SCIENCE N5 ANNALS OF THE NEW YORK ACADEMY OF SCIENCES Vol. XXVI, pp. 1-70, pll. I-III Editor, Edmund Otis Hovey GEOLOGICAL RECONNOISSANCE OF PORTO RICO BY Charles P. Bekkey NEW YORK PUBLISHED BY THE ACADEMY 3 Maech, 1915 THE NEW YOEK ACADEMY OP SCIENCES (Lyceum op Natural History, 1817-1876) Officers, 1915 Presidenl — Ceurui'J Ejikdioujuiv Kunz, 601 West 110th trStreet Vice-Presidents — Charles P. Berkky, IUymond C. Osj5URN, Charles Baskerville, Clark Wissler Corresponding Secretary — Henry E. Craiipton, American Mnsenni Recording Secrelary — Edmund Otis Hovey, American Museum Treasurer — Henry J. Cochran, 389 Fifth Avenue Librarian— Raii.fii W. Tower, American Museiiin Editor — Edmund Otis Hovey, American Musciiin SECTION OF GEOLOGY AND MINERALOGY Chairman — Charles P. Bi'IRKey, (V)luiiihia TTniversitv Secretary — A. B. Pacini, 3704 Bedford Ave., Brooklyn SECTION OF BIOLOGY Chairman — Eaymond (1. Onjjurn. 557 West 124th Street Secretary — William K. Gregory, American Museum SECTION OF ASTRONOMY, PHYSICS AND CHEMISTRY Cliairman—CiiMU.RS Baskerville, College of the (!ity of Hew Ynrk Secrekiry — Ernkst ¥j. Smith, 50 East 41st Street SECTION OF ANTHROPOLOGY AND PSYCHOLOGY Ctiairman — Clark Wissler, American Alusemn Secretary — Eobert H. Lovvie, American Museum The sessions of ^the Academy are hehl on Monday evenings at 8:15 o'clock from October to May, inclusive, at the American Museum of Natural History, 77tli Street and Central Park, West. W FT [Annals N. Y. Acad. Sci., Vol. XXVI, pp. 1-70, Pll. I-III. 3 March, 1915] GEOLOGICAL RECONNOISSANCE OF POETO EICO ^ By Charles P. Berkey (Presented in abstract before the Academy, 7 December, 1914) CONTENTS Page Introduction 2 General description 3 New York Academy of Sciences Expedition 7 Object of the expedition 9 Investigations and discussions 9 Rock formations 9 Younger series 11 San Juan formation 11 Arecibo formation , 12 Older series 17 Shales IS Limestones 19 Coamo tuff-limestone 19 Trujillo Alto limestone 21 "Shred" limestone 21 La Muda limestone 22 "Mountain limestone" 28 Corozal limestone 23 Conglomerates 23 Tuffs 2ry Volcanic flows 26 Intrusives 26 Summary 29 Petrographic range 29 Depth of decay 34 Structural features 35 Igneous structures 36 Volcanic vent complexes 37 Folding 38 Faulting 39 Large structural groups 41 Unconformities 41 Veins 42 1 Based on Ihe observations and studies of an expedition organized under the joint snp port of the New York Academy of Sciences and the Insular Governnicnl of I'orto Ifico. Manuscript received by the Kditor 1 February, 1015. (1) 2 ANNALS NEW YORK ACADEMY OF SCIENCES Page • Minor structures 43 Enterolithic structures 43 Double cross-bedding 43 Special relief features 47 Playas 47 Promontories 47 Terraces 48 Cuestas 49 Peneplain 50 Haystack hills 51 Mineral resources 58 Gold 54 Copper 55 Zinc, lead and silver 55 Iron 55 Coal and oil 56 Limerock 56 Guano 57 Road metal 57 Hot springs ^ 57 Historical statement 58 Future problems 61 Base map , 61 Geologic map 62 District studies 62 Reef -building organisms 63 Paleontology 64 Tertiary subdivision 64 San Juan formation 64 Subdivision of pre-Tertiary complex 64 Mineral resources 64 Petrography 65 Physiography 65 Thermal waters ; 65 Geologic history 66 Collections 66 Illustrations : 67 Cross-sections 67 Maps 68 Acknowledgments 68 Bibliography 69 Introduction The Island of Porto Eico has never had a thorouo-h or detailed seo- logical study. There have been, however, a number of papers written that have described special features or general conditions in a very ac- BERKEY, GEOLOGICAL REC0NN0I8SANGE OF FORTO RICO 3 ceptable manner. Most of the writings^ of this kind are widely scattered in volumes of periodicals or proceedings of learned societies or pamphlets which in most cases are not readily obtained or consulted. From a perusal of these articles, one learns that Porto Kico belongs structurally and genetically to the mountain chain now represented by the isolated islands forming the principal "West Indian group. Enough work has been done, especially by E. T. Hill,^ to outline roughly the geological history of the Island of Porto Rico and indicate on a map the distribution of some of the formations. An especially good general de- scription of physical features, also, is given by H. M. Wilson.* An intro- ductory general description, in large part along the same lines as these, will probably serve the present purpose. General Description" The Island of Porto Rico is situated in the Torrid Zone between lati- tude 17° 54" and 18° 30" north and longitude 65° 13" and 67° 15" west. It is the easternmost and southernmost of the Greater Antilles. It lies within the trade-wind belt, and the constancy of these winds gives the island a remarkably mild and uniform climate. There is an abundance of rainfall on the windward side, which in this case is the east end and the north side as far west as Camuy. The effect of the momitains across which these winds blow is to make the south side of the island and most of the western portion comparatively arid. Some districts are said to have no rainfall for a whole year at a stretch. The Atlantic Ocean lies to the north and east, the Caribbean Sea lies to the south, while Mona Channel on the west separates the Island of Porto Rico from Haj^ti. Brownson Deep, reaching the profound depth of twenty-four thousand feet below sea level, one of the deepest spots known, lies immediately to the north. Tanner Deep lies to the south, reaching a depth of fifteen thousand feet. Although the relief of the island above sea level is less than four thousand feet, this represents only the extreme top of a great mountain mass which rises above the submerged platform, from which its real height should be measured. The extreme relief differ- ence represented by the summit of El Yunque on the one hand and Brown- 2 The writings referred to, together with others that have some description of physical conditions in Porto Rico, are listed at the end of this paper. Those of most usefulness in the present investigation are certain papers by R. T. Hill and H. M. Wilson, jjesides a very few others of less extended character. 8 R. T. Hill: "P'orto Rico." National Geographic Magazine, volume 10, pages 93 to 112 (1889). *H. M. Wilson: "Water Resources of Porto Rico." Water Supply Paper No. 39, U. S. Geological Survey. 4 ANNALS NEW YORK ACADEMY OF SCIENCES son JJl'L'p cu the otlaT is ai)])ruximately twenty-eight thousand feet. Re- garded in this way, the Island of Porto Rico belongs to one of the higher relief features of the earth. It is in reality a Ijadly eroded summit of a great mountain belonging to an east-west chain or range including the Greater Antilles. The gen- eral structural features of the islands are consistent with this east-west axial trend which is expressed in the topography of the central Cordillera, extending from the west end near Rincon, where it starts abruptly from the water's edge, to Fajardo, where it terminates in El Yimque, the high- est point on the island. The momitain range, however, is not so simple as this statement would lead one to believe, for there are in reality two ranges or branches toward the east, one of which is known as the Sierra de Luquillo, culminating in El Yunquc, and the otlier, which is best de- veloped in the divide between Cayey and Guayama, is called the Sierra dc Cayey. The military road crosses this latter branch near Aibonito over a pass that reaches above 2,000 feet. To the westward, the double character of the mountain ranges is not so pronounced, but there is a semblance of it in the spurs that reach the sea rather abruptly, one near Rincon and the other near JMayaguez. The exact elevations of the higher mountains have not been accurately determined, the values given on the older ma]>s being evidently too great. The revised approximate elevations are : for El Yunque, at the eastern end of the island, 3,750 feet; for El Guilarte, wdiich stands to the west of the Arecibo road, 3,610 feet. The highest point ill the Sierra de Cayey is about 3,000 feet. Many points are ncai'ly as high as those given, and all of the roads that cross the island reach elevations on the divide that are in excess of 2,000 feet. Many of tlio roadways in the interior districts reach elevations over 2,225 feet. Althougli the island as a w'hole has a mountainous aspect, and although much of the interior is very rugged and i^icturesque, there is usually a comparatively gentle or smooth topography along the coast, and some of the marginal areas are almost perfectly flat. These are uniformly at the mouths of the larger rivers and represent river alluvium or delta-like deposits, — they are known in the island as playas. Tlie aspect of the island as a whole is moderately rugged. Although the major portion of the rock makeu]) is igneous, and although there is considerable complexity of structure represented in all parts of the range, all of the surface forms are of erosional orrg'hi. The relief is tliat of early maturity in the interior and perhaps late maturity in portions of the coastal districts. Exceedingly steep slopes are the rule in all parts of the island where tliere is any eonsiderablo relief, and one of the most surprising things is the way the soil dings to these slopes. One often BERKEY, (iEOLOdlCAL RECOXXO/SSAyCE OE PORTO RICO 5 a Ui I- \n z D 6 to 5 -1; a a Q X 2 c « o &. £3 < s a o u 3 o a a) 6 AXXALS NEW YORK ACADEMY OF SCIENCES sees slopes of this kind with angles of thirty to forty degrees from the horizontal under cultivation. The islands of Culebra and Vieques lie to the east of Porto Rico and are said to be similar in structure and makeup, but neither of them was visited on this expedition. The streams of the island are numerous and surprisingly large for the size of the areas drained. According to Mr. Wilson's description^ of the water supply there are twelve large streams flowing north, four flowing west, seventeen flowing south and six flowing east. Besides these there are said to be 1,300 small streams, and on account of the heavy rainfall on portions of the island many of them are of larger size than such an area would usually afford. The main divide runs near the southerly margin of the island, so that about one-third of the drainage is tributary to the Caribbean Sea on the south, and about two-thirds to the Atlantic Ocean on the north. This unsymmetrical position of the dividing range is an abnormal feature, the cause of which is the subject of discussion in an- other portion of the report. Because of the prevailing trade winds, the rainfall is very unevenly distributed. The east end and the north side are comparatively humid and plentifully watered; in contrast, the west end and especially the south side are comparatively arid. In order to overcome partly the shortage of water, a large system of irrigation is now being developed on the south side of the island. At the east end, north- east of El Yunque, there is an annual rainfall of 123 inches. On the other hand, at Cabo Rojo, at the other extreme on the southwest side of the island, it is exceedingly dry. and in occasional years there is said to be not a single drop of rainfall. Other parts of the island vary between these extremes. The wettest months are September and November. In most districts, the underlying rock is compact enough to discourage much deep water circulation and the stream run-off is correspondingly responsive to the rainfall. In the northwest corner of the island, on the broad limestone belt extending from Aguadilla to Camuy, there is a pre- vailing tendency for the surface waters to sink into underground chan- nels, leaving the surface very much more poorly watered than even the somewhat scanty rainfall would lead one to expect. In some cases, streams developed on the more compact rocks of the interior districts completely lose themselves in underground channels upon entering the limestone belt, and in some cases do not again come to the surface for several miles. Elaborate caves and channel-like caverns are common and, in the north- ern belt of limestone countr}-, there are thousands of such occurrences 5 H. M. Wilson : "Porto Rico ; its Topography and Aspects." Jour. Am. Geog. Soc, Vol. 32, p. 220. IflOO. BERKEY, GEOLOGICAL REGONNOISSANCE OF PORTO RICO 7 still preserved that are now wholly abandoned by the waters that for- merly occupied and helped to form them. There are no inland lakes, but there are a few coastal lakes and they appear to be related to the develop- ment of alluvial plains or playas and recent elevation and subsidence changes. The climate is strictly tropical, but it is so tempered by reason of the constant trade wind breezes from the ocean and the elevated character of much of the ground that it is usually agreeable and mild. The lack of great changes of temperature and the prevailingly moist conditions on most of the island have direct influence on the character of the rock decay and disintegration and also on the quality of the soil produced as well as its behavior as a residuary product. Other matters of climatic conditions have little or no bearing on geological problems and may well be avoided. The average daily temperature is eighty degrees; it rarely goes above ninety degrees or below seventy. The maximum temperature is ninety- nine degrees. Extremes recorded for the year indicate a range of forty degrees. The area of Porto Rico is given as 3,670 square miles, which is about three-fourths the size of the State of Connecticut. It is roughly rectan- gular in outline and in actual dimensions is about thirty-five miles in average width, and one hundred and five miles long from east to west. It is the fourth in size of the West India Islands and is one of the most productive and densely jwpulated districts in America, Because of the greater interest recently taken in studying the natural resources and natural history of Porto Eico, it was judged to be a suitable time to make a more elaborate and detailed study of the island's geo- logical framework and history. In accord with this view the Xew York Academy of Sciences organized an expedition which spent a part of the summer of 1914 on the island. The accompanying descriptions are based on the work accomplished by this expedition. New York Academy of Sciences Expedition The geologists sent to make a preliminary study or reconnoissance of the Island of Porto Eico left New York on the loth of August, 1914, Four weeks were spent in Porto Eico, the expedition returning to New York City on the 21st of September. The party consisted of Dr. Charles P. Berkey of Columbia University, New York, and Dr. Clarence N, Fenner of the Geophysical Laboratory, "Washington. Arrangements were made with the bureau of transportation of the Insular government in San Juan for conveyances, so that as much groimd as possible could be seen in the time available. More than 2.000 kilometers were covered by 8 ANNAL8 NEW YORK ACADEMY OF SCIENCES the aid of this transportation service and observations were made in suffi- cient detail to judge the general character and structural relations of the formations crossed. In addition to this kind of travel on the roads, short trips were made on foot to examine features or outcrops of rock which appeared to deserve investigation, and an occasional more extended trip on horseback was taken to points in the interior. With these facilities for travel, it was possible for both members of the party to give undivided attention to geological observations. It was possible to stop and make Fig. 1. — Relief features characteristic of the interior ruiujcs of Porto IHco Photograph taken from the Ponce-Penuelas road at K-10, looking northward across eroded formations of the older series to the main drainage divide. brief examinations along all the roads at hundreds of places, and, on several of the roads crossing the island, sufficiently elaborate data were secured to furnish a basis for geological cross-sections showing both relief and structural features. A complete circuit of the island was made and in addition it was crossed from north to south on three principal roads. This, together with numerous side trips into the interior, permitted ob- servations to be made on practically every formation of any considerable consequence in the island. No point in the whole area is situated more than seven miles from some road or other point of observation covered by the party, and, even in those cases, except in the extreme southwest BERKEY, GEOLOGICAL RECONNOISSANCE OP PORTO RICO 9 corner of the islainl. o])servations were made on all sides or completely around the unexphjred areas. Because of the extent of the reconnois- sance, it is the opinion of the writer that all of the essential, large, funda- mental geolooical formational units have heen found and their general structural relations have heen determined. OBJECT OF THE EXPEDITION" The purpose of this expedition was, first of all. to determine the nature and origin of the rock formations of Porto Rico, and to group them into series suitable for use in subsequent geological work. In the second place, it was the object of the party to determine as many of the larger struc- tural relations as could Ije seen and to unravel as much of the geologic history as such a hasty examination would warrant. In the tliird place, the physiographic features Avere studied for the purpose of determining their origin and relation to the formational structure and their bearing on the more recent history of the island. /Lastly, it was appreciated that the island was complex enough to have many problems that could not be solved without very much more extended investigation, and it was the purpose of this expedition to point out the problems that should receive special study and that seemed to give promise of important result^ Con- siderable attention has been given to the economic resources of tlie island by private. individuals and considerable money has ])een spent on various enterprises connected with their development. These problems were also kept in mind, and wherever convenient, special observations were made on them. Although it is possible to make suggestions concerning these eco- nomic resources, they are for the most part matters that should receive very much more extended special study. A matter that concerns the wel- fare of the island more directly than any of these is the question of quality and variety alid origin of the native soils. These of course are in large part geologic matters also, and although this reconnoissance is not sufficiently detailed to form the basis of a discussion of this matter, it is one of the lines of investigation connected with further work that will have direct value. Investigations and Discussion eock formations , The most fundamental thing to l)e determined at the outset of an in- vestigation of this kind is to discover and differentiate the different rock types and the structural units to which they belong. All of the prelimi- 10 ANNALS NEW YORK ACADEAIY OF SCIENCES nary work of this expedition was devoted. primarily to this question and collections were made for comparison thronghout the island. In the beginning, observations were made chiefly along the coastal margins be- cause of the greater amount of ground that could be covered and because of the apparent simplicity of the outermost and younger or more recent formations. The chief formations with their representative rock variety will be discussed in order from the younger to the older series. In the first place, such a reconnoissance shows that there are two great series of formations separated by a marked unconformity. Both are somewhat complex, but in that respect the older series is very much more complex, both in range of composition and number of units involved and in variety of structural relation, than the younger one. In spite of this discrepancy, it is still the most convenient and useful division to make, and, because of the strikingly different characteristics of the two series and the great prominence of the structural break between them, there is no possible chance for mistaking this fundamental feature. The whole lot of formational units are therefore grouped under the followi ^ two heads : 1) Younger Series. Inelucling the Tertiary shales, reef limestones and recent deposits. 2) Older Series. Including a complex group of formational units. — tuffs, aslics, shales, conglomerates, limestones and a great variety of iutrusives, all of which are probably of pre-Tertiary age. There are several possible subdivisions of the younger series, but in this discussion only those exhibiting enough physical constancy and char- acter to be useful in field correlation are taken into account. These are especially (1) the San -Juan Formation, a Pleistocene sand-dune deposit, and (2) the Arecibo Formation, a series of reef limestones and associated shales and marls. Besides these, there are more local developments that deserve special discussion, such as the San Sebastian shales, the Juana Diaz marls and sandy shales, and the Ponce chalk beds; but in a broad grouping these are all phases of the larger Arecibo Formation and it will take detailed paleontologic study to make the proper subdivisions. The older series has many formational members and their general rela- tions are reasonably well understood, but a systematic subdivision is not yet attempted. Correlation in this series is still more difficult than in the other, because of the great variation in character laterally and the influence of igneous activities that prevailed throughout its whole devel- opment. Some of the most characteristic of these types will be described. BERKEY, GEOLOGICAL RECOXXOISSAXCE OF PORTO RICO H Younger Series San Juan Formation. — On approaching the island by the usual route entering San Juan harbor, the first rock whose structural detail can bo seen is that on which the city of San Juan itself is built. One can see that the formation is made up of strongly bedded material that has all of the structural characteristics of a cross-bedded sandstone and is resistant enough so that it forms, at this point, a promontory extending far beyond the supporting mainland and presenting a cliff face of at least 75 feet in height. A closer inspection supports all of these observations as to structure and add^ tlio imnortaut oh>:ervation that the granular material Fig. 2. — Partially destroyed dune sand deposits of the ISan Juan formation These deposits are a short distance west of Arecibo and are nearlj' covered with fresh dune sands of the same material. is, in large part, calcareous in composition and organic in primary origin. The same type of material, with all of its characteristic structures, was seen at several other points along the north coast. Special studies were also made on sea cliffs of this formation in the vicinity of Arecibo, where the exposures were so well developed that good photographs of the struc- ture could be taken. One of these is reproduced as an illustration of the characteristics of this formation. It shows exceedingly steep cross-bed- ding structure that measures up to thirty-three degrees dip, and there are also occasional structural lines that are almost horizontal. The rock is exceedingly porou;;. the grains are unusually uniform in size, and the 12 ANNALS NEW YORK ACADE3IY OF SCIENCES binding material is calcareous, attaching one grain to another merely at the point of contact. The extension of ledges of this kind of rock far beyond the possible reach of swift-flowing streams, together with the fact that the distribution is limited to certain sections of the north coast, and, in addition, the evidence furnished by the internal structure of the rock itself, lead to the conclusion that the formation is essentially an old dune- sand deposit. Sand dunes are developed on the present coast line from very similar material, but none of the very recent dunes are solidified. On the other hand, the San Juan formation seems to have been developed before certain of the later elevations and subsidences that affected the island in its recent history, so that its material is fairly well cemented and its base extends below the present water level. Its outcrops also ex- tend to greater elevation above sea level than any of the modern dunes. It is judged, therefore, that this particular formation is the most recent of all in the island to act as a ledge former, and it is judged by its situa- tion and content to be of Pleistocene age. See additional description under heading "Structural Features." This is the most unusual formation in the island. It is a type seldom seen or seldom recognized, and it is one of the smallest in Porto Eico, in spite of the fact that it makes such an important showing at San Juan harbor. Because of the prominence of the formation in tbe city of San Juan it is suggested by the writer that the name San Juan formation be used for it and that this name be confined to the Pleistocene beds representing solidified sand-dune deposits. Areciho Formation. — ISText below and older than the San Juan forma- tion is a great series of reef limestones and shell limestones preceded by shales that form a belt of considerable width along the north coast and a portion of the south coast of the island. In a large way this series forms a structural unit. Above it in all cases lie the recent alluvial deposits and the San Juan formation and below it lie the older and more compli- cated igneous and sedimentary rocks. The break between these two rep- resents the chief unconformity in the Avhole geological column. The heaviest development of this formation is along the north coast between Tao Alto and Aguidilla. In this belt, the massive limestones of the Are- cibo formation attain the greatest thickness observed anywhere on the island, but no opportunity Avas formd for determining the amount accu- rately. There is in sight, liowever, certainly as much as 500 or 600 feet in the bluffs along the Arecibo Eiver. In this belt also, especially farther toward the west, in the vicinity of Lares and San Sebastian, there are im- derlying shales of considerable thickness which in places carry lignitic BERKEY, GEOLOGICAL REC0NN01S8ANGE OF PORTO RICO 13 material and which have been the object of some exploratory work for coal. It is evident, from observations made, that the shale beds of the Arecibo formation vary greatly and in some places are entirely wanting. At the Arecibo Eiver, for example, where the beds of the formation can be seen well exposed in the river bluffs, there is no shale development at all. Tlie limestones lie abruptly on the eroded and somewhat Aveathered surface of the older formations whicli at this point are represented by coarse and obscurely bedded or even massive volcanic tuffs. At the best place seen, which was on the east side of the river about opposite K-66 on the Arecibo road, there were a few feet of transition material between the limestone Ijeds proper and the unmodified tuff. It had not the structural appearance and makeup of the shale as seen at other points, however, and this part of the formation is regarded as entirely absent on the Are- cibo Eiver. Shaly beds, however, are seen again on the south side of the island and their best development is in the vicinity of Juaiia Diaz, where some of the beds are distinctly sandy and rather fossiliferous and carry petrified wood. Lignite is also reported irova this vicinity, but no ma- terial of that kind was found by the writer. It is more than likely that the shale beds on opposite sides of the island do not correspond in horizoii at all, but that structurally tliey iirc both basal beds. Tlic most striking development of the shale beds and overlying marls and softer layers of thin-bedded character, instead of the massive reef structure, is on the Jacaguas River south of Juana Diaz. The dips also of the formation in this particular locality are much greater than those observed at any other point. For considerable distances an average <\\\) of 30 to 36 degrees was estimated and the total thickness represented, based upon the width of tlic belt, must be at least 3,000 feet. At no other point on the south side, however, was there an opportunity to see whether the beds of this character are constant or of large lateral extent. As one goes eastward, a comparatively short distance, they are almost en- tirely lacking. On the Descalabrado Eiver, which is only ten kilometers to the east, the underlying older series of tuffs and intruded shales and limestones were followed to a distance of two miles south of the military road, whereas at Juana Diaz the basal shales of the Arecibo formation begin a half mile above the military road. It appears, therefore, that the formational marghi is swinging rapidly southward, and it is judged, from other observations made, that there is almost nothing of it repre- sented al^a distance of twenty kilometers to the east, or, in other words, that the formation docs not extend farther east than the vicinity of Sa- linas. At one other point on the south margin of the island, there is an unusually good opportunity to follow the successions of formations, and 14 AXXALS NEW YORK ACADEMY OF SCIENCES that is in the vicinity of Guanica. Limestones belonging to the older series occur immediately south of Yauco and are very strongly developed there. The hills in which these beds outcrop extend southward almost continuously to Guanica, l)ut at that point observations showed that the formation had changed and is actually part of the reef limestone of the Arecibo formation, although it is possible that the large fault observed west of Ponce may pass through this area and obscure the other structural relations. It looks, from the rapid survey, as though it would be favor- able for some of these additional studies of the character of the lower beds of the Arecibo formation. Between Guanica and Juana Diaz, wher- ever 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 or 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. Higher beds, forming a chalky white limestone to the west of Ponce, are also very fossiliferous, but in this area they are sepa- rated from the older rock series by a fault, so that it is quite impossible to tell how far above the base of the formation these beds may lie. It is judged that the portion of the formation seen at Guanica is a still higher horizon, but the exact age values have not been worked out. The total thickness of the whole Tertiary series on the south side of the island is very great. It was estimated that the shales and marls and limestones in the vicinity of Juana Diaz must certainly amount to three or four thousand feet. For a long distance along the Jacaguas Eiver south of Juana Diaz, the l)eds stand with a dip of approximately thirty-five de- grees toward the south and throughout the greater portion the character is notably dift'erent from the beds occurring farther to the west which are judged to overly them. To the east of San Juan, along the north coast, there is much less prominence of the Arecibo formation and after passing Eio Piedras it in no place crosses the main road. There are occasional hills somewhat similar to those characteristic of the landscape of Bayamon and vicinity, but they do not reach to so great a height and are separated by very much larger stretches of low ground. The strongest development of this for- mation seen to the east of San Juan is that along the Grande de Loiza Eiver between the railwav and the coast. In going still farther east to the vicinity of Luquillo, the inner margin of the formation passes out to sea and the older formation reaches the shore. Prom this point around the whole eastern end of the island no more outcrops of the Arecibo for- \ BERKEY, GEOLOGICAL RECONXOISSAXCE OF PORTO RICO 15 mation were seen, and this is true also of the southerly side coming from the east end through Xaguabo and Yabucoa and Guayama, and still far- ther west to some undetermined point l^etween Guayama and Santa Isa- bel. There mav be, however, occurrences of this formation underlvinj; the alluvial material along the coast at points considerably nearer Guay- ama than the outcroj) map indicates. The formation as a whole is essentially a structural unit. Although it is quite easy to distinguish the underlying shale member, especially well developed between San Sebastian and Lares, and although there are other structural changes, there is nowhere any appearance of unconform- ity or marked break in the succession. Besides, the shale member is not everywhere developed and, as a matter of fact, is seldom seen in tracing the formational boundaries. In some places, it is definitely shown to be absent, so that it seems unwarranted to represent this member as an im- portant part of the formation, so far as areal distribution is concerned. Judging from field observations already made, the shale and marl beds are more extensively developed on the south side than elsewhere. But this is based on observations in one particular area and the member can- not be traced very far in either direction because of other difficulties. The beds lying above the shales and representing the part referred to as a more massive limestone portion are probably susceptible to considerable differentiation on the basis of fossil content, and it is entirely possible that a rather complete range of Tertiary horizons may be determined after complete paleontologic study. In the field, however, and on the basis of structural factors, there is no apparent ground for subdivision. In this discussion, therefore, the whole series of beds, from the uncon- formity at the base to the alluvium and San Juan formation overlying it, is referred to as a single formation and, on account of its extensive devel- opment in the region about Arecibo, it is suggested that a suitable name would be the Arecibo formation. Some parts of the formation show the peculiarities and content of a coral reef, and these portions have the most irregular and most massive structures. Other parts show bedding structure more or less perfectly de- veloped, and throughout the whole formation here and there, at irregular intervals, and usually of only very limited extent, there are more shaly fades. It is the opinion of the writer that this irregular distribution of shaly beds is responsible for one of the peculiar topographic features de- veloped in the belt underlain by the Arecibo formation. This is the occurrence of almost perfectly flat soil-covered areas of no very great lateral extent at different levels, above which the numerous small knobs or hillocks of limestone rise, giving the peculiar haystack -like topography. 16 ANNALS NEW YORK ACADE3IY OF SCIENCES This type of topography is represented by the small level tract surrounded or dotted over with small hills, called "pepino hills" locally, standing like haystacks above the plain at many different elevations above the sea. This leads to the belief that the fundamental control in its development is the presence of a shaly bed in the series, which forms at each point the basis of the local plain. The hillocks standing above it or surrounding it represent remnants of the more massive and probably more porous and more easily destroyed limestone which has been attacked and largely removed liv weathering, and especially by solution, down to the more ■**W"*|p.'- Fig. :i. — V neon foniii III ieloiv Ike Arecibn luniuii khi This view sliows llie older iilt(';t. the rock referred to by Hill in some of his discussions as "mountain limestone." It seems to us that the several occurrences of limestone beds which have clearly differ- ent relations in the series, and the probability of being able to place them in different horizons, based on this content, makes it desirable to use more characteristic local designations for them. Such usage cannot be confusing even if it should be proven, as it may xery well be, that some of them are identical. In this discussion, therefore, wherever possible, the chief occurrences that are not clearly identical with formations already described are characterized l)y adding the name of the locality where there is especially good development of the formation. A lime- stone seen at Coqui, considerably farther east, and several occurrences near Yanco, and others still farther toward the west, have many points of resemblance to the Coamo limestone. In some cases even the brown volcanic spots are also present, but this is not true of all places. The most constant index as seen in the field is a fine meshed coralline fossil form, strikingly resembling a piece of loosely woven cloth. The limestone beds developed at Coamo reservoir, however, rarely show this type, but instead have a remarkable development of algae of the form known as Lithothamnia. TrujUlo Alio Limestone. — There are several other limestone members in the older series. One has been observed only on the north side of the island in the vicinitv north of Trujillo Alto and in the vicinitv of Loiza. This is a very dense fine bluish limestone made up wholly of fine micro- scopic organic growths. In some places it has a rough fragmeutal struc- ture, but for the most part the rock is massive and the aljundant o'rganic content, largely algoe, is its most striking characteristic. Whether it is younger or older than the Coamo limestone has not been determined, but that it belongs essentially to the same general series is quite certain. On account of its distribution it is conveniently referred to as the Tru- jillo Alto limestone reef. This member probably has a very moderate thickness and no great areal distribution. It is affected by solution de- veloping caves at the Trujillo Alto locality in much the same manner as is the Areciljo formation, but this rock is a much more compact type and its content and structural relations are quite distinct. It was probably of reef origin also, but is associated intimately with the upper shale members of the older series rather than with the Tertiary series. "Shred" Limestone. — Another limestone may be seen at several places on the Arecibo-Ponce road on the south side of the divide from K-13 to 22 ANNALS NEW YORK ACADEMY OF SCIENCES K-19. It lias no great thickness, but there are several independent beds. Its most striking character is the presence of patches of dark color dis- tributed in a shred-like way through the grayish mass. The rock as a whole is massive, exceedingly compact, of a bluish gray color, and, except for these dark-colored shreds, shows no recognizable structure whatever. They contain algae, however, which are expected to determine something more definite about their position in the series, but they are obscure forms, and doubtless considerable work will have to be put on these beds to determine their exact horizon. They are intimately associated with a series of igneous fragmental beds and a considerable thickness of very red shale or ash beds. Together these alternating limestone and red frag- mental beds make a striking structural succession which was not seen anywhere else. They lie in a position which is not far from the point where the Coamo limestone belt should be expected to cross the Ponce- Arecibo road, but no such structural development has been noted at any other j)lace. It is, of course, possible that the fine red ash represents the tuffs of the regular Coamo formation and the "shred'' limestone repre- sents a phase of the Coamo not developed elsewhere, but the striking physical difference encourages the making of a distinction, at least for the present. La Muda Limestone. — A rather heavy development of limestone in the vicinity of La Muda between Eio Piedras and Caguas has some super- ficial resemblance to the beds just described from the Arecibo road, but their relationship is not fully determined. The rock is not prominently tufaceous and is not marked in the same way. It has in places a coarse fragmental structure almost completely obscured by healing and it is, as usual, attacked by cave development. Some of the caves have collapsed, leaving a complex aggregate partly made up of igneous material filling the former chambers. A conglomerate bed lies below the limestone and shale at this point and both are cut off abruptly by a large intrusive mass. How these are related to other typical members of the older series is not known, but it will be possible to trace the beds to more definite relations. This is probably one of the oldest limestone members in the pre-Tertiary series. It is conveniently referred to as the La Muda Limestone. In addition to this there are ver}^ numerous small or thin local devel- opments of limestone layers distributed through the shale beds at various points. These are taken to be, in most cases, simply somewhat more heavily developed limy layers of the same origin as the rest of the fora- miniferal and ashy shales ; but the nature of their origin shows that it is reasonable to expect a development of calcareous content sufficient to make them more of a limestone than a shale. BERKEY, GEOLOGICAL EECOXXOISSAXCE OF PORTO RICO 23 "Mountain Limestone." — In some localities, such as that near Barran- quitas, and probably at other points along the same divide, the shaly beds, become very calcareous, as has been pointed out by Mr. Hill. These were referred to l)y liini as limestones and are probably included in his "■'mountain limestone" and estimated that the thickness of such beds amounted to fully a thousand feet. The prominence of the shaly struc- ture in these l)eds and their apparently close genetic relation to the type described in this discussion as shales, leads us to regard this occurrence as essentially the calcareous extreme of the shale series. iVs already noted, the shales are characteristicallv calcareous and some of them are predominantly so. Corozal Limestone. — At Corozal, a fragmental limestone was seen just south of the village. Its relations were not worked out and its meaning is therefore not well understood, but its structural peculiarities lead to the suspicion that it may be associated with volcanic fragmental accumu- lation. It was found that in at least one place in another part of the island there had been volcanic outbreaks through heavy limestone beds, and it is evident that fragmental material from such activity miijht therefore include a good deal of simple limestone fragments. It is hardly conceivable that they would in most places accumulate in enough abun- dance to make a limestone bed a second time, but such a thing is doubt- less possible. Conglomerates. — There is a very extensive development of conglomer- ate occurring in a belt whose general trend seems to be from southeast toward the northwest, crossing the military road between Aibonito and Coamo. There must be a total thickness of strata, including shales and interl)edded tiiffs with occasional small limestone, of perhaps several thousand feet. In all parts of the formation where conglomerate is de- veloped, the pebbles represent the same kinds of rocks as were encoun- tered in the tuffs and intrusive masses. Actual representatives of previ- ously solidified bedded material or indurated ash or shales are very rare, but in one case at least a pebble was observed that was judged to repre- sent a fragment from an older silicified tuff. As a matter of fact, the materials are practically all of simple igneous character 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 distribu- tion of material and the range of composition leads one to believe that this conglomerate represents a special state or condition whereby ma- terials of essentially tufaceous origin were, immediately after their vol- canic eruption, worn, rounded, somewhat assorted and bedded and mixed with related material. At the point examined, there was no satisfactory 24 ANNALS NEW YORK ACADEMY OF SCIENCES evidence of marked unconformity between the conglomerate above and the miderlying series of formations. But the fact that the conglomerate beds, which follow to great thickness, are jjrevailingly of simpler struc- tural 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 controlling simpler deposits which preceded and followed them, and it is possible that it may be found useful in separating the complex series of older mixed bedded rocks and tuffs into an older and a younger divis- ion by using this conglomerate as a dividing member. This is supported to some extent bv the occurrence of a conglomerate of similar character but of very much less extent on the north side of the island, several miles south of Bayamon, and also one of apparently less prominence near La Muda. If additional field work should show that the conglomerate belt could be traced from one side of the island to the other, it seems to me that it would be entirely practicable to make this division. The conglomerate is invaded by igneous intrusive material in much the same manner as is observed in the other rock formations, but the massive habit of the rock as a whole leads to a predominance of transverse dike- like masses rather than the simpler looking sills. At one point in par- ticular, however, near K-86 on the military road between Aibonito and Coamo, the conglomerate has been invaded by a magma that must have been fluid enough to penetrate the porous matrix surrounding the con- glomerate pebbles where it now exhibits a crystalline habit. This in- jected matrix is essentially a coarse diorite porphyry in composition, through which the pebbles are distributed in tl»e manner that they seem to have had in the original rock, so that there are still obscute traces of bedding structure. There are additional petrographic peculiarities in this rock that will be described under a different heading. This tendency of the dioritic magma to penetrate and incorporate fragmental matters was noted in several other places. It was most strikingly exhibited in certain intrusive members cutting through tuffs and shales. In some of these cases there is so great a quantity of fragmental matter as to wholly obscure the true nature of the rock unless one can see the structural rela- tions. In the case of the conglomerate, however, the crystalline habit of the matrix is a striking feature and it is very evident that it is wholly different from the regular conglomerate habit. J BERKEY, GEOLOGIVAL RECOyXOISfiAXCE OF PORTO RICO 25 Tujfs. — Tlie most abundant of all of the rock types is represented by a great variety oi' volcanic tuffs. These accumulations seem to be the funda- mental basis of the whole island. Whatever has been formed in the way of shales, sandy beds or conglomerates or any other of the ordinary sedi- mentary types seem to be directly derived from the same material that makes up the tuffs and the rocks directly associated with them, related in an interbedded succession. In most cases, the tuffs are essentially massive in their structural habit and are made up of a complex accumulation of large and small fragments of volcanic materials which prove to be chiefly andesitic and closely related porphyritic types. There is great variety in texture and minor structure and present condition, but on the whole there is enough uniformity of actual composition to justify classing them all as andesitic tuffs. In verv manv places there is obscure beddino- structure indicatinsr a tendency to assort and work over this material at the time of its deposi- tion. This is especially noticeable in the finer materials and some of these beds are made up essentially of ash. These ash beds resemble the type referred to as shales so closely, in some places, that it is impossible to distinguish between them in their field appearance. I judge also that there is practically a gradation from one ruck to the other, the ash beds showing transitions to shales, especially where they have developed under conditions encouraging much weathering and wt»rkiiig over of the frag- mental materials and promoting the growth of organisms in sufficient amount to make the accumulating beds somewhat calcareous in compo- sition. Tuff's and ashes are well kno^\'n to be especially liable to attack by alteration and to the ordinary changes that modify rocks. It so happens, therefore, that many of these rej^resentatives are completely modified and have become so dense that they exhibit none of their ashy or fragmental structure without microscopic examination. In this condition they are usually also exceedingly hard and as resistent to destruction as the hard- est crystalline rock. The largest develoi^ment of massive almost struc- tureless tuffs which were seen occupy the Sierra de Cayey between Guay- ama and Cayey and also the range along the military road toward Aibonito; but there are extensive occurrences in many other sections. Some of the most promijiently developed bedded tuff's and ash beds were seen on the north flank of El Ymique along the Sabana Eiver and on the north side of the divide below Comerio, and also along the Ponce-Arecibo Road both near the summit of the range and farther to the north midway between Arecibo and Utuado. From what Avas seen of this type of rock, it was not possible to form a 36 ANNALS NEW YORK ACADEMY OF SCIENCES definite conclusion concerning tlie age represented except by their rela- tion to certain interbedded shales and limestones. It appears that the underlying older portions of the series of tuffs and ash beds have com- paratively little of such interbedded calcareous material and have every- where been modified or altered or metamorphosed to a greater degree than beds that lie higher in the series. But beyond this there is little to judge of the actual age. As one goes higher in the series, however, there are occasional prominent limestone members with which tuffs are intimately associated or interbedded, and it may be possible, by reason of these rela- tions, to form a more accurate estimate of the geologic age of this later portion of the series. Volcanic Flows. — In addition to the sedimentary beds of various sorts and the related tviffs, there are at occasional places evidences of volcanic lava flows. These were seen at several places on the road between Baya- mon and Barranquitas. They are amygdaloidal in present habit and represent vesicular basalt and andesites. On the whole, evidences of lava flows on a large scale are wanting. This kind of product seems to have been very much more rare than the fragmental type. A more prominent thing as a structural feature is tlie occurrence of very numerous intrusive bodies. Intrusives. — The intrusive masses in Porto Eico occur in all parts of the island and in all of the formations except the Arecibo and the over- lying alluvial deposits. No such evidence was seen in any part of the Tertiary of j^ounger series; but the complex series of rocks representing the pre-Tertiary, here referred to as the older series, are cut in all sorts of ways by both large and small intrusive masses. The smaller intrusives are chiefly andesite i^orphyry in composition and have everywhere pene- trated the shales and ash beds. The commcmest occurrence is in the form of small sills or sheets conformable to the bedding structure and varying in thickness from only a few inches to many feet. These sills are so per- fect in form, have so little disturbed or modified tlie adjacent beds, and are so similar in general composition and appearance, after weathering, to the associated sedimentary beds, that it is quite impossible to deter- mine in all cases how nnich intrusive and how much original sedimentary rock is involved. The only thing noticed as a rule is the uniformity of petrographic structure that seems to be characteristic of the intrusive as compared with the associated beds. The simplest occurrences of sills of this kind, which at the same time show their igneous intrusive character, were seen near Fajardo, near Rio Piedras and in the vicinity of Comerio. But occurrences of the same kind are exceedingly numerous in nearly every district and in total amount form a very great additional thickness BERKEY, GEOLOGICAL RECONNOISSANCE OP PORTO RICO 27 to the bedded rocks, shales, ash beds, etc., with which they are associated. In some cases, these invading magmas have incorporated great quantities of fragmental matter, giving the rock in its present condition a very strikingly fragmental appearance. This habit associated with its per- fectly apparent intrusive relation makes a very unusual combination in the field. In many places there are included blocks of immediately adja- cent rock such as one sees in the occurrence at the quarry at Fajardo Playa, but, in extreme cases, the mass is chiefly fragmental in its make- up and one could not readily interpret its history without complete field Fig. 6. — Dioiite porphyry sills These sills are intruded between layers of calcareous shales and ash beds on the road near Comerio. The streaked or banded layers are shales, the massive portions, seen best at the left side of the print, are sills. A transgressive relation can be seen between the two layers at the extreme left. determination of its relations. Such occurrences may be seen in the vicinity of Guayama on the road about a mile east of that place, and also a short distance south of Eio Piedras. Few of these intrusive masses show anything but a rather uniform medium grain texture and larger ones have the average appearance of a rather fine or medium grain diorite. The granular appearance, however, is probably deceptive, due to the way the rock disintegrates, for thin sec- tions made from many typical intrusive representatives are nearly all plainly porphyritic in texture. 28 ANNALS NEW YORK ACADEMY OF SCIENCES In addition to these intrusive masses, ^rliich are of small size or at least of not very great areal dimensions, tliere are in a few districts large boss- like occurrences of massive coarse-grained igneous rock. The Ixnindaries of none of these have been traced out. but it is certain from the distribu- tion now known that in each case the area occupied is several square miles in extent. The most prominent one of this type of intrusive mass is that seen in tlie southeast portion of the island, including the district about Huamacao and Yabucoa and Las Piedras and Juncos. Whether or not this is all one mass belonging to a single intrusion has not been deter- mined. The variet}' of composition seen in the different samples taken at different points is consistent with the presence of more than one intru- sive unit; but it is also possible and quite as likely that the variety ob- served is wholly 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 the vicinity of Las Piedras and 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 expedi- tion show the j)resence of quartz in sufficient amount to make the rock a granite. It would, appear, therefore, that this occurrence in the south- east 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 probability it is not of quite so great an extent east and west, but these boundaries are unkno\vii. One other large intrusive mass was observed in the west central part of the island, in the vicinity of Jayuya and Utuado. In general appearance and texture this rock, in the average outcrop, does not differ much from that seen at the east end of the island which is commonly referred to as syenite. In this occurrence, however, such specimens as have been exam- ined with tlie aid of the microscope, show the presence of quartz in most cases in suffieient amount to make the rock of granite composition. In this case, as in that referred above, there are considerable differences of composition shown by the rocks which seem to be a part of the same mass. Specimens found, for example, near Adjuntas have the compositional characteristics of diorite, whereas a specimen taken near the margin of the boss on its northerly side, near K-53 on the road toward Arecibo, is a syenite. At certain other points near Utuado, the rock is a granite porphyry. The best idea of the variety of composition and textural quality repre- sented by all kinds of intrusives in the island can be gathered from an BERKEY, GEOLOGICAL RECOyyOISSASCE OF PORTO RICO 29 examination of the water-washed pehhles in the stream beds that liave come down the steeper mountain sides. An examination of such material shoM's an extremely larue varietal raniie. and. although hy all means the greater number are some variety of diorite or andesite porphyry, there are occasional more basaltic and more acid types represented. Sinnmari/. — From this description of the variety of rock types repre- sented by the leading field units, it may be readily seen that a subdivision into members of mapable constancy is no easy matter. It is the opinion of the Avriter that, for the present work and for in- vestigations immediatelv to follow, local designations or names will be of most direct usefulness, and that a correlation should be expected to l^e the final outcome of a series of such studies. For the present, therefore, it is judged best to use the terms Fajardo shales, Mayaguez shales, Jayuya road shales. Barranquitas shales, etc., as suitable names in these respective districts for the shale member of the older series, without any intention of suggesting their equivalence. In spite of the physical similarity in these cases, it is not at all likely that they belong to the same horizon. Similar argument will hold for most of the other meml)ers, — the ash beds, the tuffs, the limestones and the conglomerates. — and it is recom- mended in these cases, also, that locality designations be used in the field investigations. Those described in this report are not necessarily all that deserve special designation, but the same rule may be applied to addi- tional occurrences without in any way obscuring the ultimate solutions of the problem of correlation. On account, therefore, of the complexity of the structure and the limited amount of work yet done in connecting the separated occurrences into continuous field units, it is advisable to retain such terms as Coqui limestone, Coamo limestone. C'orozal lime- stone. La Muda limestone, Trujillo Alto limestone, Cayey tuffs, etc., for the earlier descriptions and special studies. i'i;Ti!(;(.i;.\i'iiir kaxge There is an exceedingly great variety of certain classes of rocks in tlie Island of Porto Eico. Those most prominently developed and showing the greatest variation in minor cliaracter, structure and relationship are the volcanics, especially the volcanic fragmentals. All sorts of tuffs, cin- der beds, ashes, mud flows and bombs are represented in great quantity, in very wide distribution and in all stages of alteration and induration. Observations made on thousands of occurrences of this character of materials leads to the conclusion that most of it is essentially of andesitic composition. Although there is an occasional fragment of either more basic or more acid composition, the predominant types are always of 30 ANNALS NEW YORE ACADEMY OF SCIENCES andesitic makeup. The present condition of these rocks, representing as they do nearly all stages between fresh material and either a thoroughly weathered or considerably metamorphosed condition, is a more interest- ing study than their primary composition. Some of the most dense and resistant rocks in the whole island are these older metamorphosed tuffs and ashes. ISText in point of abundance is the group of ciystalline igneous rocks. In this case there is somewhat greater prominence of varieties represent- FiG. 7. — Photomicrograph of a tyjncal thoroughly indurated andesitic ash, magnification 28 diameters A rock of this type appears in the field as a darlc-greenish hard resistant obscurely bedded layer, usually closely associated with more massive tuffs on the one hand or more strongly bedded shales on the other. The clear grains are mineral fragments ; the more complex grains are fragments of lava, cinders, glass, etc., all thoroughly bound into a complex aggregate. ing the acid and basic ends of the classification scheme, but here also the rocks of the andesite-diorite family are by all means the most numerous and most widely distributed. The greater number of occurrences are represented by members of this family l^elonging to intrusives that would be classed normally as andesite porphyries, porphyrites of various kinds and diorite porphyries. The minor variations represented by these rocks BERKEY, GEOLOGICAL RECOXNOI8SAXCE OF PORTO RICO 31 would probalily ineliule all of the habits known to this family. In addi- tion to the porphyries of this family composition, there are less common occurrences of felsite, quartz porphyry-, granite porphyry and basalt porphyry. In surface flows there is, besides the andesites, an occasional amygdaloidal basalt, but so far as observed there was no rhyolite or other very acid surface type. Among the massive larger intrusives, the com- monest and most abundant type is a granite porphyry or granite varying in some parts to the composition of syenite. A massive rock of the na- FiG. 8. — Photomicrograph of a typical iceathered shale from Fajardo Playa, magnification 28 diameters The dark field is made up of an extremely flue aggregate of earthy materials. The white circular and irregular spots are entirely empty and constitute the porosity of the rock. The circular forms of these voids suggest that they represent former calcareous content in the form of foraminifera, now completely removed by weathering. ture of a diorite is also represented, as is a very coarse rock of the nature of a giant granite. How these are related, how many intermediate varieties there may be and whether this variation represents magmatic differentiation within a single mass or instead different units of intru- sion, has not yet been determined in enough detail to make a positive statement. But in at least two cases where these large masses were seen. 32 ANNALS NEW YORK ACADEMY OF SCIENCES the hasty examination given to them leads to the belief that clifEerentia- tion effects can be traced. The closest relatives of the igneons rocks are the sediments, and be- cause of the fact that the material constituting these sediments has been furnished by the volcanic fragmental supply in large part, their character and makeup is in many cases not strikingly different from the ashes and finer tuffs. They do, lio-wever, represent an additional assorting, an ad- ditional weathering and an additional opportunity for intermixture of Fig. 9. — PJiotomicro(/r(ii)h of a foraminifeial shale from the Bayamon-Comerio road, magniflciition 28 (liameters The dark areas are chiefly earthy aggi-egates or very fine texture : the whitish areas are calcareous spots which in many cases still preserve the forms of foraminifera. It is the removal of such materials from the shales that is believed to account for their porosity as seen in weathered ontcioiis. materials from different sources and uf organic material developing at the same time. These conditions give a great range of composition and mineral makeup to the shales and sandstones- and they merge by imper- ceptible gradations from simple tufaceous or arkosic sediments to cal- careous rocks or even to fairly pure limestones. The common source of the calcareous element in these rocks is from an intermixture of fora- BERKEY, GEOLOGICAL RECOXXOISSAXCE OF PORTO RICO 33 miniferal matters representing organic growths accompanying the accu- mulation of the deposit. The limestone members on the one side representing almost pure or- ganic accumulations, and the conglomerates on the other representing the simpler destructive volcanic fragmental matters, give the range between which an exceedingly great variety of sediments are represe»ted. The shales of the younger series, represented by the Lares and the Juana Diaz shales, are more strictlv detrital and of true erosinnal and Fig. 10. — Photomicrograph of the San Juan formation indurated dune sand, magnification 28 diameters The clearest grains are simple mineral fragments ; the dark ones and the grayish ones with internal structure are fragments of calcareous organic growths. The grayish matrix Is a secondary binding material of calcific composition, in this case practically filling the interstitial spaces. destructive origin, and in places they contain lignitic material which suggests different physical conditions. This shows, however, in their upper laj'ers an increasing amount of organic content also, and finally are succeeded by limestones of wholly organic makeup. The failure of volcanic activity during and subsequent to that time gave no opportunity for the amount of intermixture that is seen in the shales of the older series, so that as a result the younger series of rocks is petrographically 34 ANNALS NEW YORK ACADEjMY OF SCIENCES more simple and less modified. The organic content is of greater variety, however, and because of the reef-building tendency the primary structure is more varied than is seen in the limestones and shales of the pre-Ter- tiary representatives. The most striking petrographic type is, probably, the solidified dune sand making up the San Juan formation. Its uniformity of grain, strong cross-bedded character, porous habit, together with its rather sur- prising stability, make it an object of some considerable interest. Sev- eral of these classes of rocks, therefore, represent petrographic series of unusual range and variety, and because of their perfection of develop- ment would seem to warrant detailed study. There are no foliated metamorphic rocks so far as yet seen in Porto Eico. One specimen of such rock, a mica schist, was shown to the writer as having come from the Portuguese river not far from Ponce, but a hur- ried reconnoissance in the vicinity failed to uncover anything even re- sembling it. The specimen probably does not belong to the rocks of Porto Eico. No evidence whatever has been seen of conditions that would be expected where such rocks occur. The most profoundly modified rock observed is a massive serpentine. Such material was seen at two places by this party, one near Yauco and another on the road to Comerio, and the same type is reported by Pro- fessor Crampton on a much larger scale near Maricao. But they are all simple petrographic cases after all, being ordinary intrusive units of heavier ferro-magnesian content than the average which have been heav- ily altered, especially by hydration j^rocesses, to the present condition. Genetically and historically, the serpentines are not materially different from the other intrusive bodies. Depth of Decay Alteration has affected the rocks at most points to consideral)le depth, but in spite of this there are plenty of outcrops, and along the roads there are many cuts exposing fresh rock. The stream beds are strewn with fresh pebbles and boulders. Although decay obscures the character of the rock in most of fhe outcrops, the sti'ucture is usually fairly well pre- served, and in most cases enough can be seen to enable one to determine the formational habit. The most striking thing about many of these badly decayed outcrops and cuts is the remarkable way they stand against destruction or removal by ordinary weathering and erosion agents. At many points, road cuts are made, witli side walls absolutely vertical, tlirough wholly decayed rock material, that stand for years without crumbling down. Embank- BERKEY, GEOLOGICAL REGONNOISSANCE OF PORTO RICO 35 ments made of earth along roads and trails behave in the same way and one is continually surprised at the steepness of such slopes and their ap- parent stability. Slopes of 40° are not rare on hillsides that are culti- vated, and one occasionally crosses divides that are mere knife edges with slopes of this kind on both sides. Such stability of the soil is a great factor in preserving the agricultural productivity of Porto Eico and in the distribution of its agricultural industries over so much of the in- terior area. There seem to be three factors of large consequence in this stability of the soil mantle. One is the clinging character of some of the vegetation which tends to bind the soil together ; another is the small range of tem- perature variation which reduces disintegration or disruption tendencies to a minimum ; and still another is the low content of inert or refractory materials, such as quartz, in the rocks whose destruction has furnished the soils; all of which factors favor the making of especially tenacious soil. Most of the soils are for the same reason exceedingly difficult to cultivate. As a direct consequence of this soil behavior and climatic control, there is comparatively little dust formed in Porto Eico. This is especially noticeable on the roads, where one is almost never in the least troubled by it. In two districts, one on the north fringe of El Yunque along the Sabana Eiver and the other on the north side of the divide near Adjuntas, deposits were seen which suggest glacial action. Very large boulders are stranded in jiositions where it is difficult to account for them by ordinary erosion means, but too little study was given and too little evidence is at hand to warrant a more definite statement. STRUCTURAL FEATURES Most of the structural features represented in the geology are men- tioned in connection with descriptions in other sections of this report. This is especially true of such structures as may be regarded as essen- tially primary, including the interbedded and intrusive forms of various kinds. There is no doubt but that the most prominent structural com- bination in Porto Eican geology is represented by the succession of inter- bedded sedimentary beds and tuffs, cut liy or interleaved with intrusivei in the form of sills, dikes or irregular stringers. The combination occurs in great varietv as to succession, relative amounts or proportions, quality of material, present condition and relations to other members, but in spite of these variables the structural feature is essentially the same and has the same meaning everywhere. 36 ANNALS NEW YORK ACADEMY OF SCIENCES Igneous Structures The most striking thing in connection with the structure is the re- markable uniformity of the sills and their close resemblance on that ac- count, after weathering, to the fragmental beds with which they are associated. The small amount of metamoi-phosing influence that they seem to have had, also adds to the difficulty. In some cases, however, a transgressive intrusion has disturbed the adjacent beds a great deal in a mechanical way. In the average case, it is judged that the intruded magma has neither penetrated the materials of the adjacent beds to a noticeable amount, nor .%,™. Fig. 11. — Shales and nxh beds rut hij a large irregular diUe and sill The dike is shown at the location of the standing flgure and the siU extends upward to the left between the plainly bedded layers. Both the dike and the sill are crowded with fragmental material to an extent that makes the intrusive have more the appear- ance of a volcanic fragmental than a true intrusive. has it absorbed or ineorjto rated a great deal of such material. But in a few cases where structural relations were indisputable it was equally clear that the intrusives, both transgressive and concordant, were liter- ally choked with foreign fragmental matter, making them resemble the real tuffs so closely that it is doubtful whether the difference would have been detected except for the clearness of the intrusive relation. Such occurrences suggest that there may well he many other apparently frag- mental interbeds that are in reality fragment-clogged intrusives. On ac- BERKEY, GEOLOGICAL RECOXNOISSANCE OF PORTO RICO 37 count of the great abimdance of the fragmental matter, it does not seem possible that these intrusive masses could penetrate in that condition to some of the positions where we now find them. It is more likel_y that a rather fluid magma has penetrated some unusually porous fragmental bed forming a matrix for it, perhaps also spreading it somewhat, and then in breaking across to another bed, in some cases it was still mobile enough to drag the mixture along into the larger transgressive structures. This idea is somewhat supported by the finding of a conglomerate bed, near K-86 on the militar}'- road west of Aibonito, impregnated with an igneous matrix in essentially this same manner. In addition to these forms, there are numerous larger intrusive masses, the largest of which deserve a special name. I see no objection to calling them bosses. The two largest occur, one between Caguas and the Caribbean sea toward the south and southeast, and the other between Ja3niya and Utuado on the north side of the divide.- Volcanic Vent Complexes A special igneous structure that has not been referred to except inci- dentally is that composite of disturbed structures which represents the location of old volcanic vents. They are essentially a complex of irreg- ular intrusive units cutting and including masses and aggregates of •various fragmental and sedimentary types in a mixed relation. In the clearest cases, such a complex suddenly takes the place of a formation of apparent promise of continuity such as a series of sedimentary beds, and after an interval these beds are again found continuing as before. For example, the Coamo limestone formation is abruptly cut off and its place is taken for a mile or more by one of these igneous complexes, the lime- stone continuing on the other side again. The Jacaguas reservoir, just above Juana Diaz, lies in one of these old volcanic-vent complexes, occu- pying, however, only a portion of the area. Another such case is repre- sented by the complex cutting the great conglomerate beds on the military road at about K-87— 88 west of Aibonito. Another is believed to be represented by the very striking basin-like area crossed b}^ the road between San German and Hormigueros. This one is now represented by a very smooth plain five or six miles across surrounded on all sides by more hilly country. The same conditions are imdoubtedly indicated by the extremely complex structures seen on the Descalabrado river two miles below, south of the military road. Some of these mark the sites of ancient craters, cloorsred or choked with fragmental and intruding ma- terials. 38 ANNALS NEW YORK ACADEMY OF SCIENCES Folding Most of the rock formations representing original bedded types have been more or less tilted or otherwise do not now have their original atti- tude. Those belonging to the younger series, the Tertiary limestones and shales on the north side of the island, are comparatively little dis- turbed, and in some cases do not have a very different dip in spite of thu fact that they have changed very much in position with respect to sea level. On the south side of the island, however, even these later beds are in many places tilted at a higher angle than they had in the beginning ■«. / ^ ■' '- 31 "1 M, ... ^— fXJ^j^^O/^^^M i r' ^^ ^^» '^J '^rS^H '•^•^^^ ™ Kii^^W^-- .._ •*V5^i«E.^aBS^iB ^^^^^^^^^^^^^s^ff*^^^ E^'i^^^SPW h Pig. 12. — Overtiii-HCfl fold and crush ^oiie in finely bedded shales on the Jayuya road near the summit of the range and occasionally show high angles and even gentle folds. This condition may be seen on the Jaeaguas Kiver, near Juana Diaz, better than at most places, but similar conditions are indicated by the relations at certain points farther west. This condition on the south side of the island indi- cates more extensive and more violent dynamic disturbance on this side, which is further supported by the presence of faults cutting and affecting the Tertiary series on its present inner margin. The older series, the pre-Tertiary, is still more profoundly affected and, in almost all districts, shale and ash beds may be found standing at high angles, in many cases practically vertical, and in occasional instances J BERKEY, GEOLOGICAL REC0XN0I8SANCE OF rORTO RICO 39 crumpled and overturned in a most complicated manner. Minor fold structures of this kind are especially prominent in the higher ranges, for example, along the Jayuya road along the divide opposite station K-24 of the Ponce-Arecibo road. At such a place may be found as compli- cated structural features of this kind as is usually present in any folded mountain region. The high angle at which such beds stand at many other points leads to the belief that similar complexities characterize a great many of the districts occupying the higlier mountam portion of the island as well as some of the marginal areas, but the great amount of erosion and the very limited exposures at most points tend to obscure some of these details. The complex way in which the igneous intrusive masses cut all of these formations also tends to obscure and modify and further complicate the simpler fold structures so that it is not always possible to properly credit the disturbed attitude. On account also of the fact that the total quantity of injected or intruded materials, including dikes, sills and bosses, is exceedingly large and miist have caused exten- sive disturbances by reason of the displacement produced by the occupa- tion by these intrusive masses, it is likely that much of the observed ab- normal attitude of the bedded rocks may be due to this cause rather than to regional folding of a simpler sort. It is fair to say, however, that a sufficient amount of data is not yet available to draw general conclusions as to the meaning of the fold structures in Porto Kico. The striking thing is that all of the older formations are disturbed and that their position and attitude, even along the margins of the island, indicate that the region affected by these movements was more extensive than the present land area. Faulting There are many evidences of faulting on a small scale, in some of which the displacements can be measured. But in most cases the direct evi- dence lies chiefly in the existence of crush zones, slickensided walls and abrupt changes of rock type; there is no opportunity, on account of the general rock complexity, to secure quantitative data. Judging from the difficulty in tracing certain formations between districts where they have been identified, it is probable that there are occasional faults of large dis- placement. jSTumerous crush zones were seen on the Comerio road espe- cially, but in this case also no system was discernible from the few meas- urements available. The most prominent fault, in its effect upon present features, is the one now marking the inner margin of the younger series of chalky lime- stones and shales constituting the coastal lielt along the south side of the 40 ANNALS NEW YORK ACADEMY OF SCIENCES island from Juana Diaz past Ponce at a sliort distance to the north, cross- ing the Ponce- Arecibo road at K-4.8, and thence westwam, crossing the Ponce-Pennelas road at K-10. This is the only large fault actually ob- served that is necessarily of recent age, although a few others are inferred. 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 with respect to the younger series forming the present coastal margin wherever this fault has been seen. It has been traced by us from Juana Diaz to the vicinity of Penelas, a distance of about 12 miles. What becomes of Fig. 13. — Crumpled shales as seen along the Jayuya road near the summit of the range it at either end is not yet determined, but it is believoil to extend much farther in both directions. 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 south- erly margin, tilting the whole mass gently northward. If this disturb- ance took place, as seems to be iiidicated by the fault described, in very late Tertiary time, accompanying the emergence from the sea, it would accoimt for the al)normalities of Tertiary rock distribution as well as the unsymmetrical position of the main drainage divide. In any case, how- ever, the fault block structure is a very late development and is superim- posed on the other more complex and older structures of the mass. BERKEY, GEOLOGICAL REGOXXOISSANCE OF PORTO RICO 41 The island is comparatively abruptly terminated at both the east and west ends. The younger limestone , margin, which is fairly continuous along the north coast and extends along about half of the south coast, is wholly wanting at the east end and is also absent at the west end except at the corners. At Fajardo. at the east end and at Eincon, at the west end, for example, the older complex bedded rocks continue to the shore line. In the uplifting of the present island mass, it would therefore ap- pear that ])reaks occurred at both ends. The included mass is therefore probably bounded roughly on three sides by faults, the east, west and south, and is as a block tilted gently to the north. That there is, besides, considerable differential movement accompany- ing the uplift and disturbance, is indicated by the warping of the erosion plain lying beneath the younger series, the Eocene peneplain, which stands essentially at sea level in the vicinity of Loiza and more than a thousand feet above it at Lares. This difference is accompanied by a much wider belt of these later limestones also in the region about Lares tlian elsewhere. Such warping need not of course be confined to the last movement : it may have accompanied the depression in early Tertiary time, permitting, as is indicated by deposits, very marked differences in the development and encroachment of the organic accumulations. Large Structural Groups Where rock formations or field units are so numerous and so closely related and so complex in primary structure if taken in detail, it is ad- visable to combine them into fewer more generalized groups. A first step of a very general sort, but in all respects sound, has been taken in recog- nizing and using the terms "Younger Series" and "Older Series" in this paper. An additional step has been suggested in recognizing certain smaller associations under the terms San Juan Formation, Arecibo For- mation, Coamo Limestone Formation, Juana Diaz shales and marls, Fajardo shales. Sierra de Cayey tuffs, Ponce chalky limestone, etc., but these are for the most part local designations, some of which may well be expected to become unnecessary after complete correlation is established. A good structural basis for sound subdivision of this sort is not yet worked out. TJnconformities There is only one marked miconformity in the structure of the island. This is between the younger and the older series. It measures the break in the sedimentary succession represented by the erosion interval during which this mountain mass, now represented by Porto Eico, was reduced 42 ANNALS NEW YORK ACADEMY OF SCIENCES to a comparatively monotonous surface for the most part at least near to the sea level. The time interval need not have been of very great geo- logic value, but it represents the time between the last violent outbursts of volcanic eruptive activity, occurring near the close of the Cretaceous, and the beginning of simple sedimentation and limestone reef develop- ment and other organic accumulations in the early Tertiary. This un- conformity is very pronounced along the northern margin of the island ■wherever the two principal series of formations are well developed. This is not easily seen on the south side. Init the relative complexities of atti- tude of the two series, together with their position, emphasizes the same fundamental relation. This break in sedimentation is not anywhere marked by a development of a basal conglomerate. In some places the new series is inaugurated by the development of shales, part of which are lignitic, indicating land conditions, but in other places such beds are entirely wanting and the upturned eroded members of the older series are followed abruptly l)y limestones of the reef type. The tirst type of succession is illustrated in the vicinity of San Sebastian and Lares and the latter type of abrupt limestone succession by the conditions seen on the Arecibo Eiver. It is entirely likely that the time value of this break is not evervwhere the same. Probablv the districts in which shale beds are developed saw the beginning of sedimentary deposition at an earlier period than those in which the shales are entirely lacking. It will be pos- sible to work out these historical and structural differences with further study of the content and distribution. A very extensive development of conglomerates in the region immedi- ately west of Aibonito and smaller occurrences at several other points, especially on the Comerio road south of Bayamon and also near the military road in the vicinity of La Muda, have a suggestion of the possi- bility of a rather important structural break, but there is thus far no conclusive evidence of the presence of any real unconformity. Veins Quartz veins are not prominently developed. There are occasional quartz stringers and in a few places they are abundant enough to make a sort of net work, but in no instance was a large persistent fissure vein seen. In some cases the stringers of quartz carry values in gold, and in all probability they are the source of the placer gold known to exist in certain districts. So far as observations have been made, there is no par- ticular system represented in the vein occurrences. Questions connected with this subject together with mineralization and possible value as mineral resources should ])e made a special study. BFRKEY. GEOLOGICAL RECOXXOISSA^'CE OF PORTO RICO 43 Minor Structures Although there is extensive development of sedimentary formations which have been subjected to much disturbance, there is comparatively little structure of a minor sort that seems to deserve such discussion in this description. Two, however, that seem to have special significance connected with the origin of the particular beds which have been found are (a) a peculiar crumpled, enterolithic structure seen in one of the ash beds near Guayama, and (b) the wind-assorted cross-bedding structure of the old dune sands of the San Juan formation at Arecibo. Enterolithic Structure. — The enterolithic structure noted in the ash beds, on account of the thinness of the bed, — about eighteen inches, — and the simplicity of the associated structure, — simple tilted beds, — leads one to believe that the structure is essentially primary rather than of subse- quent dynamic origin. Its appearance is perfectly consistent with the explanation that it is jn'eserved from the time of deposition and its be- havior at that time as a small mud flow. It is a structure siich as might be formed by slumping movement of a soft layer. It should be expected that there would be frequent behavior of this kind in the accumulation of such extensive heUs of ashy materials, which must in some cases have been deposited under conditions that would make slumping movements possible, l)ut it is not to be expected that material of this kind would in most cases be capable of preserving any of these primary movements. In the case noted, the quality of the interior makeup of the bed seems to have been more favorable to such preservation. It is the only case where such an observation was made. Double Cross-bedding. — The cross-bedding structure belonging to the San Juan formation is a prominent feature wherever these ancient dune sands are preserved. A great prevalence of steeply inclined minor struc- tures is crossed l)y fewer nearly horizontal ones. Measurements made on sea-cliffs a short distance west of Arecibo, where this rock is very promi- nently developed, gave dips of 30 to 33 degrees repeatedly. A series of these is abruptly terminated by a more nearly horizontal bedding for a comparatively short distance and the whole structure is repeated. The layers with this kind of structure are prevailingly one-half to two feet thick and no ripple marks were seen on any of the beds examined. The cross-bedding structure in this case dips always to the west or southwest, and the average strike of the principal beds is about north 30 degrees west. This is consistent with a wind direction not very different from the pres- ent prevailing winds. Very strong structural development of this kind is also to be seen in the city of San Juan at the promontory on which the 44 ANNALS NEW YORK ACADEMY OF SCIENCES Morro is built, but measurements of orientation were not taken there. An occurrence of this rock immediately to the east ^ of Arecibo, a short distance south of the lighthouse, showed structures of this kind on a much larger scale than was seen elsewhere, and with an especially inter- esting combination structure. The principal or stronger divisions are widely separated and lie nearly horizontal. A comparatively small bed lying in this position was almost unconsolidated, but those strongly cross- bedded immediately above as well as those below were compact enough to stand in a vertical cliff 30 to 40 feet higb. Tlie chief interest attaches Fig. 14.- — Cliff of the San Juan formation nontli uf the liyhthounc at Arecibo The prominent cross-bedding, extending throughout the upper thirtj- feet of the cliff, Is well shown, together with a less strongly marked horizontal structure crossing the same beds. The prominent break near the base is made by a layer of sand which is very poorly consolidated. to the strongly cross-bedded portion forming the upper twenty feet or more of the exposed cliff. The cross-bedding structure itself extends without interruption through a much greater vertical range than in any other outcrop examined, but its attitude and dip were not markedly dif- ferent; the feature that was strikingly different from the structure seen anywhere else was introduced by the presence of less pronounced but still very plainly marked horizontal structures, making an interpenetrating mesh-like arrangement in the face of tlie cliff. This can l)e seen strongly enough to show in a photograph even at a distance of 100 feet. It is evi- BERKEY, GEOLOGICAL RECOXXOISSAXVE OF PORTO RICO 45 dent that some i)eculiar condition has been in control in the development of this compound structure. There is no reason to assume any diflference of source or origin for the steeply inclined structures, or cross-bedding structure, from that assumed for all of the other occurrences of the San Juan formation. It is apparently a perfectly normal product of the as- sorting action and deposition of material under the work of the wind. But under normal conditions it would not happen that a second structure in a horizontal position should be repeatedly developed crossing the well marked cross-beds so that the whole complex combination should be de- veloped on such a scale as is seen in this case. From the nature of the de- posit and the conditions in which it may well be assumed to have been formed — that is, at or near sea level in the vicinity of or bordering upon standing bodies of water — one would be inclined to favor the explanation that tlie accumulating cross-bedded sands fell into or rolled into a body of water which liad a tendency to attack the newly deposited material and to bind the grains together. The difficulty with such an explanation is in the fact that the horizontal structure is repeated at small intervals practically tliroughout the deposit, and apparently without disturbing the primary depositional structure at all. It would seem quite unlikely that loose matters of this kind, falling into or rolling into a body of water of an open surface sort, should maintain or preserve the primary struc- ture so well. Perhaps it is more logical, in view of all the features, to connect the development of this structure which seems, from its slight influence or modification of the cross-bedding, to be wholly secondary with the subsidence of the coast which is shovsai to have been one of the late events in the geological history. As subsidence progressed, perhajis somewhat irregularly, it would iiappen that the ground v/ater level would rise correspondingly liigh in beds that were passing below sea level. At the surface of this ground water level the tendency would be to accomplisli a binding of the granular materials together. Both above and below the ground water level there would probably not be so strong a tendency to develop this binding. With the next step in the progress of subsidence, another streak or indurated zone would be established and these have been repeated throughout the whole formation during the whole period of subsidence. An action of this kind would not tend to disturb the pri- mary structure at all; it would on the contrary tend to preserve it or make it less destructible because of the improved induration. But it could, in addition, develop a succession of secondary structures through- out the whole mass which, if the binding is fairly substantial, might rival the primary structure in prominence when exposed to subsequent de- structive attack. It is possible that such a succession of horizontal struc- 46 ANNALS NEW YORK ACADEMY OF SCIENCES tures could be developed even under a perfectly continuous but very slow subsidence movement l)y reason of the natural seasonal ground-water fluctuation. From this point of view, the range between two succeeding ^ Fig. 15. — Detail af the iluiihlc strurtme in the San Juan formation at Arecibo This photograpli was taken at the same point as the one sliown in Figure 14 to bring out the horizontal structure crossing the inclined layers. There is no doubt whatever that the dark layer of less consolidated sand in the lower third of the photograph is a primary bedding structure, but the horizontal marks crossing the inclined layers in the upper part of the view are believed to be of secondarv origin. BEREEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 47 horizontal markings would measure the fluctuation range of the ground water, the harder zones representing in that case the more persistent upper level for each succeeding depression position. The cross-bedding structure shows as plainly as it does on exposed sur- faces also because of the fact that certain streaks are more perfectly in- durated than the intervening ones, and the objection might well be raised that a history of the kind suggested above woidd not be expected to de- velop such a difference of induration in layers inclined at such high angles to the horizontal. As a matter of fact, however, these sands are not simple in their makeup. They are in large part fragments of organic material and complete shells of small organisms of a calcareous nature and the primary cross-bedding structure represents an assorting action on this mixture of silicate and carbonate mineral material. It so hap- pens, therefore, that the successive structural units are not necessarily of the same mineral proportions, and in the process of induration, or of binding the grains together, certain streak's yield more readily to this influence and develop greater solidity and resistance to destruction. It happens, furthermore, from a variety of rather unusual primary condi- tions and secondary influences that both a primary and secondary struc- ture of unusual prominence and peculiar association are developed in the San Juan formation. SPECIAL RELIEF FEATURES Playas The flat areas along the coastal margin which are known as playas are all developed at the mouths of rivers and are essentially alluvial deposits of floodplain and delta type. In most cases they seem to occupy areas that must formerly have been embayments in the coast. This develop- ment is most striking, for example, at the mouth of the Arecibo and of the La Plata and T.oiza rivers. In some cases, however, there is no marked embayment and the deposit is more strictly marginal, such, for example, as the Fajardo Playa at the east end of the island and others on the south coast. Promontories In addition to tliL' embayments and playas, there are, occasionally, in the intervening spaces, promontories where the rock formations extend into the sea and terminate in cliff forms. These are neither numerous nor are they confined to any particular portion of the island or to any rock formation. They are represented by the most recent of all of the 48 ANNALS NEW YORK ACADEMY OF SCIENCES formations, essentially a silicified dune deposit such as that at San Juan, also by Tertiary limestone reefs, such as that at Quebradillas and at Guanica, or by the still older igneous and clastic series, such as that at Anasco, or by massive intrusives of a strictly igneous habit, such as that at Maunabo. It would appear from this that the former outline of the island must have been more irregular than it is at present and that the distribution of marginal formations is also not as regular as has l)een represented in earlier reports. Fig. 16. — Plaijn plain and marginal terrace View of the Pla.va plain (foreground), the marginal terrace (middlec lield) and tlie mountainous divide formed h,v the Sierra de Cayey as seen from the "Central JMachete" near Guayama. This terrace bevels across the upturned edges of shales, ashes and intru- sives of the older series and is probably due to marine cutting. Terraces At many places on both sides of the island there are comparatively smooth tracts having the appearance of bordering shelves which represent true terraces. Their location along the sea margin and the compara- tively insignificant development of similar benches along the streams lead one to believe that they have an origin connected with the wave action and attack of the sea. This interpretation is supported by the presence of these terraces along the coastal margin where stream action would not seem to have been able to reach. In any case, the presence of such terraces, which stand from TOO to 200 feet above the present sea level, indicate a former more submerged condition, so that the sea or streams, or l)otli combined, were al)]e to attempt base-leveling at that ele- BERKEY, GEOLOGICAL RECO:^NOISSANCE OF PORTO RICO 49 vation. The presence of gi-eat quantities of roughly assorted gravels clogging some of the valleys of the southerly side of the island tends to support the same general conclusion. The bearing of these features on the geological history of Porto Rico will be taken up at another point. Cuestas Both on the north side, for nearly the whole length of the island, and on the south side, over the westerly half, there is a bordering belt of limestone and associated beds that bave been developed on an eroded sur- FiG. 17. — Structure beneattt the marginal terraces Strongly bedded ash together with associated shales cut by small dikes forming a part of the terrace uear Guayama. These rocks belong to the older series and dip into or toward the mountains rather than toward the sea. face which beveled across the more complex structures of the older series of formations that fonnerly made up the mass of the island. These limestone beds are several hundred feet in thickness and dip gently toward the sea. On the inner margin of their present extent toward the interior, especially along the north side of the island, they are abruptly terminated in a very irregular line of modified cliff forms facing toward the prevailingly smoother and lower ground for some distance toward the interior. For tlie most part, this limestone margin is exceedingly rugo^ed and broken. The width of the lielt with this rugged character 50 ANNALS NEW YORK ACADEMY OF SCIENCES varies very much in different parts of the field. Its most striking devel- opment is in the district extending from Tao Alto to Aguadilla, In the district extending eastward from San Juan and also in some of the areas on the south side, this margin is very much broken and so obscure in some parts as to escape detection. In its best development, however, it is a typical cuesta, formed in the usual manner by the erosion of a for- mation 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 de- struction of a series of beds and reefs that in former times covered a Fig. 18. — Inner lowland near Baijamnn View looking nortti from the Bayamon-Comerio road toward San ,F\i;in. showing the monotonous features of the lowland belt in the foreground and the comparatively promi- nent hill remnants of the Tertiary formation cuesta in the background. large portion of the island. The road running from Aguadilla to Moca. San Sebastian and Lares extends for practically the whole distance, after leaving the coast, along the inner lowland at the foot of this cuesta or along the cliff forming the inface. The same features characterize the surface topography as far east as Corozal. This feature is much less pronounced on the south side of the island. Peneplain Beneath the limestones 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 struc- BERKEY. GEOLOGICAL REC0NN0IS8ANCE OF PORTO RICO 51 ture. Occasional profiles of more distant ridges also show a sky line that suggests the former existence 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 monntainous tracts, especially at the west end of the island, near Eincon and in the vicinity of Mayaguez. At the latter place, these remnants of the old plain are called mesas. It is the judgment of the writer that these all belong to a single base-leveling sur- face or marine-cut platform formed in the period just preceding the development of the Tertiary limestone series. Judging from beds devel- oped immediately upon this surface, it mustliave been completed in early Fig. 19. — Hupslack (pepino) hills A characteristic view, showing tlie small soil-covered flats and associated haystack hills found in the region of the Areciho formation. Thotograph taken on the road be- tween Arecibo and Barceloneta. Eocene time, and perhaps was even largely developed in pre-Tertiary time. It may l)e referred to as the early Tertiary base level or peneplain. There are many minor features giving variety to the surface relief which depend for their particular relations and character on underground structures which are as yet imperfectly understood. Haystack Hills The most striking topographic feature of the whole island is the re- markable development of small isolated or grouped rugged hills usually rising abruptly above adjacent smooth flat soil-covered areas at various levels throughout a broad marginal belt along the north coast, west of 52 ANNALS NEW YORK ACADEMY OF SCIENCES San Jiiau. They constitute a feature so unusual that even the un- trained casual visitor is impressed with them. This feature has been referred to before in connection with the de- scription of the "Younger series" of rocks, especially the Arecibo reef limestone formation. In spite of the unusual appearance presented by this distribution of "haystack" hills and intervening flats, their origin is judged to be comparatively simple. The active agents and processes have been the same as those at work on all other parts of the island, but the results differ because of the fuudamental difference of material and Fig. 20. — Care stvictiire in the hniititack liiepino) hilh Near view of the limestone hills forming the margins of the small cultivated flats in the typical haystack hills district. This view shows the cavernous nature of the lime- stone forming these hills, a structure that is regarded as the most significant feature and probably the largest factor in the development of these peculiar relief forms. structure, i^owhere are these features developed except Mdiere the later reef limestones are the underlying bed-rock formation. The essential steps in the development of these forms are the follow- ing : The reef limestones are not uniform in composition or structure. They have more or less intermixture of earthy matters which are distrib- uted irregularly, but chiefly at certain hori/ons, as more earthy or shaly beds of no verv great lateral or vertical extent. Such conditions are re- BERKEY, GEOLOGICAL L'ECOXXOISi^ANCE OF PORTO RICO 53 peated at occasional intervals in successive horizons. As such a series is lifted above sea level and suljjected. to ordinary erosion and weathering, the tendency is, (a) for the purer and more massive reef limestones to be attacked by the solvent action of percolating water with a development of underground channels, porous rock condition and actual caverns, (&) for the :nore earthy layers to resist and limit such action at the levels where this matter is present in sufficient abundance, with a development of residuary material. As this action progresses toward maturity, many of the larger ca\'es collai)se and sink holes are thus formed. AVith still fur- ther development, the sink holes merge into each other in local areas where solution has been most active, the earthy debris forms a soil in the l)ottom corresponding in level M'ith the first important earthy layer, and adjacent remnants of tlic limestone reef stand out as sharp rugged hills separated by irreguJar ]iotches that represent other smaller collapsed caves. The result of such action and conditions, finally, is the numerous "haystack'" hills standing on flat soil-covered areas or surrounding such areas as if they were just set down as bunches on this surface. This re- lation is repeated at different levels throughout the belt from San Juan to Agnadilla. but the most striking developments are local, apparently M'here the structural relations are just right, and may be seen best be- tween Tao Alto and Arecibo, especially in the vicinity of Manati and Vega Alta. It was at first thought tluit former subsidence levels might have some- thing to do with establishing the level tracts, but the observation that these tracts stand at very different levels in immediately adjacent dis- tricts together with recognition of the structural difference, lead us to give credit to the primary structural character of the formation itself as the controlling factor in the present distribution. According to this ex- planation, these hills are mere remnants left from solution attack on a reef limestone, the depressions between them representing collapsed cav- erns, the walls of which may still be seen on the sides of some of the more rugged hills, and the surrounding or intervening tracts are soil-covered and level, chiefly because of the accumulation of earthy material, left be- hind after removal of the overlying reef, now halted in its reduction at the first important less solulile beds. MINERAL RESOURCES ^ An examination of specimens of minerals and ores in the hands of local prospectors and residents interested in developing mineral resources, "A sood list or tabulation of the mineral occurrences of Porto Rico may be found in tbe article by H. C. B. Nitze listed at the close of this paper. 54 ANNALS NEW YORK ACADEMY OE SCIENCES together with observations made personally, shows that there is consider- able range of minerals and ores. It appears also that considerable atten- tion has been given in a few cases to local development. There is large variety shown in a collection of this m'aterial and in some cases the speci- mens exhibited look very promising indeed. But there is almost no reliable information touching the quantity or the exact relations or esti- mates of possible profitable development. It can be said, without danger of contradiction, that none of the developments so far undertaken looking toward the systematic mining have pi'oven profitable. Gold Only one enterprise of this kind seems to furnish any production, and this is the placer mining for gold. Gold washing has been practiced from the early Spanish occupancy to the present time, and it is not at all a rare thing to see several men digging in the stream gravels for the "pay dirt" and panning out the gold. This is done in all cases on a very small scale and with the aid of the simplest equipment, and the returns appear to be very moderate. It is claimed that in former times a much more elaborate system of working such deposits was in operation under the Spanish regime, and, according to historical statements, they were considered profital)le. More recently, there has been at least one attempt near Corozal to develop this kind of ground by the use of modern a])- pliances, but the plant has been allowed to go to entire ruin. The only places where actual placer wasliing was seen in progress was three miles south of Corozal and on the Sabana river near Luquillo. iSTear Corozal, also, some work has been done in an attempt to discover the veins or lode which may have furnished the placer gold. There are several pits, trenches .and shafts, in some of which quartzose stringers were seen which appear to fulfill the requirements of a source of supply. Some free gold was found in panning a little of the weathered material at one of these spots. Tliere is little doubt but that these veinlets or stringers, which were numerous at one of the cuts, are in part the sources of the placer gold of this locality. But at no place examined was there to be seen any "vein" of apparent consequence or any structure suggesting the course or extent of the mineralization. Of course the rather mixed state repre- sented by the residuary matter, seen almost everywhere at the surface, does not lend itself readily to the tracing of veins, and it may therefore liappen that conditions would prove, after thorough exploration, to be better'than the first brief examination indicated. There are said to be some old abandoned workings dating back to Spanish conquest times at BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 55 the same locality, but such evidences are very obscure and would seem at best to have very little bearing on. present prospects. Copper Some very good specimens of copper ores were seen in the possession of Mr. Henry D. Sayre, of Corozal, who assured us that there were several localities represented in the collection and that in no case had the real value of the occurrence been determined. It was understood that exami- nations have been made by engineers sent to Porto Rico in the interest of American mining companies, and that some exploratory work has been carried on by Porto Rican companies or groups of individuals, but so far as known there is at the present time no development work being con- ducted, and the possible value of these deposits has not been thoroughly proven. Zinc, Lead and Silver One prospect, on which several himdred feet of underground work has been done, was visited at Barrio del Carme in the Sierra de Cayey, on land owned by Pablo Vasques, several miles northwest of Guayama. The country rock is chiefly andesitic tuffs cut by porphyritic intrusives. A quartz vein carrying sulphides, pyrites, sphalerite, galenite and chalcopy- rite has been followed and there is some ore on the dump. The vein vanes from a mere streak to a width of two feet. The mineralization is irregular and the values are said to be chieflv in lead and silver. The second-class ore is essentially mineralized tuff. The exploratory work has been done in large part on side slips and streaks quite outside of the vein proper. In all of this side work, there was apparently no new min- eral-bearing ground discovered. The first-grade ore is heavy and the dis- tribution of values is not determined. There is no doubt of the existence of a real vein or of the ore in this case, but there is need of more intelli- gent exploratory development along the vein proper, together with a study of the possibilities of separating the chief values by some sort of milling operation, before a reliable conclusion could be reached as to pos- sibility of working the deposit as a mine. Iron One magnetic iron prospect was visited. This occurrence is on the divide about ten kilometers west of Naguabo. It is reached by driving out on the road from Naguabo toward Torres to about this distance and then takino- saddle horses to the divide, a distance of about two kilometers southward. On the expedition we were accompanied by Mr. Arturo Gallardo. Jr., Alcalde Mmiicipal of Naguabo. 56 ANNALS NEW YORK ACADEMY OF SCIENCES There are many surface bowlders of magnetic iron of fine quality in this vicinity. A little underground working is evident at one point but this is now caveil in. Surface observations, together with a few magnetic observations, failed to show any very extensive deposit at that point, but the quality appears to be good in iron content. The ore carries a little copper and is associated Avith an igneous rock essentially andesitic in composition. It could be traced with a fair degree of certainty about fifty feet east and Avest just below the crest of the ridge. Considering the associations at this place, it seems necessary to conclude that the ore is igneous in origin and that it probably accompanies one of the porphyrite intrusives. Other occurrences of similar ores were mentioned to us in this same region, 1)ut none were visited. Coal and Oil There is no good ground for believing that valuable resources of these products exist in Porto Eico. Some prospecting for them is carried on, however, in a desultory way. The only basis for the hope of finding coal is the occurrence of lignite and lignitic material with the shales lying at the base of the younger series of rock formations, below the Arecibo lime- stone member. Lignitic material was seen in these shales near La'res, and similar or better material has been reported from near San Sebas- tian. From what, has been seen, there seems to be no promise of very valuable deposits of this kind. The structure is simple and a very little exploratory work done in a systematic manner would determine the prob- able value of every occurrence known. There is no promise at all of such content in the older series. No oil indications were observed. The only formation to be considered in investigating the prospect of oil is also the basal shales of the younger series. Lim erode A particularly porous, granular and uniform limerock is obtained from the small island, Icacos, just of? the northeast coast, and is used in sugar refining at the Central, owned by the Bird brothers, at Fajardo. The rock is organic, largely foraminiferal, and is probably structurally of the same origin as the San Juan dune sand deposits, — comparatively recent. Such materials are doubtless to be found in large amount, but not always so pure and so imiform in quality and structure. Limestone suitable for lime burning or for cement mixture is certainly not rare. Limestone of a quality that would permit its use in structural work is also found at some places, but apparently very little native stone is used. BERKEY, GEOLOGICAL RECONXOISSANCE OF PORTO RICO 57 Guano Bat guano is found in some of the caves in sufficient amount to be a source of local fertilizer. Road Metal Several kinds of stone were seen used extensively in road improve- ments. The particular variety used depends largely on the local supply, but the most common are the Arecibo limestone and the massive syenite and granite porphyry. There is no lack of these as well as other types suitable for such use. There are other mineral substances that will invite investigation, but no others came under the writer's personal observation and no facts re- garding them are in hand. HOT SPKINGS Thermal springs are known in the vicinity of Coamo. Judging from their location and apparent relation to other physical features, they are believed to lie along a fault weakness. The district is also one of com- paratively late igneous activity, and this, coupled with the other factors, leads to the suspicion that the springs are directly connected with the dying igneous activity and. may actually represent juvenile waters. At Coamo Springs Hotel, one of these springs has been developed and controlled for commercial purposes. The water is hot as it comes out of the side of a small ravine and runs down over the slope, which is covered to a moderate extent with deposit from these waters. The immediate bed rock is not well exposed, but it is judged to be either a tuff or an igneous complex and the field relations in the vicinity show that there is a thick series of beds both above and below. No doubt critical field study could determine the actual relations with considerable certainty. An analysis of these waters, furnished by the proprietor of the springs and made at the agricultural experiment station at Mayaguez, is as fol- lows: Fixed elements j)er litre of loater Free carbonic acid 0 . 01296 Sulphate of lime 0.79902 Sulphate of soda 0.52531 Chloride of potassium 0 . 00031 Chloride of sodium 0.2.3054 Silicate of soda 0.08127 Carbonate of soda , 0.03503 Carbonate of iron 0.01114 Total 1.68559 58 ANNALS NEW YORK ACADEMY OF SCIENCES Gases in solution per litre of vmter at 0° of temperature and 760 mm. of pressure Nitrogen 13 ec. 740 Oxygen 1 761 Sxilphj'dric acid 1 967 Total 15 2468 Historical Statement A complete or even a reasonably full account of the geological history of Porto Rico cannot be written at this stage of the investigations. Such a statement is necessarily the end product or climax of the whole series of studies that are proposed, but it may not be out of place to outline some of the leading and most clearly marked steps as a rough sketch or a preliminary attempt. At the outset, it is well to appreciate that the Island of Porto Rico is geologically young. There are no traces, so far as known, of any of the so-called ancient rocks. It is quite true, of course, that the older series of formations is largely a volcanic complex whose exact age may never be accurately determined, but there is no occurrence of profoundly meta- morphosed members or other evidences of great geologic age. Besides, the series, complex as it is and difficult to group into suitable divisions as it may be, undoubtedly forms a very closely related succession of minor formational miits whose uppermost members are determinable as to age with reasonable accuracy. It would appear also from the nature of the deposits and their structural relations that the accumulation must have been, for the most part, a rapid i)rocess. There is no good reason, so far as any of these facts are concerned, why the whole of the "Older series" could not have been accumulated in a single geologic period. The fossil content of the upper members of this series indicates that this period was the Cretaceous, as used in the broader sense in geology. Whether or not the older members date back to an im- mediately preceding time cannot yet be definitely stated, but whatever there is, is clearly so closely associated with the Cretaceous beds that they can all be treated as a single historical unit. This earlier period is characterized by volcanic and other igneous activ- ity on a very large scale. Beds were accumulated both above and below sea level. There seem to have been oscillations of level accompanied by recurrences of similar beds, and apparently much shifting of the supply of material accompanied by great variation of character laterally. There is good evidence that succeeding volcanic outbursts broke through these beds nt many places. BERKEY, GEOLOGICAL RECONNOISSANCE OF PORTO RICO 59 An occasional more prominent change of conditions, more or less clearly marked in the structural relations and character of material, ma}' possibly be used as a basis for epochal subdivision. It is quite clear, how- ever, that there was no profound change of geologic control throughout the whole of this earlier time, — it was strictly a volcanic period. The succession of disturbances by which it was affected is represented in part by dynamic modifications of the nature of folding, crushing and faulting, but this is probably an accompaniment of the more profound igneous activities also, and need not be regarded as evidence of any strikingly different causal process. If there were contributory causes of a regional sort controlling the folding, they are essentially simply superimposed upon or introduced into the larger, more profound and longer continued igneous activities which prevailed both before and after that time. This long period, characterized by great complexity of formational development, including tuffs, agglomerates, conglomerates, shales, lime- stones and immense numbers of intrusives of great variation in size, form and composition, finally came to an end by the dying out of the volcanic energy, and greater stability of the whole with respect to elevation and subsidence was established. Erosion cut down the exposed formations, the sea attacked the margins and in time most of the projecting moun- tain mass was reduced near to base level, the sea encroached far onto the former land areas and a new historical chapter was begun. It is not possible to say, with the data in hand, that the entire island was reduced to a peneplain, or perhaps a conoplain, but there is good evi- dence, from the traces still left of former planation and from the dispo- sition of the remnants of overlying formations still preserved, that the greater part of the present area was worn down to base level and sub- merged. The process of base leveling was going on before the close of igneous activity and it ■\\as continued long enough to bevel across rocks of all sorts with marked success, but there is no necessity for regarding it as a very long geological time. As erosion proceeded, sediments were deposited unconformably around the margins of the island of that time and perhaps also in some of the marginal valleys above sea level. These constitute the earliest shale beds of the "Younger series'" and are believed to be of Eocene age. They are at least early Tertiary. \\'here more simple marine conditions came into control, as would happen when submergence or planation had masked or destroyed the more elevated sources of supply, the deposits became almost wholly reef limestones and shell limestones, with only minor amounts of strictly detrital material irregularly distributed. This gave a succession of somewhat irregular beds which are abundantly supplied with organic 60 ANNALS NEW YORK ACADEMY OF SCIENCES remains and which bear evidence of the continued depression favorable for the growth of these accumulations for a considerable part of Tertiary time. There is some suggestion in the relations shown in the eastern portion of the island that this end was not wholly submerged and that differential subsidence gave to this portion less prominent development of the heavy, massive limestone beds. In later Tertiary time there was marked reemergence from the sea, accompanied by warping, so that the later limestones and reefs were lifted to very different elevations in different parts of tlie island margin. Since that event, the whole has been again subjected to erosional attack of the sea, and to wind work, with the result as now seen in the physical feat- ures. The comparatively easily destroyed shales, marls and limestones of the Tertiary series have been extensively removed, leaving only a fringe of these formations along the north coast and a part of the dis- tance along the south coast, and developing all of the topographic forms characteristic of the erosion of emerged coastal deposits, together with some very special forms due to the peculiar makeup and attitude of the rocks themselves. Since this first emergence there have been minor oscillations also, the record of which is observable in marginal terraces, deeply trenched flood- plain deposits, and thoroughly indurated wind deposits of presumably Pleistocene age. Apparently the latest movement has been one of slight emergence. A summary, therefore, of the larger items in the geologic history in- cludes the following: 1) A long geologic period of volcanic activity, accompanied by marginal at- tempts at assorting of fragmental and detrital material and organic accumu- lation disturbed from time to time by renewed or extended igneous activity. 2) A dying out of volcanic energy, greater stability of the mass with respect to elevation and subsidence, and erosional attack continued long enough to re- sult in extended planation and partial base leveling with final extensive sub- mergence. 3) The development of an unconformable overlying series of shales, reef limestones and related deposits chiefly of organic origin, brought to an end by final re-emergence. 4) The development of present surface features under stream erosion and marine marginal attack, with modifications arising from oscillation of level. The geologic column forming the basis of this outline, avoiding minor details that are properly the subject of further study before specific state- ment should be made, is as follows : BERKEY, GEOLOGICAL REC0NN0I88ANCE OF PORTO RICO 61 Recent alluvial deposits. Submergence with flood-plain deposits. Younger Series. San Juan dune sands (Pleistocene). Submei-gence with terrace cutting. Post-Arecibo emergence and erosion. Organic limestones, marls etc. (Mid-Tertiarj^. Arecibo reef limestones (Oligocene). Lares shales etc. (Eocene ?). Marked unconformity. Older Series. Interbedded limestones, tuffs and shales etc. (Cretaceous), Coamo tuff -limestone, Trujillo Alto limestone, Aibonito conglomerates and shales with many intrusives. Interbedded fornminiferal shales and ash shales with tuffs, cut by many intrusives (Early Cretaceous ?), Fajardo shales, Mayaguez shales, Barranquitas slialy limestones, Sierra de Cayey tuffs, Comerio consoli- dated ash beds and tuffs, etc. Future Problems One of the objects of this exploratory study was to discover and define the problems that should be investigated by this organization. It is not supposed, in enumerating this list, that these cover every possible subject of special study, but they do indicate the fields in which there is promise of immediate and valuable scientific returns, and at the same time will add to the fund of usable information to be put within reach of the people of Porto Eico. BASE MAP One of the fundamental things as a basis for all sorts of detailed geo- logic work is a good contour map. The whole island ought to be mapped in the same manner as is done in the United States, using the same quadrangle system. On accoimt of the density of population, the com- •plexity of structure and relief and the variety of agricultural uses of the soil, the scale should be approximately one mile to the inch, or 1 : 62500, so that these maps could be used as base maps for all sorts of special purposes. The maps now available are chiefly those of the Interior Department of Porto Eico, made to illustrate the various reports of the department and representing the progress of public works such as railways, telegraph and telephone lines and highways. On account of the care with which the different classes of roads have been shown, and the general accuracy of locations, these maps are especially useful in the present investiga- tions. One of the most useful is a map of the Bureau of Public Works Q2 ANNALS NEW YORK ACADEMY OF SCIENCES on a scale of approximately f inch per mile which has even the kilometer distances along the roads indicated. Until some sort of a contour map can be secured, such maps as these will be found eminently serviceable. Along the south coastal margin within the region of irrigation devel- opments, there has been some special mapping with contours. In no case do they cover much ground beyond the outer lowland and terrace border, and because of this limitation they are not so generally useful for our purpose as the Interior Department maps. They are, however, ver}^ much more accurate and detailed and for the territorv covered are emi- nently suital)]e as base maps. GEOLOGIC MAP A geologic map of the island should be one of the results of this series of studies, whether a relief map is secured or not. Such a map of the whole island is necessarily an ultimate rather than an immediate product, but district maps can be undertaken at once, with no difficulty whatever. These preliminary districts can be selected so as to include some of the most promising investigation problems in special lines, and both kinds of work can thus be carried on at the same time. This therefore leads directly to the next item, which is district studies. The only geologic map thus far attempted is that by E. T. Hill. DISTRICT STUDIES It is possible now to select areas which are known to contain geologic features of special interest and significance, and it will generally be con- venient, if not indeed necessary, for the investigator to make a detailed geologic map as a secure foundation for his special studies. One of these is the Coamo Springs District, which may be made large enough to ex- tend from the Descalabrado river on the west, to Salinas on the east, and reach as far north as Aibonito. It will include as features of special importance for investigation, in addition to the mapping, the hot springs, the great conglomerate series, one of the later of the great volcanic vent complexes, the genetic history and lioi-izon of the Coamo limestone which is a striking mixture of volcanic and organic matters, the high floodplain deposits of the stream valleys and their bearing on late geologic history, and certain physiographic studies connected with the coastal terraces. This district promises, as can be seen, an unusually large range of topics inviting special study, all of which will be illuminating to further devel- opment of the geologic survey of the island. Another district of equal promise in a very different manner is on the north coast extending from the Quebradillas to the Arecibo river and BERKEY, GEOLOGICAL REC0NN0IS8ANCE OF PORTO RICO 63 reaching from the sea to Lares so as to include a strip of the older com- plex rock series be3'ond the inner margin of the Tertiary series of reef limestones and shales which constitute the greater part of the bed rock of the area. Beside the mapping and detail of structural relations, this district presents the best opportunity to investigate the question of exact age of the basal beds of the Tertiary series, the transition from lignitic shales of perhaps fresh water alluvial origin to massive limestones of reef type, a subdivision of the Tertiary series, the meaning of the thinning out and disappearance of the Lares shales toward the east, and a paleon- tologic study of the beds, all of which are fundamental in any additional study of other districts containing the Tertiary rocks. There are besides good opportunities to study the meaning of the high terrace-like shelf coming abruptly to the sea at Quebradillas river and the meaning of the deep embayments now occupied by such playas as that at Arecibo. This is also one of the best localities for a detailed studv of the structural and petrographic features of the San Juan formation as well as the behavior of modern dime sands along the present coast. Another district of still different features, and giving foundation for special studies of quite a different bearing, is that lying between Caguas and the Caribbean sea and perhaps extending as far eastward as Naguabo. This will include the largest massive igneous unit in the whole island and promises information bearing npon magmatic differentiation, origin of the magnetic iron ores, relation of the great intrusive masses to the other igneous representatives, petrographic range of the igneous rocks, and marginal metamorphic or other effects,^studies fundamental in a final statement of the igneous history of the island. There are other districts which have special problems associated with the regular areal geologic work, but these are sutTicient to indicate the range of such district studies and their variety. Certain special investigations are of a sort requiring comparison and summary of many different localities, and for these it will not be wise to handicap the investigator by limiting work to a single district. Some of these are suggested below. REEF-BUILDING ORGANISMS The limestones of Porto Iiico are remarkable for the great prominence of algte and corals and other closely associated organisms lending them- selves to the construction of reefs and accompanying deposits. These forms belong to practically every limestone formation of both the older and the younger series except those most closely related to the shales. It is a study requiring the training of a specialist in such lines. 64 ANNAL8 NEW YORK ACADEMY OF SCIENCES PALEONTOLOGY The total organic content is much greater than is intended to be in- cluded under "Eeef-huilding organisms." There are immense numbers of splendidly preserved fossil species of organisms belonging especially to the Tertiary series. Probably a great many are ne^y to science. There are probably few places in America or within territory belonging to the ITnited States where the marine Tertiary succession is of more promise than in Porto Eico. This problem or line of investigation is closely related to the next topic, that of Tertiary subdivision. TERTIARY SUBDIVISION A faunal and structural summary will naturally lead to the establish- ing of subdivisions and the determination of horizons in the younger series of rocks culminating in a statement of the complete Tertiary his- tory of the island. SAN JUAN FORMATION A study of the characteristics and detail of origin and historical steps associated with the Pleistocene fossil dune sands, referred to as the San Juan formation, is another problem. * SUBDIVISION OF PRE-TERTIARY COMPLEX A discussion of this kind is one that will properly follow upon the completion of areal work in several of the typical districts. It is, how- ever, one that will necessitate investigations throughout the interior of the island, and will include a summar}^ of the characteristics of all of the prominent local formations. A grouping and correlation can no doubt be made in due time. MINERAL RESOURCES On account of the interest taken by the people of Porto Eico in the question of possible mineral resources, it is desirable to undertake an investigation of the kinds of products, their origin, distribution and prob- able economic value. In connection with this, because of the small amount of exploratory work that has been done, it would be especially useful if suggestions were made at, the same time about the methods of exploratory development and the people cautioned concerning wasteful methods. This work should be done so as to cover the whole range of mineral possibilities in the island regardless of location. There are known deposits of copper, iron, gold, lead, silver and zinc among the BERKEY, GEOLOGICAL BECONNOISSANCE OF PORTO RICO 65 metals as well as a number of non-metallic products. But in no case is tlie real value, or the probable extent or the geological relation, known sufficiently well at the present time to serve as a basis for a discussion. This should be one of the first undertakings of this survey, both because of the fact that its value is fully appreciated by the people of Porto Eico and because its conclusions do not materially depend upon the other in- vestigations or mapping progress. PETROGRAPHY On account of the great number of igneous rock occurrences and the very great variety that is certain to be shown in quality and minor petrographic character, and because of the considerable range in compo- sition already known to characterize the intnisives, there would seem to be an ample and promising field in this line for a special investigation. Tt is possible that some genetic relationship is exhibited in the distribu- tion of these variations and that a thorough comprehensive study would throw some light on the more obscure problems connected with the gen- esis of igneous rocks. This is a problem that can be taken up at any time, and that need not be regarded as dependent upon special district studies, although it is evident that the finishing of work on certain dis- tricts would facilitate a study of this kind. PHYSIOGRAPHY Enough is known of the physiographic features and their meaning to appreciate that a great deal of the detail of the later geologic history of the island is more or less intimately bound up in the physiographic de- velopment. The broader or larger physiographic featiires have already been suggested, but there are certainly many details, some of which may well be of much significance in understanding the geologic history, which will require the special attention of a trained physiographer. Porto Eico is a unit of geologic history, of geologic structure and of physiographic form. Each is of sufficient complexity and unity to be made independent subjects of investigation. THERMAL WATERS The hot springs in the vicinity of Coamo Springs suggest from their situation and reported composition the possibility of being representa- tives of juvenile waters. It is believed that a study, planned especially to investigate the origin and character of these waters, together with such others as may exist, would be a very suitable special investigation. On 66 ANNALS NEW YORK ACADEMY OF SCIENCES account of the fact that the principal occurrence of this type of water is very local, it wonkl l)e possible to combine a study of this kind with a district study such as has been referred to in a preceding paragraph. GEOLOGIC HISTORY Tlie complete geologic history of the Island of Porto Eico cannot be written until all of these and perhaps other more special investigations have been made. A complete historical statement must be regarded as an end product of the whole range of studies carried out for more special ])urposes. It is, therefore, the final topic and may well be deferred to a time when most of these already suggested have l)een carried far enough so that the data of special importance secured by them are available for this general summary. It is evident from tlie appearance of this list that there is a very great amount of geological A\<>i'k iiwaiiiiig tlte investigator in Porto Pico, and that it is \ari('(l and coinplicatf'd enough to riMjuire several years of study in large pari by. experts or specialists in all branches of Ihe su1)ject. It can be seen also that the Island of Porto Eico is a geograpliic unit of more tha.n usual complexity and scientific interest and gives ])roinis(' of results for cH'oi-t ex|)ended in researches along geologic lines. Collections A beginning has been made toward securing a representative collection of typical rocks and fossils. Several hundred specimens were brought to New York for use in formulating the accompanying description, and as a possible basis for further more special investigations. As a first step in this direction, about a hundred thin sections of the rocks have been nuide for microscopic study and detailed comparison. They will form a basis in planning the S])ecial ])etrogra])hic investiga- tions which may l)e undertaken. In like manner a large number of fossils have been gathered aiul their general relations are being studied. Additional investigations along paleontologic lines will be in large part outlined or suggested by the trend of these studies, for although the collection is very fragmentary it is nevertheless characteristic and fairly representative of the principal for- mations. More than a hundred photographs were taken of strictly geological subjects illustrating typical physiographic features, structural detail of rock formations, structural relations, etc. These are all s,\titably labeled and form the beginning of a collection of illustrations M Poi^tq Rican BERKEY, GEOLOGICAL REC0NN0I8SANCE OF PORTO RICO 67 geology. In addition to tlie regular pliotograplis, a number of photomi- crographs have been made from the thin sections of typical rocks. Some of this material will finally serve as a foundation for the geo- h)gical section (if what it is hoped may Ix'conic a Natural History Museum of Porto Rico. Illustrations cross-sections The note ])ooks of tlie party and field maps carry a record of field de- terminations and detail of structural relations and comments greatly in excess of what can be published in sudi a report. They arc the ])roperty of the organization iind -.wv of partifular service as guides in plajming furtlier woi'k and in giving each lU'W investigator his bearings, together w ifli sonic suggestions .-iliont tlic cliaiiictci' of his own district or the dis- tribution of (lata bearing upon his special iuNcstigatiou. The note books contain observations along some of the principal roads in sufficient detail to serve as a foundation for complete generalized geologic cross-sections of the island on two esi)ecially important lines. Cross-sections, therefore. hav(> been drawn to illustrate the kind of sur- face relief, the grade of tlu' rond. the kinds id' rocks or rock formations and the geologic structurnl rtdations. and arc I'l'produced to accompany this report. An inunense amoniil (d' detail is necessarily omitted or com- hincd into giMicralizations in oi'der to bring the sections within the scope of a publication of this kind. It is judged that some of these details will i)e suitable illustrati\(' matter foi' future i'e]>orts hased on studies of spe- cial districts. One of the sections is hased on data gathered along the roail from Ponce to Arcciho. The line is di'awn from Arecibo to Ponce diiV(;t and the data are [)rojecti'd to this line. This method tends to obliterate the windings of the I'oad and secure practically normal propor- tions and i(dati\(' positions for the associated formatioiuil units. The other section line is drawn directly from San Juan Point to Santa Isabel. By projecting to this line all the data gatliered on the Bayamon-Comerio- Barranquitas-Coamo roa. : "Evolution of the .Vntilles." Hcot. (Jeog. Mag., vol. 18, pp. 369-376, 1 pi. ]9(}2. Discusses general geologic history of America, and especially that of Central America and West Indies. GuppY, R. J. L. : "Geological Connections of the Caribbean Region." Trans. Canadian Institute, pp. 373-391. January, 1909. Discusses general re- gional relations and the probability of an Antillean continent. FisKE, Amos K. : "The West Indies." G. P. Putnam's Son, New York. 1899. Folded maps, plates. Physical characteristics of Porto Rican history etc., social and economic conditions. Hamilton, S. Harbert : "Notes on Some of the Ore Deposits of Porto Rico." Eng. and Min. Journal, vol. 88, p. 518. September, 1909. Hill, Robert T. : "Cuba and Porto Rico with the Other Islands of the West Indies." New York. 1898. A book of over 400 pages containing a very large amount of information covering many matters in addition to the physical features and the geology. : "The Yalue of Porto Rico." Forum, vol. 27, pp. 414-419. June, 1899. : "Porto Rico." National Geographic Magazine, vol. 10, pp. 93-112. 1899. One of the best articles on the physical features of Porto Rico. : "The Geology and Physical Geography of Jamaica." Bull. Museum Comp. ZooL, \o\. 34. 1899. A study of a type of Antillean development. : "The Forest Conditions of Porto Rico." Bulletin 25, U. S. Department of Agriculture. 1899. Quoted in part in Public Opinion. Contains very brief description of physical features and has a good relief map. Newberry, J. S. : "Geology of the West Indies." Trans. N. Y. Acad. Sci., vol. 1, pp. 23-24. 1882. NiTZE, H. C. B. : "Investigations of Some of the Mineral Resources of Porto Rico." Twentieth Ann. Rept., U. S. Geological Survey, Part 6, pp. 779-878. Gives a tabulation of the mineral occurrences, and a very brief note on the principal ones. 70 ANNAL8 NFAV YORK ACADEMY OF SCIENCES Spencer, J. W. : "On the geological relationship of the volcanics of the West Indies." Victoria Inst. Jarately, each in its own cover, antf are distrilmted in hundles on an average of three; [)er yeaf. 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