierra de Cayey and it !''i>ses through this district from the northwest to the southeast corner. ^1" Sierra de Cayey forms the divide for all the rivers in the Coamo- 120 ^WJENTIFW SURVEY OF PORTO RICO Guayama district. Nowhere in this district is it cut through, but it has been maturely dissected, so that out from it to either side extend many sprawling ranges, some of which have been completely severed from the main range. Though its elevation in no place exceeds 4000 feet, never- theless because of its ruggedness and heavy vegetable covering it is a most imposing and beautiful feature. The Coamo Springs is a homoclinal range (Fig. 17) which extends from the west central border of this district, with a general southeast- ward trend, to Salinas Hill, just northwest of Salinas. Aguirre Hills are thought also to be related to the Coamo Springs Eange and are so mapped in this report, though this relationship has not been proven. This corre- lation of the Aguirre Hills is based upon their alignment with the Cerro Raspado and upon the fact that they are formed of a limestone of similar character. The Coamo Springs Eange rises as a solid, abrupt wall 500 to 1200 feet above the surrounding lowlands, having a steep northeastern slope and a gentler southwestern, with a width of about one-half mile. Its crest is notched by many wind-gaps and it is cut through by several large superimposed rivers, which serve to divide it into divisions as shown by the topographical map. South of this range lie many smaller and lower hills created by the erosion of subsequent valleys by tributaries of the Coamo and Descalabrado Eivers. HEIGHT, RELIEF, DISSECTION The average elevation of the Sierra de Cayey is 2500 feet in the eastern and 3000 feet in the western portions of this district. The relief of all the ranges is, perhaps, 1500 feet. The surface is dissected to early maturity and the texture in the interior is exceedingly fine. The slopes of the Porto Eican Mountains are always steep and deeply dissected. Thus slopes of 40° are common. The mountain sides are always cut by numer- ous gullies producing a very fine texture. Knife-edge crests are common and are barely wide enough to give footing to the small Porto Eican pack- horses as they follow the trails upon them. The cause of these knife-edge crests is the character of the soil, which will stand up as vertical walls 5 to 10 feet for a year or more without caving. Such a soil does not creep readily after streams have cut the slopes, but remains standing upon the divides until finally carved away by running water. Headward por- tions of streams on opposite sides of divides, therefore, are not separated by a zone where soil flowage assists, but erode until they actually meet in a knife-edge divide. HOBiW, aKOLOilY OF THE (J0A3t(M;UAYJMA DISTMICT 131 CO.NC0KDAKT ELEVATIONS Tlio ])i'iicplaoo (Icscnhcd by l^orkcy in Iris rwoirnaissam'O siudv (sf olio Rieo in li)l-J is well shown iu the Coanio-yiiayiiina distriet. As one ;mcl$ in the lowlands and looks np to tbe ?<r!niinit>:, it seems inipogsible I ima.iririe any sni'l'sice moi-c brokrai or any slopes more steep. V^iewcd, v.m a .-iinnniit, the peaks blend into a solid nias.s with a^ rcioarkably k'vel iri/.on line, regardless of the diroetion in wJdeh the distant peaks are '•I'-weiJ. Not only are the mountains in the interior uf nearly nniforni iK-ight, but some of tbeni have flat tops, which oCieu al!or(i suflkaent "f'^f'^e ,Cor a iialf dozen native hiit,«. Wlien one coti?idors tliat the rnonn- ^•nns arc composed of rocks of almost infinite variety and exceed iii.edv "''iai'le strnctnre, tlieir concordant elevation and the existence of an ''''^•i>i<>nal flat area argue that tlic wduile mas? has 'lieen beveled by erosion '"' f'trm a jsencplanc surface (Figs; 3 and ?'), 122 SCIENTIFIC SURVEY OF PORTO RICO Interior Valleys There are two large subsequent valleys of greater importance than any other in this district. The Military Eoad from Ponce to Albonito follows a great lowland which continues eastward into the area under considera- tion. If one could remove the Coamo Springs Eange, at the point where both the Military Koad and the Eio Descalabrados find passage, this valley would continue eastward to the Coamo Eiver. East of this passage or gap the valley is very large and open. It extends eastward from the great white hills of the Coamo Springs Eange lying just outside the western boundary of this district to a mile beyond Coamo Springs. The southern boundary is formed by the wall-like Coamo Springs Eange and the northern boundary is not sharp and distinct, but changes gradually from a flat lowland through many small hills to a large and low range. Beyond Coamo Springs and a group of low hills 400 feet above sea-level it continues eastward and finally merges into the valley of the Salinas. The floor is uniformly 300 feet in elevation. The largest lowland, carved in soft tuffs and limestones, is found north of the Sierra de Cayey, in the northeastern part of this district. The shape is irregularly ovate, with the apex pointed toward Aibonito and the base widening northward so as to include Cidra. The principal outlet is north of Comerio, where the Eio de la Plata has cut a deep gorge in the highlands to the south. Another low pass or col lies beyond Las Cruces and it is taken advantage of by the Military Eoad between Cayey and San Juan. The floor is not flat, but is broken by numerous low hills composed of resistant rocks which rise as much as 300 feet above the general level and have a trend conformable with the lowland as a whole. The average elevation is 1500 feet above sea-level. Drain^age relation of streams to trend of valleys and mountains The larger streams do not lie in the larger valleys, nor are the major ranges parallel to the larger streams. This feature indicates a super- imposed condition of the larger streams and the development of large subsequent valleys by their tributaries. A portion of the Eio de la Plata is an exception to this rule, for it descends from the northern slope of the Sierra de Cayey and flows northwestward across the floor of the Barranquitas-Cayey lowland to Comerio, where it turns northward and escapes through a narrow gorge. On the other hand, the Eio Descala- brados, the Coamo and the Eio Jueyes cut narrow water gaps through HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 123 the Coamo Springs Eange. Eelated to the gorges through the mountain are gaps through which streams do not always flow; for instance, the Cerro Kaspado is cut at its eastern extremity by a deep notch through which a road leading northwestward from Salinas passes to the low- lands lying behind the range. The width of the stream valleys is directly related to the character of the rock over which they flow. Where the streams flow across lowlands, their valleys are wide; where they flow through ranges their valleys are narrow. There are also minor variations in width which apparently are due to variations in the character of the rock over which the stream flows. Some of the streams flow over broad, flat-bottomed, loam-covered valleys for perhaps a maximum distance of half a mile and then cut deep gorges beyond which they may develop plunge pools. This condition is repeated many times in the length of every stream. It is only possible to give a few illustrations of the innumerable cases. For instance, three miles north of the Kio Jueyes Water Gap the Eio Jueyes cuts a gorge not more than 10 or 15 feet wide, while just beyond this gorge a valley one-quarter mile wide and over 3000 feet long opens up. Just beyond the open valley occurs another constriction and then another wide valley. In the gorges the stream flows on bare rock and through the wide valleys upon thick beds of silts and pebbles. ENTRENCHED STREAM TERRACES Near the inner margins of the coastal plain all the streams are more or less entrenched. Thus the Coamo Eiver, near Coamo Springs, cuts vertically downward into alluvium (Figs. 1 and 23). Other streams are similarly incised, but to a less extent; the degree of incision seems to decrease eastward. The streams which show the best entrenchments also show the best terraces ; thus, on the Eio de Coamo three excellent terraces are developed. All the larger streams are provided with terraces, but the smaller streams lack them. The largest number of terraces cut, one below the other, is three, and these are found near the Banos de Coamo. The terraces are always local, yet the higher terraces are sometimes very wide, as, north of the Coamo Eeservoir, along the whole lower half of the Descalabrados, the Jueyes and the Eio de La Lapa. COASTAL STREAMS Streams which flow across the coastal plain are modified in various ways. Many of the streams meander, others wither away and practically disappear ; the Salinas is an excellent example of this latter phenomenon. ;|^24: SCIENTIFIC SURVEY OF PORTO RICO Long before the coastal plain proper is reached this stream takes on a braided appearance and by the time it has gone a short distance beyond Salinas it disappears in the soft porous alluvinm. Some streams, of which the Cimarrono is the best example, exist only on the coastal plain. CAPTURE The Salinas wind gap mentioned above is a good example of recent capture. At one time a stream must have flowed through this gap to the Caribbean Sea ; indeed, a small brook is developed there now in very wet weather. Water-worn gravels occupy this gap. Apparently a sub- sequent branch of the Kio Jueyes has captured the headward portion of the stream which formerly occupied the gap. GENEEAL GEOLOGY Difficulties in Obtaining Data Geological data are not easily obtained in the Coamo-Guayama district. Contrary to a generally prevailing belief, the larger part of Porto Eico is not readily accessible. Aside from the excellent roads which pass through the Coamo-Guayama district, travel in the mountainous interior is over miserable trails where one must often hew his way with a machete. In addition to the difficulties of penetrating the thick tropical growth, there are the difficulties resulting from the heavy mantle of soil. Out- crops are so rare that in field-work it was necessary to wade the streams and examine the rocks laid bare in the stream bed. Exposures in the streams, in the cuts along the Military Eoad, and the occasional outcrops at the crest of the mountains form the chief sources of the information contained in this report. General Physical Character of the Eocks The greater proportion of the rocks of the Coamo-Guayama district are of sedimentary origin and consist of tuffs, agglomerates, shales de- rived from volcanic ash, conglomerates, limestones and cherts. The tuffs and conglomerates are decidedly massive and the determination of their dip and strike is impossible. The conglomerates show little or no bedding. Occasional beds of limestones, shales and cherts furnish the only clews as to the geological structure of this terrain. The types of rock derived from volcanic sources dominate and of these few have passed through more than one sedimentary cycle. For the most part, the sediments are loosely bound tuffs, ash shales, or calcareou^ HODGE, GEOLOGY OF THE GOAMO-GUAYAMA DISTRICT 125 tuffs. Despite the fact that the igneous rocks are quantitatively negligible, vet they are of great scientific interest because dikes, sills and flows occur in almost infinite number. If all the igneous outcrops which have been observed were plotted on the accompanying geological map, the entire surface would be dotted, and for that reason only a few of the major igneous bodies have been mapped. Another factor adding to the complexity is the rapid alternation from cue type of rock to another. For instance, in traversing a stream for a kilometer some fifty different and distinct occurrences of igneous and sedimentary rocks may be observed. In the few places where it was possible to trace a definite stratum laterally its character changed so completely that specimens collected a mile apart would not be recognized as belonging to the same bed. Occurring just as often as the above changeable transition is the pinching out of the strata because of their lenticular shape. It is, therefore, no exaggeration to say that the Coamo- Guayama district in particular and Porto Eico as a whole consists primarily of volcanic ash in many thousands of sedimentary beds, in part reworked and in part alternating with marine strata, all of which are cut through and through in a most varied manner by igneous rocks. General Structure The grouping together of such varied rocks into formations would not be such a difficult matter if the structure were simple and if it were possible to gather more complete data concerning the structure. Tropical vegetation and deep residual soil conceal the bedrock. It was the ex- perience of the author and of several other geologists who have worked in Porto Eico that frequently in the course of a whole day's traverse only one dip and strike could be obtained which were considered reliable. Such conditions explain why the structural relationship of the bedded rocks in the Coamo-Guayama district are not well understood. In the western half of the district all of the dips recorded were to the south and varied between 20° and 55°. The fact, that the dips in the older series are not in all cases steeper than those of the younger, indicates local folds; nevertheless, because of insufficient outcrops, it was impossible to determine the detailed character of these local folds. Eastward, ap- proaching the borders of a batholith which lies just beyond this area, Hiore variable dips were noted. Aside from local irregularities, it appears t hat most of the northern half of this district is folded into a flat, broad ^vncline. The south limit of this syncline begins to develop at K. 12 on tlie road to Guayama. South of this point to K. 24 the rocks are folded ][28 SCIENTIFIC SURVEY OF PORTO RICO SHALES The shales consist of bedded ashes and reworked tuffs. The former have suffered so little reworking that in thin sections their pyroclastic nature is easily made out. The latter have been so thoroughly disinte- grated and sorted by water that the original character of their material is determined only by a few associated volcanic fragments. The perfec- tion of the bedding is best developed in the younger rocks and most of the shales were accumulated near the shore, as is indicated by the presence of organic remains in them. Some of the shales are silicified and others are epidotized. The silicified shales are thought to be due to the organi- zation of radiolaria and diatoms, and the epidotized shales to hydro- thermal influences. LIMESTONES Between tufis with a few organic remains and limestones with a few tuff fragments, there is every graduation and in like manner the lime- stones are related to the shales. Some of the tuffs, andesitic sills, and andesitic flows have so thoroughly altered to carbonate that they now consist of as much as 80 per cent carbonates and have usually been mis- taken for limestones. The true organic limestones can be divided into two classes, those with no algal content and those containing alga^, and the latter, are found only in the middle of the uppermost group of rocks. The other limestones are made up chiefly of foraminifera with some con- tributions made by pelecypod or gastropod forms. In all cases the bulk of the rock consists of reorganized calcite derived from organic remains. Types of Igneous Eocks Present Although the range of igneous rocks is wide (Figs. 24 to 40), the most common variety is augite andesite. The following list shows the range rhyolite, granite, biotite granite, feldspathic andesite, granodiorite, augite andesite, hornblende andesite, diorite, hornblende diorite, hornblende- augite andesite, basic augite andesite, olivine-free basalt, and olivine basalt. Porphyritic rocks are uncommon and granitoid types extremely rare. The massive rocks appear usually in small dikes and flows; espe- cially vesicular varieties were only observed in a few places. As pointed out above, the tuffs, volcanic breccias, and agglomerates contain so m^ny igneous types that in a thin section of a tuff as many as ten to fifteen varieties may be observed. These fragments consist of rhyolites, feld- spathic andesites, augite andesites and olivine andesites, and of these, as in the larger masses, augite andesites greatly predominate. After the HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 1^9 augite andesites, feldspathic andesites occur in greatest abundance, and are followed, in point of number, by the basic varieties. The fact that the same type of rock appears in tuffs, flows, dikes and sills strongly suggests that these formations are nearly contemporaneous and indicates that while tuffs and breccias were accumulating on the surface liquid matter was being injected into the basal portion of the pyroclastic and sedimentary series. Hydrothermally Altered and Mineralized Eocks Nearly all the rocks studied in the Coamo-Guayama district show some mineralization and many of the rocks are intensely hydrothermally altered. Basis for an Attempted Division of the Eocks The rocks of the Coamo-Guayama are very hard to classify into series because of the variety of outcrops, complexity of the structure, extreme variability of rock types, and absence of fossils. The few fossils which liave been observed are of no assistance in making correlations within this district, although they are of value in giving a clew as to t'le .i^eier il age of the strata. The only practicable method of making correlations, which is not free from possibilities of error, consisted in noting the out- crops with their occasional dips and strikes; then, with a knowledge of the general structure, the hand specimens were compared and the out- crops of the same kind connected. Because of the similarity of the type, the divisions, except in a very few cases, between one series and another, are simply arbitrary lines, which attempt to separate sedimentary rocks of one general dominant character from those of another. While the correlations within the district are lithological, the ages of the various series rests upon a firmer foundation. Thus the structural data is sufficient to work out the succession and so determine which are the older and which are the younger. Again, there is some evidence of uncon- formities indicating diastrophic breaks of magnitude, and it may be fair to correlate these breaks with similar ones occurring at the same time in other parts of the world. Finally, there are fossils which indicate rather closely the age of the particular stratum in which they occur. Thus, with a known sequence of rocks, broken by a few unconformities, and with an occasional stratum, the age of which is more or less definitely known, it is possible to divide the rocks into major series. The following series of sedimentary rocks, named after type localities ^rom the northeast to the southwest, have been established : 130 SCIENTIFIC SURVEY OF PORTO RICO Eio de la Plata series. Mostly tuffs with a few agglomerates, shales and limestones. Barranqnitas-Cayey series. Mostly shales with much limestone and a little tnff. Sierra de Cayey series. Mostly conglomerates with a little tuff and some shale. Guayama series. A complex series, mostly of shale, but with much chert, some tuff, limestone and conglomerate. Eio Jueyes series. A complex series, mostly limestone, but with much shale and some tuffs and conglomerates. Coamo Springs series. A heavy bedded limestone. Eio Descalabrados series. A thick series of shales with a little limestone and chert. Arecibo formation. Mostly limestone. Santa Isabel series. Mostly alluvia. And the following intrusive series : Early intrusives. Batholithic rocks. Later intrusives. In many cases the above divisions are areal and descriptive rather than distinctly stratigraphical. Eio de la Plata Series The rocks comprising the Eio de la Plata series are named from the river whose headwaters flow across them for many miles. The series is composed chiefly of tuffs with a few agglomerates and several lenses of limestone. The tuffs weather rapidly and are easily eroded. Into them the Eio de la Plata has eroded a great lowland (Fig. 3). The formations composing this group form a broad belt which occupies the northeastern portion of the district, and structurally forms the flat crest of an anticline. The strike of the rocks is generally ¥. 40° W. The northern limit of this series is presumed to lie outside of the Coamo-Guayama district. ISTo definite line can be indicated at its upper contact because the variety of rocks characteristic of the group changes by an insensible graduation into those classed as the Barranquitas-Cayey series. IJOIHU'I, atHUAiC.Y OF Tin: ('(fAMJ'huVAYAMA BIHTRICT j ;|1 T'.illV iiwki! Up ilio greater pari of this group uiid mv, a^ a rrrlc, iiii Mnitilicd. They vary frojii ereatri yeUow i<» rt;(l, a,re Iretpieiitly mottled ;uiil weather ciiKily tu a sticky ehiy soil Tlie ^truetiiral detaik of sojir nrciirj'iiig at K. n5 m tJie :Militiiry litiail, west .if Uayey, are shown ii i(-ure d. MieroPcM.pit^aHy they are ehicHy lithic tiili>. The i'rHgiiioii^ iire eiiher au.gitc HiulesiteH oj* aiigitc-oJiviiic ainlesites. I'hc aiigite noi-ur !"'h'i\< of iiuigiictiti; grains, Aiigiie, :i< ph,eiUK-r\\<t>, coiii|irises ijlidut !<' r e.-nt ef ihm^ tirlfs. The rohj>pars, whieli are oC the uligeehi^c-ajirlesiiie 'icty. arc frequently enibayed by the groiUHhiuiKS and largely altererl hj '"rile ami ealejt<; and enirrpri^c nhmii M) per eent of these tultn. A hw '•oie i-rj^stalH were present, but arc now iaitireJy altcriHl to >crj>e«tiiie, '^•'iii'iite, liuioiiile and iddingsite. 'Hie gronndiuass is eonipesetl en- •!v of alteraticMi prodiiets, such as raleite and^ linionite and a tittle ^iie. 'Vhe Invk of bedding and the angiihir eharaeter of tlie fragnienls ^'''iile tliat these tnffs aeenniuhited directly fre.m vulcanic «Tiipti<ei '-"lit anv nreeoss of reworking. 132 SCIENTIFIC SURVEY OF PORTO RICO LIMESTONES Two beds of limestones have been observed. One begins just south of Cidra and outcrops every two or three miles till Just south of Comerio. It is a dense, light gray, fine-grained rock. The microscope proves it to be composed of numerous flat fragments of pelecypods and gastropods. The shells are triturated and oriented, showing that they have been wave- rolled and finally deposited in a bedded condition. Among the forami- niferal forms observed were : Textularia gibbosa, d^Orbigny. Textularia conica, d^Orbigny. Textularia tiochus, d'Orbigny. Plecanium {Textularia) speyeri (Eeuss) ? Operculine sp. Orbitoides papyracea. Orbitoides faujastii, Sowerby. The shells make up about 60 per cent of the rock and the matrix is com- posed of crystalline calcite, recrystallized from the shell fragments and their detritus. A second small lens of limestone, of essentially the same character, occurs about three miles due south of Cidra. AGGLOMERATES Thin beds of rock, composed of angular or rounded andesite or of augite andesite prophyry boulders varying in diameter between 1 to 6 inches, appear frequently as lenses in the tuffs. Where the boulders are angular they are undoubtedly agglomerates and where rounded they ex- hibit the effects of transportation, probably in mud flows. The matrix of both types is always tuffaceous. Barranquitas-Cayey Series The rocks of the Barranquitas-Cayey series are respectively named from the two villages situated upon them, in the northeast corner and in the eastern margin of the Coamo-Guayama district. The series extends from the north central part of this area southeastward to the east central border and forms a belt averaging about three and one-half miles across the outcrop. This group consists chiefly of shales and shaly limestones with numerous limestone lenses and an occasional bed of tuff and con- glomerates. The anticlinal structure of the Eio de la Plata series changes in its southern margin to an equally flat syncline. This syncline continues southward and gives the structure to the Barranquitas-Cay v series. The dip of the southern limb of the syncline increases in steei^- IKiDdE. (}E(H.(K!y OF THE eoAMO-dVAYAMA DI^TIUVT ];',;] iH's^ to tbe scHithwanl until, near tho upper roiitiict with Uu' Hierra de I'avey series, the dip amoiuits to 30" io 10" K. Tlie strike of "X. 40^ W. Ill ilie western part chaii.iris.s tu almost due east, in tlie ea^^t-eeid-.ral part. Subordinate to the inajoj- sfiaietirre^: are several faint aiitielinal flexures, espeeiallj in tdie northwest near Conierio. Thf thickness is between 2000 and 3000 feet The lower part gTadi>s into the Ilio ck> la Plata siuuen ariil thi' npper portion fornes a sharp (Hiniaci with the Sierra de I'^oyey series. The sJniles gra,de on the one hand into niassivi- tuir.< and on tlie other uto pore limestones. Tlie nnijority are eak-areons and tlie elnin^i-o frnin -ilit-eons, as, tor instane<', several of tlic .m'liy nu?ud)ers whii-h extend, nninlnrruptedly from I^H,rranquilas tn Aibonito. TJu- color va.ries from n-ht gray and ^^reen to a, very dark nmy, almost blaok. ^Jdie beddiui( i^vcnyuvs two to tliree iiiehes. ddir; rrx-ks break vciw easily into I'bonibs :^l>out live iiu-b(^s long. The silieotuis varieties nntb-r the jnieroseopo •r>.'ve to })e very diMise, sln^aky shaky eontaininu about 20 per cent of ^'i'ly splierienl area.s, a;veraaing; 2 mm. in rliam(>ter; a few of these are "ili'b with silt and most of tboni are empty, bul in one was found a 'liolariaii bcluipffin^i- lo the gr-nus LiOincarn pi\ so that it is reasonable to 1-pose that tbev iiro all dni> to b.rauiiuircra wbieh Inue been dissolve! 134 SCIENTIFIC SURVEY OF PORTO RICO out (Fig. 5). The ehaleedoiiie cement of the rock may have been de- rived by recrystallization of the siliceous Lithocampe shells. Some of these shales are intensively mineralized by pyrite which has come in along bedding planes and has metasomatically replaced the shales. LIMESTONES Beds of limestones occnr frequently among the shales^ and of these three are of special importance. A series of interbedded lenses occur near the base of the series and outcrop frequently along the road from Cayey to Aibonito. They are black or dark gray in color and very dense, and when examined microscopically are found to consist of innumerable round bodies about 3 mm. in diameter, which are varieties of radiolarians and diatoms. The diatoms are: Paralia (Melosira) sulcata, Ehr. Conscenodiscus elegans, Grev ? And the radiolarians are : Porodiscus concentrica (Flustrella concentrica, Ehr.) (Fig. 41). LitJiocampe sp. (?) (Fig. 5). The tiny shell bodies are all stratified and about 30 per cent of them have been recrystallized to form an interlocking crystalline of cement calcite. Occurring in streaks and parallel with the bedding in these rocks is about 5 per cent of carbonaceous matter. There are also many angular crystals of augite and feldspar which are altered to calcite. It is evident that these limestones were ^formed either in the neritic zone or upon land by a gradual accumulation of foraminiferal, diatom, and radiolarian shells which had drifted landward and that, as they accumu- lated, wind-borne pyroclastic fragments were dropped among the shells and so added to the limestone. These limestones are, therefore, recrystal- lized tuffaceous shell limestones. Another limestone occurs near the middle of the formation tw^o and one-half miles south of Cayey. This limestone varies from a dirty gray to a dull purple in color, and the microscope shows it to be composed of shell fragments. The fragments are bounded by curves which are not in harmony with the organic structure and which indicate that the com- ponents have been rounded by erosion. This wear was developed under oxidizing conditions, as is shown by the heavy hematitic stain bordering each fragment and the entire absence of this stain in the interstitial cement which is pure calcite of secondary origin. The rock also contains some introduced sulphides. Apparently shells were blown from the strand ijitiuyi-:, oi-:oij,)(iY of the ho \M()^«iU wama ihhthict 135 ,r,l:,iul ami iiccuimiJiitod in lieds. Kn roule they w<to worn ami uxidizcnl III. I 8al)?cHHiei)th- th«;y were coiiu'ntctl l»y cjilrite, iiilp:>diice(1 |)rol»al>ly by ihe auT'iicy of niin wat.crs ( Fi^'. ti). Anotlier liwivy belf of llmi^<tolHS, "The ^luuiitrtin liiiie<toiies" of II. T. Hill, ueciira eloso t(. tlio upper eoiitrtci ul' ihis grrmp a.iul cxiuiids in a l„rii. fjwu about one mile we^t to ^^ix miles enM of llir> ¥ilitary Iload ft ;.sstimaiod to be a thoijsan<l re.-ttbi.-k. '['be relatiuii^liip between the sbalen ami the liiuestojies k very iriti- pnilr. Tbe furnior aci-urniilaied as estunrine, delta, or luocl-tlat .leposit«. =«s:"" -^- f'ifjctlirr to fin-m one of the timef<t<mes of latter ciiher just iM-yoiid tbein in tlie ucritic z;on,e or upon them who '\l> were windnlrifted inland a nliori dislanee. Tiie ^linles eoiitn: !iv niiiriiie oru'anisnis and, the liinestoiies much foia-eHtrial niaterial. e western two-lliirds cd' the ,t*.arran(piila.s-i'ayey scric*^ is chiefly % hat in, tbe casteru portion a tidTaeeeus phase develops nniil, at .iH,erri border ami bey«aid, the series is eonrpoSfd n! tufTs with a, ,u;y:lonierares. A sinrilar grfrdatitm exists Itctween ttie tuffs and liine- > or shal(»s. Sona- are very fine .u'rairanl and woidd be called shales '• field but for an oeeasional _<jTain indicating a pyroclast.ic origin. 'afTs varv from tin* t:exl4ire of ashes to volcanic bri'ccias and coaj'^e 136 SCJESTIFW SURVEY OP PORTO RICO agirloiin; rates. The color i.-^ black, dark gray or olive green; where ihu grained, and, as the te^xture becomes coarser, tJie (H)b>r becomes mottle! hecauKi' of the fragiiients, wliieh may be black, gray, green, dark piirpU' •:.! dull red. .Many of the lava flows contain tulTaecous nu-itcrial which iiiii\ ainA.ujif. to as much as dO per cent, laidcr tht* niicrostiopc thx' tuffs prev.-. to-be divisible into two classes, eryslal tuffs ajid lithic tuffs. The crysini tulTs (Fig. 12) have the. composition of an augitc andesite j)orpliyry suiil eontain, taking tlic average, about 20 per cent augiie, 4t) per cent andcsin. ftddspar, 20 per c;eni magn,etite and 20 per cent of secondary prodiieis, Homctijnes tiiey show a faintly streaked, diaraerer, as it they wi-re <!• ]'»ositcd, un.dej- some sedimcntajT eontrol. Tlw augite is fresh, fractniv^ ami pale green. The bddsf)ars are Inghly altrTcd, usually to i;hl(»rite ai some epidoie, wbieh Inive, l>y tlieir dr\elopmeiit, created the binding ni: lerial of the roek. At times there arc small |)Hen(h)vesieular spaces lid' with zeolites, proba.bly natrulite. Tfiese spa<'cs arc about 1 mm. in s^ and are pruhablv produced by inhltration ami metasomatie rcplac(;jni-' by zeolites. In eomposiiion, the litlde tulfs (Fig. Kt) arc rather siujplc. They n osiwlly conrpo-ed of rehlspathic andesitcs or augite an.(lesiles. The 1-. tnral tvpes. however, are numerous and as many as fiftet^n may l)0 jnv-' HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 137 in one thin section. They vary from glasses to felsites of many textures and structures. PIIYSIOGRAPIIIC EFFECT The shales of the Barranquitas-Cayey are more resistant to weathering tlian the Eio cle la Plata group. The spurs which extend out from the Sierra de Cayey are carved in them. Other hills occur in the lowland, such as the Cerro Gardo and Cerro Eabanal; their occurrence is due to tlie presence of more resistant shales. The topography thus produced is illustrated in figure 3. In this picture the greater part of the foreground, extending beyond the village of x\ibonito, is occupied by the Barranquitas- Cavey group and beyond the first crest is the lowland cut into the Barran- quitas-Cayey series. GENESIS The composition of the formations and their succession gives some clew as to their origin. Apparently at the time that the Barranquitas-Cayey series was being formed a volcanic center existed to the northeast and fur- nished the larger part of the material of the strata. In its immediate vi- cinity the sediments were mostly pyroclastic in origin. Farther away the volcanic ejecta were reworked and deposited close to the shore of the Carib- bean Sea. The offshore zone was almost outside of volcanic influences and in it strata of biogenic origin with slight addition of pyroclastic material were chiefly accumulated. Compared with the Rio de la Plata series, which was formed during a period of intense vulcanism, in which there were a few quiescent intervals sufflcient for a slight invasion of the sea, tlie Barranquitas-Cayey series is formed largely of reworked ma- terial indicating distant or less violent vulcanism. Sierra de Cayey Series distribution The rocks of this series are mostly conglomerates, with a few tuffs, and 'ire found cropping out along the crest of the Sierra de Cayey Mountains, Hlter which they are named. The road from Barranquitas to Barros, 'H\\()nd K. 16, follows the lower contact of the Sierra de Cayey series with the underlying Barranquitas-Ca3'ey series. The bends of this road, which ^^viug northward, are upon the shales of the Barranquitas-Cayey series; tliose which swing southward expose the conglomerates of the Sierra de ^ <ivey series. The Military Road between K. 85 and K. 88 crosses the Sierra de Cayey Range througli a wind gap carved in this conglomerate. 138 SCIENTIFIC SURVEY OF PORTO RICO The conglomerate is well shown north of La Lapa, where the Rio Cayev cuts a narrow gorge^, and again where the Jajome cuts through it west of Jajome bajo. The conglomerate phase is not dominant in all parts ol this series. In the eastern portion tuff occurs instead of the conglomerate and for this reason the Cayey-Ouayama Road between K. 15 and K. 21 shows comparatively few exposures of conglomerates. This transition from conglomerates to tuff is excellently shown along the trail, wliicli leads from K. 21 westward to Barrio del Carmen. STRUCTURE The synclinal structure found in the Barranquitas-Cayey series changes to anticlinal in the area of the rocks of the Sierra de Cayey series. Thus, O o ^o° o o ^-o?42-q. Fig. 8.- -Character of the unconformity hetiveen the Barranquitas-Cayey and the Sierm de Cayey series a fiat anticline exists between kilometer 15 and kilometer 24.5. Going westward the strata begin to dip uniformly southward at angles between 30° and 40° and the strike varies between IST. 40° to 60° W. A break of importance occurs at the base of this series^ as indicated by the great conglomerate beds in the Sierra de Cayey group and by a distinct uncon- formity visible at two places. Thus between K. 86 and K. 87, on tlie Military Road west of Aibonito, the basal conglomerate of the Sierra de Cayey group rests upon the eroded surface of the underlying shales and contains pebbles of these shales (Fig. 8). Dips taken along this section show the shales to be gently folded and the conglomerate to lie upon tlie truncated portion of a faint anticlinal fold. Again, just west of where the Rio de Cuyon changes from a southward-flowing to an eastward-flo^v- ing stream, the heavy conglomerate lies upon a truncated portion of an anticlinal fold in the underlying shales. The upper contact is gradational with the overlying series. The formation is estimated to be between 3OO0 and 4000 feet. HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 139 COMPOSITION AND CORRELATION The series is composed almost entirely of conglomerate with a few assorted beds of tuff. The basal portion lies nnconformabl}^ upon the older formations, as shown by evidences both of erosion and folding before the conglomerate beds were deposited ; and the upper portion changes by a gradual gradation into shales. Beds of shale begin to appear at infre- quent intervals in the conglomerate, and then become more frequent, until shales finally constitute the entire group. Beginning where the shale beds predominate, another series, called the Guayama, is established. CONGLOMERATE The pebbles and boulders vary greatly in size, from a tenth of an inch up to seven feet. The largest boulders observed were exposed in the gorge of the Cuyon, and at that place boulders a foot and larger were very com- mon. Most of the pebbles and boulders are very well rounded. The tor- rential bedding of the finer materials and the orientation of the rounded boulders indicate stream control at the time of deposition. The pebbles are mostly andesites or porphyritic augite andesites. The dikes and sills which cut through this conglomerate are of a coarser nature than the pebbles and some of them are of quite different composition. The matrix for the pebbles is of two types. One type is water-sorted sandy material derived from tuffs or massive igneous rocks. Under the microscope this matrix proves to be composed of angular, sorted, and laminated grains chiefly of feldspar. These are bound together by sec- ondary products in the following proportions : Per cent Carbonaceous matter 5 Calcite 20 Chlorite 20 In part, however, the matrix is igneous, as in the case of a locality east of Barros where there are many conglomerate beds, some of which have an undoubted igneous matrix, and again southwest of Aibonito, especially in K. 86, where the conglomerates have an igneous matrix, and still again at the Cuyon gorge and westward along the Cuyon. The best exhibition of the igneous matrix is seen along the Eio de Cuyon, where for more than a mile the stream bed is formed by the water-polished surface of a <'onglomerate with a green andesitic cementing material. Structural fea- tures, such as bedding of .the conglomerate, were not disturbed by the invasion of the igneous matrix, which must have entered the conglomerate in the form of an extremely fluid magma (Fig. 9). 8Cn-]NTJFI0 8URVKY OF I'OBTO RICO ^K'^.'I'^-S] ^^^^lUnujhunr.rale n-lth «tt (V/ik'oh.s mnf**. ;ra|»liMl by rrofcswi.r rhaii.'^ I*. r..>rk.-i EODGE, GEOLOGY OF THE COAMO-GVAYAMA DISTRICT 141 Study of the microscopic character of the igiieons matrix shows it to be a massive rock of hiatal fabric. The oligoclase phenocrysts are turbid and lath-shaped, averaging from 8 mm. to 14 mm. and comprise over 40 per cent of the rock. Tabular, almost eqnant, pale green, and exceedingly fresh augites frequently occur as chadocrysts in the feldspar ; and magne- tite occurs in skeletal grains. The minerals of the groundmass are all less than .01 mm. in size and frequently they are all altered to chlorite; when not replaced they are oligoclase and augite. Dr. Charles P. Berkey (1915, p. 23), speaking of these conglomerates as seen by him on the Military Eoad, southwest of Aibonito, says : There must be a total thickness of strata, including shales and interbedded tuffs with occasional small limestones, of perhaps several thousand feet. In all parts of the formation where conglomerate is developed, the pebbles repre- sent the same kind of rocks as were encountered in the tuffs and intrusive masses. Actual representatives of previously solidified bedded material or indurated ash and shales are very rare, but in one case at least a pebble was observed that was judged to represent a fragment from an older silicified tuff. As a matter of fact, the materials are practically all of simple igneous char- acter and the matrix in most parts of the formation is very abundant, or even predominant, the particles of which are of the same igneous material. The distribution of material and the range of composition leads one to believe that this conglomerate represents a special state or condition whereby materials of essentially tuffaceous origin were, immediately after their volcanic eruption, worn, rounded, somewhat assorted and bedded and mixed with related ma- terial. The fact that the conglomerate beds, which follow to great thickness, are prevailingly of simple structural habit, as compared with the calcareous shales, ash and tuff series immediately below, suggests that there may be a break here of larger consequence than is observed in other parts of the pre- Tertiary or older series. The development of so extensive a series of conglomerates doubtless does represent a considerable change in physical conditions, compared to those con- trolling the simpler deposits which preceded and followed them. PHYSIOGRAPHIC EFFECTS The conglomerates of the Sierra de Cayey play an important part in the formation of the Sierra de Cayey Mountains. Where the conglom- erate beds are most numerous the mountains are higher and in straighter lines, and where the conglomerates are replaced by tuffs the mountains become less definite and more scattered as, for instance, in the eastern [portions of this district. The Sierra de Cayey Eange may be due to sev- eral causes, such as the erosion back of a tilted fault block, but its present H'Hef is no doubt due to the resistance of the conglomerate. Streams ^v'hich cut across the series do so at right angles and with narrow canons 142 SCIENTIFIC SURVEY OF PORTO RICO such as those of the Cuyon and Jajome. The roads which pass over the mountains choose abandoned caiions or wind-gaps. GENESIS Because of the presence of heavy beds of rounded boulders, the origin of the Sierra de Cayey series is thought to be due to the rapid erosion of uplifted mountains and to the deposition upon their flanks of great allu- vial fans. The materials of this group were not necessarily derived from the beds immediately below, because the strata beneath are not deeply eroded. Their site of deposition may have been a slope of small grade and they may have been deposited after transportation from loci of ero- sion a great many miles to the northward. While they were being accu- mulated it is possible that volcanoes were active a short distance to the east and threw out great quantities of tuffaceous rock. Under such con- ditions the strata on the east would have been predominately tuffaceous, and this view is supported by the fact that the tuffs, in the earlier portion of the series^ contain a great number of associated volcanic flows. The boulders found in the Sierra de Cayey series were, no doubt, formed in streams with gradients no steeper than those of the streams in Porto Eico at the present time. In the bed of the Salinas, for example, large boulders are to be found today. The fact that these boulders are found just north of Salinas, and far from their source, indicates the enormous transporting power of small streams, if favored by occasional torrents. These boulders could not have been formed by wave attack because the waves of an advancing sea will grind into silt any boulders broken off unless they happen to fall into deep water and beyond surf action. Other features which argue against a marine origin are the great petrographiv' variety, the discontinuity along the strike of the beds, the absence of marine fossils, and the absence of a marine succession characteristic of an overlap. GuAYAMA Series GENERAL STATEMENT Lying above the Sierra de Cayey series is the Guayama series, the strata of which are named after the City of Guayama, located upon them in the southeastern corner of the district. The average distance across their outcrop is about four miles. The series is the most complex of all those studied in this district, for it consists of shales, limestones, conglomerates, sandstones, tuffs, volcanic breccia, agglomerates and cherts. Of these, HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 143 shales are by far the most important. Lenses of limestones are numerous and there are distinct and separate beds of conglomerate. In the western part of the Coamo-Guayama district the strata appar- ently all dip southward between 30° to 40°. To the east a syncline is formed between K. 24.5 and Guayama. The lower contact is gradational with the Sierra de Cayey series. If an unconformity exists between the Guayama series and the Eio Jueyes above, it was not observed in the limited time devoted to this study. Moreover, it cannot be said that the rocks differ as radically from the overlying group as do the other series. The total thickness is about 2500 feet. SHALES The shales constitute about 75 per cent of the rock in the group, but their continuity is broken near the middle of the group by numerous large lenses of conglomerates. Many of the shales are limy and in places they pass over into true limestones, a condition which is particularly true near the top of the group and a short distance above the bottom of the group. In the western portion of the district shales replace both con- glomerates and limestones. In the central part they have the least devel- opment and toward Guayama they are replaced, in part, by tuffs. The physical character and mineralogical composition of these shales is varied. For the most part they show excellent closely spaced bedding. Where the bedding is best developed fracture cleavage is also shown best and causes them to break up into small two-inch rhomboid blocks. The color varies from very dark to light gray. These shales were water-sorted and depos- ited under water, as is shown by their streaked appearance, which is brought out in a decided manner upon weathered surfaces. All of the material composing these shales is volcanic and it is common to find every graduation between massive tuffs and bedded shales. Hence most of the shales should, perhaps, be called finely bedded ashes. The true character of these shales is best shown by their microstructure. The fragments range between .05 mm. and .01 mm., and occasionally larger angular fragments, of clear undecomposed oligoclase or andesine feldspar without orientation, occur which look as if they had been dropped into soft silts after having been carried by the wind. Some of the larger frag- ments are rounded. Feldspathic niaterial constitutes 60 per cent, car- bonaceous material 20 per cent, secondary chlorite and calcite, derived in part from the feldspars and in some shales in part from organic material, make up the remaining 20 per cent, and some secondary chalcedony is present at times. 1 j |, SClEXTUnC SUKVI'JY OF PORTO RICO A few of these mcka contain sphorical voids aj-ratigecl in Inics parallel to the bedding, some of wlricii contain Orbiiolites sp. (?) (Fig. 10), but whieb, for tlio raoHi |»iirt, are filled with seeondary ehlofit.e iiiid i.-aleiti'. Some of these sliales, especiallj near the top of this series, are hard and dense, duo in part to the baking influence of intrusions and. in part due to introduced silica and epidoti*, l)nt which in larger part is due to reor- ii:anization of organic siliceous remains. The cherts are ol varied ct>lors, a eundition whJeh is de-pendent 'apon tlieir genesis. For instance, tliosc due to baking or reorganization are If/ Orhll gray in color like tiu' softer shales, and ilmse whicti contain nnich iutro^. diiccd nniterial are either n. blood ]-e<l or <lull green. Under the niicro- scope ihe genesis of the rod and greon cherts show that at first th(! ordi- nary tulTaccious shale was inipregniilcd by epidote giving rise to the green chert, and later some oC tliese green cherts were fractured and specular hematite was introduced as veins which, in part (u* cntirtdy, r«placed the rock forming the red cliorls. Subse(|ucntty both green and red chert'^ have been veined with wdiite quartz. The i-eorganized chert shows traces of the original shale, which is com- posed chiefly of fehlspar with streaks of carbonaceous matter bound to- irether by secondary chlorite and calcite. Tiny spherical holes arrangef* HODGM, (JPJOLOGr OP THE VOAMO^^dVAYAlLA IHSTRICT 1/15 ill lines parallel to the liedding iir(3 niiiiicroiw. some oJ' wliicli cojitain ealcite, others elialciHloiiVj and in still otiier< the two minerals are iritcr- lTowji. Tlie occurrence of caloite and ((oa^iiz iiiforgrown indicates a con- teinporaneous deposiriori. whir-li is cheiuiiraJly po.«siblc, since both are solnldi; in an alkaline solution. Fortunately; not all of these areas are tilled with, reorganized jidnerals, but a fe^\' arc filled with siliceous radio- laria or cah:mreoiis I'oramini feral remains, .sonH> of Miiir-li show only partial reorganization. This appears tt) indicate thai ilie sdieiiieatioii is due to t)iu migration ot silica from reorsmized radiohiria. These rocks also aM ; i .;.';.':: "^n:'«-:1 rlJ}:^. -.:. ' ^" \\ ' "'. ' ; -. r.^ »., v.; -: . J '"' * -: ""-iilfciiV i : i'-t * ' f - ' " ' ' ' -"; ;\1 J;:-; :«' ' . ". - : * .>:;; • • 1- ^ '-» ^- * ^ .•...• . * - sliuw micro-desiccation fractnri'S, smin: of which are (^-emented by migrated chalcedony (Fig. 11). Although the in lis oecui- largtdy in llie easii-rn fxposuo> of ibis grutip. 3'et thin beds freifiiently occur among the shales, limestones, and con- k'loracrate formations. This is to be expected in view of the fact that the >hah,'s are only reworked idiases of a iniT series. 'Jdie prc^sence of tlie tuU's 'Vilh the limestones indicates tht> pmnodie recurrence of intense volcanic iictivities. Purely pyroelastic types cannot he sliarply divided into crystal ^'H(l litJiic tuffs, because tlie two types usually occur togetlier. Litbie ''liaracter, liowc'ver, is tlie must. commi>ii in wiiicli the rock fragments are :'ll andcsitie and extreme! v angular and show as many as ten textural 14(5 HCIHSTIFIC HVIIVHY OF PdHro HIVO Viirit'ties <»t' rock in a single tiiiji scetioii. I'lic fragiiU'iitH! are hound Uy gethiT l)y st'cdiidttry clilorito, hoinatiie iiiul calcifce. The production of Hccondai'v oaltite in luany of iIicko clastic foek.-? has proc'CHidi'd to siieli an extent tJuit tiioy arc cuuioioiily iiiistakoji in i\\v firdd for liniestoiies. Tho orvHtal tuffs arc composed of about 40 per cent of liighly altered plagioelase fragments, ureas of palagonitc frequently auioujiiing to 5 per <-e!it, and 55 pcT cent of secondary caleite (Fig. j3). The tulTB weather down to a red idaycry s^oil fr<»m which all the caleite has been leaeliecl and^ in wliit^i j,uj hedded strueture is apparcsnt. Spheroidal exfoliaii<ni is rather charaeteriKtic, not only of the tutTs in tliis group, luit of all tutfo in the island. Tlu* Hoodoo strut:tu.re. which frecpu'utly oexairs, resuK-s from f.he sticky, eornpaet natu,re of tlu> tutf soil and from the prtisence of small ro(;k rragnanits, whicli serve as protct^'ting caps to the soil below tluvni arnl prevent tla^ vertically falling tropical rains from eroding away the underlying tuff soil. The result is a multit.ude of piniuieles ttiree to six inches high covering the whole face of aji exposed tuff i'orniatiou. The formation, of the Hoodoo structure is doe in part to th(^ same Jactor which develops tJic knife-edge ridges- that is. to the compact, sticky. dilTKniltly erotied nature of the soil. N'ot all of the tuffs are nnissive; some are faintly hedded. 'L'lu"' latter are in smrn! types composed of rathr-r c;oarse, 7 to 10 mm. ecpiigraniilar., roiHided lithie fragments, which an* usually oxidized upon the peripliery. HODGE, GEOLOGY OF THE COAMO-GVAYAMA DISTRICT 14'/ Siich fragments indicate a period of water sorting and rounding, followed by a period of oxidation. The cementing material is introduced calcite. Corresponding to the lithic is a crystal type, which is composed of rounded feldspar grains with associated carbonaceous streaks and a cement of sec- ondary calcite, chlorite and quartz. Such characters indicate a water sorted and rounded rock. Other varieties of the bedded tuffs are com- posed of extremely angular grains, which are sufficiently well sorted and oriented to produce a bedded structure. The bedded structure is probably due to wind, because moving air within the limits of the size of the frag- ments which it can carry is more sensitive to differences of specific gravity than water, and, in addition, wind possesses a greater range of velocity. Moreover, a grain is always subject to resorting by the wind until it is finally dropped into a body of water. Most crystal tuff fragments are elongated; hence, when a tuff fragment falls to ground, if perchance it strikes upon one end, the fragment will fall over and lie upon a prismatic face. The accumulation of tuff grains all lying upon prismatic faces will result in orientation of the constituents and a development of ohscure bedding structure. The essential components of all the lithic tuffs are oligoclase-andesine feldspars (40 to 50 per cent) highly altered to calcite and chlorite ; also clear, fresh, fractured, equant, pale green augites about 5 per cent, magnetite 5 per cent and a few lithic fragments. The sec- ondary cement consists of calcite and chlorite 20 per cent. Still another variety is formed from tuff fragments which fell in the sea and became incorporated in the accumulating organic remains, such as foraminifera. Among the organic remains occur from 10 to 30 per cent of crystal or tuff fragments, some of which are rounded and evi- dently water-carried and others are angular and must have been wind- borne, but in all specimens a few well rounded grains testify to the ac- cumulation of the organic fragments close to the littoral zone. LIMESTONES Akin to both the tuffs and the shales are the limestone lenses, wliicli are, as a rule, only a few feet thick, although a few exceed 100 feet in thickness. Since they extend along their strike rarely more than a mile, the geologist is constantly bewildered by the abruptness with which they pinch out. Most of the limestones are found near the top of the group, in the central part of the district, which may indicate a former embay- nnent of the sea. In color they are light to dark gray and in texture fine irrained, and the thin sections show that these limestones are composed ^*f clastic organic fragments, chief of which are foraminifera. 148 SCIENTIFIC SURVEY OF PORTO RICO CONGLOMERATES The best development of a conglomerate occurs near the middle of the group in the central part of the district, though isolated conglomerate beds are found frequently in other parts of this group, as near K. 99 and K. 94 on the Military Eoad. The pebbles and boulders are usually sub- angular, though some well rounded ones are to be found. Very large boulders, three feet in diameter, were seen north of Santiago. The boul- ders are chiefly andesitic igneous rocks with rare limestone fragments. These conglomerates in part have an igneous matrix, as, for example, those near K. 99 on the Military Eoad and of some north of Santiago. AGGLOMERATES A few beds of agglomerate occur in the tuff. PHYSIOGRAPHIC EFFECT In detail these rocks are affected by erosion in quite a variable manner, but considered as a whole they are carved into great spurs which extend southwestward and southward from the main axis of the Sierra de Cayey. The Military Eoad from Coamo to Aibonito after crossing the Cuyon Eiver near K. 93 follows one of these spurs up to the Aibonito Wind Gap or Pass; the road from Guayama to Cayey follows another of these spurs to the crest of the range. In the great lower valley of the Salinas Eiver there are many ridges oriented with the structure, such as Monte Sabater, west of Coqui. South of the Guamani Canal, extending from the Salinas Valley to Guayama, is a group of hills 200 to 300 feet high, developed upon resistant strata belonging to the Guayama series. GENESIS The origin of the Guayama series is doubtless similar to that of the Sierra de Cayey. The rocks of this series may simply be the peripheral portion of the Sierra de Cayey alluvial fans. Another and more probable view is to consider the Guayama series as the reworked portions of the Sierra de Cayey fan. After the first impulse of rejuvenation in the Sierra de Cayey had spent itself, erosion of a moderate character devel- oped upon a lower grade. Eocks of varied types developed synchronously, because many agencies were at work. While rivers were building up a conglomerate inland and delta deposits seaward, volcanoes were active, spreading far and wide pyroclastic material, and here and there lava was flowing down a mountain side. Eepeated periods of intense volcanisn) HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 149 followed by long periods of gentle volcanic activity developed heavy beds of tuff and agglomerate and added to all the other formations some ejected volcanic material. During the dry season tuffs, perhaps gathered in the stream beds, cut into the various alluvial fans in sufficient quanti- ties to divert the streams from their original beds in the times of flood. These periodic diversions would have started great mud-flows down the mountain sides, causing pebbles and boulders to be swept along with the mud-flows, all becoming more or less rounded en route. Periodic encroachments of the sea gave rise to marine lenses. To do this a very slight subsidence would have been sufficient because (1) the delta phase of the piedmont plain would not have extended much beyond the shore zone and (2) in tropical regions, as shown in Porto Eico at the present time, limestones are laid down not only in the neritic zone, but commonly in the zone between high and low tides, and also some are formed beyond the reach of the sea, such as deposits of tepetate. Eio JuEYES Series GENERAL CHARACTER The Rio Jueyes series is named from the river which flows over their central exposures. The series extends in a broad belt with an outcrop 4 to 5 miles broad and reaches from the west central part of this area southeastward, passing beneath the sea south of Guayama. Its one out- standing difference from the Guayama series is the number of limestones contained within it. The limestones, however, do not exceed in volume or number the shales, tuffs, or conglomerate beds. Lenses of agglomer- ates and volcanic breccias are numerous. STRUCTURE The rocks of this series differ from adjacent series sufficiently to make them worthy of a separate grouping. Their upper contact is sharp and clean. In fact, in many places the point of the geological pick may be placed where the Eio Jueyes series ceases and the Coamo Springs lime- stone series begins. The beds generally dip S. 30°. The total thickness may be as much as 5000 feet. LIMESTOlSrES Two types of limestones occur, the white algal limestones and the dark gray crystalline, fine grained, highly bedded limestones. Limestones begin to occur in the lowest formations and in Santa ^^abater Hill reach their best development. This is a long, narrow, lionw 150 SCIENTIFIC SURVEY OF PORTO RICO cliiial hill north of Central Aguirre^ which rises as a mendip, or morro. through the thin coastal plain deposit. The limestone beds composing this hill are in themselves variable ; some have a cream tinge, are porous, and have an evident organic structure ; other beds are highly crystalline, fractured and cemented by white calcite. Interbedded with both lime- stones are thin beds of tuifs. Both develop a pitted and cavernous sur- face upon weathering. It is remarkable how this limestone, soft and porous as it is, stands up as a resistant ridge. Everywhere in Porto Eico the limestones are resistant to weathering. Some form great ridges like the Coamo Springs limestone described below, others remain only as gentle swells, but everywhere they rise above the shale or tuff beds. In thin section the white limestones are seen to be made up largely of honey yellow or colorless crystalline calcite. Rounded outlines, but with- out identifiable structures, indicate the former presence of organic sub- stances such as algae, which are present in tiny fragments, and foraminif- era, such as Glohigerina cretacea, Glohigerina hulloides and Truncatn- lina lobatala, all of which indicate a very early Tertiary age. Other limestones are fine grained, showing light and dark shades of gray and buff, and occur as thin, discontinuous lenses. Some may be 100 feet thick, but all those whose thickness could be determined are only a few feet thick. Most of the lenses are found in the eastern half of that portion of this group which rises above the recent coastal plain deposits. One rather continuous series of lenses, traced from southwest of Guayama across the district, occurs south of La Lapa, south of Santiago on the Rio Jueyes, south of Coamo on the Coamo River and again on the Des- calabrados River. The lenses always occur among the shales into which they grade. Megoscopic volcanic material is present in many of them. Under the microscope they are seen to vary from a tuffaceous limestone to a tuff with many shells. The organic fragments are comminuted pelecypod and gastropod shells with numerous foraminiferal remains, and diatoms, such as Miliolina seminulum Linne? The foraminifera ob- served are : Orhulina universa? Orhitoides papi'acea (Boubee). Textularia gihhosa, d'Orbigny. Glohigerina, d'Orbigny. Numnlites sp. ? Gaudryina sp. ? Polyrn.orphina sp. ? Miliolina sp. ? BilocuUna sp. ? IIOIMU-:, aKOIJMSY OF I'HH VO:\M()-(U: AY A M A IH STRICT 151 'nu' .rotliolariaiis art' cliictiy I'ot'i'xliscm coiiecfitricvs. The ort:;rtiiit> re- imim Ii}iv(3 all. been iriorc or Icsh riMTystallizcd and no (•rtlcar<3uu.s algal siriifiiire was noted. The tnJTaeouii>; iMaterial is n.sn.ally very liigbly al- rrrvd feldspar latliH with oceasional lithif fraginents. The feldspars and 111 hie rra.<£mont,« alter te caleite and chlorite. 'he shale<. Wkv th<'- linustoneH. are h.est dev<du]>ed jiisi west of tlie inas Kivtn-, when- liioy are Jinely hedded, siliecoiis irnd occur -.m IVe- lit lavers iiinon<.^ the argil lac<*ons phases. The c.der varies from but! in.^^^^Litliir hitf ' :iray. In lliin section fhcy are sccji to he nuide up of a streaked dense eitcrial, chiefly quartz and some earhonaceoiis material. Little spherical cli-s, which are arranged In lines parallel to the bedding, make up ahmit pi-r cent of tlie rock. Most of the tiny cavities are vacant, a few have a iii'in- filling and an oceasicjnal one has a diatom or radiolariaii in it, (•'li as MelfMpinf. artntariii, ilah.. ajul fUm:f.f)()(lkem iNHrgmarus, and ii'clar types. 'rUFFS i''^"ls of tuffs arc ext'cedingly commem. ]k'<ls of limestone are often '^-^'ply divi<le<l by a tldn hiyer of tutl*; iiMKst of the conglomerates have 'ilfactMuis matrix and some of llie shales grade into tuffs. The pyro- 152 SCIENTIFIC SURVEY OF PORTO RIOO clastic formations are best developed in the western portion of the group. As in the older group, the tuffs may be divided into the lithic and the crystal groups. The lithic tuffs are frequently very beautiful rocks, since they are composed of angular fine-grained rock fragments of extremely varied colors, such as black, red, green, brown and purple. An excellent exposure of this type is found in the stream bed east of Coamo. The rock types present in the lithic tuffs are always andesites and as many as ten different varieties based upon texture may be present in one thin section. The fragments are cemented by calcite and chlorite and some are well bedded (Fig. 13). The crystal tuffs do not differ from those described in the older series. They consist of lath-shaped altered oligoclase andesine feldspars, clear, fresh, pale green, embayed and fractured augites and grains of magne- tite, all cemented by secondary calcite, chlorite, limonite and some zoisite. These secondary products are all derived from the feldspar and a former volcanic dust matrix. CON^GLOMEBATES AND AGGLOMERATES In the Eio Jueyes series frequent beds of conglomerate and agglom- erate occur. The beds are never thick nor continuous along the strike and as a rule they are formed of well rounded pebbles and boulders of andesite and andesite porphyry. An interesting formation occurs about two miles north of the Eio Jueyes Water Gap, which consists of numer- ous well rounded three-inch pebbles of limestone, dark gray in color and highly fractured, with the fractures cemented by calcite. Among the well rounded pebbles also occur frequent three to six inch angular agglom- erates, all in a tuffaceous matrix. Beds of this conglomerate six feet thick alternate with two-foot beds of shale. Two factors alternated to produce this formation: near-by volcanic eruptions, which gave rise to the tuffs and agglomerates, and stream erosion, which produced the conglomerates. The great falls of volcanic debris, probably diverted to streams first from one side of the alluvial slope down which they were flowing, and then to the other, and in this way the alternatiiig beds of shale, conglomerate and agglomerate were produced. PHYSIOGRi PHIC EFFECT In part the Eio Jueyes series j>artakes in the formation of the spurs extending from the south flank of the Sierra de Cayey, and in part assists in the formation of an independent landmass south of the Eio Cuyon and west of Salinas. A series of great subsequent valleys is carved out of il^c HODGE, GEOLOGY OF THE GOAMO-GUAYAMA DISTRICT 153 southern half of the western third of the Rio Jueyes rocks. The eastern half of the series lies beneath the Santa Isabel coastal plain deposits and, therefore, exposures are to be found at only infrequent intervals. GENESIS From the facts stated above it appears that the conditions of accumu- lation were much the same as in the previous period. Mountains com- posed of older strata were being dissected and out of them alluvial fans of deposition were formed. Those portions of the fans which dip into the Caribbean Sea gave rise to shales, while conglomerates were formed in the ever-shifting stream beds. Between the projecting prongs of the fan the embayed sea was the site of calcareous deposition, and so lime- stones accumulated at stratigraphic horizons equivalent to the shales. Volcanic vents, of which a large number have been found in this district, gave forth moderate amounts of tufiE continuously and at times excep- tionally large quantities of both tuff and agglomerate. The result was that a series made up of alluvial fans alternating with local lenses of marine limestone was formed. CoAMO Springs Limestone Series This series is named from a thermal spring located in the Coamo River Water Gap. It extends as a homoclinal mountain from Central Aguirre northwestward to the Descalabrados River and thence to the Jacaguas Reservoir. The Central Aguirre Hills are thought to be part of the same formation, a conclusion based upon the close similarity of the rocks and the alignment of these hills with the general trend of the Coamo Springs Range. If these hills are formed from the Coamo Springs limestone, then this formation continues eastward beneath the Caribbean Sea. The total length is at least thirty miles and the thickness about 800 feet. This group is composed essentially of a single formation, a heavy bedded, steeply dipping limestone, which forms one of the most interesting petrographic types and erosional features in Porto Rico. In addition to limestone, however, there are many thin beds of tuff within the lime- stone, into which the limestone grades at its top and bottom. Near the top and bottom the tuff beds get thicker and more numerous until the limestone ceases entirely. STRUCTURE The limestones and associated tuff beds of the Coamo Springs group J^ave been tilted to rather high angles and in places faulted. In the 1 rr-1 HCIKSTlFiV mJUVEY OF I'OJl'I'O RIOO Aixnlnv Jlills the ilip is 4^,r B., at Salinas ilill 4^' S. miil al Jiuiyi-: Water i'iap tluM'e is a<-han<^\' in dip from tlic top, wlierc it is 10" ,S., tu tli; bottom, wiw.re il is -l.">" S., which iiidi<iitoH tlie soutti Miiib of an aiilicljii" West of the Coamo River the dip vnrios bctweeii Xr H. an. I 50^' S. aa- in the vii-initv ul* tin; Rio Doscalahnnhv^ antii-liiial .structure is well show, (FiK- 11). The AW'l oviflenci' of anticlinal structure to the east, aii' w.'si of the ( oaiiio River Wal«'r (hip su.ii;a-ests thai the structure at tli a-an is also antielinal. even thouuii it is not evident there Iwearise of th nui.ssivc eliaracter of the rocks. An antieliruil structure at this po wouh! explaJu the great breadth ot cAposuri: (rf tlie limestone. The whole range is ai'rano-ed m erhe.hn. The ecHitiunalion on t westwartl side of the Eio de Coamo is almost a mile soiddi of a line c< tiimed Ircmi the ('Jeri'o I^imontol wxistward,. and in like manner tlie C'c' C'ia,via. is tu-ukcn in tw<» phiee.'!. These are features exj)laiiiahh; only faultin;«r. Tlu' contacts of tlie t'oamo Springs limestone series are ! Ijest inarked of auv roeks in. th.e district. IIODG-E, aEOLOaV OF THE V0AM04S-UAYA MA IH^^TIflCT ] fi The appearaiico of the IJiiif.stoiic which forms this series is viiriahlo, •:vi ecri.aiii fe}itur{3s aiv. s() ccxiistiint tliat the rock may l)o I'lu^'Ogiiized nt a ii^laiute and can be eimly (listingiiisliod froiu all other hriiestoiioF oi-eiirriiig III }*ort() Bico. The c«iIor varies from oliaJk' white to ereaiii or to pale lilac, 'rhc grain is verv Jiiie in most oe(;urrejii;es, but iii part it is medium, .Hid the size of the grain increases to (he westward. Some of the rock is made up w-]iolly of crystalline liniestruu:', but bu- the uiost part it. is mottled w-ith organic aud lufT I'raguicnts wdiicli are brown, soft and uiiiy ii-y in size frctui 1 to ;5 nim. arul are creani-colorfd, Tiifir iM'gauic elmr- •tfT is a.ppareut even in hand specimens, as shown hv concentric zones whicli a, fine mesh structure can be seen. Oilier varieties have a line- iinieil iilac-eohu-ed material stained wuth hematite, binning a matrix >r the organic bodies. The ihalli of the a1ga> of wlih-b this rock is ryc'ly eomposcrl are sliown in figures 15. li'j arid 18. Idle uiicroscopic character of this rock is most interesting. It is bmn-l • consist of at h'ast three varieties of alga-like strueture. Borne of i'"«e arc irndoubiedly of the red ealcare<ous alga:; type. Figure lb shows ''*• b>rm that lias a fucoi<l appeararu-e, wdiich is apparcrdly the spore H' of a thallus in longitudinal section, and 15 a tballus in cross-section. addition to the alga> structure, fcvraminiftu'a of several speeii's arc 156 ^VltJSriFIV SURVEY OF PORTO RICO |>n\<t!iit:. sujiie of which ai'u Ani-plvisleyinu sp. (.Fig. 19), Discorbina ghhukirk, <ri)j-|)iguv. Orhitoide.H sp., dhbitjerina sp. (Fig. 43) ; ostrac<)i«l> jilso occur, all ol whicii testify to a Tci'tiaij age. The interstitial siiJ)^ stance is made up of intcrloeking ciilcitc grains averaging .05 inni. in iljaiuoter, w]ii{;1i have evidently heen derived from the organic bedie^ (especially the alga;) by reiTj-stallizaiioii. Tliis is shown hj the transition contact between tlie two and by the ph,adowy oiitlitics of thalli in crystal liy^cd ealcite. Tlie orgaiiii: structures have heen highly fraetuj-cd, broker and cemented by limtaiiie before the iodiiratioii of the roek took place. This snggests dry-land conditions. The tnltaceoos matter consists of friictiii'cd crystals of the «digocerie-andesine varieties. Chlorite areas arr- Fig. 16. Lithuthumnmn thnllu. ^IH'ingn limestone also present, snggesiing foimor pyroi green glaneouitie-Jike snbstanee occurs. The tiiirs have no featii .ifl'eroJit from those describerl above. I'lTYHKHnr-UMlK: Becanse limestones are not eroded with ease in tlie tropics, the O.tan Spring's litnestoiie stands np as a ridge of sontli ward-dipping rock. Agulrj'o Hills are only a few linndred feet bJgh (Fig. 17) and a S(a,)araied 'from the rest of this homoelinal rnonntain hv the broad vail HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 157 of the Salinas Eiver. Salinas Hill^ the eastern extremity of the Cerro Raspado, rises to four hundred feet, and from this place the elevation increases till just east of Coamo Springs, where the elevation of the Cerro l^imentol is one thousand feet. Just west of Coamo Springs the elevation is again onl}^ six hundred feet, but it rises westward to over one thousand /tVo xJueyes Wafer Gap. Coamo Wafer Gap. tSqlinas Hitl. /\<^uirre HiUs I Sa/ina'S Wind Gap, Fig. 17. Coamo Springs Range The sky-line is formed by the crest of the Coamo Springs Range. The middle portion of the sketch is occupied by the present coastal plain. feet at Jacaquas Eeservoir. To the north a great subsequent lowland has been carved, and to the south and east of the Coamo Eiver the coastal terraces, carved in a later formation, reach to the foot of these mountains. AVind-gaps in its crest are common. In places the crest is double and in others triple, due to the intercalated and easily eroded beds of tuff. GENESIS Specimens selected either at random or by intent from various portions of the limestone formation universally show algal structure. In some parts, where only little shreds and fragments are to be found, the former presence of alga? is attested to by the faint outlines of their former thalli preserved in the recrystallized calcite. From top to bottom and through- out the entire length the formation was originally composed of about 40 per cent algas—an estimation based upon the percentage found in the many thin sections studied. In some of the thin sections and hand speci- mens the ramifying branches of the former plants can still be seen, but for the most part the algas are in fragments. The fragments are more or less rounded and, of course, this rounding is largely the result of surf- work, but in part it undoubtedly was due to terrestrial agencies—wind and flowing water—as shown by the oxidized borders of many of the worn algge fragments. Some of the oxidized and other fragments possess fractures which, cemented by calcite, must have been formed at a time Iiill .sriHM'i Fli' XI'h'VHV OF i'ORTO Rli'Vt loraimmr.T;! .HiMirnn.u^ !i-su<-!ai.'<I wiih it nir- iils«, of slialluu-Wiiirr ..n-i!). or fliai runimiiiifiTii ciiii be <lrirtcd slioivwiin! and Hrrmuiihili..! uiKli- <halli»w-walcr i-cihIiI ir.iis. Siiu-.= \\w. rornnum fcra in tli.' {'uiiin.. S|iriii-> li!IU«>inilr ar<' I.k-lltlrill IT sillllliir With tho,.,. fmuul 111 nl.l.T illld Xnuu^v^ Hiiic.^tuiHs. nol cniitamin.u- al.u^a- n-tiiaiii^, it is |.r.,hal.l) saff !<. tiMH-lmi.- I1iai ili.-sc liiiK-sioii..^, t..,,. W..IV Iuii.hmI in s:|,aH.nv ..iiil.a vmcnK uf th.^ siMiihcrn ^^mM. T-. ii<-cc|.| tlii.- vi.'w as,>is!> in lli<. r\|»laiiat mii uf ili,;^ ..r^ riirfciHi'. clia.rarh.r atal rdat ini)slii|. of the linicsfuiH. (...is.-. fi al- Miuwii l.y llif. iivcra; Ifianirrio), sclorf.'ri fr HO.;^!i |.(.r cciil an. I i In tlin pnnrssns n rirlunniii .ha- (n tin- a dnioniitic lininsl..i a ixvvni dnal of nia-nnsiuin rarhnnaf... 'Hii- i^ r.'iirl.N'n analv^<'> i fIooi„„„, iHUl) of /,,7/,"'^ I part^ ..r the world, wlm-h is ralnniii .arl...nal.^ ^iiini rari»..i,al.. K.l I pr-r rnnl^ Isillizalinii tlwrv iiiav 'have linm a residual rih snludlr .-liarart..!^ Hf'thn niK-inni i-arbniialn and ^^ liavc Ih.,.11 r..nn,.d. .\k:v were andonbt-dlv lit. Inrni nf plain liln in tlnMairlicr ,<ivul<.u-u-al p,.ri..ds ; uUVrnil llial thn duluniihs of Ihn rarlv Palnnzni,. 1 HODGE, GEOLOGY OF THE COAMO-aUAYAMA DISTRICT 161 Rio Descalabrados Seiues general character The series is named from the river which flows across them at their widest exposure. All the strata of the older series occurring above the Coamo Springs limestone belong to this series. It is largely covered by coastal plain deposits; hence outcrops are few and isolated and are found chiefly in a few road-cuts. AVest of the Coamo Eiver a series of: parallel hills rise through the coastal plain deposits aud produce the ex- posures upon which this discussion is based. This group is composed of* Hmestones^ shales, cherts and tuffs. STRUCTURE The rocks all dip seaward at angles from 20° to 50° and the strike is uniformly K. 30°-50° W. Between the Coamo and Descalabrados Rivers the rocks are broken by a normal fault, which is thought to be the same fault that offsets the Coamo Spriugs limestone. The lower contact is sharply defined, but the upper contact is unknown because it lies beneath the Caribbean Sea. The thickness of this series is at least 8000 feet. LIMESTONES Three distinct limestones occur which resemble each other very closely in a petrographic way and which are separated from each otlier by thicK beds of shales, cherts and tufl's. The variations which do occur are of a linear character—that is, they grade into each other from top to bottom or along the strike. The color is gray except where the limestones con- tain much included tuffaceous material, which gives them a green or yel- low shade. The grain is medium or coarse crystalline where the rock is homogeneous, except where large quantities of tuffs are present which ap- pear to have prevented complete recrystallization. The lowest limestone formation is the most impure aud resembles the (Joamo Springs lime- stone very much. This is especially true in that outcrop forming the hill to the east of the Coamo Springs Eeservoir. AH these rocks are highly fractured and cemented with calcite. In this section these limestoues are seen to be made of three tv})es of material: organic, volcanic and recrystallized calcite. The organic ma- terial consists of algcT, which are most numerous in the Ijottom limestones. 1^'oraminifera are common in all three formations, some of which are Nummulites sp., Orhitoides sp., Biloculina sp., Miliolinas sp. and Glohi- fierina sp. The tuffaceous material consists of oligoclase—andesine feld- Hi3 HCI I-ISTIFIC HVII\1-:Y of PijHTi) RKU) <\HM' :\\w\vx\ tu ehloi-itf, ralfilc and !imoHil«?. DoiH'n.lin.u- iii.oii tlic jilaHHlfinii' of oik; or the ritln-r ol' tli<. sccnndarv niiiienVlH, the t-olor oC ilu- rock is oli^^c g-recn or a Iniif vellow. (iraiiis of iiiiioMiciJic an? coimnon and ii few pieces of dovitrificd pjilagonitc oc-<-iir. Tho hhidinj-- of caloito has l.con «h'vi'lopod from inlorsiilial linio silt ajid from \\w. largr.r organic-, roniaijis. In ,nia,nv wa.vs tlio orig-in of liu'so liniestoiios wa?< ideiitii-al with lliat of the Coain,o Hiirin.u-^ formal ion. SIIALI'B AND CiiKUTS 'Idle shales anjl (duTls fonu fin.dy bedded deposits oeeorriiig between I be HineKtoiies. Most are silieified, a few <'pi(U..tized, and some are eal^ xi\ others are aradlh •lie expi brines uiit in 'relitd'. The roh>r.s are hlaek. dark o-ra,y or g-reen. Miero'^ seopiciilly these shah-s and eherts are of a (haiso material tlic eompoHition of whi IS impossibk' to determine, hut very hi oinhirv ipiariz with rarhona<:(>onH stri'aks, serit-itie flakes and limonite hands. .Most of rliese roeks oontain, small spherieal boles arranged ill lines, some of whu-b cavities are filled with ipiartz. olhei's with quartz can 1h' seen hi alh-r HH-ondary ftmirt.z. tho eahate imist he tertiary and JIOIHiK, r.EiHAH'.y OF 'irilK VOA Mt)A}V W A M A IKIsTRIVT Jill] ^Mir.M.hKiMl. prolmhl.v. iimlrr ilicniiiil ciiiidit ions. SciiU' cf thi! shah-r^ are huciiin'il niid tlu' I'rIctiircK {;i'iueiil.cd by <'i>iil<>tt' which fh'vdups oiitwaixl iVnni fhu frai-ivuros ami in sonu- ••ntiivJy r.'])liUH' the roek. The Hilieiliea- :inn is lh<m,dit to Im' duo iMifirt-lv to ror-iTstallizalioii ol' the radirJariii juh! .liatonis, somo of whieh aro 'llvlw,lmii^ luu„l>ol(Hi Whr. ami Poro^ ,./."xrM,v' fi)w(v'/i/r/fw.>- Khr. ( l''ig. "^O) and diaioins of tlie type of Mfladra. >M, TJic hij.-hly lu^diml ohnruoicr of tho shaleH hi-ltai.iidiig: to ihin uimip is Jiuwii in, ficruro 21. rvsio l.inrtH.d iMl,ir,.B of !»o :j)osealalH-ado>= ^tw%. A tVw hi'lls. dn.» in ijie -nsliint linicsione hods, am sicparated hv a sori.w ol" small sid.s,.,|,„.|it ^vs npcai towan ('<wtno and tlio I H'^alsdiradoH liivy iH.'sn hills an" divid.-d into Ihiw parts l)y (he Cnamo and the iHseak- ^:idoK. {)\ Ihew the small liniestone hill east of the (Joanin Restjrvoir is ' ni.i'thea.sternmost di\is,ion. In addition to tlie ihree major divisions h.rmed. thoiie hills are apiin divided hv simill rowTaient vallev^. 164 SCIENTIFIC SURVEY OF PORTO RICO GENESIS The view is held that these shales are entirely of delta origin, which ls indicated by the excellent bedding, the carbonaceous streaks and lenticular character of the beds. During the period at which this group was formed there was an intermittent subsidence, so that after delta shales had ac- cumulated offshore limestones would develop. The subsidence was sufficiently graded so that before the limestones could reach a notable thickness a newly formed delta advanced seaward and covered them. A later subsidence gave opportunity for a new limestone deposit. This condition was repeated at least three times. Arecibo Formation This formation has been described by Berkey, the name being derived from a river on the north coast where the limestones occur in great thick- ness. The Arecibo caps a hill 100 meters in diameter in the southwestern corner of the Coamo-Guayama district. This formation once covered the district over a much greater area, as is shown by the erosional border lying Just west of the district, by the isolated remnant of erosion above mentioned, and by the superimposed drainage. Because of its importance in the interpretation of the geological his- tory of this region an extended quotation from Berkey (1915, p. 50) is perhaps admissible : Both on the north side, for nearly the whole length of the island, and on the south side, over the western half, there is a bordering belt of limestone and associated beds that have been developed on an eroded surface which beveled across the more complex structures of the older series of formations that formerly made up the mass of the island. These limestone beds are sev- eral hundred feet in thickness and dip gently toward the sea. On the inner margin of their present extent toward the interior they are abruptly termi- nated in a very Irregular line of modified cliff forms facing toward the pre- vailing smoother and lower ground for some distance toward the interior. In its best development, however, it is a typical cuesta, formed in the usual manner by the erosion of a formation representing a recently uplifted coastal series. The series of formations involved formerly extended inland very much farther than they do now. Only the outer margin remains from the erosional construction of a series of beds and reefs that in former times covered a large portion of the island. ... In a large way this series forms a structural unit. Below it lie the older and more complicated Igneous and sedimentary rocks. The break between these two represents the chief unconformity in the whole geological column. For ;i considerable distance an average dip of 30° to 36° was estimated and the tota 1 thickness represented, based upon the widtli of the belt, must be at least 3(KH) HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 165 feet. As one goen eastward, a comparatively short distance tliey are almost entirely lacking. ... Between Guanica and Juana Diaz, wherever the inner margin of the Arecibo formation was seen, it was bounded by a fault which brings the upper beds abruptly against the older tuffs and shales of the pre-Tertiary. The formation furnishes an abundance of fossils. The lower portion of the lower beds on the south side of the island, as seen at Juana Diaz, seem to be the most promising for a determination of the age of the beds of the formation. . . . The formation as a whole is essentially a structural unit. . . . These rocks are separated from the older ones by a discordant uncon- formity or structural unconformity. This is shown by the manner in which these gently dipping Arecibo limestones rest on rocks tilted to high angles and by the fact that they rest on a peneplane surface. The existence of this peneplane was first observed by Berkey, who says : Beneath the limestone constituting the cuesta and representing the Tertiary series, there are, in numerous places, traces of a former plain that represented the results of erosion on rocks that had a complex structure. Occasional pro- files of more distant ridges also show a sky-line that suggests the former exist- ence of such a plain, and in favorable localities it can be traced directly to the «foot of the limestone cuesta. Occasional traces are also seen on more moun- tainous tracts, especially at the west end of the island, near Rincon and in the vicinity of Mayaguez. It is the judgment of the writer that these all belong to a single base-leveling surface or marine-cut platform formed in the period just preceding the development of the Tertiary limestone series. The peneplaned surface has been elevated and tilted. The elevation is 2500 feet in the eastern portion of the Coamo-Guayama district and 3000 feet in the western. It descends toward the north. Dissection has cut down far below the level of the peneplaned surface to the south, and so its former trend can only be inferred. Were it not for dissection the former peneplaned surface would attain at least another thousand feet at the southern coast. That the Arecibo limestone once covered the entire district is shown hy the superimposed condition of the major streams. In the above para- graphs the general strike of the stratified rocks is described as northwest and southeast, but the large streams, excepting the Eio de la Plata, flow f^outhward. It might be argued that the original drainage was parallel to the structure and that coastal streams have worked back and captured tliem. Such problematical coastal streams would have received their initial supply of water from the most arid portion of the island, while the captured streams were supplied by the heavy daily rains of the interior. To capture under such conditions is impossible and the theory must be aiumdoned. The large streams have courses discordant with the struc- ture. This is shown in most striking fashion by the Rio Descalabrados. 166 SCIENTIFIC SURVEY OF PORTO RICO Coamo and Jueyes, which pass at angles of ninety degrees and compara- tively narrov, water gaps through the Coamo Eange. PHYSIOGRAPHIC EFFECT One result of the superimposition of the drainage across the structure of the rock has been a numerous series of captures. A good illustration of a recent one is cited. A channel filled with pebbles lies between Salinas Hill and the Cerro Easpado. Back of this latter range a subsequent branch of the Rio Jueyes flows^ and it seems evident that this latter stream has captured the headwaters of this nameless stream and diverted them eastward. As a result a wind-gap, which is so low that a wagon road passes through it, has been formed. Santa Isabel Series occurrence This formation is named from a little village situated upon the coastal plain deposit in the southwestern part of the district. Deposits of the Santa Isabel series cover the southern border, extend as embayments up all the rivers flowing across the coastal plain, and occur as isolated patches along the various rivers. GENERAL CHARACTER For the most part this series is composed of a fine black alluvium of varying thickness. Thus, west of La Lapa the Eio Cerro shows 20 feet of bedded silts interlarded with boulder beds. North of Salinas Hill it is 25 feet, near the Banos de Coamo the river flows between vertical silt walls 50 feet high (Fig. 23), and at Guayama the silts are only a few feet thick. All of the alluvium found in the valleys of this district cannot be classed as a coastal plain deposit (Fig. 2). Some is due to a local base- level created by a hard rock partially damming some stream. Streams dammed in this manner have aggraded their valleys with alluvia to con- siderable depths. Thus, three miles north of Eio Jueyes Water Gap is a deposit 25 to 30 feet thick composed of excellently bedded silts with an occasional lens of pebbles, and in this stream there are also alluvial beds at 660 feet elevation. Again, in the great Barranquitas-Cayey subsequent lowland is a deposit of silt 10 to 15 feet thick, due to a local base-level located just south of Comerio which is about 1500 feet above sea-level. Such deposits are classed with those of coastal plain because they have been formed in Recent times. HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 167 The coastal plain rests upon the upturned edges of a series of tuffs and shales and upon these planed surfaces wave-rounded boulders are fre- quently found, as shown in a cut of the road southeast of Guayama. Be- tween and on the top of the Aguirre Hills such boulders occur in profu- sion and represent the time when the sea was awash with the tops of these hills. Some of these perfectly rounded boulders are of Arecibo limestone which must have been transported littorally a great many miles. In the beds of many of the streams deposits of chalky white, porous limestone are being formed. These tepetate deposits form in the shal- lows of the intermittent streams, where the waters evaporate rapidly. They are also formed at points where the water cataracts over a small ledge and loses some of its carbon-dioxide content and is thus forced to precipitate its heavy load of calcium carbonate. PHYSIOGRAPHIC EFFECT When the coastal plain was in process of creation the streams previ- ously developed upon the old land entered the sea at the inner margin of the present coastal plain. With the retreat of the sea the rivers flowed over this coastal plain and some of the streams, like the Coamo, sought out their old courses and became resurrected, while others, like the Des- calabrados and the Rio de la Lapa, were not able to cut through the coastal plain upon which they meander to the sea. These streams meander upon reaching the coastal plain because they lose much of their water through the silts and gravels. This lost water, upon reaching the surface of the wave-cut terraces, seeps along the top of these terraces to the sea. This seepage furnishes the only zone of water to which wells may be driven in the whole Coamo-Guayama district. Still another class of streams was created for the first time upon the gently inclined surface after the emergence of the coastal plain. The lower portion of the Guamani belongs to this classification and the Rio Cemarrona is another good example which by headward erosion has begun to eat into the old land behind the coastal plain. The superimposition by the coastal plain of some streams has caused marked changes in drainage. Thus, the present course of the Guamani is either due to capture or to the superimposition of a stream which for- merly flowed down the wide-open valley east of Guayama hills, but now flows down to a point marked by arrow (Fig. 22) and then turns abruptly "^vestward through a narrow cleft in a spur of the mountains and then flows southwestward to the sea. The terraces found at the lower end of the Coamo, and to a less extent associated with the other streams, are due to the dissection of the em- U)H SVIHSTIFIC HURVKY ftF PORTO RKU) bayed portions of tlie coastal plaiti. Tlie ('lovation of about: 250 feet wliich brought tlie constal plain to view revived all tlio streams. Tlw n'viviii<r inthieiice of uplift took plaee long' enough ago that the effect is now aetivo in the headward eourses of sueh streams as the Hio tie la Ijapn. Ill the upper reaebes of the streams degradalJoii is eutting into the old f'omfJex of volcanic reek. Figure 23 sh(tws how the Rio de Coamo lia:-' <'Ut^ a }'onng \'alb'y 50 feet deep into the a.lluvium accumulated when the Coamo stood at tlie highest levtd during recent times. Most of the streams in this district have cut laterally faster than they <'ut downwai'd. I'his hiteral eutting flattens tlie floor of tlie valley. T]i"c '2-2.-^ Van. en fif tlie (Hi by Pro£.>ss<»r CIi cut eoiitiiiues to one side imtil tiic strtnims meets a re.sistant roek vrliieli reflecis it to llie oppcsife side, and a delleetion, in a stream (mee atartc<i c'oiitifiues until dctleett'd agaim Associated with lateral eutting is dowi^^ sti'cam eultitig. The combined eil'eet is to ]iroduee a terrace. Hlow uplift <.f Htnuims which are pro\d«led witli rocks in tlu'ir hanks to defiect theiii will r>au.<c a, series of terraees to he out one below the other. Ledgers of rocks pn.jt'cting into the slream, are ideal for this purpose. The llio dc Coamo is deilccted by the Coamo Springs limestojic and as a result thn-' terraces arc forured. Continued uplift luis caused tliese to he dissected h\ e<iiiHftquerit streams int(} a dendritic pattern. The highest terrace of tl'f HODGE, GEOLOGY OF TILE C0A3I0-(JUA¥AUA DISTRICT 160 t'oaiiio in dissected to a depth of 50 feet in this way—a testiiiKrtiy to the rapid rejiivejuitiou which h«s 1rtk(!ii place. Tlie coastal pattern appear,-; to l.ie due in piirt t<j the (;uastal deposits lu shewn by tlic present growth of the 8outh coa^^t,. Ofl'sliore coral reefs art* heiiig l)nilt. Inside ul' these laiHl-derivcd silts are acciimiilatcd until tlie W'rtter is sLialluw enongh for rortn«i,i'ove hnshes. These form veritahle traps fur siMliment and the sea floor is rapidly transl'ornied to dry land. All along the st^onth slunv this is an active process. If one conhl remove all cEmRo anvils Photoffraphcfl by Froft'issor If. I Ibis hiiilt land the angular eharaeter of tlie south coast would he vei-y pronoiunx'd. The projections seem to liave no relation to present streams: tliey ai-e not deltas; in faci. tliey are eom|:)osed of trvmcated rock (;over«Ml with u thin^ Teneer of coastal |)lain. A teniative theory is proposed—the >onthweHt shore of each <d' these ])rongs lias a trend practically parallel '•ntli the general stniftture of the region. .Moreover, if the varions rcsist- iiiit strata were continued to the sea thc>y would in each ease follow these I'i'ojcx-tions. When the region was suhinerged. snl)se(ineiit valleys w^ere drowned and the ridges hctween were planed hy the advancing sea. Then 170 SCIENTIFIC SURVEY OF PORTO RICO the uplift of 250 feet came^, which was not sufficient to fully resuscitato the drowned portions which form the embayed portions between each pro- jection. The combined results of drowning and marine planation have produced the angular coast lines. GENESIS The withdrawal of the sea to its present position exposed a series of terraces covered by a mantle of sediment. This mantle is called a coap.tal plain^ though it is commonly misnamed playas (Grabau^ 1913). The coastal plain is a heavy black alluvial silt deposit and is looked upon as delta material deposited beyond the cutting edge of the advanc- ing sea which carved the southern coastal terrace. After being deposited as deltas^ littoral currents swept the material along shore and so spreaii a thin veneer over the whole wave-cut terrace. The mangrove bushes undoubtedly were an important factor in the formation of this coastal plain. Their ramifying branches caught the silts and caused the sea bottom to be rapidly brought to the surface. Thus part of the Central Aguirre Harbor near Pta. Carchones within the last five or six years has had a fringe of land built out from the shore over 200 meters and the mangrove belt has advanced seaward an equal distance. Early Intrusives, Flows and Pyroclastics occurrence The earliest intrusive igneous rocks occur as innumerable dikes and sills cutting the older series and as occasional flows and as fragments in tuffs and volcanic breccias. The rocks of this series will be discussed in two divisions. The first described will be the composition rock types, and the second the struc- tural types. COMPOSI TI ox TYPES Tbe following composition types will be described : rhyolites, feldspatliic andesites, augite andesites, olivine andesites, basic augite andesites. olivine-free basalts and olivine basalts. Rhyolites.—Ehyolites are extremely rare and occur widely separated as thin flows. Most of the rhyolitic types w^ere observed as occasional fragments in volcanic tuffs and breccia in the younger rocks of the ol(h"' series. The hand specimens are mostly aphanitic, though a few sliow^ quartz grains as large as 1 mm., and flowage structure is well developed. T!ie HODGE, GEOLOGY OP THE GOAMO-GUAYAMA DISTRICT 171 eolor varies from gray, bluish-gray to butl' and all are more or less mottled by iron stains. In thin section a few of tliese rocks prove to be glasses. Patic suh- >tance varies from 75 to 90 per cent, which is colorless, shows excellent How lines and contains many blebs and bubbles. In most of the thin sections the quartz cannot be certainly distinguished from the feldspars. hi some, however, small .03 mm. euhedral quartz crystals occur. Tabular clear feldspar of the sanadine variety is the rule, but ortboclase and (>ven albite are frequently found, which are usually kaolinized. Magnetite and titanite occur. Secondary minerals are limonite, kaolin, sericite and loucoxene. Some of -these rocks are devitrihed, as shown by tlie niicro- apbantic but not isotropic groundmass, and these rocks are much affected \)\ epidotization and carbonation. Fehhpathic Andesites.—The feldspathic andesite rocks occur as small (Hkes, as thin sheets or flows and as parts of volcanic tuffs and breccias throughout the entire district and cutting all the sedinieutary fonnations of the older series. While their distribution is universal, they do not occur in equal abundance in all parts of the area. Thus, in the eastern part of the district they are about the only rocks to be found, but to the westward, andesites of other varieties increase in number uutil in the extreme western portion feldspathic andesites are found infrequently. In other words, feldspathic andesites are more numerous in the proximity of tlie granite batholith and appear to be diaschistic dikes. Tliese rocks, as found in the fiehl, show grains whicli are rarely larger than 3 mm. and very few of them are aphanitic. Where glassy substance is ])resent it is only found by microscopic study. Most of the rocks are hiatal—that is, show crystals of marked larger sizes—but few show a perfect gradation between tlie grains of various sizes. The minerals as a nde, have a tabular or prismatic habit and some have a true granular shai)e. Only phenocrysts of plagioclase appear, which are usually tabular, thougli at times tliey are prismatic. Earely do the plagioclases occur in .groups, but when they do, two or more are crossed like tlie lettei' X , and some are striated. The colors of these phenocrysts vary from pure white to pale green when fresh. The color of these rocks as a whole is de- cidedly light green, tliough darker green is common and other colors are irray and buff. Oommonly the green color is not uniform, but within the pale green spots of much darker green occur. These types upon weather- ing simply get a darker green, thougli the feldspars at times take on a ''ddish color due to iron stains. Epidote areas are common and these '>''gin to develop just about the feldspars. The rocks are frequently liactured and the fractures cemented Avith calcite or in some places by n3 SdEXTIFIV t<Um'EY OF PORTO RICO opidoti'. Srjinc {•((rilaiii vesicles which are usually alMtut I mm. luii^- an.'-: lilled with caJciic, which, is wliito or piiik. Miitrosectpie study shows ihc st riH-turcf^ of these rocks to he (;xtreniely variable. Soiue are (-M|iiigTaiJiilar. othoFH are <le«;idc(lly liiaial, l)Ui most of theru occupy pOHir!on,s hetwecni the two extrenie.s. By far the h;irgest mrinher are ft<M'infe porphyroid. 'rh<- lar|4"or *£raiiis rai'cly get ahove '4 jjini. iii size, (h(! averag'c Uirger crvstaJ- or pheiiocrysts are ahout ;> to "> nun. and the smallesi; grains in tJi-- «ii'ouiidjuass ar<> .05 iiuii. The pheiiocrysts schloni make up more thai! 2"> per eeiit of the rock: tlie average is It) per eeiit. lu some rocks l,hn e-rysials ha\H' im <h'(iiiitc' arrniigojiieirt, hut in uiosi the crystals in the- groundiiiasH li«\'e a true pilitic ttsxture. liaJ'er tyi^s luive diahase textri and extreiuely rare types liavc line granular texf.ure. aiidesinc^ variety. Tlu'V are tidjular to prisiuatic, dull, at times high allcred even to conipleti' rephiceiiient, rrequeidJy eniJ)ayed hy tlie greuir mass, and <d'ten <'Outrtiiiirsg inclusions of the groundniass. d'hc iwiiiuii hues ai'o irregular and uneven. Very <-<»nunonIy the feldspars arc eriiii h>|)hyric. The feldspars are extremely suseeptilde t(! alterati<ui and fact it is a rare thing to find a fre.di one. The change lo chh»rite tak place readily and especially in the ease of the snuiller crystals. In th variety of andesite, where the grouiidmass is highlv feldspathic, it usually entirely altererl to ehlr)rite: Inuu-e tlu> green color of all' the HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 173 locks. The feldspars also alter very readily to calcite. In some this alteration has proceeded to such an extent that they are commonly mis- taken for limestones. Other alterations affecting the feldspars arc changes to secondary quartz, epidote, actinolite, and zeolites which are 1 lydroth ermal effects. The feldspathic andesites are almost a single primary mineral type. The only other primary constituents are magnetite and apatite. The magnetite is often euhedral and averages 5 per cent in amount (Fig. 24) . Tlie iilliiigs of the amygdules will be discussed under hydrothermal eirects. Augite Andesites.—Eocks of the aiigite andesite variety are very rare in the eastern portion of the Coamo-Guayama district. In the central part, compared to the feldspathic andesite, they occur as two to one an*! ill the western third as three to one. They are not confined to rocks of any particular age, but occur as small dikes, sills and flows in rocks of all ages. Sills especially are abundant ; augite andesites occur most profusely as components of the tuffs. The two best occurrences of columnar rock are of augite andesite, one of which occurs about three miles north of the La Lapa and the other on the Aibonito-Comerio Eoad at K. 40. Tlie several occurrences of pahoehoe lava are of augite andesite. Augite ande- sites are found about the several volcanic centers and are especially ahimdant about the volcanic center east of La Lapa. The augite ande- bites tliereforc are by far the most abundant igneous rocks in the district and make up the major part of the pyroclastic sediments. The size of the grains in these rocks is characteristically small, usually below 3 mm. ill their largest dimension, and a few of the rocks have components so small that their crystallized character cannot be determined with a hand k'lis. The few types wliich are in part glassy were only so determined by microscopic study. Over 80 per cent are porphyritic, and of these but a small portion are augite porphyries or with phenocrysts 5 mm. or larger. The fabric is most variable; some are decidedly hiatal i. e., composed of two types of grains differing greatly in size. In some the grains are of uniform size. In most of these rocks the grains are tahular or prismatic •uid have pilitic or diabasic texture. In a few rare types the texture is tndy granular. In some of the rocks of tliis type only augite phenocrysts are to be observed. These are usually equant or equant-tabular, of dark- .i^ieen color, and the augites are fresh and clear and more or less perfect 111 form. The feldspars when observed are usually tabular or tabiilar- <''lMant, rarely prismatic, and frequently many are grouped together and <*'nirdonly two or more feldspars are crossed like the letter Z, Striations 'I'e rarely present. The color is sometimes white, usually gray or green- IT f<€lE:sriFIt' MHiVJ-]Y OF PORTO lll€0 • ]n:i'<vi\\. Iiitroihioed zeolites, aim pi'<"<loiiiiji<itin>^- iiiiiierals, present, Iiitroihioed Kcolites, epiJotc. iuitij iiJid .|i!iii1z an; iViMpicii! ly s.-eii. TJk; ii'lash}- Vriritilie.- ol" rlie aiigili! iinde.sites tu'ciir only as pt.riiojis of luffs and volcanic bri'ceiiiH. In thcsi>. tlic sl.r'iictn.re. is diMjidcdly porpliy- ritic. Dark brown glass fVrnns 70 per (XHit of tlie rock, which froirtirins ol)undant vesicles and crystal litcs niid shows a well-ileYelopcd^ Ihnv strnc- tnj-e. Tlie feldspars lu'rin 10 per cent of tlii.s rock and tire ial/ular twiniHMi. clear and are of llie olio;(.claHe^»ndesitie variety. Angite up to 15 per (*ent occiirK in small enhedral |.tale green, elea.ved. fractured fresh u:r;nns. Apatite and titanite also oeeiir. fjeneoxene and chlorite arc de~ velopcil in small aniuunts. These pafog-anites or tiugite anjh-sife vitro- |)h}Tes caj]ni)t he ollnn* ihan tlie furiner glassy scnni I'ormed on the sur- Incc «d: some lava'dlUerl ci^iter (Fig. 2i ). (Jlitinc Andcdlr.i. Andesitic rucks eontaiiujig olivine are not nhww- dant, tliough several notaldi^' oceiirrcnees of rliem wqtc ohsorvcd. ^JdnJ-' ncnih of La Tjapa. is a large exposure oceujiyiiig the Ited <d' tlie E;io de h Laf)a lor half: a jrulc and^ in ihe vicinity <d' a, volcanic ceiitc'r cast of I.a liapa they arc common as dikes and liicy occur tlirmighoiit the central and western lialf of the distidet as dikes, sills, flows, or as fragments in tutfs and volcanic hn-ccin. The hand spcMnmens of these rocks hiok very much like the aiigUv ;i.ndcsites. from which ihcv dilTer ordy by the darker colors of lJic gromnt^^ JIODGK, GFJUAHiy OF THE iiii.l Mie Miv<ciu'.= of oli villi- iiih 1 MfJ^^f;UA YA MA IJI,STIII(:T | ", J iiistciid of uiigite ery^tiik. T !{ja.-?H irf .UTii.y. dirty .uTct'ii or cliocolati' iiiul jruii .-Jtrtiii.- art' vt'i Wlioir thf (,ili\iii<; i< prcKoiit it oceiirK us tiny 1 to "^ hiiil. ooIhhV l.nl i!>Tially flu; oli\'iiie is iiltercd and its foriner proHfiico i = 0 is shnwn by rinjt!n> c.f olivi-iic Hliiifi,. lillcd with liiiionii.'. Tile jiiierol'iibric vi' iIh'W rui'ks is sorijito iiilcr.st'iirtl to seriate porpliy- !'t';it*1i -» mm. 8iju!(' auu'itc }ils«^> oc-erirs in tlic?*? rooks rlioy might be ooii- -bhMvd as basaJts, but wo pla'-.e in this g-noi|) all the rocks in wlodi the niiaidiiv of t:lio Wdsiiars is tlirce tiiiK's ih.- f-dia^f eonsiiiiioiits. The nU< •itii sonif lat.radoriti'. As iduaK.ory , siiH.T tbrv arc r-xirornoly iaiiiiidaiit :<'i)lrt-.. is ;<>. The oW^ndim'^^^nruh'^ r twmiiiii.o- buiHdla.. Tlu-y la.iutc. I'lic aii-iio i. th/iis sis lilt- i:i-fiiiiidiiiass I'hc'ir rtViTaii:<' piToci) uc(;ur> ill ]irisiiiadic crystals witli re aJtercd to cah-ito and imich staijiei! h jrdc iriToii, frt'sli, Twiiiiicd. cloavcd, rraoiurcd. oinmdojihyrio ctrvstals wdtli riiadoiTYsts of magTiotite and mak<;s up 10 por I'oiit of tbo r.H-k. 'Vlv t:rysts. Whoii frcsli IJioy arr; oulanlral but usiiallv tboy nm cujiipU;hdv obysi,; and uoda; np A per o,oit. M,agiiotiu> ciOciuv ii,s lar^- irrcuiPar o'Jiins. ih-o(|iieritlv iidcr*jTowii witb aiigito. aiiioiinlin^ii' to o piT oi'io. liwtiHiiiii, liirioJiitc, H'riH'iitim\ sefoudar^v in«giM;tit«' iirc ihc altiTaiioii miiierab (Fig-. 28). B(m,(- Angitp J it.lesllfx.^^Onlv two occnrri'iu-es of basic augite an.Io- yiti* were ohHorviMl. best!) as dikon culting^ t'lic biwcr piirts of the older «orii>s. 'l'b«' matrix of this nick is aphaiiitic and dark dull glayi^;h gTOcn. Small 2 mm. dark gn'oti. almost black. «H|iiaiit augiies and 5 mm. lailis of pal(3 grctiii f('kM|iar forjii tin? idieiiocrvsrs. The mieroHcopie aj)pO!)r^^ aiisY' of tla«8c rocks is very similar to the aiignie andcsitcs tlnscidbcd above, from which they differ in the greatisr pereeiilage of femag raiiU'Tals. Thus in the aiigite aiickvites the ratio of fehlHpa.rs and femags runs TO twin mati( ht pr pleoe Fig. 2ik^^ -^Olirinc frt:e bnmiU .weiiient uf the crystids is diabasie. rJienocrvHtH form about ¥) ])er of tlie rock. 'J'lie fehlsjmrs are <d' the aiidesiiH-labradorite variety. . oligoeUise also oecuj's. The feldspars are tabular, irregrdarly led and '/a)Wa\, an<! contain inelusions of brown glass. They oceiir oiips and are strongly alTceted by chloritic alt»rations. As plamo- «; the feldspars bnnn ;'.0 per cent of the roek and, tlie angito 13 per The pbenocrvstie aiigites are tabular to et]uant, snbhodral, fresli 'Uiimlopliysie. In the groundrnass the feldspars are extremely pris- '., fresh and form |)cr eiait. The aiigil.es of the groundrnass ("orm T eeot, are csquaiit, fresh, liighly ehaived, pale yellowish green, non- iroir* and ijit(\rstitial. Magnetite in large 4 mm. ecpiant grains IIOlHll'l (tFA)L(H!Y OF Till-: i'OAMO^lUAYA M A ItlSTRiar \]<) :i.niiK S |H'r ctMii. nml n tew o-rains of siparits- ucnir. Chhriti' <U'Vt'lu|Hnl aloiiM i-l('avrt.u-c plains in tli(> feldspar is the only «H-oii(la,ry mineral. <nriio of Ihose rocks are jiiHy<iilal(ti(lrtl willi vnhiiv (illing-s. fiiirine^firc Ihmilis.- The «Hily ueeuriviiee is a vesicular How neiir a l.o-s of graiiotliorites iioHlu'asf of ('(Hiiii. ( 'oiit^idered iiiegai-eopieally it }< slightly ]>orph3Titie. with light io dark gray IVUIspars oceurrijig iji a diirk'griiy malrix. Small aiiivgduU. areas lilled witli white ralci'te are hr.'seiit. Tlie fahrii- of this r.iek is ..eriale purphyritie. PheiuKnTsis of aiide^iiie-laJjradorite. whie-li are lahyjar ervstals made up of frroad twins of variahk wulfh. As pheuoerystf^ au«l as grouudimiss tlie fiddspars make ap CO per eeni of the nndx. Ahout 'M) per oiart of hytownite iiiHuiiiite ni-cairs, whieh is simihir to tbi' otlu;r felds|airs except in extinctirju. Xearly 10 per cent, of .i>5 mm. ecpiant grain of angitc are prcseni. CaK .ilc. <-hlorite ami hennilite oc^cur as sceondary minerals (Fig. 2'.)). Oliriiic Bmalh. ^The nlivine hasalt.s are found frc<|nently among the luffs or as Hows, dikes or sills of the idder series. The eolor of the matrix ^^ \crv dark gray, purple or reddish. 'Hie ])heiioerysts are :l iimi,. e.|uant = '> tahular dark greiui ang'itcs, o mm. talnilar. striated, light grei-n Cchl- -pars. and rn.s1v pits of olivine shape. Most of these rocks arc either "'sieular or amygdaloidal (Fig. ?A)). A study of the thin sections shx.ws ^..me of them to l„- basalt it- xitropliyrcs which have a d<>eitledly bialal e 180 SCIENTIFIC SURVEY OF PORTO RICO fabric. Phenocrysts make up 50 per cent of the rock. In part the groundmass is microaphanitic. The basic glass is gra3dsh yellow, though in part it is greenish, probably due to chlorite alterations. The yellowish glass is crowded with tiny bubbles, globulites and perlitic cracks. Th' phenocrysts consist of large 1 mm. crystals of bytownite-anorthite, 10 per cent of andesine, and 10 per cent of labradorite. All the pheno- crysts are tabular, broadly twinned and altered along twinning planes to chlorite. Clear, fresh equant pale green euhedral augites occur, amount- ing to 10 per cent. Olivine entirely altered to serpentine reaches 5 per cent and a few grains of magnetite are always present. The holocrystalline types of olivine basalts have a seriate porphyritic fabric and possess phenocrysts, which make up 35 per cent of the rock. The feldspars which range from andesine to bytownite are tabular, l)roadly and irregularly twinned, dull, zonal, resorbed and at times com- pletely altered to chlorite and carbonate. The augites are clear, equant to tabular, euhedral, pale green and cumulophyric. The euhedral olivine is dull or entirely altered to limonite, magnetite, chlorite and iddings- ite ( ?), The groundmass is made up of ^^fern magnetite'^—that is, mag- netite crystals in parallel growth and untwinned laths of feldspar. Li total the olivine makes up 12 per cent, augite 10 per cent, magnetite 15 per cent, and feldspar with secondary products the remainder of the rock. SPECIAL STRUCTURAL TYPES Some of the above-described composition types take on structures whicii deserve special mention. There are many occurrences in Porto Rico of volcanic flows filled with pyroclastic materials. These flows are either augite andesite or feldspathic andesites. Scattered throughout the igne- ous matrix are lithic fragments and sedimentary fragments. Some of these rocks are crowded with introduced fragments, as, for example, the beautiful rock exposed in the stream bed just east of Cayey. The frag- ments show little or no absorption, but they do show at times contact phenomena. Thus euhedral .05 mm. pale greenish-yellow garnets form rims around the feldspars and occur scattered irregularly throughout the groundmass (Fig. 31). Other examples of flows filled with breccia are foimd east of Coamo, just south of Cidra, near the northward bend of Rio de Cuyon and just west of Comerio. True flow breccias were found which belong to the augite andesite or feldspathic varieties. In thin sec- tion they are seen to be composed of angular fragments lying in irregular positions, in a matrix of the same composition. The best example wa?^ foimd south of Cidra (Fig. 32). jjouai:, aiUiijH;v of the co i i/fi-r;rj v 1.1/ i dihtuivt igt TlHi mm\. iiiifrcsliiig- ivpcs oi; , of «1 rcessiblo oceiirreiiee i? lietween ,lv. Hi auliicr is found wlieru the Uio (U^ C ..wt (.xfeiulK ill \\w. licit of ihi,' Mm , .: I'j.H^ 0). (^rilir;il iHivcs HoliH-ifd a,ii<j exiuiiiiUHl iiiienxcupjcaJl} -lu.u i!'.u.| igneous Htnichiiv ( Fi-, :;:!). Thi. slnirtiiiv mj-lii bnvo Ikh-d <!- '•If.'|)ed by iiM»laiii()r]j1nsjji. Iml ilii^>:: im ili«|>!'iivi'(] by ibi; uiiiiU'i^iwiorplidsJn HiimK;i(;iM)f tlu! UTKJ.T 01; ovcr l.viiig hcV. 'il<)re.A,.r. ibc wune cnuglnni^ •t^^ib' traeec! alonii the hirikc chaiiEos b. (nniirtlirMis with ii <m]im,'nbin ir^:i(rix. The ninirJx is imfhTiibl.'cIly j.rijijarv \<i\m)m rf.rk iuid wan Incnll^, im-ucliiwA, wbirli Wf.iil.1 bf. |)us<ihb; in sinerul wavs. A vnUcv (ilbd viiii -••nirb*iTionib) hoviblors or n. Ian of cnn^loiiierdes miixbi bavc had n Ib.w .,l ^^= a pacH ovor it. This lava w-mld siidv iiitu and lib lh(> inlaivli.v. and -' .1'nietd' ibo liordoiv.. In view cf tha i.criWi; sodina-iiliirv matrix o) di' >a,ini> coiigbniiarab; beds fb-mvla-iv'. ir is nnbki'l\ Hial tlio pnidicHi '^'^'h a. eonudoinfa-alo nuitiix ^\a« frr-o .d' inlT .a- iiii.T^tii iiil silr iriair..T, 184 SCIENTIFIC SURVEY OF PORTO RICO will be seen that near the end of every flow there must exist a posiiioi where the columns will be bent, as shown in the figure. GENESIS All the rocks described above are considered early differentiates of the batholith which is exposed in the eastern end of the island and which extends westward beneath the Coamo-Guayama district. If all the types could be averaged the composition would probably be that of an augite andesite, a rock which may be considered to represent the composition of the magma reservoir from which they were derived. Since the sediments in the Coamo-Guayama district were derived from rocks of the same com- position^ the view cannot be held that the composition of the underlyiiiij "jrzrSX^ .-sr-'^- I II''. ' J.--^- / -_^- f-- / / / ' \"y--7--'-f"j-j-;a^-j^ -----. .6 Fig. 34. Relation of isothermal surfaces to andcsitic columns magma was due to their assimilation. The composition was either ])!•!- marily augite andesite or made so by assimilation of rocks not exposed in this terrain. Batiiolithic Eocks occuerence The greater portion of the batiiolithic rocks lie beyond the limits of this district and enter it only as a somewhat detached mass occupying tlie hills just west of Guayama. GENERAL CHARACTER The general character is described by Berkey (1915, p. 28) : The most prominent type of intrusive mass is that seen in the southeast vox- tion of the island, including the district about Huamacao and Yabucoa ami Las Piedras and Juncos. Whether or not this is all one mass belonging to a ^single intrusion has not been determined. The variety of composition seen in the different samples, taken at different points, is consistent with the presence of more than one intrusive unit ; but it is also possible and quite as likely tliat HODGE, GEOLOGY OF THE GOAMO-GUAYAMA DIBTRIGT 185 tlie variety observed is wlioUy due to magmatic differentiation. The southerly portion of this mass, especially that near Yabucoa, is represented by a very coarse, very quartzose and almost pegmatitic granite. Farther to the north, in tlie vicinity of Las Fiedras Juncos, the rock has the appearance of a syenite. Although a part of the rock does show the composition of a true syenite, by far the greater number of specimens collected on this expedition show the presence of quartz in sufficient amount to make the rock a granite. It would appear, therefore, that this occurrence in the southeast portion of the island is essentially a granite mass and that it is of unusually large size, reaching practically from the coast at Maunabo to Caguas. The distance across this mass is, therefore, not less than about 12 miles north and south. In all proba- bility it is not of quite so great an extent east and west, but these boundaries are unknown. The portion adjacent to this district is not coarse enough to be a true granite. Micropegmatitic structure is characteristic. Traced toward the Coamo-Guayama district it becomes dioritic in composition and the grain becomes finer until at Guayama the extreme border is an andesite in com- position and texture. Ilornhlende Graniie.—Hornblende granite is the only type which en- ters the Coamo-Guayama district. The hand specimen appearance is mottled, due to grains which are of equal size and irregular form. Areas of dark green hornblende 1 to 3 mm. in size are separated by larger areas of quartz and feldspar. The feldspar is either white, pale pink, or pale green. Tinder the microscope this rock is holocrystalline and equigran- iilar. The average size of the component crystals is 2 mm., which is al- most too small for a granite. The rock consists of 30 per cent orthoclase iiitergrown micrographically with quartz. Feldspar of the albite-oligo- clase variety occurs to 20 per cent. The feldspars are faintly kaolinized. Quartz occurs as part of the microscopic intergrowth and as euhedral grains. The latter contain numerous lines of bubble inclusions. Quartz makes up 40 per cent of the rock. The hornblende occurs in prismatic crystals, is cumulophyric and makes up 5 per cent of the rock. The horn- blende is in part bleached and in part altered to epidote and chlorite. Other primary constituents are titaniferous magnetite altered to leucoxcne and apatite. Some limonite stains occur and the rock is highly altered in part to epidote, and needles of actinolite (Fig. 35). GENESIS W'lietlier the l)atholith is a satellite or the peripheral portion of the piiiiiitive batholith, it must represent the residuum left after the separa- tion of the augite andesites and basalts. Whether it is injected without "^'•Uition or whether it is invaded by dissolving the country does not mat- ISli ,v(7fAV77/-7f,' ,^'|.7n7-;>' (IF l>(HlTit HlVit let'. I'or ill thi' Jatl<M- viWi\ it would only assiiiiiliitt! n wuall portion of Hi rooks^, t!liiiiijiat.*'ii sih intrusions jind extrii^^ions in iin eiirlicr ))erio(l. Siir assiiiiilatioji would ineroasc its liasicity Init little; il would return to stage in its nniginatif liistorv a litth» previous to its present one. P>e(•ill|^ the apophyses of Ihis hatholitii are of internu^'diate eomposition and quini titatively snnill to return all apophyses to the hatholitii would inerea< Hh'ghtly its hasieity; therefore, at the lime of its injeetion tlie compositi,:. of this Imtholith was prohahly oTiuidiorilic or nionzonitie. Heiiee, it v turn we add tiie large nnndier of augite andesites an«l the few hasahi :'.:.. JKfnhleiiile anniit types, (he enrnposition of rhe priniiti' he dioriiic. nblained. whieh woiiJ<i liAi'Kt! Ix'ri.'csjvi-: tiooKs o(i"Uin;i-:\er: l{oeks <d' this series oeeur as hirgt; intrusive dikes and stocks tlirougl out this district. Tlan- hear undouhiedly a diasehistie relati(Hi to tis hatholith. The (Huiiposition rajige in this gr(uip is from biotite grate diorite to liornhletide aieh-site, which is a greater range than e.Y,isfs lt« tween the extreme of Hie <'ar]ier iiitrusive.s. liiolUe (rriiiioiliorile.—^thut north id' where tlie Eio de ('hiyon turns an th'ws westward is a hoss of granodiorite whieli eu!:s the Barran<iiiita; IIOIXIH. ilFAilJJUV OF THF CiKl MOAH: AV A U A niHTHIFT jSi rayoy fierics. A iinn\\) of hills easl of ('<M|ui i> luriiKMl by a boss of thi^ r.Kik, the iiuir,triiia,l pha.-e oi: which is dioritt' and m-sir its ha^^c a dike of iHiirito atidositf and a How of vcsiciihir oljviiio-froc 1m«ilt are found (Fig. M\). Mega.scopicrtlly \he iwk eonsist^5 of 2 lo .'i Jiiiii. mn^m.^^ of dark .hivoh i.K.tite and occasional ^^pcckn oC fiornbl.-ndc or pvroxeno in a wliiu' to light ;i:r.H!n, fiidd of feldspar, lii sonic varieties tlie qurtrtz is iiof apparent in flii' hand sfiecinien. fn thin section the rock is (MiidgTanidar ami made np r.f fiTains whieli average o mm. in size. OligtM-lasc in dull, tidinhir. snle liedral, twinned crystals amoniits to .">.'» per cent. Some of the oligoelase K twinned aeeonh-ng \u hotli the alhite and ('arlsha<l laws. The Inotite lo 15 per cent. .Mueli of the hiotite has gone over into the chlorite. The lirsi oecurrenee inentioncd above has .1 ]>er etnit of idear, colorless aiigite, ilie second a small area of green hornblende. In the former tdie rpiariz "fi:»rs in iotcrstiiial material in the latter as mnJtit'orm grains wilh irumy strings (tf inelusi«>ns. In both, (piartz is prcstnd; to ahont 2t) per <r!d;, EqiuiTit grains of nntgnetile and indsnioid.-i of apatih' ami zircon 'ii-'an-. SerieJto is seeondary and epidote an ijitrodnecd rnini-ral. llomhletule lHorU(>.^^^^^^^Ah)vkH of this type oe<mr thronghont the older -^"rics as l)osscs or thick ininisions. They ai'e mi)re common than their '-er^grained, eqnivalents, tin- hornldcnd<. andesitcs. snggt'stirig a genoth- e iation existing ])etween drij^th and the occurrence of hornblende. As 188 SVIHSTIflV i^'Uin h'l' of I'Oh'TO KJiH^ ob$eTV(Mi ill till' ik'h'l tln' >izo of the grain riiiigei^ bciweoii 2 mm, to 5 {in-, em., tlie average Imng ;> mm. With few exee|>t.ioiis the rodiA a,rc coiu posed of gfiiLiis oC equal nmi am] these are tJioruughly iiitergrowri. h .iom,e, liowever, ihe h«.rJihU-fi.Je crystals gr-i wry hirge; a lew have bet:! observed wliicli were () em. leug. Tbi' Iioriihk'iuh'H are dceichMlly pris iriatic, blach and J'resh. The feld.s]iar,< n.re taimlar, rarely striated, am \'ary in color between vrliitf and pah- green, (ieiu'ratlv tbio hornljlcrn'h'; occur as interstitial matter between tlie feld8pars. A J'ew grain.s of niui.; nctitc are common. Tlie miereseope shuws th(> (isseiiiial stniei-ure of tliesi rocks to be seriate lunueoid, and comjjose*! of grains averaging 1 nnn. ii size. Ahont 50 per cent ol' this rock is made up of tat,)utar 1 mm. enhe TlnnMenilc lUori dral, broadly twirineil (-ryslals ol' andesine. ^lust of tliest^ a.n(lesiiie.< iw^ frcsli ajid clear, thougli: a few^ are >ligld:ly altereil to ehkH;it;e and zeolik=- A lew crystals of labradorite, k>ss ol* ortlioelase and many of oligoela- occnr. The hornblende is pale green, subhedral. arul pkH)ehroik>., eiinir inkTStitially and cinniilopliyi'icnJly, ami contains elunlocrysts ol i'eldypar Aronnd tlic borders of the }M>nddendos are large mrmbers of nnignetit' grains. Tlic hornl>leudes h,a\'e very raggYMl bnrd(>rs and tke magnetite occur VYithin the shadowy former Ixn-der of the crystals. A])pare!ddy Ih. iinignctitcs are left as a rcsidnnm after the ilissohdng awa;y of tlu) liee-p blende borders. Idornldcmk; makes n,[) 25 per cent of tlie rock. Ab«n; tlie border of tke hornbheidcs ami occnrring apparently as reeryskdli/:- tkm suhstanecs are tinv angile and ealcite crystals. Magnetite ocenrs i /lOJJOE, (LtOLOdY OF THI-: UitAMO^dUAYAMA^ DISTKWl' 189 :i!gi> iircab with ehrtdncryste of feldspar. Apatite ako occurs as a pri- HHry iiiiiieral. Tin; foldBpare are altering tei calcite, scricite and sccoii- iiry (piartz (l''ig. 37). !Iornhlfmde-Au(jii.e JJioril e.-^^>hmt south of Ln Lapa several dikes of ins variety of roelc wen; oh<erved. Considering tlio large iiinnber of Aigjle aridcsites, it is reiiiarkahli; thai; no smgitc diorites were foiind : or, view the matter from aiioth,er point, (lie large juuiihor of lionibleiide iiorites is in striking contrast witli the very few lioriibleiido aiidcsitcs. •rystalline, cqiiigJ'anular, iiitergrown, 3 miu. areas of dark green horii- fipude with sinnllp-'* ipliifn i\.|!l«|->,;jr fir-ofH. TTi.flpr \ho pd"r('.scopc this rock :fe*,,'ii; '^JW'^ ''4:gg!|^«.:»-:^^:- 'ipears seriate |}urphyroid witli the largest grains reacliing .05 nrni. Tlie 'Idspaj's ai'c chictly aiidcsine with soim^ (dJ,gi»clase and make irp al:iuut 50 •r cent of the J'ock. Tlie liornhleode is all altered io (dilorite and in and f:ir tlie diloritie areas, still retaining tlie h<vrtdilende sliape, are small "vstals of augite. Some of these augitos an> cmmi'lopliyrie. 'J'lio aiigite ji'refore appears to have been din*iv(»d fi't»nj the liornlilende. Lorjg 1.5 iii'i. needles of afsatite ocenr. Tlie feldspars are zeoliiizetl and secondary "•iftz is present (Fig. 38). Ifornblendc Aiidi'slle, OoraJdendi; andesifes occur as iliiek dikes. A <• one is exp»tsed on the Aibonito-^(k)anio Military Boad, betwer-n K. !)7 -i K. i)8, and several dil<r.s o.-.-nr sonili ef the fuann. Reservoir dam. 190 kifUENTIFKJ 8UlfM-]Y OF I'ORTO Rl€i III fill oecnin-oiicos the i*«m%s aro eoar.-^f', almoBt lioriililejule diorites. ^^'.. s]>ecimoris were ohscrrvod in lln- v(^»lcanie iuils. Iliuid s|)cciiiieiis sliov grains wliicli may approach I nun.; the avci'agf^! is about 1 imn. exei'pt in a rare occui'rcnce whicli lias a clivitiifuHl grouniJmaHs. The lal)rie is, a> a riilo, hiatal, with large; |)henoer\'st:s of plagioelaso and horiibleiide occur^^ ring ill tlio gi-oiiiulmsiss, whi<jli rnaj ho light or ilark green, gray, pal*^ l)hie or dull gi'ay. Tlie plagiocdasc; cryslals aro tabular, j*arely striat-'Nl and somctimoK crossed. Ilje hornblendes a.rr- hlaek. shiny, and Irigtdv prismatic. Ihider the inieroseope tlio fabric h. nsnaJlv f(?ujul to be Si'riate porphy^^ roid, though a few .may he seriate [MH'phvritie. The larger crystals, conr^ Fig. no. ^MomUeii'ie <ui<fi.xii S?/ir? niorily in tJie case of h<o'nt)lende, reaeh 5 iiini. in size and the fcldsp/uv .3 mm. The ciystals of the gronndiiuiss may be so small that the texture is nricrocryptx^crysialliiie, as in the r'asc of one luiirnhlende andesitc wi'i- a di vitrified matrix. Idie m<tft. eoinniun textur<j is diabiiKJc. The pheoe^ erysts arc horuhlemles and feldspars. The hornblendes are euhedral O' swbhedraT prismatic, dark gretoi, usually <:niunlophyric, and highly af^ tered to chlorite and epiilote. Huine eoiitain eliadocrysts of magjudif-. which commonly sliows octahech'al |>a.rting. Magnetite amounting h- •• per Cent also frequently occurs sectuKhii-ily as a product of rcorganizaii h; alionf the ragged b(u-ders of the hornhl..ii,h.s. The feld.<])ars are of ) ' oligoclasc-andcsine variety, with the envptiasis on^ the anrlesine. and a'- -"^ JIOtKJfl, GEOLOGY OF THE CiLiMiXlUAYA MA, DlnrillVT \<)\ ;i<£C ili per i!OJ)t. Om; or two per r-.mit of urlluH-hii^e DCeiirs at iiims aii«t an occasional crystal oC pla,gioela«;. Tln' feldspars luv irregularly twiiiiied iriid zonal. 'J'hcir shape is labiiia.r, as plioiHu-rysts. ur |)i'isinatie in tlu> U-roiiridniass. Tlio pliouoei'vst.ic felds|iiii's a,ri' coiiijiioiilv' fniintilophyric, crossed, iuid liiglilv altoivd to .'hlurilf! ami ealcitc. 3fagiiclito oeoiirs m nqiiaiil t-rystals up lo 5 \w\: t-eiit. DtJier primary minerals a,ro a|)aiite, pyrite auxl ilmeuitx!. The gremudruass is eoiiiposod ot i'okispars aud Magnetite and gfuicrally is ontij'oly alieriM.l. Tjit; ratio ol ground - mass to pliouocrysts is OCI to 3 L Oiher secondary minerals are zoisite aud leiieoxeiic (i*'ig. ?•!)). 1l(mihlcn(k--AwjUp. AnMmh'. l)ike< uf ihi< rook .soeur auioiig the aii- pie andcsites and are particidarly commou near l.a Lapa ami north of lie Bafios de ('oamo. Tlicsc rocks in hand speoimmi show H mm. plaaio^^ :rysts of hornljlende. These are set in a light gi-een nuitrix of feldspar iiths. Iji tliiii section they are serialoHtd^ersertid. Augite crystals form he largest phenocrysts, iimoiudJng to 10 per i-ont. They are pale green, d.ear, fractured, ami ehanxid; the liornWeiidcs arc prisuiatio ami green Mid are altering rapidly to tdiloriio, 'Idio liorid)lon<lo t.cears both as plieiav s'Ysts aud tlircaighont tlic gnnuuhnasH as shreds. The total aniowrt of I'irnbleude is 10 per ohjP., Oligoolaso-andesine in euhedral, zonal- 'vinued, ]udsiuatie. dull crystals make up OO 'per cent. Magnetite- an<1 -aiite oeeur (Fig. dO). 192 SCIENTIFIC SURVEY OF PORTO RICO GENESIS The rocks discussed above are thought to be diaschistic dikes given off by a batholithic mass which underlies the Coamo-Guayama district. The range in composition is greater than in the earlier intrusives, which is due to the fact that a batholith which ascends to higher levels will show greater differentiation than at lower levels. This is due to the greater time for differentiation and the wider range in temperatures and pres- sures. The larger intrusives are more acid; the largest observed were of biotite granodiorite and the smallest were of hornblende andesite—that is, the nearer the intrusives approach in size to the batholic rocks the more acid becomes their composition, or the nearer the diaschistic dikes are eroded to the batholith the more their composition approaches that of batholith. The succession of these various intrusives was not determined. An;ong the older andesites were a few hornblende andesites. The horn- blende diorites are always thick dikes and much more common than horn- blende andesite. 'No augite diorites were found, but a few hornblende- augite diorites were observed. In all these, as described above, the augite is reorganized from the hornblende, and the hornblende often shows re- organization to magnetite and secondary augite. Such facts show that hornblende as a mineral is genetically related to higher temperatures and pressures, and augite to lower temperatures and pressures. Conclusions as to the Ages of the Various Series sedimentary rocks In the above paragraphs two pronounced unconformities have been de- scribed. One exists between the Baranquitas-Cayey and the Sierra de Cayey series, beneath which some fossils were found and which give a clew as to the ages of the older rocks. A short distance below this uncon- formable contact two calyxes of a coral were found in a limestone which have been identified as Cladophyllia furcifera. Ordinarily this form is considered a typical index fossil for the Edwards division of the Freder- icksburg group of the Comanche in the Gulf region. Its presence here should indicate that the rocks are at least of Comanche age. Below the limestone occurs a thin bed of bog iron ore and in it were some leaves. Dr. Edward W. Berry and Dr. F. H. Knowlton identified these leaves as follows : Nelsonia, an old Mesozoic order of cycads. Proforhipis, a fern with same range as above. Another species of Mesozoic fern. A dicotyledon. HODGE, GEOLOGY OF THE GOAMO-GVAYAMA DISTRICT 193 The above authorities stated that these plants were not critical for the Comanche, but that they tended to support the evidence of the corals at least they indicate Mesozoic age. In addition to the above, at several horizons some radioliaria, foraminifera and diatoms were found^ as fol- lows: Textularia gibbosa^ d^Orbigny. Textularia conica, d^Orbigny. Textularia triochus, d^Orbigny. Pleocanium (Textularia) speyeri (Reuss). Operculina sp. Orbitoides papyracea, fragments. Orbitoides faujasii, Sowerby, fragments. Paralia (Melosira) sulcata, Ehr. Coscinodiscus elegans grev f Parodiscus concentrica (Flustrella concentrica, Ehr). Lithocampe with seven joints and flatly compressed edges (Fig. 5). Orbulina universal, d'Orbigny? Textularia gibosa is a form well developed in the Miocene aiul earl\- Tertiary and Textularia conica, d'Orbigny, is common today in the coral reefs of Porto Rico. However, Textularia, as a whole, is characteristic of the Lower and Upper Cretaceous ; Operculina and Orbitoides occur also in the Cretaceous. Eadiolarian forms, though very numerous in the Mio- cene, are well known to occur far back in the geological scale and Orbulina universa is common in the Upper and Lower Cretaceous. Thus while some have great range, yet in view of the fact that the most common form, Textularia, is generally Comanche or Cretaceous, it is probably safe to say that these smaller creatures also support a Comanche age for this system. I conclude, therefore, that the above rocks are Comanche in age. Immediately above the Sierra de Cayey and Barranquitas-Cayey con- tact the only fossils found were several Orbitolites forms, which signify but little. About two miles north of the Rio Jueyes Water Gap and close to the Rio Jueyes River, in a very calcerous tuff, was found one speci- men of Venericardia alticosta.^ In the Gulf States this is considered an index fossil for the Jacksonian and Claibornian formations of the Upper Eocene. But some fossils which are indices of certain horizons in one place are often found to belong to beds of older age in other regions. This may prove to be the case with Venericardia alticosta. Large numbers of foraminifera, radiolarians and diatoms were found in these rocks. In all cases the minute organisms were only discovered' in thin sections and for that reason their determination in many cases was ^Identified by Rtifus M. Bagg. * Identified by Prof. A. W. Graban. 194 SCIENTIFIC SURVEY OF PORTO RICO found impossible. Sufficient were determined to testify very strongly to an Eocene age for these rocks. The types observed follow :^ Foraminifera : Orhitoidea papyracea (Boubee). Textularia gihhosa, d^Orbigny. Globigerina huUoidea, d^Orbign}^ Truncatulina lohatula (W. and J.). Meliolina seminulum, Linne? Textularia sp. Gand7"yina sp. Polymorphina sp. Miliolina sp. Biloculine sp. Numulites sp. Orbitoides sp. Amphistegina sp.? (Fig. 19). Discorhina globularis, d^Orbigny. Globigernia sp. ? (Fig. 42). Ostracods sp. ? Heliodiscus htimboldU, Ehr? Porodiscus concentricus, Ehr? (Figs. 20 and 41). iJiatoms : Melospira arenarm, Bab. COScenodiscus marginatus. Mel sira sp. Among the above fossils Porodiscus concentricus is of early Tertiary age (Fig. 41), as is Amphistegina (Fig. 19). As a whole, this group of minute forms and Venericardia indicates Tertiary and some point directly to the Eocene. Apparently the Eocenic is in the older series. That this is so is further shown by the age of the next succeeding group of rocks. The Eocene age of the uppermost rocks of the older series has some bearing on the problematical presence of the Cretaceous. For, in view of the universal presence of Cretaceous rocks in the islands of the West Indies which have been studied, it seems fair to assume Cretaceous rocks in the Coamo-Guayama district. There may be an unconformity be- tween the Eocene and Cretaceous ( ?) which was not found in the limited time devoted to this study. The next great break is the discordant unconformity separating the Rio Descalabrados formation and what little remains of the Arecibo in the Coamo-Guayama district. The age of the Arecibo is rather definitely fixed. Fossils were collected from it on the north coast by Mr. E. D. •' Idontified by Riifiis M. Bagg. JIODGB], UKOIJXIY OF THE t'OA .HO-a l\\ VA MA JHSTMJOT 195 •rentiiciift in i,iiJ(i,^e<niH xhale ,*iprrr .^^ 196 BGIENTIFIG SURVEY OF PORTO RICO Semmes and from it in the south coast by Prof. Charles P. Berkey. Both collections were identified by Dr. W. H. Dall. On the north coast were found : Metis trinitaria (Dall), an index of the Bowden formation, Upper Oligocene of Jamaica. Pecten sp. Haminea sp. Very abundant. Cypraea sp. Abundant. Amauropsis sp. Conus sp. Ampulina sp. Stromhus sp. Orhicella aff. cavernosa (Linne), Fringed coral, not determinable, but similar to those found in Oligocene of Antiga, Anguilla and Cuba. Dr. Dall considers this an Upper Oligocene fauna. Metis trinitaria^ which is typically Upper Oligocene, is stated by Mr. K. D. Semmes to occur at the base, as well as in the upper portions of this formation. Professor Berkey collected his fossils west of the mouth of the Descala- brados Eiver, just outside of the area under discussion, and west of Ponce. Care was taken to gather them from the base of the Arecibo limestone, which is the younger series of Berkey. These were identified as: Lucina sp. Lucina sp., much like forms from the Chattahoochee. Pecten sp., like Pecten thetidis (Sowerby). Fasciolaria. Amauropsis ocalina, similar to forms in the Ocala limestone of the Gulf States. Mr. K. T. Hill (1899) states that the Arecibo limestone is in part equivalent to the Arecibo on the north coast and though largely Pleisto- cene, the lower strata may be as old as Oligocene. Of the older series Hill (1899a) says: ^''The rocks of the central mountain region are Cre- taceous and possibly of early Eocene age ; at least no evidence tending to establish other dates has been as yet discovered.^^ Since these limestones of Upper Oligocene age lie upon a peneplaned surface, they indicate that the Eocene exists either in the great unconformity or is to be found in tlie older series. The presence of Venericardia and the large number of Tertiary foraminifera diatoms and radiolaria seems to suggest that tbo latter is the case. If the age of these formations is correctly determined, then a great break lies between the Eocene and the Oligocene in Porto Eico. The HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 19 'i character of the Eocene-Oligocene contact in North America is poorly understood. A summary (Cooke, 1916) of the views taken regarding the contact in the area where it is best known shows that the various au- thorities differ by as much as a whole geological period. Could this divergency of view be explained by the existence of an unrecognized unconformity ? The Santa Isabel formation is undoubtedly recent in age. Fossils collected from railway cuts and from terraces south of Coamo Springs were compared with specimens washed up on the south shore of the island and for every fossil species an identical one could be found among the types still living. The stratified -character of the fossil horizons, the presence of corals and sponges, and the total absence of human artifacts led to the conclusion that the fossils found were deposited by natural agencies and that their presence indicates an elevation of very Recent origin. IGNEOUS KOCKS The igneous rocks appear to be divisible into three age groups. In the first group they occur mostly as dikes, flows, sills and volcanic fragments of which augite andesites were found in greatest number. This is shown hy their dominance in the early tuffs, by their presence as flows in the older series, where they occur as intrusions, and by their comparative rarity in the vicinity of the granite batholith. Types sucli as basic augite andesites, olivine-free basalts and olivine basalts, in so far as these rocks occur, are to be found under similar conditions. Feldspathic andesites are to be found in the older tuffs, but in view of the great number bear- ing a peripheral relation to the granite batholith, it is more reasonable to postulate a later period at which a great many of the intrusive feldspathic andesites developed. The second group is younger. The granite batholith cuts Comanche and Cretaceous sediments and probably, though it cannot be proved, cuts Eocene sediments. The point which is important is that the igneous rocks classed in the first group occur as component parts in sediments which are cut by this granite batholith. The top of this batholith is truncated by the peneplane developed in Porto Eico and the invasion of the batholith is, therefore, antecedent to the peneplane. The direct evidence that a third stage exists are the dioritic and hornblende andesite dikes which cut the batholith at various places, liotably north of Virella. Throughout the Coamo-Guayama district biotite granodiorites, hornblende diorites, and hornblende-augite diorites occur. These are so different in character from the rocks known to belonsr to 198 SCIENTIFIC SURVEY OP FORTO RICO the first stage and so like those which cut the batholith that the inference is that they are genetically related to the granite batholith and later in age. The feldspathic andesites are best developed close to the batholith^ and seem to be, therefore, apophyses of it. Not all the angite andesites- can be proved to belong to the first stage, given off in the third stage. Some of them may have been SUMMARY OF AGE RELATIONS The following tabulation summarizes the age relationship of the various rocks : ' Eio de la Plata series } r^ i, Barranquitas-Cayey series f ^^ommche. The Older Series. Sierra de Cayey series Guayama series Early intrusives Eio Jueyes series Coamo Springs series Eio Descalabrados series Batholithic rocks Later intrusives ^, ^^ rx • ( Arecibo formation The Younger Series. ^ g^^^^ j^^^^^ formation y Cretaceous to Eocene. Oligocene to Eecent. HISTOEICAL SUMMAEY Under the heading of '^genesis^^ and elsewhere, the geological history of the Coamo-Guayama district has been described in detail. Here only a summary of the more salient events will be given. Early Period to End of Comanchio Time The very existence of Porto Eico is dependent upon a volcanic past. This much can be said from the character of the sediments which are of pyrogenic materials, slightly reworked or not reworked at all. That the island has been built up from the ocean bed by the accumulation solely of volcanic debris is to be doubted. The more probable view is that the vulcanism which produced the sediments was only a superficial expression of the major organic forces which elevated the whole West Indian region in early Comanchic (?) time. The embryo Porto Eico may even have stood above sea-level when the first volcanic activity began. The practi- cal absence of glassy types in oldest known pyroclastic rocks, or, for thai HODGE, GEOLOGY OF THE COAMO-GVAYAMA DISTRICT 199 matter, in any of the rocks, seem to indicate relatively slow cooling, which would not take place in the cold water of the profound ocean depths. The earliest record in the Coamo-Guayama district shows intense vidcanism. The materials ejected or poured out had little or no rework- ing. Quiescent periods occurred during which the sea advanced over former sites of deposition, as is shown hy the several beds of limestone which are free of tuff fragments. During the latter portion of this era viilcanism apparently decreased^ for the uppermost rocks are reworked tuffs and ashes. Cretaceous and Eocene The deposits accumulated in previous times of intense volcanic activity were reworked and redeposited during this period. The Sierra de Cayey period was oue of sudden and pronounced uplift. The uplift, which was either that of a broad arch or a fault block, revived all of the stream^^ and the result was the creation of great alluvial fans which sloped down from the mountain toward the south. This uplift was probably not very great. A small change of elevation will cause decided erosion. At the time under consideration the elevation was probably not much greater than at the present time. The essential factor in rejuvenated erosion is that at times the stream becomes torrential and carries down a mixed load of boulders and silts, followed by a period when the stream is in- capable of removing the boulders, but can separate out from among them the finer material. Thus in the present flood-plain deposits and in the course of the Salinas Eiver large boulders occur in great profusion, many of which are as large as any occurring in the older measures. The same conditions are exhibited on the Coamo Eiver and several streams on tlie south coast. The alluvial fan condition, initiated by the Sierra de Cayey period, continued through the Eocene, After the first impulse of rejuvenation had worn itself out, the streams eroded and transported finer materials on a reduced grade. Periodic outbursts of vulcanism developed tuffaceous measures. Slight oscillations of the Porto Eico block permitted the sea to advance inland for periods long enough to develop thin limestone beds. As a whole, however, the data testify to a very little diastrophic move- ment. Most of the limestones were formed in embayments between great idUivial fans and developed synchronously with shales brought down by ''ivers or tuff intermittently thrown out by volcanoes. As the island was gradually worn lower and lower, the batholith underlying the Coamo- ^'Uayama district, except in the southeastern corner where it outcrops, ^•'1^ .i^radually working its way upward. Long before the end of the 200 SCIENTIFIC SURVEY OF PORTO RICO Eocene the upper portion of the batholith had come so close to the sur- face that it froze and was partly eroded. At the close of the Eocene the general baseleveling, which had been in progress since the end of the Comanche, eventually reached its highest degree of perfection in a peiie- plane and the land surface became so low that the sea was enabled to advance over it. Oligocene At the time of the marine transgression vulcanism must have been absent; otherwise the basal portion of the coastal deposits would contain much volcanic debris and the limestone would contain many tuffaceous inclusions. ISTeither of these conditions are conspicuous enough to have been observed ; on the contrary, the base of the coastal plain deposit is a carbonaceous shale. This carbonaceous basal shale indicates that the sea advanced rapidly over an area pretty well worn down. If the sea had transgressed slowly a perfect sorting of materials would have ensued from marine-planed old rock and this would have given conglomerates, sand- stones and shales. But if there existed a peneplane, covered with a deep residual soil and supporting a heavy growth of tropical vegetation and the sea advanced rapidly, then only a carbonaceous shale, followed by limestone deposits, would have been formed. The result of the over- lapping sea was the deposition of a heavy limestone formation. Miocene to Pleistocene The first event after the deposition of the limestones was uplift. Berkev (1915^ p. 40) writes as follows: The physiographic habit of the island as a whole tends to support the view that the fundamental structural form is that of a large fault block, with the principal fault displacement and uplift along or near the southerly margin, tilting the whole mass gently northward. If this disturbance took place, as seems to be indicated by the fault described, in very late Tertiary time, accom- panying the emergence from the sea, it would account for the abnormalities of Tertiary rock distribution as well as the unsymmetrical position of tbe main drainage divide. In any case, however, the fault block structure is s. very late development and is superimposed on the other more complex aiKl older structures of the mass. The next event was the removal of the Arecibo and dissection of tln^ underlying older series. So completely has this been performed in tlie Coamo-Guayama district that only one outlying hill remains of the Coastal Plain limestone deposits. Dissection has continued to the present time and brought the district to a state of maturity and has been accenii- HODGE, GEOLOGY OF THE COAMO-GUAYAMA DWTRICT 201 panied by stream adjustments^ but without equal success, for the streams are about as imperfectly adjusted as possible. Pleistocene ( ?) and Eecent Judging purely by analogy, Porto Kico suffered first a major uplift in the Pleistocene (?). This first movement, intended to satisfy internal earth stresses, probably not only compensated these stresses, but over- reached its mark and produced others of an opposite nature. To relieve tliese latter stresses, oscillatory movements of the island are now taking place. The frequent earthquakes throughout the West Indies are part of this phenomenon. One effect has been to cause a recent elevation of the island. But before this recent uplift took place the island remained sta- tionary for a considerable period, during which the Caribbean carved a platform far into the island. Thus west of the Coamo-Guayama district a terrace is cut into the Arecibo Oligocene limestones. However, the total cutting in the whole island was so little that nine-tenths of it still re- mains mountainous. When the sea had finally finished its carving the shore stood much farther inland than at present and the site of the Guayama was beneath the sea and those of Salinas and Santa Isabel were between 50 and 60 fathoms deep. The surface of the Caribbean was nearly awash with the tops of the Aguirre Hills, Salinas Hill and Monte Sebater. Great bays were formed in the valley of tlie rivers, one of which extended up Guamani to about K. 24, another reached La Lapa in the vicinity of Salinas, a third reached as far north along the Eio de Coamo as the Bahos de Coamo, and the great subsequent lowland behind the Coamo Springs limestone range was a salt marsh. Those hills which stood as islands during the sea^s farthest advance, or as reefs 25 feet or more above the sea bottom, became surrounded on all sides by the coastal plain and some sediments now all eroded away may have been deposited upon them. The isolated hill southwest of Coamo Eeservoir, the low hills southwest of Salinas Hill, Salinas Hill itself, Monte Sabater and Aguirre Hills and the hills near Melonia Eeservoir are now islands in the coastal plain technically called mendips or morros. The high hills east of Guayama were at one time mendips, but erosion of the coastal plain to the north has converted them into peninsulas extending into a coastal plain. After the sea had advanced as far inland as described above, uplift occurred in two successive stages. The result was the formation of two terraces. The second uplift followed closely upon the first, because the ^ea -cliff cut is small and the size of the lower terrace is much the larger. 202 .SCIENTIFIC SURVEY OF PORTO RICO VULCANISM Volcanic centers are typical features of the Coamo-Guayama region and of many other parts of Porto Kico. In this district volcanic vents were observed in the following places : South of the Coamo Eeservoir, about a mile south of the Rio Descalabrado Water Gap, between K. 98 and K. 99, and between K. 87 and K. 88 on the Military Road, several on the Rio de Cuyon east of the bridge on the Military Road crossing it at K. 93 and a very large and complex one east of La Lapa. The one, south of Coamo Dam on the Military Road, can be seen in cross-section. There the strata are cut by numerous intrusions, mostly of a dioritic character; the sedimentary beds have been shoved, crowded, broken and thrown out of their former attitudes and the intrusive dikes contain large irregular fragments of the sediment, which are hydrothermally altered. Tuff accumulations occur irregularly between the sediments and the dikes. On the Descalabrado, where a volcanic vent is to be seen in plan, the same features are shown and in addition the rocks are highly fractured and cemented by epidote and calcite. The center east of La Lapa shows aii old vent, now filled with coarse volcanic breccia, with much tuff and intro- duced epidote and calcite. Surrounding this vent, within an area of about two miles square, are hundreds of augite andesite intrusives and flows and less numerous dikes and flows of feldspathic andesite a^d a few hornblende diorites. These numerous intrusives, cutting shales, whicli are intensely altered hydrothermally, form a complex the structure of which is impossible to decipher. In the bed of the Cuyon a limestone formation is broken through by a vent, which is filled with limestone fragments, andesite tuff and augite andesite lava. With the one excep- tion, between K. 87 and K. 88, all vents cut Eocenic sediments. Older ones undoubtedly existed, but their presence has now been effectively concealed and the only conclusion to be drawn from the location is that vulcanism continued into post-Eocenic time. Of course all the super- ficial physiographic features produced by vulcanism were wiped away by the post-Bocenic peneplanation. IIYDROTHERMAL EFFECTS Mention has been made of the highly fractured character of all the Porto Rican rocks. These fractures are usually cemented by introduced minerals. Some rocks, in addition, show metasomatic replacement and a few others are almost entirely replaced. Whether the minerals are simply introduced or in part replaced, all are considered hydrothermal. There is direct evidence in some of the addition of water; in others only HODGE, GEOLiKIY OF THE COAM<i^r,'U_\YAM.\ JH8TBIGT 20:; certain constituents liav(> hccn liiHsolvc*! nwiiv, a})purt>!itly by wat.srr. utid III many the state of llie iiiiuorals iuilu:iil(>s tnily tlie presence of sojin* iigejit, sucli as watcir, wliicli lacilitate<l clHMni(;<il reorgajiizatioii without iLic jntrocluctioii or subtraction of any component. TLie Mk)wiiig types aC hydruthcrmal alftiralioii will he considi-rod : (1) Those in wliiiili one or mors- oC calcih'. iiiiartz, chlorite ami epitlote wcTi; developed. (2) Those in which z«ditps. ealcitr. and ddorili; were fomied. (3) Those in which epidot-c, finartz. ehlorilu and actiriohte. one or more, were iritrodiieeih (4) Those in wliioh lieiiiatitc and qnarlz wvyo ini.rudoeed.. Fia. 43. CMnrilc 'Jvfi-!in>iii;i fn.m J,:hlHi, Ih fresk aut/iie' (1) Weathered surfaces of Vurtu Un-dn meks show jiartial or complete Ireration of tlic h-mag minerals, especially of tlie ]»yri«i'ncs. This is in I liking contrast with tlie clear, fi'csli, unallfred conilitJou of the same nnurak within^ the hr)dy of Ihe J'ock whoro wnatla-ring inllnmcs hnw <vt rcachech Feldspars, on flie otlier Inind, oven in tlie ,mu>i, durahh^'. rm and uirweatliered rodks, art; generally altorrd. heldspars aro com- louly entireJj replaced in parts of tlio rocks far heyornl any influence of 'gathering. Tlius in n prospt-ct <ni ihe slupe< <>( the LinpiJllo Mountain ino-e a sh'aft Inid Ix'.ui >unk AO h'ot IjcIuw Hm- w.-afdieivd zone, the hdih^ purs were all altered. Ihie foldspars am univi-rsally doll and Jiist«oi,.s> 20+ HVJILXTIFIC t;VR\l-]Y (IF I'OKTO RKJO iiltcrcd iJiaii in tlie iutt'j'ior of iiowrtfliered ^<H;k^5. The above facts leaf; to tlie eoijcliisiou that the ultenitioii of the fehlsparn is relatcfl to seine ijiteruai agency. M'any of tlio fehlspars show little patches wiiliin them of calcitic alter- rttj'ons; others are replaced by pseii<loiiior|>hs of eahriii?. The feldspatliir .aTfiiindiiiasst'S alter I'rtsily, frecpienily and eompletely to ealcitc. Thi- JVhk^pars arc all ealcie and only tlie athlitiuii uf (.'O, is rcqnired to alter tiicni. Almost tivery J'uek Khowt- (•artKwiate and Htnie are altered so com- plctelv thai, in the ca^c of sills or Hows, they are easilv nd^taken for lime- shnies. Tin's alteration is not eonfincd to igneous rocks, but is found in sediments which eontaiji ealciunnbc-siring niiii<n'als. It is not an exa«>-<i"t'r^^^ atiori to say that 20 por eoit of all llic roclis in the (Joa:uio-(Juayama, dis- trict arc altered to cart»onate. Iror sucli an intensive alteration CO^^ bearing water soliitioris nmst Ijavc been coming up from the depths in greai; A^(dum(>s and for long pcrio(Ls of tJmc. Tlie ealeite so formed dc't^-s not always remain in ])jaee, Imt migrates to othur rocks or is dissolveif out of the riteks entirel.v. Thus the deposits (d* tepatate, frecpicntly ol)^^^ served in the stream Iteds, result from the preeipitation^ of the d;issolYe<! eaJcite and apparently the streams arc carrying concentrated solutions «e' concentrated ealcinm (;arbonate to the sea. llartlly less eomnnxn than the ealeite is chlorite. The flrst place lien rhiorite develr»p- '- ah>rig cleavages or twinning planes in, the feldspar^^ HOUai-J, (IHOlAHir of the COAMHAiVAYAMA nwrRIVT 205 \\1icre well (Ic'vcJupcd it i> alway;^ in ;.t^^.><o<'iati(»n with tlio CeldsparH aiul <iiow< jliroot evidence! of h(>iiig ii-cnetieiilly relaft'd to tl»c JVldspars, Jii the rof'ks where; elilorile is best developed the ferromagiiesiiim miiu'ral aii»it(; will be iAi-AY, fresh and irnaJI;4M-e(l (Fig. 4;]). In th(> ease of chlorite we iiiuBt grant tlie introdjicticvn of mag'riesiiim. ]:'rol)ably water solutions .(L niaguesiniii earboiudx' reaeted with the foldspiirs to i'orm CaCO... and rlie liydruus magnesium alriniiiiiuni silicates. The ealcitc and ehiorite are. as a I'ule. iiitiniately associated as if thJs wert; tJic ease. Seeou,<la.ry .[iiartz woidd be rifleased by sneli a reaetioji. and line-grained secondarv Miiartz is, tlierefore, very eoniinon in these roeks. lud'lofe at times rc- I'liices till! feldspars and oeeiirs aloin* or in associatioti with tlie above iiiitieralsi. It can be formed from the calcite feldspars direetlj by hydra- tion (Fig. -kA). (See figure 45.) The phenomena, cliseiissed in t:he above paragra,p]i ar(! principally tliose 'r reorganization phis slight additions of inagnesimn, ( 'O, and IF,,C). Ifot 'va,ter, however, was the stiniuJani. which oeeasioncd the reorganization. (2) Closely related to the aliove alterations are the zeolites. Strange ! = • say, the zeolites wer(> iwi fonrid tilUng ainydaluitlal cravifies nor are the ' "lites eonnnon ajul iliey ahvays (jceiir intimab;ly associated with ehlcn'ite. \''oiind the periplun-y ol ilK!se zeolile-chlorite areas is, commonly, a :-r.1er of ealeitc. 'Hie shape aiel the assoeiafion whh more or less altered 20(1 saii-JXTiFic !Si^/n'i-:Y of i'oiito jikjo feldspars is siicli as io prove a <liroci flcivelojiinent from tlie calcic feld spars. Kcorgairizatio]!, siiiriulatod t>3' tJicrninl wat(?rs, bcuiring in additioj to niagricBiiiin carbonates, also potassium carbonates, wi)idd pnjdii.,-. (jhloiite, zor)lites, and ealcite from calcic fe](ls|)ars. It appears that n the secondary minerals formed ealcite was the most mobile, for it trav elcd to the pcripbery of llic groups of secioiidary minera.ls so formed <> even fartlier (Fig. -15). (B) The greater portion of tln^ liydi'otlierrnal rocks contain the quart/; epidotc, chlorite, actinolite and^ zeolites, one or more, in rdiJirir vesieidii cavities or fractures ami flic boirndaries of tla^se cavities show little or e- repbu'crnerrt (Fig. HI). In a few eases the repbu^emeni lias proceeded i such an extent tliat tins vvbole !)ody of tlic rock is a mass of ej'o'dofi 'bbough epidotc was not tbe first mineral to eomc in, it was in Bia.ny rock tbe ord^y one and (piaMiJi:ativ(dy it plays the major rob}. In t^lic i.v|)cs •:. roeli dcscri 1)0(1 above epidole can^ l>c dtdlidtcly proved to have clevelopr-- originally from tbe t:eldsprtrs lyv rcorgauizaiJon (bb'gs. 1:4^ and 47). 8ivn) ol: tlie (ypidote so duveloprnl niay tiave udgratetl and tilled the veiidtd,s aiv vesicilcs in the rocks of tliis tyfx:-. if so. tlie epidote in some of its occu- nmees came in from distant, eb,'c|)-seated sourctjs. for it occurs in veins ;• in vesicles in rocks wbiefi are stJ'ietly Iresb and mialtered in all otber p:e tieulars. M^oreovcr, ifs intimate as?(M>iation with actinolite ot nndonltt-' introduction argues very strongly foi' a deep-s<'ale<l source (bbs". ifi). ' HODGE, aiJOlJXJY (kF TflK COAJKhdlWy \MA DIHTJIIVT 20 T it^ tlifficiilt to expliiiii tile noiuplotc rc])I,a('i'iti<>iit unless jiitroductioii \sm under intense hyclrotlicfnial coiniitinns. M\ the iriiiie]-a1s did imi jK-cessarily cuine in n. deliniti' .^nccessiuji. [ji -oiiH' roeks (Fig. dO) ciiiajdz, epidt^.tc am! adinolite si-ejo to have e(nrn' in aJiiiust HinrulurnoouslY. WJiorc the snci-essiori is best; shown, tlie order is .iniirtz, epidole;, cldoriti', aetiiiolite and zuulit.-s. In a few rucks epidolu v(%-\iv^ on tli(3 walls cd: veins whicli have a (piartz liHing, showing rpidote to Iiave been iotrodneed first, but tlio balance ut the cvidenoc indiertte^ liiat quartz began to tlcvcdofi first and continne.l to t-unn' in with the lan-r Aetinolite ncxnirs as isohited needles, as needles pejjetratlng .pumz, ami iis rsidiating oT(»wths a.ini the occurreiH;*; of net i noli p.. in .mudi rchiri,onslii|)S is peiierally regartied as sure proof of igneous ennmaJJons, The aetirioHte i> iHii, stalde, lint readily liydrat(w and ciniverts to (Jdorite. It is rnetase^^ iiuitie only in so far as it pencti-aP-s puailz ^ a ph.'iHunenon, very eonmion 01 roeks altereil by ignj'ous ematuilions, Aji interesting feature, rattier eonunr>n. is th,e assor-iation >.>!' zeohd.e-^ '^vilii ibf; aetiiKdite frerpjent!}' in intimate intergrowi:h. IT the aetinolite - an igneons emiination. Uien the zeolites nHi<r also have been iidi'odueed I'V tlieruial waters at Idgli temperaturt;. Anutlier feature is tlie extreme ' •vi'lopiueut of secondary quartz. Shales are converietl enlirelv to ehecfs ' ' fill- (fuartz. 'Ilir- association of the quartz with, tlic oilier niinerids of •im ^Vl KSTIFK; f^VKVEY OF POh'TO Rli'O llii.s groii}) pro\es it ti* he livdrotluTiiialiy iiitrixluei'il hiuI iwi the rt'Hiili of reorgiioizatioii ol' radiolarifvii and diatom sliellF^^. a.s is tlu> eaf^e in suni^ of the sedirrlCTlt^:. Tlie scdijiiciits sliow the cikureoiiH sludlh of foramijiil' I'ra rcplnecd by .soeoudnry (iimrtz. jii otljA-r rt»eks eiilciro of hydrotlHirmnl ui'i.a'iji ])JainJy repIiieoH tlie (iiiai'tz. Kvideiitly inert caJcitc Ciui lie riiplac'i'd Ity hvdrotlierinal quartz, hut wliuu 'hoih, caleite aM (luart./ arc hydmth.-T i!i;d tlie qoartz has a greater sfdutioii prcHsure aud is readily replaced ev ejih'ito. In the andesiti' sei:(uidary quartz will devehtp iii au.recdes an. I fheii by rcorganiznrion take «»n the trapezuhedral \in'm. When lirst nh^ h:ervefl the writer eoiisidfircd Hueb rocks to lie qiiart;z andesites. Further :rl(lit!)' firoictllH r,f i study shows that tlic (puirtz is aH secoruhvry. It is ])<issihlc that nijin\' *:< the quartz audesih's described in the literature were ol this origin. (^^1) T}w. four til ty])e (Jcals with scune eli't^cts wliieli ari; irdxvrestiug he CfUise tlicy sliow a liydrothenrud iniroduolhai ol hematite anj.l Imhuus of the possibilities involved il thi' process \vvni carried to eoiufdeiior J^hales are silieifjt>d hy spheruliti(! growths of ehalceduny. ^Ik! growii of these ehalt'edonie libers was spluu-ieal au<l oearly ecjiial in all direct i<'C aud tho growth was frecpieutly interrupted, probably due to tlie interini! taut supply ut fecdiug sohitdous. Syuchrouously wiih the growih of \h (lialcedony sptierulib's. specular hematite was intTodnced. It was d.c posited iutorstitirtlly, r'hi(-fly hetwciai the sphernles. aud wdiou the sphere HODOE, GEOLOGY OF THE GOAMO-GUAYAMA DISTRICT 209 lites were not -growing formed a hematitic shell about them. After a period of continued growth of the spherules, another shell of hematite developed, and repetition of this process formed bodies of spherically radiating fibers of chalcedon}^ containing within them and around them concentric shells of specular hematite. After the cessation of chalcedonic growth hematite continued to be introduced and in some cases has meta- somatically replaced the fibers of spherules. Where this has taken place bodies formed of a succession of concentric shells of hematite separated by radiating fibers of hematite are cemented together by hematite—that is to say, a typical oolitic structure is formed. The oolitic structure is emphasized in thin sections by the hydration of parts of the shells to limonite (Fig. 48). The petrogenesis of this rock was not worked out until too late to collect critical samples. It is thought, however, that the continuation of this process to completion in a large shale bed should produce a rock that could not be distinguished from oolitic iron ores. Or, to go even further, why could not some of our oolitic iron ores have been produced by hydrothermal action ? The phenomena described in the above two paragraphs are developerl best in the vicinity of old volcanic centers. The two best occurrences were found about a volcanic center near the Quintana thermal spring, north of Ponce, and about the La Lapa volcanic center. MINEEALIZATIOK Mineralization is a feature considered to be genetically related to the hydrothermal effects and is just as common. ISFearly all the rock of the Coamo-Guayama district contains introduced sulphides, mostly pyrite, though chalcopyritic pyrite is frequently present, as shown by the copper carbonate stains on the rocks. THE COAMO THERMAL SPRINGS LocATioisr AND PuRrosE OF Study The Baiios de Coamo is a thermal spring located just north of the Coamo Water Gap and issues through a bed of conglomerate and tuff, which rises as a boss of rock through alluvium (Fig. 49). The point at which the spring opens is about 50 feet above the beds of the nearby Coamo River. Practically no deposits are being formed by its waters. The thermal and mineral properties of the water have for many hun- dreds of years been regarded as possessing medicinal properties and it is KiJlPLXTIFlfJ *N'l7/i!l7';i' OF POHTO 11100 from tlijs fact tliat it \m> of il,.s 'waters is tlio suln. ved tiie name jiailos ile ('oauio. The < discnsHion in tliis section. OCCUUHKXC'K G« AND IVl-JJ.S llceoirnaissaiiee studies o?er tlic greatcf part ot the is?la.ii(l i.iy the write n.l obdervatioiiH loade l»v wveral geologists over all of Porto llieo ijid :ite that, with this on*- exwptiun, no springs oi: aipv kind an' to i; )U!id in the <ddcr series of rooks. Hicre iirc iiuuiv s<u;pagcs at th /^e G'iyoJ ^e:C»afr. TiiulOKfai.lied hy tip toil. rains. Thesti raiinui he. fonsid(>red springs and do not afleet our dis- riissioii. Nh.t; only iw springs absent fi'oin the (dder series, hnt wells Slink iid:o them have never been suceesstuh 'In the x'oirnger sej'ios, how^ e.ver, springs do oeeiir at tinuiS, and wolls arc oceasiouall}' located. Tlr> i-aaise of this rehitioiis'hip is simple. I'he rainfall upon th<; interior nioniiiains, fornied of the older rooks, does n<d: sink into them, 1)ijt fof^ lows sriperfieiol cK)nr.ses down their flanks nntil the wahn;5 rncet llie e-iiesiri edge of the coastal plain deposits, nnder whivh a small part of the wat.- HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT ^H seeps. The waters working between the coastal plain deposits and the bed-rock of the older series give rise to springs or wells. Thus the best wells in the Arecibo formation have been found near thin coastal margins and the best wells in the Santa Isabel alluvium are found adjacent to streams. Does it not appear significant that in the older series there is only one spring and that a thermal spring? Location Near a Fault The direct location of the spring is probably due to a fault which strikes north very close to the Coamo Elver. Berkey (1915^ p. 39) de- scribes another fault which may continue into this district^ and if it does extend eastward in the same direction it would lie very close to the Coamo Springs : The most prominent fault, in its effect upon present features, is tlie one now marking the inner margin of the "younger series" of chall^y limestones and shales constituting the coastal belt along the south side of the island from Juana Diaz past Ponce at a short distance to the north, crossing the Ponce- Arecibo Road at K. 4.8 and thence westward crossing the Ponce-Penuelas Road at K. 10. . . . It must be of very late Tertiary age, because the chalky Ponce beds are abruptly cut off by it. The older rocks of the pre-Tertiary are lifted w^ith respect to the "younger series" forming the present coastal margin wherever this fault has been seen. . . . What becomes of it at either end is not yet determined, but it is believed to extend much farther in both directions. Source of Water The Coamo Springs may obtain their waters from one or more of six sources^ as follows : (1) Meteoric, (2) Connate, ( 3 ) Entrapped, (4) Strayed magmatic, (5) Secondary magmatic, (6) Primary magmatic. (1) Meteoric waters are the waters of rainfall which have descended to sufficient depths to become heated and have then ascended again to the surface. (2) Connate waters are sea waters which became entangled with ma- rine sediments at the time of deposition and remained locked up in these feodiments after the coastal plains had been uplifted and more or less consolidated. The best known example of this is found in the copper mines of Michigan (Lane, 1908). 212 SCIENTIFIC SURVEY OF PORTO RICO (3) Entrapped waters are waters of any type which were formerly following some underground channel and as a result of faulting, in- trusion of a dike^ or the occurrence of some similar event were cut off from an avenue of escape and prevented from reaching the surface. Waters at Butte may belong to this category (Lindgren, 1913). (4) Strayed magmatic waters are those given off from a magma for the first time and which follow channels not leading to the surface^ and so have remained stored underground until tapped by a fault, a valley, or by mining. Such waters, depending upon the length of their sojourn, may develop a composition reflecting the country rock through which they have passed. Some of the waters of the Comstock Lode may be of this type. (5) Secondary magmatic waters are waters of any type which, as a result of assimilation of rocks containing them by an invading magma or as a result of diffusion into the magma from the country rock, have become incorporated into the magma and have been subsequently given off from the magma. (6) Finally waters, which were part of the earth^s interior magmatic reservoir, may be given off and reach the surface for the first time. Such waters are primarily juvenile. Where there are so many sources or combinations of sources from which a spring could obtain its waters, it is difficult to decide which is the more probable source. Possibility of a Meteoric Source seasonal rains and variability The meteoric origin will first be considered. If the waters are meteoric or derived from the rainfall, they should show some relationsliip to the seasonal rains of Porto Eico. That the rains are seasonal is shown ])y the following record of a typical station, the Toro Negro station near Juana Diaz.^ Inches 1911. September 8.60 October 21 . 03 November 6 . 25 December 18 . 14 53. G2 « From the records of the Porto Rican Irrigation Service. HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 213 1912. January 2.82 February 2.07 March 3.38 April 7.50 May 1.89 June 1.51 July 9.46 August 8.81 37.44 1912. September 14.71 October , 25.67 November 15 . 06 December 2 . 59 57.83 1913. January 2.54 February 2 .91 March 5 . 06 April 9.97 May 8.44 June 4.03 July 2.18 August 3.04 38.17 1913. September 13.35 October 11 .80 November 5 . 80 December 2 . 59 33.54 This is a typical station as regards variability of rainfall^ and it shows that during the fonr months nearly twice as much water falls as during the eight arid months. Yet despite this great difference in precipitation the Coamo thermal spring shows no noticeable change in volume year after year. Legend tells us that at the time of Columbus it gave forth more water than at the present time, but within the last two generations the flow has been just the same as it is today. The southern coast of Porto Eico is arid and it is within this belt that the Coamo-Guayama district lies. In 1897-98 for a period of 13 months not a drop of rain fell in Guayama (Wilson, 1899), but the thermal s))ring, a few miles distant, did not show the slightest variability in it:^ tlischarge. 214 SCIENTIFIC tiURVEY OF FORTO RICO FLOODS xlND UNDERGROUND CIRCULATION After one of Porto Kico's torrential rains the streams rise rapidly to floods sufficient to carry boulders, but within a few hours after the cessation of rainfall the streams quickly fall and within twenty-four hours the beds are dry. This phenomenon is particularly noticeable on the south side, where rains are infrequent. On the north side daily rains occur and the larger streams receive new contributions of water before the previous day's rain has been entirely carried away. JSTevertheless, upon the tributary streams, which are more sensitive to daily deluges, the intermittent character of the stream is striking. It would appear that if there was an underground circulation some of the rainfall would be delaj^ed underground to feed the streams a day or two after heavy rain. Mr. P. H. N"ewell, the hydrographer of the Porto Eican Irrigation Service, stated to me that "the discharge figures show a peculiar condition to exist. They show that almost as much water runs off of the various drainage basins as enters them by rainfall.^' This does not seem to be an anomaly. It only proves that practically none of the rainfall enters au underground circulation. That rainfall cannot enter a subsurface circu- lation is not to be wondered at, in view of the character of the soil, which is an extremely unctuous, tenacious, sticky, red, heavy clay. In the in- terior of the island, where the rainfall is heaviest, this impervious clay is as much as 50 feet thick in places. It adheres to almost vertical slopes despite rains of intense character. Any channels of circulation open at the surface would quickly become sealed by this clay. The arguments presented above will appear to most readers as quite unnecessary, in view of the fact that meteoric waters to become heated would have to descend beyond a thousand feet or more. The accumulated evidence is now overwhelming in proving that deep rocks are dry rocks (Lindgren, 1913; Kemp, 1903). TEMPERATURE The temperature of Coamo Springs is 44° C. Cold meteoric waters would have to descend to over 3000 feet to reach a ground temperature as high as 44° C, unless, perhaps, they came within the influence of volcanic rocks of very recent origin. LOCATION NEAR A VOLCANIC CENTER Just south of the Coamo Springs Eeservoir dam an old volcanic center is shown in cross-section and in the vicinity of Coamo Springs former post-Eocenic volcanic centers are very common. There is one between HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 215 the springs and Coamo^ another on the Kio des Descalabrados, and to the eastward^ in the Jueyes drainage system^ are several more volcanic complexes. This spring is thus intimately related to volcanic centers. The close proximity of this spring to young volcanic centers must have a genetic significance. How recently these volcanoes were active is not known, ^o evidence of the volcanoes cutting the Arecibo rocks has been found. The volcanoes are at least post-Eocene and may perhaps be post- Oligocene. CONSTANCY OF COMPOSITION The following table gives the results of four chemical analyses.^ Num- ber 4 is the analysis made for this study. I am inclined to regard analysis number 3 to be devoid of accuracy and there is much doubt about the other two. Neglecting number 3 and considering only CaCOg, MgSO^ and SiOg;, there seems to be a rather constant value for them all—that is to say^ the figures are of the same magnitude. In a limited way analyses numbers 1, 2 and 4 argue for a constant composition over a period of many years. I II III IV HCl 00.84 00.852 H2SO4 44.08 36.710 H2CO3 , 27.88 33.433 0.76 NH,C1 11.31 Nh.COs 2.07 Na^NOs 00.38 NaNOa 06.93 Na^SiOs Na^SO^ NaCl KCl LiCl CaCOs 00.20 ('aSO^ 00.41 ^IgOOa 20.07 ^IgSO, 00.84 FeC03 Fe.HCOa FeSO^ 04.59 04.801 ^iO^ 01.09 01.095 03.71 4.79 30.98 13.60 07.72 0.02 02.43 00.34 00.390 00.01 00.413 47.12 63.90 21.460 00.846 00.63 02.38 0.66 00.26 100.00 100.00 100.00 100.00 'I. J. J. HeHer, 1847. II. Seiior Gunenez, 1899. t° 45.3 ; average temperature of air near springs, 27.3 ; and constant sulphurous odor ; sp. gr., 1.233. ^11. 1.6855 grains per mile made at the agricultural station of Mayaguez. ^V. Louis Hernandez, 1916. t° 44° 1.59970 grains per mille. 216 SCIENTIFIC SURVEY OF PORTO RICO Jiiver waters are solutions of calcmm and sodium carbonates and aiiiea. it is an axiomatic fact that river waters reflect tHe composition of the rock over which they flow, and one should expect spring water which has traveled through such rocks to have a similar composition. These analy- ses show that this is not the case. The location of Coamo Springs, their constant flow, the temperature, composition, constancy of their composition and the physical character of the rocks of the older series all argue against the direct meteoric origin for their waters. Possibility of Connate, Entrapped and Strayed Magmatio Waters The presence of a new loci of heat, such as an intrusive magma or a volcanic center (and it is to be remembered that a batholith underlies this district), would stimulate entrapped or connate w^aters and rejuve- nate strayed magmatic waters and cause them to rise to the surface anfl there form springs. Apparently the only way these waters could be dis- tinguished one from another would be by their composition. Lindgreii (1913), speaking of connate waters, says that "the presence of bromide is almost characteristic; traces of iodine and boron are often found." These critical elements are lacking in the Coamo Springs waters. There are some features concerning the composition of the Coamo Springs waters which show their dissimilarity to entrapped meteoric or connate waters. As they issue at4he surface they are mostly solutions of calcium sulphate. Their present composition is undoubtedly due to re- action of sulphuric acid with the very calcareous rocks through w^hich the waters pass. Acid waters of meteoric origin are always low in sodium, potassium and lithium, three elements which occur in notable quantities in these waters, and, on the other hand, these alkalies are very common in waters of volcanic origin (Clark, 1916). The ammonia radi(^le, which is also peculiar to volcanic waters, is strikingly high in the Coamo Springs waters. If the assumption is made that the sulphuric acid has resulted from the oxidation of hydrogen sulphide, then the composition of the Coamo Springs water at depth is an alkaline solution of sodium, potas- sium, and lithium chlorides, ^ammonia hydroxide and hydrogen sulphide gas. Such a composition is, indeed, unlike any entrapped meteoric or connate water ! The composition indicates that the Coamo Springs waters are neither connate nor entrapped meteoric waters, but it does not dis- prove them to be entrapped magmatic waters. Moreover, strayed mag- matic would have a composition identical with entrapped magmatic w^aters, or, for that matter, secondary magmatic waters. Strayed ma.u HODGE, GEOLOGY OF THE COAMO-GUAYAMA DWTRICT 2V1 matic waters or entrapped inagmatic waters could be rejuvenated so that they ascended to the surface and there formed springs. The waters of Coamo Springs may have such an origin. Possibilities of Secondary and Primary Magmatic AVati<:u8 Two other possibilities of origin that remain to be considered are secondary and primary magmatic waters. Waters contained in rocks invaded by a magma are considered by some WTiters to be able to enter a magma either by assimilation of the country rock or by diffusion from the country rock into the magma. After the waters had entered the magma their composition would have become changed, so that tliey could not be distinguished from primary magmatic waters. The problem, then, resolves itself into a consideration of the possibilities of diffusion and of assimilation. ASSIMILATION Even the most enthusiastic exponents of magmatic assimilation admit that the process of assimilation does not take place in the zone of howage, and w^e need only consider in this zone absorption by diffusion. DIFFUSION In order to diffuse, water must be present in the rock adjacent to the magma and it must be available. Bocks in the zone of flowage do not have more than 0.5 per cent by volume of pore space. The pore spaces that do exist are closed and completely separated from one another by crystalline substance. There is absolutely no evidence, experimental or geological, that gases can diffuse through solid crystals. A minute pas- sage must always be present. Granting that a little water exists in tliese pores (though the experience of deep mines and bore-holes is quite to the contrary), would it in any manner be able to create fractures or develo]) cleavages through which it could pass to the magma? Of course the water in the pores adjacent to the intrusive magma would be absorbed, but the quantity would be infinitesimal. Adjacent to the magma the temperature is 1000° C. or higher. From the walls of the magma out- ward the conductivity of the rock is so low that within a few feet the iomperature is lowered to 500 degrees, and from that diminishes rapidly outward. There is about the upper surface of the magma a zone 10 feet thick at most which has a temperature sufficient to raise water above its critical point of 364.3° C, and throughout this zone all the water stored i^' the pores is in the vapor phase. In this state and brought in contact ^18 SCIENTIFIC SURVEY OF PORTO RICO with solutions of minerals composing the magma, water will rapidly diffuse and become incorporated into the magma. Tliere are two factors which would prevent this diffusion taking place : one is the cubical expansion of the minerals upon heating, and the other is the immense outward pressure of the gases contained within tiie magma. The minerals of the rocks next to the intruded magma would expand, when heated, in all directions. If the pores were empty the}^ would be closed. If the pores contained water vapor, this vapor would either be compressed or forced to move along some plane of weakness. If the water vapor moved in any direction it would necessarily move out- ward and away from the magma, because in that direction the tempera- ture would be lower and the cubical expansion less. Any fractures pres- ent or separation along cleavage planes would be closed tight and the pressure forcing them tight would be greatest near the magma and less far away. Thus the magma in the zone of flowage hermetically seals itself on the outside by a layer of rock, ten feet or more wide, through which gases cannot pass and out of which water vapor, if it must escape, wall be compelled to travel away from the magma. All authorities on igneous rocks, such as A. Ilarker, A. Lacroix, Joseph P. Iddings, E. A. Daly and numerous others, ascribe to magmas enormous quantities of gases, such as water, carbon dioxide, boron, fluorine, chlorine, and others in lesser amounts. Whether these gaseg are able to free themselves from the magma in the zone of flowage or not, the tendency is there and the outward pressure of these gases is sufficient if released to drive outward and away from the magma any water which might exist. These gases probably do not escape in the anamorphic zone be- cause of the hermetical zone surrounding the magma, as discussed in the above paragraph. As magmas approach the upper limits of the ana- morphic zone and the outward pressure of the dissolved gases becomes greater, due to crystallization, it is probable that these gases may open up veins for themselves, but in so doing the velocity of their outward move- ment would not permit any water contained in rock pores to diffuse toward the magma. Assimilation or Diffusion in Katamorphic Zone In the katamorphic zone of the earth^s crust, where fractures may exist and pore spaces are larger, cubical expansion of the minerals, due to the heating by the uprising magma, will not be sufficient to form a hermetr*' shell. Any water which by some unexplained manner may get down m thousand or two thousand feet, if brought in contact with a magma, would diffuse into it. Smaller cavities and fractures close to the magma HODGE, GEOLOGY OF THE GOAMO-GUAYAMA DISTRICT 219 will be closed, but larger fractures will be filled by injected masses of rock. The peripheral portions of these injected igneous rocks will allow diffusion into them of water vapor, but the area of the terminal portions exposed to diffusion will be small and allow of no contribution directly to the magma. If these injected bodies succeed in completely surround- ing a block of rock and this block sinks into the magma, because of greater relative density, or if a slab of rock is loosened by shattering, any water contained will be incorporated. The speed of marginal shattering or block dissection will be a slow process and will always be preceded by an outward radiation of heat from the magma. The heat will convert the water to steam^ which will move in the direction of least resistance. The effect will be like that of the emanations from the magma, for the steam Avill drive any water contained in the rock outward and away from the magma. This steam will not be able to move toward the magma, because of cubical expansion close to it, because of the outward traveling injected bodies, and because of the ema- nations. Before, therefore, a block could be assimilated, all the water contained would probably be driven out of it. As the intrusive works its way into the fracture zone, the indigenous gases will escape outward. They will drive all other gases before them. Their outward velocity is great and penetration power most complete, which is illustrated by the way in which these gases follow out and open up the cleavage planes of calcite grains of a limestone in the phenomena of contact metamorphism. The waters which the outward moving gases meet with are dissolved, but the speed of diffusion in the gaseous solution is undoubtedly slower than the speed of the outward moving gases, and all waters in the invaded rocks would be carried along the crest of the first wave of emanations. The upward movement of the magma is slow and this process will act continuously until all the gases are spent. Apparently, then, only an unimportant fraction of water contained in the invaded rocks could dif- fuse into a magma and we must conclude that the probabilities are highly in favor of the theory that the waters of the Coamo Springs are appearing at the surface for the first time in their history. MINEEAL EESOIJECES General Considerations The possibilities of metal mining in Porto Eico are discouraging for several reasons: One is the absence of cheap power. Coal and oil must ^>e imported and water-power sites are limited. Another is due to the J act that the ore could not be smelted locally ; hence only high-grade oro 220 SCIENTIFIC SURVEY OF PORTO RICO could be shipped. This would mean the building of railroads on the island and long ocean transportation. The few railroads built on the flat coastal plains have cost between $5,000 and $10,000 per mile. As the mountainous interior is extremely rugged and broken, railroads, where they could be built, would have to rise on steep grades and make very sharp turns. In many places railways are practically impossible and long, expensive aerial tramways would be the only means of transporta- tion. An ore deposit would need to be very rich and large to warrant exploitation under the above-mentioned difficulties. The island of Porto Kico has been explored for centuries, but up to date not a single paying mine has been developed. 'No large area con- tains a more dense population. ISTearly every nook and corner has been observed, yet no ore deposit of importance has been discovered. Despite popular legend, there is no evidence that the early Spaniards, though they sought long and zealously, ever found a deposit which could be worked even with slave labor. For centuries the Spaniards and later the ever enthusiastic Yankee prospector searched for ores. Nevertheless, the few prospects opened have never developed into mines. Metallic Minerals In the Coamo-Guayama district near Barrio del Carmen several pros- pects have been opened, which I have had opportunity to examine. The character of the larger one, situated just south of Carmen, will be de- scribed (Fig. 50). Several openings have been made in a hill composed almost entirely of a conglomerate with a tuffaceous matrix. The workings follow miner- alized crush zones and true veins, and are extensive enough to show the general geological relations. Conglomerate is cut by an intrusion which varies in composition from an augite to a feldspathic andesite. The con- glomerate is found in the main drift and the first lateral ; it is also found in the various prospect pits on this hill and in the pits north and west of Carmen. The intrusive rock appears in the major part of the workings and a dike cuts the first 20 feet of the first lateral. Mineralized faults of small displacement are common in the andesite and are accompanied by a green andesite gouge, which may be a few inches to two feet thick. Tho andesite is highly fractured, which may have been due to cooling or to resurgent movements of the subjacent liquid portions after the peripheral portions have solidified, or to tectonic movements. Whatever the cause, the fracturing took place while the magma was still giving off emana- tions, for the numerous small fractures have a calcite filling and into some of the larger veins pyrite came and filled the central portions. TIk mHH,f!. CF.aiJiCY iiF TUF. FO \ MO-i H:AY A }L\ I>lf<l'lllFT :: iijHJfsit*' i.< iiicniHtxd l.v calt:iti' :if. its fontHct witJi ihc euimtn' mek. a at)|>aiviin\ at tho tani*^ ot thv iiiti'ii>it»n ot llis> aiulHsito (M'O^ hc-.nn n-atoiv w^.Tf givcji olf in givat rjnaiihij, ajiiif ul' which followed tJa- oii <vall I'l: thu aii(h^^i1i« and prwipifatcd calcikn wliilo nth.TS work<al iJinain-h the frneliiriK ain'l ffmlls aiul IVwiikhI calcirt; vriiii.:. "" :,., 'ife--. 3 ''"'v~'^---. -*^#''^^-> <^ «.'':; . ..•••:fe'<?;^-': . ' . ' .• ^""">"' u-^i: ' voiii> ccTiir. «hi(;h an- (aaislifi.-d and show .WtM'al .uy-ii oral; ions of dcposilion. The pru<],n-l p\i< norlh aaid west of ClariiKMi opr-ii up vains <.r i.lantical I'liarach^a:. Svnr aaeh, ajidcsitit- rock mmr?, hi dia vciiKs are rrndouhtr'.lh.- rohdx-d. The rjiiariz voiiis oontaiti ]iyriio Iciia : Iho sTieecssioii. wdiioh can 1;h! ]da,iidy made out. is (iiiartz or [na-ite and ea1eih\ Not ad of the fiuartz w.lm aro mint^ralized. 222 SCIENTIFIC SURVEY OF PORTO RICO galena was formed first, pyrite always followed. Those veins which leave the andesite and penetrate the country rock always show a preponderance of pyrite over galena. Apparently, then, pyrite was the more mobile of the minerals and traveled farthest. Since the galena is genetically related to the andesite and is the least mobile mineral, it should increase with depth. Considering the difficult transportation problems, the veins of galena exposed in the present pros- pect make it unworkable unless the galena is rich in silver, which as far as I know is not the case. Much of the present workings have followed slips and gouges and not the true veins. If, in all the exploratory work, the galena-bearing veins had been followed in depth and the slips and seams neglected, the presence or absence of galena in paying quantities would have been completely demonstrated. Coal A few coal beds have been reported. One near San Sebastian and an- other near Juana Dias were examined and in each case the ^'bed" proved to be merely a thin seam of highly marcasitic low-grade bituminous or lignitic coal. In the Coamo-Guayama district one small seam was ob- served south of Cayey near the crest of the Sierra de Cayey Mountains. Oil Shales and Petroleum An investigation was made of the oil shale and petroleum possibihties of Porto Rico. Shales were gathered from many localities in Porto Rico which were reported to carry oil or were considered by the writer to have oil possibilities. The results of many analyses of the shales collected were all negative. Detailed study of the structure of the older series was impossible in the limited time devoted to study. In most places, how- ever, it is apparent that the strata have been folded to too high angles and eroded too deeply to preserve intact an oil reservoir which may have existed. There are a few localities, however, where the beds show the gentle folds suitable for oil storage, as, for instance, between Cidra and Cayey. Perhaps a more careful study would reveal structures worth testing for oil. Road-metal Limestone outcrops are numerous, most of which are suitable for road- metal. Along the San Juan to Ponce Road outcrops were noted at the following places : HODGE, GEOLOGY OF THE COAMO-GUAYAMA DISTRICT 223 Kilometer .9 Kilometer 55.8 Kilometer 61.1 Kilometer 61.4 Kilometer 67.0 Kilometer 73.2 Kilometer 78.2 Kilometer 84.0 Kilometer 85.1 Kilometer 87.7 Kilometer 90.9 Kilometer 94.2 Kilometer 99.5 Kilometer 100 . 3 Kilometer 104.1 Kilometer 105.3 Kilometer 106 . 6 Kilometer 107.5 Kilometer 108.2 Kilometer 111.9 West of Cidra two good limestone outcrops exist. Limestone is almost absent from the Cayey to Guayama Eoad, but andesite intrusives occur almost continuously. The Aibonito to Barranquitas Eoad shows many outcrops of silicified shales suitable for road-metal. The limestone of the Coamo Springs Eange serves as a convenient source of suitable rock for the Coast Eoad. Salt Two salt pans exist Just southeast of Salinas. About 30^000 pounds of salt have been obtained from these pans by evaporation of sea water. Clays The surface of the mountainous interior of Porto Eico is universally covered by a heavy, sticky, ferruginous clay akin to the gumbo found in the United States. This clay is too plastic and too ferruginous to be used for structural brick, paving brick or sewer pipe unless properly mixed with shales and carefully burnt, and fuel is too expensive to make elaborate burning feasible. Such clays, however, are suitable for low- grade tile and pottery. Some building bricks have been made in Porto Rico. In the Coamo-Guayama district low-grade red bricks were made near the following places : Coamo 20,700 brick, valued at $140 ikiavama 250,000 brick, valued at $2,000 '"^anta Isabel 50,000 brick, valued at $300 221 SCIENTIFIC SURVEY OF PORTO RICO The bricks manufactured are extremely friable^ as would be expected, considering the primitive methods of manufacture employed. The clay is mixed and ground in a circular pit around which a wagon wheel is pulled by a horse. The wheel is attached to a long beam which is fixed to a pole in the center of the pit. The prepared clay is hand-molded and dried in the sun, after which the green brick are built into a rectangular kiln. Charcoal is used to burn the brick. Lime Stone suitable for lime burning is probably not present in this district. As shown in the earlier part of this report, all the limestones are high in tuflfaceous materials, which makes them unsuitable for a lime as pure as required by the sugar industry. ISTatural Cement The presence of the tuff, however, makes tlie limestones of a composi- tion required for natural cements. This fact was demonstrated by the Central Aguirre. This Sugar Company burnt some of the limestones in the Aguirre Hills hoping to prepare a pure lime, but the product they obtained was a cement. Structural Limestone Limestone is the best tropical building stone. The lieavy vegetation keeps the carbon dioxide content of the rain and ground water so low that solution has but little influence. This is shown by the San Juan fortress and castle, which, after standing for centuries, are as good and solid as the day they were built. The best limestone in the Coamo-Guayama dis- trict is that of the Coamo Springs Eange, which offers an inexhaustible supply. This limestone in most places is heavy bedded, solid and uni- form. West of kilometer 108, on the Military Eoad, a quarry is being worked in a desultory fashion. The limestone is not only easily quarried, but is accessible from the Coast Eoad along nearly the whole of the district. Thermal Springs Three thermal springs exist in Porto Eico. The Quintana Spring is located in a subsequent valley, lying north of Ponce, just at the foot of the mountainous interior, where the rocks in the immediate vicinity indi- cate the former loci of volcanoes. The Coamo Spring has been describeik ISTorth of Arroyo and situated far out in the coastal plain is the VirelLt HODGE, GEOLOGY OF THE GOAMO-GUAYAMA D18TRJGT 225 SpxM-ig. The following analyses of the waters of the above springs were made for this report by Mr. Louis Hernandez^ of the Porto Rican Service of '(anitation : Quantitative Analysis COAMO. Parts per million. Nitrites 002 Nitrates 03 Free ammonia 048 Albuminoid ammonia 072 Total ammonia 12 Potassium 0184 Sodium 14908 Lithium 003861 Sulpliates 00892 Magnesium 00409 Calcium 41976 Chlorine 13184 Iron Iron and aluminum 0233 Silicon dioxide 0596 Carbonates as CO3 01787 Bicarbonates as HCO3 0O3O2 Ferric oxide VlRELLA. Parts per million. .00 .02 .0432 .048 .0912 .0697 .819 .00287 .4598 .0012 1.6877 1.35796 1.60374 .3678 1.06577 .00402 5.84702 QU INTAX A. Parts per million. .004 .02 .012 .012 .024 .012 .26517 .00389 .125 .0437 .0846 . 16269 .002 .037 .00178 .78583 Amount of water used, (c.c.) 100 100 1000 500 500 10000 1000 1000 1000 100 1000 1000 1000 100 100 COAMO. VlIlELLA. QUINTANA. Sodium nitrite 006 .00643 Sodium nitrate .1108 .0739 .0729 Ammonium chloride .1809 .1374 .03615 Lithium chloride .00542 .00343 .00467 Potassium chloride .0389 .1469 .025 Sodium chloride .12346 2.1991 .3476 Magnesium sulphate .01023 .004 .1094 Calcium sulphate 1.02254 1.1088 .1439 Ferric carbonate .03797 .00759 .00378 Ferric bicarbonate .00418 Silicon dioxide .0593 . 3678 .037 Calcium carbonate 1.7721 1.59970 5.82102 .78683 Total solids 1 .6048 5.8242 7914 226 SCIENTIFIC SURVEY OF FORTO RICO Qualitative Analysis COAMO. ViRELLA. QUINTANA. Bromine Negative. Negative. Negative. Iodine Negative. Negative. Negative. Hydrogen sulphide Negative. Negative. Negative. Carbonic anhydride (Free) .... Negative. Negative. Negative. Free sulphurous anhydride Negative. Negative. Negative. Phosphoric anhydride Negative. Negative. Negative. Boric acid Negative. Negative. Negative. Lithium (spectroscopically) Positive. Positive. Positive. .Specific gravity 1.003 1.006 1.004 Temperature 44° C. 30° 0. 34° O. Coamo Water: No reaction on litmus; alkaline to phenolphthalein, but on boil- ing the color disappeared. Virella Water: Alkaline to litmus and phenolphthalein. Quintana Water: Neutral to litmus, but alkaline to phenolphthalein. The high sodium chloride content of the Yirella waters is probably due to wind-carried salt spray of the near-by coast and to the fact that the waters issue through the recent coastal plain deposits, which have not as yet been completely freed of their connate sodium chloride. The high magnesium chloride content of the Quintama Springs makes their waters unsuited for drinking purposes. The waters of the Coamo Springs are good to drink and a trade in bottled waters should be developed. The temperature of the Quintama and Coamo Springs has encouraged their owners to build comfortable bathing pavilions, which will attract more and more tourists as their fame spreads abroad. HODGE, GEOLOGY OF THE GOAMO-GUAYAMA DISTRICT 22? BIBLIOGRAPHY Berkey, O. p. 1915. Geological recoiinoissance of Porto Rico. Ann. N. Y. Acad. Sci., XXVI, pp. 1-70. Clark, F. W. 1916. Data of geochemistry, PP. 199, 200. Cooke, C. W. ' 1916. The age of the Ocala limestone of Florida. Journ. Wash. Acad. Sci., VI, No. 1. FOSLIE, M. 1907. The lithothamnia. Trans. Linn. Soc. London. Zool. (2), XII, pp. 177-192. Communicated by J. S. Gardiner. Grabau, a. W. 1913. Principles of stratigraphy, p. 602. New York. Hill, R. T. 1899. Porto Rico. Nat. Geog. Mag., X, p. 109. 1899a. Non-metals of Porto Rico. Ann. Rept. U. S. Geol. Survey, XX, part VI, p. 771. HOGBOM, A. G. 1894. Ueber Dolomitbildung und dolomitische Kalkorganismen. Neues Jahrbuch f. Min. Geol. u. Pal., I, p. 272. Howe, M. A. 1912. The building of ''coral" reefs. Science, N. S., XXXV, p. 837. Kemp, J. F. 1903. Igneous rocks and circulating waters as factors in ore-deposition. Trans. Am. Inst. Min. Eng., XXXIII, pp. 707-711. Kjellman, F. R. 1883. Algae of the Arctic Sea. Konig. Sven. Vet. Acad. Handl., XX, No. 5, p. 96. Lane, A. 0. 1908. Mine waters. Lake Superior Min. Inst., XII, pp. 154-163. LlNDGREN, W. 1913. Mineral deposits. N. Y. Weber, van Bosse A., and Foslie, M. 1904. The Corallinaceae of the Siboga Expedition. Siboga-Expeditie Monographie, LXI, p. 4. Wilson, H. M. 1899. The engineering development of Porto Rico. Eng. Mag., XVII. 228 SCIENTIFIC SURVEY OF PORTO RICO GEOLOGIC MAP Explanation Recent-Santa Isabel formation; mostly alluvia. Oligoeene-Arecibo formation ; mostly limestone. Eocene. . Comanche to Cretacic. . . Comanche. . .^ Kio Descalabrado series ; a thick series of shales with a little limestone and chert. Coamo Springs series ; a heavy bedded limestone. liio Jueyes formation; a complex series, mostly limestone, but with much shale, some tuffs and conglomerates. f Guayma series; a complex series, mostly of shale, but with much chert, some tuff, limestone and conglomerate. Sierra de Cayey series ; mostly conglomerate, with a little tuft' and some shale. Barranquitas-Cayey series ; mostly shales, with much limestone and a little tuff. Rio de la Plata series; mostly tuff, with a few agglomerates, shales and limestones. 1^ Survey OF Porto Rico and the Virgin Islands Volume I Part II Pto. De Arroya Ot C^yMrtS (/^certain Contoicts ^^•^ ^ss^w- n'se' THE UNIVERSITY OF MICHIGAN DATE DUE .t UNIVERSITY OF MiCHIGAN 3 9015 03458 5607 BOUND UN I » ' it (^"ii<^H» DO NOT REMOVE OR MUTILATE CARD H