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NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY Porto Rico and the Virgin Isk VOLUME II—Part 1 The Geology of the Lares District, Porto Rico— Be/a Hubbard NEW VOKK; k A !^*B^"'' NEW YORK ACADEMY OF SCIENCES ^'lfll^^ SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands VOLUME II—Part 1 The Geology of the Lares District, Porto Rico— Bela Hubbard i - Ml. %^ J'- NEW YORK; Published by the Academy 1923 I THE GEOLOGY OP THE LARES DISTRICT, PORTO RICO By Bela Hubbard CONTENTS Page Introduction 2 Nature and purpose of the work , 2 Acknowledgments 3 Previous work 4 General description 4 The Antilles 4 Porto Rico 5 The Lares District 6 The Cretaceous formations 6 Petrography 7 Igneous rocks 7 Sedimentary and clastic rocks 15 Stratigraphy 25 The Rio Culebrinas Series 25 The Rio Blanco Series 26 The Rio Yauco Series 29 Structural features 30 Age 34 Correlation 35 Llthogenesis 36 Geologic history 37 The Tertiary formations 38 Stratigraphy 39 The San Sebastian shale 39 The Lares formation ^ 42 The Cibao limestone 44 The Los Puertos limestone 45 The Quebradillas limestone 46 Summary of formations 49 Structure 49 Age and correlation 51 The San Sebastian fauna 53 The Lares fauna 57 The Cibao fauna 58 The Los Puertos fauna 59 The Quebradillas fauna 60 Summary of the faunas 64 Correlation with the south coast , 64 2 SCIENTIFIC SURVEY OF PORTO RICO Page Comi)arison with important xVntillian localities | 71 Oligocene or Miocene 72 Tertiary history 75 Magnitude of the submergence 70 The uplift 77 Physiography 77 The Complex Mountainous Oldland 78 The peneplane surface 79 Drainage features , 81 The Elevated Coastal Plain 83 The Coastal Plain belts 86 Drainage 88 Origin of the Pepino Hills 91 The Playa Plains 93 Features of the coast line 95 Terraces 95 The San Juan formation 98 The consolidated dune sands , 98 The consolidated beach deposits 99 Evidence of uplift 100 Age of the San Juan formation 100 Conclusions 103 The Playa deposits 103 Summary of Pleistocene—recent events 104 Mineral resources 105 Iron 105 Copper 105 Kaolin 108 Brick clay 108 Lime 100 Building stone 109 Road metal 110 Lignite 110 Guano Ill Oil Ill Summary Ill Bibliography Ill INTEODUCTIOJST ]^ATURE AND PURPOSE OF THE WORK The present paper gives the results of a survey of the geology of the Lares District, Porto Eico, made during the summer of 1916 under the auspices of the ISTew York Academy of Sciences and the Insular Govern- ment of Porto Eico. It is one of a series of reports, each covering the f I HUBBARD, GEOLOGY OF THE LARES DISTRICT 3 geology of a portion of the island.^ The Lares District comprises the northwest corner of Porto lUco and Desecheo Island, a total area of about 500 square miles. The survey of this district involved the study of the rock fomiations, mineral resources and the making of a geologic map. As the field work was done in three months time, methods of the recon- naissance type had to be used. Before entering the field, a base map (3 inches= 1 mile) was prepared from available sources and this was divided into quadrangles of 5 x 2 miles. The field work consisted chiefly in running traverses across the general strike of the rock formations. All traverses were made by pacing or by the time elapse method. Elevations were obtained by aneroid. In recording the data, two methods were used 1. Cavalry Sketching Case. The blue print quadrangles of the base map were used on the sketching case. Contours were sketched in for a distance of one-half mile or more on either side of the line of traverse. 2. N'ote Book Traverse, using Brunton compass and protractor. This method was found best for the more detailed work in limited areas, and during rainy weather. Photographs were used repeatedly as a means of gathering data for the topographic map. In portions of the area not covered by photographs or traverse, the topographic features shown on tlie map have been generalized. The first two months of the field work were spent among the Tertiary rocks of the district, while the final month was spent in the area of Cre- taceous rocks. Subdivision and correlation of the Tertiary formations was the most important problem to be worked out, and hence required the expenditure of more time than a well balanced survey would ordi- narily warrant. A large number of fossils were collected in the Tertiary area, which have been d.escribed in Volume III, Part 2, of these reports. Acknowledgments Acknowledgment is made particularly to Dr. C. P. Berkey for assist- ance and advice in almost every phase of the work. Dr. A. W. Grabau, Dr. D. W. Johnson and other members of the Columbia University Geol- ogy Department have given valued suggestions and aid. In the field work, every courtesy and assistance was given by the government officials at San Juan, and acknowledgment is made especially to Colonel George K. Shanton, Chief of Insular Police, Major Basil H. Dutcher, U. S. A., and Judge Bonner, Auditor. In Mayaguez, Mr. D. W. May, Director of the Agricultural Experiment Station, took very great interest in the work 1 For an account of the plans and progress of the geological survey of Porto Rico, see Scientific Survey of Porto Ilico and the Virgin Islands, New York Academy of Sci- ences, vol. I, part 1, pp. 1-10, 24-20, 1919. 4 SCIENTIFIC SURVEY OF PORTO RICO and assisted in every way. SefLor Narciso Rabell^ of San Sebastian, a student of the geology of the island, aided in collecting many of the fossils, in giving valuable information, and in showing a lively interest in the progress of the work. The aid of the Insular Police in every town in the Lares District was a big factor in carrying on the survey. Eesi- dents of the district not only displayed the warmest hospitality, but showed a keen and intelligent interest in the purposes and results of the field work. Phevious Work The list of articles dealing with Porto Kico is given in the Bibliog- raphy. Prior to 1916, little geological work of a detailed nature had been done in northwestern Porto Kico. E. T. Hill made a reconnaissance in 1898, and published several articles. He visited the Lares District, and made observations along the Lares Road, where he collected many fossil corals, later described by T. W. Vaughan. In 1914, in the course of their reconnaissance of the island, C. P. Berkey and C. R. Fenner covered the main routes of travel in the district, and noted many of the geologic features characteristic of this part of the island. During the same year, certain fossil localities in the district were visited by C. A. Reeds, where a large amount of material was collected. In 1915, E. T. Hodge made a brief reconnaissance of certain localities in the southern half of the district, to investigate the reported presence of oil shales. GENERAL DESCRIPTION The Antilles Porto Rico, forming the easternmost member of the Greater Antilles, is an almost completely submerged complex mountain chain having a maximum elevation of about 28,000 feet above the adjacent lowlands or submarine valleys of the Atlantic and Caribbean. This mountain chain is more or less continuous in an east-west direction, its highest portions forming the islands of Cuba, Haiti-Santo Domingo, Jamaica, and others. Lateral branches of this chain form the Lesser Antilles and minor groups of islands.^ The rocks of this x\ntillean chain are chiefly of Mesozoic age, and comprise volcanic flows, intrusives, and sediments of marine and continental origin. These rocks, as a whole, show evidence of having been formed during a more or less continuous period of volcanic activity, which reached a climax in extensive orogenic movements at the close of '^ An excellent illustration of the sub-sea relief of the Antillean region is given in Bull lOa, U. S. Nat, Mus., Plate 73, 1919. HUBBARD, GEOLOGY OF THE LARES DISTRICT 5 Mesozoic time. With the Tertiary, began a differential submergence, not affecting some portions of the region until Oligocene time, and resulting in a deposition of reef and shell limestones. Subsequent uplift in Mio- cene time, has exposed the fringing beds of Tertiary limestones on nearly all the islands of the Antillean group. Finally, there have been minor crustal movements and fluctuations of sea level during the Pleistocene Epoch. The crustal movements have continued to the present. Porto Eico As a result of the major geologic events just outlined, modified by minor events peculiar to the vicinity of Porto Eico, the following are the chief geologic and physiographic elements or units found on the island : 1. Complex mountainous oldland area, which makes up the core of the island. The rocks so far as known are all of Cretaceous age and com- prise intrusives, flow^s, tuffs, ash, shales and other sedimentary rocks. The igneous rocks are predominantly andesitic, though other types occur. The structure is highly complex, marked by many thrust faults and over- turned, folds. The predominant strike of the structure is northAvest- southeast, as it is in most places throughout the older rocks of the An- tilles. This oldland area is characterized by its mountainous aspect, steep soil-covered slopes, with marked absence of cliffs or other exposures of unweathered rock. The present cycle of erosion has reached maturity. The existence of at least two previous cycles of erosion has been shown by Lobeck (1922). 2. Elevated Coastal Plain. Marked by nearly horizontal deposits of reef and shell limestones of Oligocene age, deposited along the north and south coasts of the island (see Berkey, 1915). These limestones with basal shale members rest unconformably upon the Upper Cretaceous rocks, the time interval covering the Eocene period and the early Oligo- cene. The total vertical uplift reached a maximum of 1500 feet. The uplift on the north coast was differential, accompanied by gentle warping. On the south coast it was accompanied by local faulting and considerable tilting of the beds to seaward. Thus the north coast beds dip at angles averaging less than 5° in a seaward direction, while the average for the south coast is considerably greater. Erosion of the present cycle (post-Oligocene) has affected these Ter- tiary limestones, chiefly by surface and subterranean solution, and has stripped the oldland area of a portion of its Tertiary covering. 3. Pleistocene to Eecent Coastal Deposits. These include alluvial plains or playas at mouths of the large streams, elevated beach gravels. 6 SCIENTIFIC SURVEY OF PORTO RICO and indurated dune sands. They rejaresent the effects of Pleistocene variations of sea level, accompanied and followed by local crustal move- ments, which, over most of the island, have resulted in a series of uplifts totalling at least 200 feet. The Lares District This district contains a portion of almost every major geologic and physiographic unit to be found on the island. The Older Series, or Upper Cretaceous rocks, occupy the mountainous area south of the Lares Eoad. In this area there are remnants of the earliest post-Cretaceous or late Cretaceous peneplane, the first erosion cycle. Evidence of the second cycle, however, is not found within the district. The entire area north of the Lares highway is occupied by the overlapping, relatively undisturbed limestones, marls and shales of the Younger Series or Oli- gocene, resting upon an irregular surface carved in the highly disturbed Cretaceous rocks, and dipping gently seaward. This is the most complete development of these rocks to be found on the island. Post-Oligocene uplift has raised these massive reef limestones to a present maximum elevation of 1300 feet. Solution with extensive development of under- ground drainage has produced in this limestone belt a peculiar type of Karst topography with its pepino hills. The Pleistocene and more recent uplifts are well shown along the extensive coastline of the district. They are marked by marine terraces and elevated fossil beaches. The typical playas are developed at the mouths of the largest streams, particularly on the west coast. THE CRETACEOUS FORMATIONS As a whole, the rocks are more nearly similar to those of the Ponce District to the south than to the rocks elsewhere on the island. When compared with eastern Porto Rico, the most striking feature in the Lares District is the predominance of shales and volcanic elastics, with an ab- sence of intrusives of the large batholithic type. The strike of the beds is northwest-southeast, as is the case throughout the rest of the island. In the Lares District, the average strike is north 50° west, although there are variations from this over considerable areas. The prevailing dip is to the southwest, and the northernmost beds are therefore the oldest, except locally in cases of repetition by folds and faults. In the eastern portion of the area, volcanic tuffs predominate, while in the western half, shales predominate. Of igneous rocks, none of the deep seated, batho- HUBBARD, GEOLOGY OF THE LARES DISTRICT 7 lithic types occur, although some coarse diorite was found in stream gravels near Aiiasco. The prevailing igneous rock in the district is andesite porphyry, occurring as sills, laccoliths, and irregular intrusive bodies. Petrography igneous rocks The igneous rocks are characterized by a small petrographic range. Andesite porphyry makes up about nine-tenths of all the igneous material in the district. The textural range is from felsites to porphyries. Glasses are rare or not easily recognizable because of the devitrification and weathering. The following types have been determined, and their distribution shown on the geologic map Table 1 Name of Rock Usual Occurrence Quartz diorite porphyry Irregular intrusives. Andesite Surface flows, usually amygdaloidal. Andesite porphyry Surface flows, sills, dikes, and lacco- lithic or elongate intrusive bodies. Augite andesite Surface flows, amygdaloidal. Augite andesite porphyry Surface flows, sills, dikes, and irreg- ular intrusive bodies grading into Gabbro porphyry. Gabbro porphyry Irregular intrusives, grading into Augite andesite porphyry. Serpentine Dikes and irregular intrusives of small size. Quartz Diorite Porphyry. The only locality in which this rock is found is in the Eio Blanco valley south of Lares. It occurs as a boss of considerable size, and is associated with one of the most conspicuous centers of former volcanic activity in the district. The rock is compara- tively resistant to erosion, and forms the prominent peak of Mt. Torre- cillo (Fig. 1). 8 fiCIENTlPIC SURVEY OP FONTO RWO Tile principal minerals are pkgioclase (aodesino-oligoflase, aTitlcsiiic, labradorito), h()Tiil)l(mclo, and quartz. Tliese iiiincirals form the plieiio- CTvsts, many of tliem large and in some cases occupying much more space than the gromidmiiss. The plagioclase occurs abundantly as niicrolites in the? fclsitie gromidmass. The qnartz crystals are not idioniorpliic, hut occur as sub-romid grains, in some cases quite conspicuous in hand speci- mens. The accessory minerals are magnetite, apatite, ruiile, and prob- ably some orthoclase. The apatite crystals occur characteristically in the feldspar grains, and the ratile is confined to the quartz. Quartz is ordy m^M Fig. 1.--r/cM- Inoki'i he l)Hirkgr(Himl. wItJi K. P, LarcR-IHo rH(tu< ' left. Cordlllei cntral of the Ponce •alley of tlie Rio Bl ail awM'ssory eoiiHtituent in some portions of the rock. Tlie elrief altera- tions aj-e, hornblenjJe to chlorite and serpeutijuy; plagioclase to kaolin, sanssnrjte, and carboiuite: and magnetite to limonite. Th<' rock grarles into audesite porphyry, and locally has a higli content of. triffaceuus material, and Tesembles a true tuff. Portions which are (!X{!eedijig]y high in tuifaeeous material are generally Jdgldy kaolinized. Ojie of the pockets of white kaolin is shown in^ figure 2. The typi(:al phases of the rock are readily recognized iji tlie field by the freshness, resistance to erosion, light gray color mottled with black liornbleiide plienocrvsts, and the conspicuous grains of quartz. It is <pdte differeul. in general appearance from the mon,otouous audesite porphyries found throughout tlie district. HUBBARD, (iEOLOflY OF THE LARES DISTRICT <} Andesite. Andesites of the type foiincl in the Ponce District are rather rare. They omnr in limit<Kl areas along the Rio Blanco east of Ailasco. The most typicaJ occurrence is on the north side of t)ie Bio Bhrneo valley, one mile east of the teriniiiiiB of the sugar railroad. This ro(;k is locally amygdaloid al, and luidoubtcdly a surface flow. It is one of the few large exposures of andesite in tho Lares District which is not porphyritie. In tfiiu section, it shows miero-diabasic structure, produced hy inicrolites or minute latli4ikc crystals of plagioelasc (apparently oligoclase), with an interstitial groundmass of ferromagnesiaii mineral, probably entirely hornl)len(le, though some of this may be secondary frcini auy-itc. The rock has undergone! considerable alteration frojn^ weathering. Andesite Porphyry. This is l»y far tlic most al)uudant type; of ig.iH^njs rock found in the district. Its chief occurrences as )utrusi\'c bodies are sliown on the geologic map. It occurs as: surface flows at other points, notably south of-Aguada,, and at Iv. ;10.7 on tho Miiyaguez^Las .\Iii,rias Koad. Its (>xtnisive origin in these instances is showji t>y tiwi al)ijinhiii(-'e of amygdaloidal ca.vities. In a few localities typical palisade structure is develoijcd, as, for example, uu the Kio JManco, Ih^ miles east of tlie end of the Lares-Rio Blanco Road (Fig. :]). Dikes of andesite ptyrphyry intruded in massive tuffs are found at various points along tlie Rio 10 SVIEXTIFIV SUMl'HY OF PORTO RICO Blaiieo sugar jiiilway, cast uf Ailaseo. This rock also or-ciirs \m sills iii the tuffs and ash beds, but many oC tlieso are small and not shown ou the geologic inia.|j, Plagioclftse is nlwavs the cliiol* mijieral and most persistent p1ieoo(n-\'st- forming constituent. Of tlie plagioclases, andesine and andesiue-oli«-o- clas(3 are bj far the most common. In many cases, labradoritc is present as a principal mineral with the audesine. Oligoclase rarely occurs as a pririci|)al mineral. Hornblende is the ever present ferrojiiagnesian coii- stitiieiit. Where it is not abundant, th(» ro(!k is |)ink or light grgy in HUBBARD, GEOLOGY OF THE LARE.S DWTR/CT H color due to the preponderance of feldspars, but where the ferromagne- sian element is present in abundance, as is more commonly the case, the rock is gray or dark gray if labradorite is present. The accessory min- erals are angite, magnetite, apatite, pyrite, and rarely ilmenite, albite^ and quartz. The common alterations are, hornblende to chlorite, ser- pentine, epidote, limonite, calcite, and quartz; plagioclase to calcite^ kaolin, quartz and saussurite. The plagioclases occur also as microlites in the groundmass, and ex- hibit a marked parallelism in their arrangement as a result of flowage of the magma during crystallization. Fractured or fragmental pheno- crysts are very common, also indicating flowage. Strain effects can in many cases be observed in the phenocry^sts, as might be expected. The zonal effects of the plagioclase phenocrysts is one of the most typical features of nearly all of the andesite porphyries. These zones of growth are of variable composition, as shown by their slightly different extinc- tion angles and differential alteration. In some crystals the interior is completely altered, while the border is fresh. In other cases the altera- tion has affected alternate zones, so that the concentric structure is vis- ible in hand specimens of the rock. The abundance of carbonate as an alteration product of the plagioclases is illustrated by the fact that many of the weathered andesites effervesce with acid. From a study of slides of the andesite porphyries certain associations of minerals are evident. Either andesine or andesine-oligoclase is always present as a principal mineral, and in some cases both varieties occur. With these either oligoclase or labradorite may be present, but oligoclase apparently does not occur with the labradorite. Where augite is present as an accessory mineral, labradorite usually occurs as a principal mineral. Ilmenite is associated with magnetite but does not occur unless mag- netite is present. It is not nearly as common in the andesites of the Lares District as in similar rocks of the Ponce District. Quartz and calcite are frequently found filling joints and small cracks. In a few localities pyrite and chalcopyrite occur in the quartz veins. The best example of this is found on the Eio Blanco 3iear Alto Sano. The quartz veins occur in andesite porphyry dikes cutting massive tuffs. With the chalcopyrite are minor quantities of bornite. The veins are nowhere more than 3 inches thick, and most of them much less. In the amygdaloidal andesite porphyry in the area south of Aguada, and on tlie Rio Blanco sugar railroad, 3 miles east of Ahasco, introduced matter is found in the form of amygdules, partially or wholly filling the cavities. The amygdules are cluefly of amorplious silica and zeolites. In the 12 SCIENTIFIC SURVEY OF PORTO RICO locality south of Aguada, massive veins of banded agate with core of crystalline quartz, occur abundantly, cutting the andesite porphyry, and apparently associated with nearby intrusive bodies of serpentine. The amygdaloidal cavities are elongate parallel with the ilowage alignment of the plagioclase phenocrysts, and are lined with banded agate or other amorphous silica, the interior being filled with crystalline quartz. Where they are only partially filled, the quartz pyramids form small geodes, with incrustations of prehnite and other zeolites. More or less tuffaceous material is present in practically* all the ande- site porphyries. In thin sections, this included matter is seen to consist mainly of broken crystals of feldspar, hornblende, and other minerals. It is often difficult to determine whether these are merely phenocrysts broken up by flowage during crystallization, or whether they are tuffa- ceous materials of extraneous derivation. Not infrequently fragments of devitrified glass, or lithlic fragments of more than one crystal are encoun- tered, and these indicate the true nature of the broken crystals associated with them. In some cases the rock is so crowded with fragmental ma- terials that it is impossible to tell whether it is of igneous or of clastic origin. In examining thin sections, one may be considerably surprised to find fragmental foraminiferal shells in what otherwise appears to be a typical andesite porphyry. The difficulty is increased by the fact that many of the tuffaceous crystalline fragments, especially the plagioclase, are so regular in form that they are not distinguishable from phenocrysts. It may truthfully be said that there are all gradations between a tuffa- ceous andesite and an andesite tuff. Augite Andesite, The only known occurrence of this rock is in the southern half of the area of lava flows shown on the geologic map south of Aguada. It is a dark massive amygdaloidal to vesicular lava, in some places very ropy and scoriaceous. The amygdaloidal cavities have no regularity of occurrence which might indicate successive flows. In some places, the cavities are filled with amorphous silica, zeolites, and native copper with associated copper minerals. Elsewhere the amygda- loidal cavities are empty, and the rock is light in weight due to its ex- treme porosity. In some exposures it resembles a coarse pumice. The mineralized areas are associated with points and small fault crush zones. The latter are filled with calcite, stilbite, native copper, malachite, and other minerals associated with the copper. This mineralization fills both the crush zones and adjacent cavities in the rock. This locality is de- scribed more fully in the chapter on mineral resources. In thin section, this lava shows a dark ferromagnesian groundmass HUBBARD, GEOLOGY OF THE LARES DISTRICT 13 with fine lathes or microlites of plagioclase (chiefly andesine) exhibiting perfect flow structure. Occasional small grains of ferromagnesian min- eral show the presence of augite, but whether hornblende is present also was not determined. Augite Andesite Porphyry. 'Next to andesite porphyry, this is the most abundant type of igneous rock in the Lares District. Like the former, it occurs characteristically as elongate intrusive bodies, appar- ently sills or laccoliths. It is also associated with the larger and more massive bodies *of gabbro porphyry, of which it seems to form lateral off- shoots. Examples of this relationship are found on the Mayaguez-Las Marias Eoad between K. 8 and K. 9, and south of the Lares Road near K. 37. In some other localities it is associated with, and seems to grade into andesite porphyry, as in the Rio Blanco valley south of Lares, and in the same valley farther west, near east terminus of the sugar railroad from Anasco. This rock is distinguished in the field from andesite porphyry by its darker gray to greenish color, the latter being due to alteration of augite. The large phenocrysts of augite are another distinctive charac- teristic. There are all gradations between augite andesite porphyry and gabbro porphyries, depending merely upon the relative prominence of phenocrysts and groundmass. Both varieties of rock may frequently be found in the same outcrop. The principal minerals are augite, andesine- oligoclase, labradorite, andesine, and oligoclase. It is seldom that more than two of these are found occurring together as .principal minerals. The commonest association is labradorite with andesine. The usual accessory minerals are magnetite, hornblende, one or more of the above named plagioclase varieties, apatite, and pyrite. Hornblende is present as a principal constituent in a few of the specimens. Occasionally what appears to be primary quartz is found in small quantity. The common alterations are, augite to chlorite, uralite, serpentine, and carbonate; plagioclases to saussurite, carbonate, kaolin and quartz; magnetite to limonite. Of introduced material, calcite occurs in minor quantities in some specimens. Texturally, the rock is a typical augite porphyry, with the plagioclase phenocrysts generally smaller than the augite, though considerable of the latter is disseminated as small grains in the groundmass. The plagioclases are characterized by strain effects, zonal growth, and usually by parallel arrangement due to flowage. They are in many instances fractured, with the cracks filled by groundmass. A poikilitic habit is occasionally seen, with inclusions of ferromagnesian minerals in the plagioclase phenocrysts. 14 SCIENTIFIC SURVEY OF PORTO RICO The outcrops of this rock are, as a rule, fairly fresh, but thin sections show considerable alterations. Secondary carbonate is often so abundant that the rock will effervesce with acid. This is especially true where there is a considerable quantity of included tuffaceous material. Gabhro Porphyry. This rock is found on the Mayaguez-Las Marias Koad between K. 8 and K. 9, and south of the Lares Eoad near K. 37. As previously noted, it is associated with the less porphyritic augite andesite porphyry. It forms rather large intrusive bodies which should be classed as bosses or volcanic necks. They do not exhibit the elongate form characterizing the andesite porphyries. In the case of the occur- rence on the Mayaguez-Las Marias Eoad, the intrusive relationship to the adjacent shale beds is evident. Mineralogically, the rock is about the same as the augite andesite porphyry, the chief difference being in texture. The commonest prin- cipal minerals are augite, andesine, and labradorite. As accessories, magnetite is present in every specimen, with usually some hornblende, apatite, and andesine-oligoclase. The characteristic alteration products, as in the augite andesite porphyries, are chlorite, secondary hornblende, serpentine, epidote, kaolin, carbonate, saussurite, quartz, and limonite. Strain effects, and evidences of fracturing and flowage during crystal- lization are characteristic. Likewise, the usual zone effects are seen in the plagioclases, with apparently the more acid variations making up the outer zones of the crystals. The groundmass is characteristically dark, due to the predominance of ferromagnesian constituents, and is usually marked by microlites and plagioclase. Quite often very little ground- mass is present. In some specimens, the augite phenocrysts are very large, and the augite is greatly in excess over the feldspar. These phases are typical augite porphyrites, like those found near Maricao, in the Ponce District. Serpentine. No outcrops of this rock were found in the Lares District, but its presence is shown by its surface weathering product, the typical red limonite soil with limonite concretions, like that covering the Mesa^ southeast of Mayaguez. It is presumably derived from an olivine-bearing rock. The distribution of this material is shown on the geologic map in the volcanic area south of iVguada. It occurs in small irregular patches, lying wholly within the area of andesite lava flows. The serpentine prob- ably occurs as dikes in the andesite. The limonite ore is similar in general character and origin to the Mayaguez ore, which has been de- scribed by Fettke (1918) and by Mitchell (1922) and shows that the Mayaguez serpentine body is undoubtedly intrusive into the adjacent HUBBARD, GEOLOGY OF THE LAREIS DWTRICT 15 shales, and hence represents one of the later phases of volcanic activity affecting Porto Eico. The Aguada serpentine is doubtless the contem- porary of the Mayaguez intrusive. SEDIMENTARY AND CLASTIC ROCKS The sediments are almost entirely of clastic make-up, and derived in large part from volcanic sources. There is a total absence of sandstones and conglomerates, and very few of the fragments in the tuffs show even the slightest suggestion of roundness. Limestones are also lacking except in a few isolated instances of no importance, and even these contain minor quantities of tuffaceous material. The following types of clastic or sedimentary rock seem worthy of separate description : Table 2 Name of Rock Occurrence Massive tuffs and agglomerate Chiefly in the eastern part of the district, and in the central zone called the Rio Blanco series. Bedded tuffs Interbedded with shales and ash. Ash Fnterbedded with shales and tuff", beds usually not as thick as the tuffs. Ashv shale Do Black shale Southwestern portion of the district. Tjime shale Wide spread, but characteristic of Pt. Jiguero region and the Atalaya Range north of Aiiasco. Tilnipstoiip M ••• South of Las Marias and on Desecheo Island. As lenses of small extent in shale or tuff. Ohert North of Mayaguez and on Desecheo Island, as lenses in shale. Massive Tuffs and Agglomerate. These rocks are so deeply weathered that very little can be found out about their structure and composition. 1(5 SeiKN'JlFIV miRfEY OF PORTO RICO In many places wt^athcrod exposures will show the lithic fragments ap- parently well preserved, \mi with the entire rock so badlv cleeiwed thai, it can be cut into sliees with a knife as easily as clay. In the more material. Thiis rork ) time. ma-^isive CX| Khsure.' fragm(!ii:tsc11 Ji l)e up of fla.ginent^; < <:;h,ief w(athering labs of li c o£ the , IHn Biftnvn iiear m w shale imbedded In uth of mo PrieAo I. matrix of finer pyrocln Ivity rluflog late Cretac< (it the slightei^t trace of bedding or assortment of 1. Iti eoinpo8ition, they are for the most part made tmhiHite p<)rj)hyry. Kiiolin and iron oxides are- the thieliS. The more reddish areas of clay soil on the HUBnARD, (IKOLOOY OF THE LARKt( Dlf^TUICT l'^ mountainous ^^lopes in the southeastern pr)rtioii of tlie Larci^ iJistrict are derived chiefly from tliej^(> iiiaKsive tuffs. The coarse a,iJ;gioinera.t;ic phases are of iiMjre limited oe<:-urrctie(\ usually lieiug fount! elose to the centers where volcauie activity was particularly stTong. One of the best ex- posures of this tvp(! of rock is fourn] in the Kio Blanco valley soiitliwest of Lar(!s, elose to the mouth of the Rio J'l-ieto. I.arge slahs of lime slnile are foirnd ijnbedded in a massive, ecnnpaet agglomerate, mad<> \ip largely of antlusite fragments (Fig. 4). The lime shale oecur.s nearby in a rela- ti\'ely undisturbed condition, and alternates with lliin lavers ol' tuff. ^#*.wli'^*^-*^-- ^X'^-^- *' -*W*-.iS*"s^^ :*. - ^»-' : V- ^?*:^'^^'^-^.,£.;-. - .^ * - ^'L Ajiparently tlie lime shale eousolidatetl soon aftef deposition, leaving the inierbedded tuff: bods still uneonsoLid,ated, so that vvlieu the voleauie dis- ruptive action tt)ok phice, la.rgc slabs of tlie slialc were thrown up and ind)ed<led in tln^ I'esulting agglomerate. 'J'ho tulf layers, being uneoo- s(didated, were completely broken up and distributed as groundmasH in the agglomerate. This grouiulmass is composed of fragments of felds))ar. (juartz, ealeite, magnetite, I'erromagues ian minerals, pieces of andosite por|ihyry, and lime shale. Another type of agglomerate, in which the fragments are chiefly large blocks of an,desite porpliyry is well e.vptJSed south of K. 8, Lares Itoad (Fig. 5}. This unit<,'ria,l lies innnediately adjacent to a s^mall Ikj.-s or volcrtisie ncek of im,<,ntx' aiulcHito |)(>rphyry, and in turn k j^urruuiided by thin \mh uf totl* and Fhale B-hii'h have been dilfererifially indurateil and alt.ored in proximity to tlie intrusive body and aggiomerate (Fig. 0). A sihort distance from the area of volcanic activity, they are rehitively undistniiiod and are raialtered. Bedded Tuffs. TJiese are, for the mo.«t part, fine grained, and gradt^ into typical ash beds. They occur characteristically interhcchh;^! with shales and ash, and are distrilnited tltnniglioof^ the district, thongJi more ahumjaiit in the western, part. Whih;' tlic coarseness of the njatcrial is tilnilcH ami tlihi-bedunl tulf, Hhowirijj ulteratiun i'ffff<i.<i pm<lm-r<l !>» a m-tn-hii qnite variable, even in short distances, few of the fragments are hirger tlian one-half incli in diameter. They are composed of broken, pbeno- crysts or erystalfin,e ;particles derived from andesite porpliyries, and lienco contain tlic nsual nn'nerals, sncli a$ plagioclasc, t^rromagiicsians, ami magnetite. Wlicre finely intcrhedtk;d with sliaJcs, they contain a (ronsid- erablc percentage of ealcite and ipiartz graiiis. Some of the cpiartz grains show slight traces of rtninded edges, as thoiigli subjected to current action. Wherever the tnt! is iiiterl)edded witli bbick shale, the black sliale forms a matrix for the angular grains of the tulL Where interbedded with linie shale, iJmw have a Jiigh content of ealcite and fragments of the lime HfHUfAh'TK i!B(UA)(JY OF T!IK LM{Et< DISTllKJT HJ sliiile, and .have many veins of r-ahMte in Irx-alities of close folding or faiiltin^n The ImUUHl tuffs r-an usually bo n'cogiiizfMl in tlio field by their cliaracteristic Hiilun-oidal m'i'atlieriii_g (Fig. T) whieh, distinguishes them from tile oecai^ional thin^ si Lis <jf andosite. This type of weathorijig is also to be found in^ the more massive tiilfs (Fig. 8). It is sometimes ditlieiilt to distinguish soeli oeeurreiiees fron.i congJomerate. .\s has been^ pointed out by Berkey. Aliiehell, antl others, the tutfs are much more suhjei't to surface weatliering than igneous nn'ks of the same mineral compositinn. It is only where the tuff lias been iiKhirnted. rlue 1#^...-' y. ",. ..:.:-- . Asiuitla-Uinwn llima. to the action of a nearlyv intrusive body, that it is more resistant to ero- sion than the shale witli which it is interhiMhhid ( Fig. 5). The tut! beds are (iuiractcrisiically thicker tliiui the adjacent beds of shale. Tliey range from 6 inches to several feet, and have no minor stratification. alt.h.ough some beds sh,ovv a slight assortment of coarse and fine material Aj<h. Beds of ash occur witli the shales and Ixnhled tuffs. Th.rv re- semble the shales, and where; the outcrop is weathered tht; two are not distinguishable. Typical ash beds are not abundant. The most <'omm,ou <K!eurrein:!e is shale high, in ashy niaterial, and there an> all gradatiruis between a typical sliale and a typical ash. Where the roc-k is exceedingly iine grained, it is sometimes impossible to deterrnine the eoint»osition. 20 i^C/KNTlFJC iyURVEY OF POUTO RICO Anj fine partj'<'le? of volcttiiie glai?B wliieli may have been present are usually not rceogJikiible in tliiii s(X;tioiis, owing to devitrifioatiou and alteraf.io;ii. Jn some ease-s, iniiiutc rod-like particles are present, siia;- g(»sliv(» of fibrous glass or pele's; hair, but tbese are invariably found badly altered l)y weatliering. Tlie ash beds grade, in texture, to fine grain tnifs whit;!i do not. <liffer otherwise froui typical ash. In, thin sections, ash may usually be distingirished from sluile by its lack of assortment of nm Expo . pla: paTticb?« and absesice or >eareity ol; b)raniinil'eral shells. In baud speci- mens it i.s usually distingiiishal)le by its fracture. Berkey has pointed out the fact that the shales gen(*rally break with a bloeky fracture, espe- cially in th(! case cd* \veather(;d bbick sfiale. Tlie lime shales liave a emooti), curved or almost coneboidal fractur(i. Typical aslies, liowever, havi' a ronghi'r. more irregular fraetirre, dn.<^ to tlie uuevenness of the weathering which tliey have undergone. As a rule, the typical ashes arc ol slightly coarser texture tliau most of the sbales_, but this dilTereuce is not evident in band specimens. HUBBARD, GEOLOGY OF THE LARES DISTRICT 21 The ashes, and ashy beds in general, are divisible into two types, calcareous and non-calcareous. The former are chiefly shales high in ash content, and carrying a few foraminiferal remains and grains of calcite. The calcite grains are in some cases slightly rounded and pitted as though by solution. The non-calcareous variety is made up chiefly of andesitic material, minute angular crystalline particles, and an indeter- minate groundmass, containing possibly devitrified glass and altered crystalline fragments. They contain no foraminiferal shells and but very little calcite, except as introduced matter filling veins. Of the two types of ash, the calcareous is the more common in the southwestern part of the district, while in the northwest portion of the Cretaceous area, the non-calcareous variety predominates except in the area south of Moca. Ashy Shales. Shales high in the ash content can usually be classified only by the use of thin sections. The depth to which they are weathered increases the difficulty. The weathered ashy shales are generally non- calcareous, and resemble the weathered black shale, but are higher in kaolinized material. They are typically reddish brown to gray in color, and at some places contain white patches or streaks of kaolin (Fig. 9). Black Shales. These have been described by Berkey and Mitchell, and similar shales have been described by Semmes. Mitchell shows that the red, blocky shales exposed around Mayaguez are the weathering product of a black shale, found exposed as such, only in quarries or stream chan- nels. Fresh exposures of this shale occur south and northeast of Con- sumo and at K. 3.7, Mayaguez-Consumo Eoad. There are all gradations between a pyritiferous black shale and a very dark lime shale. The black shale exposed near Consumo shows in thin sections, a dark opaque groundmass, apparently in part carbonaceous matter, in which are im- bedded many foraminiferal shells, a few minute angular fragments of quartz and feldspar, and comparatively large well formed cubes of pyrite. The pyrite in crystallizing out has crowded the surrounding carbona- ceous groundmass. Carbonaceous films, apparently small plant remains, are seen in some hand specimens of the rock. The only calcareous ma- terial visible in thin sections consists of foraminifera, but these are sufficient to cause the rock to efl'ervesce with acid. K microscopic strati- fication or assortment is visible in thin sections, but is not evident in hand specimens. A more calcareous phase of this shale occurs at K. 3.7, Mayaguez- Consumo Eoad. The rock in this locality is an alternation of thin bands of the dark foraminiferal shale with bands of fine grained andesitic tuff*. The contact between the two is very sharp. In thin sections, the shale 22 fif'll-]NTIFlC ^^URVI'jY of PORTO RICO sliows a (lark carbonaceous gr-oinuliiias.s witlt foraniijiiferal nliolls in aljuii- (lanec. The adjacent tiilT layers are cuinpused of angular fragiiients of andesite and fragmeiital crystals, all of whieli are imlieddcd in tlie j^ame dark groiiiidmass wliieh forms the l)ulk of the shalt;. f^'oramiiiiferal shells are ft)uiHl in the tuff layers with the carl>onac:eoiis matrix. In addition to foraminifera, the shale coiitains an abundaiiee of radiolariau Hl'BBAlilf, OEOLOflY OF TIIK LAMES lUsTHIVT 23 reiuaiiis, markccj by touihIcm] Ixxlien (»f amorphouK or ?;j)lioriilitii' <;ili(ii. Tlowevor, a few iiiialtereil ra^ijiolariaii sbclls (teeiir. lAme Slialen. Tliese arc miicli commotH'r tluiii the hliK-k shale. Tvf)- k-al. eAposiires of litne slialo are shown in ii^ure^ 10 uud U. Many of ilic layi'i's are liigiily ealcareoiiH, and have the eharaeter of a tliiii-beflih^e! or handed, hard, conrp?i('t limestone, dark hliiish gray in erdor. Expo.<iire.^ of this type arc to be soon in fjuarries along the autrjinobilo road erosf^ing Ihe Alabiya Kange northwest of Afuiseo. In thin section, thbs nsek proves to be made up eldetly <d' c'ak'"-ito grains with a variable amomit of voh-anio nuittcn-. sricli as amndiir i'rao-ments of ^£^. -^^H^. M-.^-^y^^' foldnpar, hornblemlo, and magnetite, ('onsiderahle reerystallizatiuji of the caleite is evideirt, and this gives tlie rock its liigii density and eojn- piietness. Iroramini feral sliells occur, oficn^ in considerable (juantitj, hut in some specimens they arc absent. Tlriis this roek difl'er.s from the ])yritiferons black shale in that its calcareous content is not dcpetident upon the presence of foraniinifera. Tlie foraminiferal shells are all of microscopic size, and include indeterminate forms rescnd)ling (jlohi<jm'imi and Rotalia. In some cases their structure has liecn entirely destroyed by subsequent crystallization to aragonitc. Some of the shells are frag- 24 SCIENTIFIC SURVEY OF PORTO RICO mental, as though having been transported before deposition, but this feature is not common. Owing to its high lime content, lime shale is more resistant to erosion and weathering than any other rock occurring abundantly in the Lares District. Along the Atalaya Kange, north and northwest of Aiiasco, their influence upon the topography is rather marked. Other, less extensive, developments of the same type of shale occur on Desecheo Island, in the Eio Blanco valley near the mouth of the Rio Prieto, at certain points in the hills south of Moca, and along the automobile road northeast of Rincon. Limestone. True limestone is almost unknown in the Cretaceous rocks of the Lares District, although one might class certain portions of the lime shale as a limestone. A small but prominent inlier of limestone occurs southeast of the Mayaguez-Las Marias Road opposite K. 22. This rock is a hard, compact fine grained crystalline limestone, gray to pink in color, and white on weathered surfaces. The structure is massive, with no bedding visible in the outcrop. Bedding, however, may have been obscured, since the out- crop shows faulting and fracturing to an extreme degree, with numerous calcite veins. In thin section, the rock appears as a fairly pure limestone, made up almost entirely of recrystallized calcite grains with a very small admixture of tuffaceous material. It contains many foraminiferal shells (chiefly Nummulites and Glohigerina) and fragments of RadioUtes. Lithologically, the rock resembles the San German limestone of the Ponce District. A few thin beds of similar rock occur with the lime shales on Desecheo Island. These exhibit distinct stratification, and are banded by zones of tuifaceous material. The fossils are chiefly foraminiferal shells (Glohigerina and others), in great abundance, and a few fragments of Badiolites. Chert. Chert beds are of very local extent. They occur as lenses in the shales north of Mayaguez. Mitchell found an outcrop on the north side of Mayaguez Bay. Another exposure is to be seen just west of K. 2, Mayaguez-Aiiasco Road. Chert is also found in small quantity, inter- bedded with shales on Desecheo Island. In its field occurrence, this rock is conspicuous because of its relatively fresh, unweathered condition. The color varies from gray to pink, or reddish brown where stained by limonite. It is extremely hard throughout, but extensively fractured, and breaks into sharp, very irregular blocks, not having the conchoidal or curved fracture surfaces characteristic of most cherts. In thin section, under crossed nicols, it appears as a fine aggregate of amorphous silica. HUBBARD, GEOLOGY OF THE LARES DISTRICT 25 crowded with microspherulitic portions^ which are very probably the re- mains of radiolarian shells. Tuffaceous material is present in very small amounts. Stratigraphy Owing to difficulties of field work and the limited time spent in the field, it is impossible to correlate rocks of similar lithologic habit occur- ring in various portions of the district. It is possible, however, to make larger subdivisions, each doubtless containing several formations. The geologic map shows two belts of shales, with a central belt of tuffs, intru- sives, and extrusives. The northern or oldest belt of rocks comprises tuffs and intrusives in the southeast, which grade into shales to the north- west. This belt is called the Eio Culebrinas Series because of its develop- ment along the Rio Culebrinas valley. The central belt, predominantly tuffs and andesitic rocks, is called the Rio Blanco Series, from its ex- posure along the Rio Blanco. The Rio Yauco Series, in the southern portion of the district, is named from the Rio Yauco Shale, a term ap- plied by Mitchell to the black shales which form a prominent part of this series between Mayaguez and Consumo. The relationship of these three series, as well as the complexity of the structure, is shown in the cross- sections (Plate I, Figs. 7-10). It should be noted that the boundaries of these subdivisions are marked more or less by gradational change from one prevailing type of rock to another, hence the boundary lines as drawn on the geologic map are somewhat arbitrary. On this map no attempt has been made to show all the occurrences of each type of rock. Thus where shale is the predominant rock, the entire area is mapped as shale, and only the larger tuJf beds or intrusives are shown. THE RIO CULEBRI^^lS SERIES In the region south of Lares, this series is represented by a wide area of tuffs. Exposures of unweathered rock are so few that the true char- acter of the underlying formation cannot be determined in most places. At least one small area of shale occurs east of Lares, and there are doubt- • less many others. Intrusive bodies of andesite porphyry, augite andesite porphyry, and gabbro porphyry are found, chiefly in the vicinity of Lares. To the northwest, the tuffs grade into shales. This transition is well shown by the alternation (interfingering) found in the section south of San Sebastian (Plate I, Fig. 7). Still farther to the northwest, shale is the predominant material, but is interbedded throughout with ash and tuff. A small area of intrusive, niniHAIlD, IIFAHAKIY OF THH L.t/?/vK DIHTIflVT the prevailing dip in to the s<)lltllW('^i. Most of the old voleaiiic vmr< are in this 1)elr. Aiuoiig these, the two luoHt important ones oeenr f^outh of A^ruHda and smitli of Lares in tlio I{io Blanco valUn-. The former is marked hv the largest area of surface flows in the district, eosisisting of amygchdoidal rocks (an(h^site and aiiu-ite andcsitc). It is in the angite aiidesite that native copper occurs. Jn this hirgv area of surface flows are siiiidl intTOsive^ of serpentine, marked on the snrfacte hx patclies of iron ore like that rsecurring oji the Mesa at Mnyagucz. An<itlier srirface flow of an,gile andesite is found in tlie llio Anasco valley al)oiit tlirec miles ea.st of Ana^co. Tlicre is no niitivc co[»per, liowevcr, in tliis roek. w«>st. Tho wnsKive Uii»-k-l«'d(l<.r:i (nlTs are seen on tlw rljrht. dijipiiig s^twiil.v t<» tlic rast Tills dlscor(lan<>o in dip tin«1 strike was louud upon eXiiHilnatton to l.e the result <»£ a tliriwt fann uf considerable iiiiisijltude. Tlw area, south of Lares in th(> l{io .Blanco valley k an. old volcanic complex and ;|.n.'obahl.y represents the location of several former volc.-uioes. Only one area of iindouhted. surface flows is .found in the R.io Blanco Series, l)ut andesite ptvrphyry showing eohimnar structure occu.rs along the river al)ou.t I.14 miles east of the 'end of the Larcs-lio Bla..nco Hoad (see I/i.g. 3). Tlie (.'ohimns exhibit radial arrangemeiit and prol)ably mark an old volcanic throat. The size of the plienoerysts seems to show that this roek was not a surface How. .Northwest of this point is a rattier large in.eluded area of sliales, tull's, a.nd lime shales, lithologieally similar 2S SCIENTIFIC SUR'VEY OF PORTO RICO to those of the Eio Taiico Series. Tliis iiilier of acKiimeiits lias bcori badly iiuiltx'd and exit up by tlic surroiuuling iiitrusives. In the Eio Bbiiieo valle)' lit tht! nioiith oC tiie Kio Prioto, tht; liirie sliaJos are fauJteil (Vig. 12} and l;)rokeJi up, IVirmiiiff in places a vidcaiiic a«'giumeratt; of cUKlositic cojiteiit in wliidi large slabs of the sliale are imbedded (Fig. 4). ^'he ijitriLsioii of quartz diorite porphyry, sliown on the map in this volcaruc area, is best exposed in a higfi peak known as M;t. Torrecillo (Fig. I), located at K, 48-^9 on the Lan>s-Eio illaneo Eoad. Tlw b(\st: exposures of the liio Blaiieu Series are foioid along tlie sugar railroad east of Ailaseo in liie Rio Bhmco valley, (loiug upstream, one passes from tlie h(4t of Eio Yaueo Series shalcis into a su(;eession of tuJfs, andesite thiws and andesite porphyry intrusivos, throiiglioiit whicli verv litth'i uf the stru('tTire ean be seen. C)<;(-'asionalIy ohsenre hediling may be fouud in tlie tuffs, or tlie dip of a fh.w or sill of andesite may he dislj'n- guished. Ileri; ami there are small included patetses of shale of verv limited extent. 'Fhe most curions au.d njiexptK^ded feature cd' ilw Rio J^hineo Heries is tlie pivsonee of an isidated ouierop of limestone, rising from a monotonous aJ-ea of deeply weallicred roek, all a'pparcntly tuff. As shown on ilie jiiap, this liinestojie lies 1^, mihiS southwest of Las ifarias, and may be seen from a point near K. 23 on tJit^ Mavaguez-T^as Marias Road. It is similar lithologieally to the San Oerman limestone HUBBARD, GEOLOGY OF THE LARES DISTRICT 29 of the Ponce District and contains Radiolites sp. and many foraminifera, including indeterminate species of Glohigerina, Nummulites and an Orhitoides or similar form belonging to the Orhitoides group. THP] RIO YAUCO SERIES This series, as previously noted, is predominantly an area of shales, often interbedded with ash and tuif, and with occasional andesitic flows and intrusives. Most of the shale is much less weathered than that of the Eio Culebrinas Series. This is due to the relative abundance of the hard gray-blue lime shales which are much more resistant to erosion and weathering than are the more ashy, easily kaolinized shales of the series to the north. It would seem that the Rio Yauco beds have undergone greater folding and faulting than the underlying rocks of the other two series, but this difference is probably only apparent, not real. The details of structure in the other two series are usually obscured by weathering. The northern, or stratigraphically lowest, portion of the Rio Yauco belt contains much shale of the ashy type alternating witli tuffs. Excel- lent exposures of these are found along the automobile road northeast of Rincon. A very characteristic feature of these beds is the spheroidal weathering of the tuffs (Figs. 7 and 13). Southeast from Rincon, and forming the range of hills known as Atalaya Peak, are developed the blue-gray, hard lime shales, often banded, and resembling the Peiiuelas shale of the Ponce District. In the almost total absence of true lime- stone in the Lares District these lime shales are among the ridge-form- ing types of rocks to be found. Southeast from Ahasco these lime shales grade into the pyritic black shale of the vicinity of Consumo, and its weathered product, the red kaolinized shales, are well exposed in the vicinity of Mayaguez. Tuffs of the more massive type are more abundant southeast of Ahasco Playa. An intrusive body of considerable size composed of gabbro porphyry, grading into augite andesite porphyry, is well exposed at vari- ous points from K. 8 to K. 10 on the Mayaguez-Las Marias Road. It is clearly intrusive into the shales. North of Mayaguez, near K. 2 (Maya- guez-Ahasco Road), are several small areas of chert, occurring as lenses in the surrounding shale. They are apparently of radiolarian origin. Desecheo Island, lying 15 miles west of Pt. Jiguero, contains more features of geologic interest than any area of equal size on the mainland. It is a continuation of the belt of lime shales of the Rio Yauco Series which form the back-bone of the Cordillera Central, known on the west coast of Porto Rico as Atalava Peak. The arid climate of Desecheo has •io sciKxiine sritvEr of i'orto meo preserved the ntck.- in a rclat.ivcly iiu weathered state, heuee tlie striicf.urc can be well .^eeri. The I'liormoiis amoiiiit ol faulting and t'raeturiiig m w(dl i^howri in tigiire.< VI and 15. Amtojg tli<» special fcatiir(!S should be mentioned the presence of eliert beds like those north of Majaguez, and i)t A {(nv thill beds of limestone of the San German type, containing RndiolUeh- and an alnindauee of furaminifera. STinTf:Ti:«AL Features The strike of the beds is, as prtn'iously stated, north west-stnithcast. ih\ tlie map tiu; lines desii^natina- Hhah» have lieen drawn so as to iiidi- p^ ^k-^' t eate the sti'ike wherever known. Tlic dip is to tlie southwest, at variabh', Irut almost nniversally Idgh angles, Varial>lc or northeast dips denote folding or fanlting, or both. The ebaracteristic structure of the series is sh,own in tlie four sections (Plate J, Figs. T, H, <), 10). Figures !• ami 10 are generalized or i<h>al sections, and illustrate the dilTerence between, the east and tiie west parts of the Lares District.. Figures T and 8 are ba.se<l on traverses nuide acrijss the series. Wherever the course of the traverse has deviated Crcmi the line of tli(^' section, tlie data has bficn projected on the section. Tlie l<K:a- tions of the sections are not shown on the map, but may readily be se:Mi from the desigiuitions of towns througli wJdch they pass. Structurally, the rocks of tlie district are of two kinds,—massive and bedded. Tlu; massive rocks include t,lie andesite intrusives, of laecolithic Scientific Survey of Porto Rico and the Virgin Islands Volume II, Part 1, Plate I FIG. 3 FIG4- FIG.5 FIG. 4 HA TO ARKI8A- ISABELA SECTION DiRCCTtON - NORTH 5* EAST TOTAL TillUHtSi 3250 rZET ft T~~~ 1 MltfS C0UA20- Ql/EBRADILLAS SECTION JDiRtCflON - NORTH- SOUTH TOTAL THICKNESS iSlSft.U LARES-CAMUY SECTION DIRECTION- NORTH |0° EAST TOTAL THICKNESS 3850 TEET sr FIO CAMUY-HATILLO SCCTION DIRECTION - NORTH t-OUTK TOTAL THlCKrJESS 3700fEET SAN SteMWM RIO Cl/UEBftHM ^ ILAIRIES MSTEECT THE NUMBERS ON Pl^OflLES REFER TO FIELD STATIONS AND DIP OF THE BEDS JPILATM RIO BLANCO SERfCS HUBBARD, GEOLOGY OF THE LARES DISTRICT 31 or irregular form, some of the thicker surface flows, such as those south of Aguada, and the massive tuifs. The latter cover large areas, and are almost universally too deeply weathered to show their real structure. The few unweathered exposures show either very obscure bedding of massive proportions or else none whatever. The bedded rocks comprise chiefly the shales, with interbedded tuffs, the smaller andesite sills and flows. It is chiefly in these bedded rocks that the highly disturbed con- dition of the rocks can be seen. Intrusives. These are characteristically of the laccolithic type, or oc- curring as sills in the bedded formations or massive tuffs. Their origin is indicated by their occurrence as elongate bodies following the general strike of the surrounding beds. Many of them may be surface flows, but their porphyritic habit is so nearly universal, and the phenocrysts of such considerable size that it seems safer to interpret them as intrusives. It is nearly everywhere impossible to solve this problem from the nature of the contact with adjacent sediments and volcanic elastics. The reasons for this are chiefly because of the deep surface weathering and absence of extensive exposures of fresh rock. Another factor is the similarity in mineral make-up of the igneous and clastic rocks, and the consequent absence of contact metamorphic effects. Some of the large and irregular igneous bodies, such as the gabbro porphyries, are obviously intrusive into the sediments. They cut across the beds and are not elongate parallel to the strike. Folds. At various localities in the shale areas are zones of exception- ally strong folding and faulting, marked by crush zones and extreme variability of dips and strikes. Wherever the folds are exposed to view, they are seen to be overturned folds, usually pitching at considerably high angles. A typical example is shown in figure 11. These minor folds are superposed on major folds which form the largest and most continuous structural units of the shale areas. The best example is the Anasco Synclinorium , shown in the section on Plate I, figure 10. This great syncline in the Eio Yauco shales underlies the Anasco Playa. Its northeastern limb and a portion of its southwestern limb are exposed along the south side of Atalaya Range, northwest of Aiiasco. From Aiiasco Playa it extends southeast, across the Mayaguez-Consume Road between K. 6 and K. 12. Other large synclines and anticlines occur northeast and southeast of Rincon. The axial planes of most of the minor folds dip with the adjacent and less disturbed beds, hence to the southwest. The pitch of the folds is to the northwest, though in some instances in the opposite direction. 32 SCIENTIFIC f^Umi'JY OF PORTO RICO In aclditioij to this northwest-southeast systoni ol' major and minor folds, there is evidence ot another, and probatdy hiter systom. more of tlie nature of broad, ext.onsive flexures, whose ax(>s seem to extend in approxi- mately a north-south direction, rieajiy n,ormal to tlie strike. Evidence of this ma\' he seen in the very gra<hial and progressive (-liange of strike of the shales over eerlaiii areas of considerahle size. This feature i?* shown on the uiai) in nunierous plaeen, notahlv in the area (d' liio Yauco sliales lying south of Aiiaseo and north of Mayagnez. Tlie same move- ments wliich produced these north-soiitli tlexures fn-otiahly were respon- sible, at least to some extent, for the plunging of the overturned anti- clines and syneliues a!>o¥e noted. HUBBARD, GEOLOGY OF THE LARES DISTRICT 33 Faults. Both normal and thrust faults occur in great number. Usually^ evidence of faulting is to be found only in the presence of crush zones. In such cases very little can be determined as to the nature or extent of the movement. They are probably for the most part thrust faults of considerable extent. Most of the normal faults seem to be of relatively slight movements, tliough very numerous, and in aggregate, doubtless represent displacement to be measured in thousands of feet. While the data are insufficient to make a positive statement, observations seem to show that the normal faults more commonly cut across the gen- eral strike of the formations, and hence are to be associated with the north-south flexures previously described. The thrust faults also cut across the strike of the beds, but on the average at lower angles, and sw / / / //// ^77 ' ' r*"^ NE Fig. 16. Thrust -fault in Rio Yauco shale K. 13.85, Mayaguez-Consumo Road. Strike of beds, N. ment, several feet. 75° W., dip 45° S. W. Displace- hence are to be associated with the earlier, or mountain-building move- ments, which produced the intensive folding. Except in the cases of minor faults, the amount of displacement could not be determined in any of the faults observed. Figure 12 shows a thrust fault which probably indicates a movement of some hundreds of feet. The view shows massive tuff beds almost normal to the adjacent shales, against which they have been faulted. 'No marked folding occurs in the vicinity, thus the discordance shown in the illustration could not have been brought about by any minor displacement. Thrust faults are well exposed on Desecheo Island, as shown in figure 14. The faults are accompanied by extensive fracturing, and the numerous joints thus pro- duced have been filled with quartz or calcite. The extent to which the fracturing has gone is well shown in figure 15, illustrating an exposure on the northwest coast of Desecheo Island. 34 SCIENTIFIC SURVEY OF PORTO RICO A minor thrust fault of special significance is exposed at K. 13.85 on the Mayaguez-Consumo Eoad. iVs shown in the illustration^ figure 16, the fault plane dips at a very low angle and shows a displacement of only a few feet. It is located on the north limb of a large syncline, and is apparently a break thrust, produced during the folding. 'That it is not a normal fault is sliowm by the very slight dip of the plane. It, there- fore, serves to emphasize the point previously made, that the thrust fault- ing was probably contemporaneous with the intensive northw^est-southeast folding. Thickness. The complexity of structure, absence of continuous out- crops, and limited time spent in the field make accurate determinations of thickness out of the question. In the section of Eio Culebrinas Series south of Moca the shales and tuffs measure over 3000 feet in thickness. This probably is near the maximum figure for this series. In the Rio Yauco Series, about 6000 feet of shales and tuffs are exposed along the Mayaguez-Las Marias Eoad from K. 13 to K. 19. k traverse across the series northeast from Eincon shows a total thickness of over 8000 feet. This measurement makes considerable allowance for duplication of strata by folding and faulting, and probably represents the maximum thickness of the entire Eio Yauco Series. The thickness of the Eio Blanco Series cannot be determined, owing to the massive character of the tuffs and the many intrusive bodies. These estimates of thickness include the interbedded flows and intru- sives, which, although individually small in the sections measured, doubt- less make up a large portion of the total. The measurements take account of the larger folds and allow for many of the minor folds. The effect that faulting may have had in duplicating the succession of beds is, how- ever, impossible to estimate, since the amount of movement along fault planes could in no case be determined. It should be pointed out that many faults, both thrust and normal, occur along the sections measured, and their effect has doubtless been to greatly increase the apparent thick- ness in each case. Nevertheless, it may be stated that the total thickness of clastic rocks exposed in the Cretaceous series of the Lares District is to be measured in thousands of feet. A conservative estimate for the entire group would be about 10,000 feet. Age The age of the Eio Yauco Series is Upper Cretaceous, as shown by the presence of Radiolites, and the same foraminifera characteristic of the Upper Certaceous beds in the Ponce District. The Eio Blanco Series is HUBBARD, GJEJOLOGY OF THE LARES DISTRICT 35 also probably Upper Cretaceous, though the presence in it of RadiolUes is not as reliable evidence as in the case of the Eio Yauco Series, since the limestone which contains this fossil may be an in-faulted remnant from another series of rocks. The Eio Culebrinas Series is the oldest group in the Lares District, but there is no evidence to show that it is older than Upper Cretaceous in age. It has beds of lime shale which contain the same foraminifera as the Eio Yauco lime shales, but these minute organisms are hardly determinable specifically. There is no evi- dence of unconformity between or within the three series of the district. Mitchell has found the same to be probably true in the southward con- tinuation of these beds in the Ponce District. There is no evidence of Eocene sediments in the Older Series of the Lares District, and likewise none in the Ponce District, as Mitchell has shown. Correlation^ The Eio Yauco Series is probably the equivalent of most of the shales in the Ponce District. Mitchell states that the Pefiuelas shale is probably . equivalent to the upper portion of the Eio Yauco shales. The lime shales of the Eio Yauco Series of the Lares District are lithologically similar to the Pehuelas shale, and may be equivalent. The presence of RadiolUes cannot be used to correlate different members, since all the limestones as yet found in both the Ponce and Lares Districts carry this fossil, and have similar foraminifera. From stratigraphic and structural consider- ations, the following statements seem probable : 1. The Eio Yauco Series is equivalent to most of the sediments occur- ring in the Ponce District. 2. Other formations in the Ponce District of doubtful correlation will very likely prove to be younger than the Eio Yauco shales. 3. The Eio Culebrinas shales are older than anything in the Ponce District, but are probably not older than L^pper Cretaceous in age. 4. The entire range of sediments from the lowest members of the Eio Culebrinas to the uppermost members of the Eio Yauco Series represent a continuous succession, unbroken by any unconformities or disconformi- ties of appreciable magnitude. Further correlation with areas of Porto Eico to the east is practically impossible at the present stage of investigation. The strike of the forma- tions in the Coamo District, if projected, would seem to show that they pass northwestward across the northern half of the Lares District, cov- ered by Tertiary formations. The presence of Radiolites in the Coamo SCIENTIFIC SURVEY OF PORTO RICO 36 tuff limestone is significant in showing that at least a portion of the formations exposed in the Coamo District are of Upper Cretaceons age. LiTHOGEXESIS The origin of the various types of igneous rocks has been pointed out in the discussion of the petrography of these rocks. The origin of the clastic rocks has likewise been referred to, but deserves further elabora- tion before attempting to outline the Cretaceous history of the district. It has been noted that practically all of the clastic rocks contain ma- terial derived from volcanic sources. From the tuffs, which were entirely derived from such sources, to shales and limestone, which contain minor amounts of this material, the entire succession of beds show that they were deposited during an almost uninterrupted period of vulcanism. As Mitchell has pointed out, the massive tuffs, which show no bedding, were doubtless accumulated on land surfaces, while the shales and stratified tuffs and ashes were accumulated u.nder water. The lime shales and black pyritic shales contain so many foraminifera that their marine origin seems highly probable. In the case of the limestones, interbedded with these shales, and carrying RadioHtes and abundant foraminifera, their marine origin cannot be questioned. The alternation of massive unfossiliferous tuffs with shales and limestones containing marine organ- isms thus shows a frequent oscillation of level during deposition, in which arms of the sea frequently encroached on the land area. The sparsity of marine life, and the singularly small number of types making up the fauna, illustrate the very hazardous and unfavorable conditions for the development of living organisms. A glance at the geologic map will at once suggest, in view of the above interpretations, the significance of the predominance of massive tuffs in the eastern portion of the district. The land mass apparently lay to the east, and the invasions of the sea came from the west and southwest. The predominance of calcareous beds on Desecheo Island, and their relatively greater foraminiferal content, sup- port this view. The greater abundance of limestone in the Ponce District to the south is also significant. The lens-like habit of the tuffs inter- bedded with the shales seems to indicate that rivers played an important part in the deposition of both the shales and bedded tuffs. In regions subjected to periodic violent volcanic eruptions, we should expect the animal organisms inhabiting the adjacent sea to be killed off periodically in great numbers. Consequently, we should look for occa- sional strata crowded with fossil remains, with intervening beds contain- ing exceedingly few fossils due to the rapid accumulation of clastic sedi- HUBBARD, OEOLOGY OF THE LARES DISTRICT 37 ment and the scarcity of marine fonns living under such continuously unfavorably conditions. This is precisely what is found in the sediments both in the Lares and Ponce Districts, as the following evidence shows 1. The limestones of the San German type are in places crowded with the remains of RadioUtes. 2. Certain beds of the lime shale show an abundance of foraminifera, while others show very few or none. 3. The black pyritic shale is indicative of the rapid accumulation of organic remains, chiefly foraminifera, and possibly plant matter. 4. The occasional chert beds seem to be derived from sudden and rapid accumulations of Eadiolaria, and possibly other silicious organisms. Foraminifera do not thrive in impure, sediment-laden waters. Their abundance in some of the layers of calcareous ash and lime shale con- taining ashy material, and the evenly banded character of these rocks, points to the following special conditions of deposition : 1. Quiet waters, free from strong currents, and presumably not very shallow. 2. Pure water, except during showers of volcanic ash, which resulted in a sudden pollution of the water, killing off of the foraminifera and other planktonic forms, and the rapid settling of both ash and shells on the quiet bottoms. Geologic Histoky From the above data, including the descriptions of the various types of rock wliich occur in the Older Series, the conclusions regarding their age, and the interpretations of conditions under which they were formed, the following steps are given in the Cretaceous and early Tertiary history of the Lares District 1. Pre-Cretaceous.—Unknown. The basement of the Upper Creta- ceous volcanic flows and elastics has not been found in the Lares District, and probably will not be found. 2. Upper Cretaceous. Period of almost continuous volcanic eruptions of the explosive type, marked by the ejection of enormous quantities of lithic fragments and frequent showers of ash, resulting in the rapid ac- cumulation of great thicknesses of clastic deposits. This action was accompanied throughout by lava flows of andesite and augite-andesite, and intrusives of the same material into the surrounding clastic rocks during their formation. There were' frequent oscillations of level with invasions of arms of the sea covering portions of the western part of the district, and on one occasion, at least, extending as far east as Lares. 38 SCIENTIFIC SURVEY OF PORTO RICO 3. Late Upper Cretaceous. Final phases of vulcaiiism^ marked by minor intrusions of magma of extreme composition^ such as peridotite, now altered to serpentine. These final intrusives were accompanied by the beginnings of great orogenic movements. 4. Close of the Cretaceous. Orogenic movements^ comprising extreme folding and thrust faulting, with the maximum forces applied in a north- east-southwest direction. This was probably the local phase of wide- spread crustal movements which outlined the present sub-sea mountain chain, the basement upon which the Antillean islands rest. 5. Close of the Cretaceous or Beginning of Eocene. Further crustal movements, involving north-south warping, normal faulting, and pos- sibly accompanied by further uplift. 6. Eocene. Continuous erosion, and reduction to oldland surface of comparatively slight relief. 7. Early Oligocene. Kenewed uplift, with dissection of the oldland surface to a region of considerable relief. Deposition of coarse gravels in valleys of torrential streams (seen southeast of Moca). 8. Middle Oligocene. The beginning of a partial submergence of the island with deposition of the San Sebastian shales in embayments along the north coast of that time. Further events are given in the discussion of the Tertiary and Pleistocene. THE TEKTIARY FORMATIONS The most complete development of the Tertiary formations in Porto Rico is in the Lares District, and occupies the area between the Lares Road and the north coast. These formations are a structural unit (Berkey, 1915, p. 12), resting unconformably upon the highly disturbed Cretaceous rocks, and overlain unconformably by Pleistocene and Recent consolidated dune sands and beach gravels, limited to the coast line. The Tertiary beds are a series of limestones, in small part of coral origin, underlain nearly everywhere by basal shales, clays, marls, or conglom- erates. The maximum thickness of the entire group is about 3800 feet, with the strata having an average dip of about 4° seaward. The range in age is from Middle Oligocene to Upper Oligocene. Table 3 shows the subdivisions and names applied to the Tertiary for- mations of the north coast by previous writers and in the present paper. Hill's term Pepino Formation, although having priority over the term Arecibo of Berkey, will not be used in the present paper, because (a) the corals collected by Hill from his Pepino formation occur also^ in the R. T. Hill (1899 c, p. 15) Pepino formation Miocene "At least 100 feet thick" Greensand marl Eocene or Oligocene Lignitic shale and clay Eocene Cretaceous Table 3 SUBDIVISIONS OF THE TERTIARY OF THE NORTH COAST, PORTO RICO C. P. Berkey (1915, p. 17) San Juan formation Pleistocene to Recent Unconformity. Arecibo formation Eocene? Oligocene Miocene? "At least 500- 600 feet thick" Quebradillas reef limestone San Sebastian shale Probably Eocene .Unconformity. Older Series Cretaceous C. A. Reeds (1916, 1917) Arecibo formation Upper Eocene- Oligocene Collazo shale Upper Eocene D. R. Semmes (1916, p. 434) San Juan limesand Recent Tertiary formations B. Hubbard * (1917) San Juan formation Pleistocene to Recent .Unconformity. Arecibo Group Quebradillas limestone Upper Oligocene (Bowden) Los Puertos limestone Cibao limestone Lower Oligocene Lares limestone Lower Oligocene San Sebastian shale Lower Oligocene or Upper Eocene T. W. Vaughan (1919, p. 260) "Pepino formation of Hill" Middle and upper Oligocene, possibly some Miocene" Older Series Unconformity. Older Series C. J. Maury (1919, p. 214) Arecibo formation Quebradillas limestone Lower Miocene (Bowden) Aguadilla limestone Upper Oligocene Lares limestone Upper Oligocene Rio Collazo shales Middle Oligocene D. R. Semmes (1919, p. 55) San Juan consolidated dune sand Pleistocene Unconformity. Arecibo formation Upper Oligocene (Bowden) "At least 500 feet thick in San Juan District" San Sebastian shale Lower Oligocene Subdivisions used in the Present Report t San Juan formation Pleistocene to Recent Disconformity. Quebradillas limestone Upper Oligocene (Bowden) 700-875 feet Los Puertos limestone Upper Oligocene 550-1000 feet Cibao limestone Middle Oligocene 250-1000 feet Lares formation Middle Oligocene 350-1275 feet Arecibo Group Unconformity. Older Series San Sebastian shale Middle Oligocene Max. 700 feet .Unconformity. Older Series Upper Cretaceous Proposed subdivisions, based on field work in 1916, and presented before the Geology Section, N. Y. Academy Sciences, in 1917. t Same as were made by the writer in 1917. The ages assigned to the formations have been modified, as the result of a study of the fossils made during the summer of 1919. HUBBARD, GEOLOGY OF THE LARES DISTRICT 39 Upper San Sebastian shale^ and (h) Pepino refers to the litliologie and topographic character of the formation, and is therefore an undesirable term. Stratigraphy the san sebastian shale The existence of a basal shale was first pointed out by Hill (1899 c), who did not apply a formational name to it, but merely mentioned its position relative to the Pepino Formation. Berkey (1915) was the first to suggest a formational name (San Sebastian shale). Subsequently, Eeeds gave the name Collazo shale, because of the excellent exposures and numerous fossils found at Callazo Falls, east of San Sebastian. Fol- lowing Eeeds, Miss Maury has used the name Rio Collazo Shales. In the present paper the name suggested by Berkey will be used, because {a) Berkey's name has priority, and {h) it is a more appropriate name than Collazo. A glance at the cross-section (Plate I) or columnar sec- tions (Plate II) will show that the maximum development of this basal shale is near San Sebastian. The columnar sections (Plate II) show the various types of material of which the formation is made. Although it is predominantly a shale, it consists in large part of dark bluish clay carrying seams of lignite with pyrite and marcasite, conglomerate and pebble beds (in most places unconsolidated), red calcareous sand or lime sand, green marl, and im- pure limestone. Many of the lignitic clays contain fossil leaves and fresh or brackish water molluscs. Sharks' teeth and vertebrate bones occur in many of the beds. The marls and red lime sands contain marine fossils, and as shown in the Collazo section (Plate II) alternate with the hrackish or fresh water beds. The marls at the top of the formation carry an abundance of large coral heads. Limestone (argillaceous) is the only material in the San Sebastian formation which is firmly con- .solidated. In other strata, the lower the content of lime the less consoli- 'dated the material. Most of the ^^conglomerates," which predominate in the basal part of the formation, are merely loose gravels or pebble beds. •Ground water has easy access in all strata except the dark blue clays, and it has leached out large quantities of calcium carbonate. Most of the fossil molluscs are found as molds with but small portions of the shell Temaining. As previously noted, the San Sebastian shale is thickest in the vicinity -of San Sebastian. From this point it pinches out on the west and thins to ]00 feet at the east end of the Lares District. Farther east, in the SCIEXTIFia SUirVEY OF I'ORTO RICO fiio Arecibo cajiYou, it is absent (Borlicy, 1!>15, p. IG). Tbiis, as com- pared with overlying format ions, the SaJi Sr^bastiiiii is of local occ.iirrciiec. Tliat its relatioiisliip wltli tlie Crctaeeons ronk^ h iinccail'ormMble bas been sh(jwii by Berkey, and is too evident to require furtber demonstra- tion. Tbe actual tiasal contact cannot be ncen in any of tlic loealities visite<l, but exposures close to the contact ran be seen in several places, the most significant of wliicli are in tbe immetliate vicinity (>t 141 res. From a survey of tbese exposures, tJic Atdowing- features are to be; noted : .• . ,:*;.j';s* ^/"v;;J':. : •':.„''.,;!* " •%,.#•:- % is™ - « ', ' '%/". 'i c- .i '''.:;''"-:\';°-i'' -'"''..'- - '^ '^ --.Ai y. *"" ',. :;% J. ''"':' •* " - ' .,.' i-^ '>s#ir W&' »* w , . !. r. , • ; ,'=. -* !-•;.• ' „ "rf^.:: ^'-^ - •K^.:-.*. *,, m./.K^i,;-^^:;^^.f::''":^ 'm Fi«. ;17. fSmiMrr Hini uwl < I fit biifie ttf thr Terliarj.i K. S.ft. Lares Koad, near Mocr roeksi, wlJicii in lliis le»caltty This deposit lies nne«nr<.rmMliIy on !' nmiat ut slmles, a!«h, and angih-nnKk«it J. Tlie surface upon wliicb the San Hebastian sbiile was deposited was one of considerable relixd' (probably two or tJirec^ liiindred feet). 2. Tlie Ban, Sebastian rests not u|ron fresh, cleanly lu-CKled ('rctaeeons rock, Init ii])on, badly decayed rock or residual soil. J.'ortions o'f tbe basal Han Scbastiaji at Lares art; made up of reworked residual soil, difteriug in, no respecrts from modern residual soil of tlie interior mountains, exce])t tliat it contains, here and there, tbe molds of Tertiary fossils. At K. 8.9, LartiS Koad, there is an ijit<:'rcsting deposit of boulder clay and coarse gravel (Fig. 11') resembling glacial till in its general appear- ance. This material is unconsolidat(>d, and composed of boulders of Cretaceous rocks imbedded in a kaoliriized matrix. It contains absolutxdv HUBEAR IK fiKOLOOY OF THE LAKK& PJSTRICT 41 ,110 material wJiieli coiiLl }iavc been derix-ed from any of thci Tertisiry for- matiojis. To the iiortliwest thin deposit piiiehes out and is r<>;plaeed by bayal Tertiar}- pebble beds and shale (Lares formation) near ^foca. Thene large boulders probably mark tlie cbamiel of an Eoeeiu! early ()1i- ir(teene river. Sneh eoarse gravel would probably not be conunetii in ebannels of old streams Jlowingoii a peiieplaue. On the other hand, sueb deposits sliuuld be eonimon in a region of considerable relief, sueh as is sliowri by the rehitionsliip of tlu^- basal 'rtTtlary at Lares. At the top of the San Sebastian sliab\, the eontaet with tbc overlying- Lares limestone is a a'radatioiial one. The ehaiiy-e from marl to limesbme by an alter- Kridge at Co: mition (inter figure IS. '^From tlie sections (Plate II) it will l,e noted that pebble beds liguJiie elays predominate in, the lower part of the San Sebastian s' Tliese irulividual beds, liowever, carrnot Im eorrelated in the diffe sections. In the section at Ijares, the upper mendier is a rcrl lime ,' eontaidng f'eclcn laresense. Tfiis bed is tracealile as far oast as (Jamuy Kiver. but <-annot be correhited ivitli certainty with anytliiii sections to the west. The most coid.iniious ineml)er in tlie fonnatio tlie zone of grcKjn marls witli abumlaiit eorals. which forms tlie t'op of San Sebastian formation at C'ollazo. This same zone, with the s the ;iiiie 42 SCIENTIFIC SURVEY OF PORTO RICO corals, is found at the top of the San Sebastian in the Hato arriba sec- tion, west of San Sebastian. Some of the same corals occur in the basal portion of the Lares limestone in the cnesta just north of Lares, hence this portion of the Lares may possibly represent a limestone facies equiv- alent to the upper San Sebastian marl beds. In the Collazo section, im- mediately below this coral zone (zone C of Vaughan, 1919), is a zone of marl containing Clemeniia dariena and Turritella to7mata. This zone likewise underlies the coral zone in the Hato arriba section, but the rock is much more calcareous than the same horizon at Collazo. To sum- marize, there are only three zones, of any considerable continuity in the San Sebastian shale 1. Upper Zone (zone C of Vaughan). Marls with abundant corals, grading eastward into limestone and red fossiliferous lime sand. 2. The Middle Zone. Marls and other types of material marked by abundance of dementia dariena and Turritella tornata var. portorico- ensis. 3. Lower Zone. Predominantly lignitic clays and gravel beds, with some marine marl and limestone, the latter containing numerous tests of a small Scutella (like 8. mississippiensis) . The extremely local nature of most of the beds and fossil occurrences makes it inadvisable to attempt a finer subdivision of the formation. THE LARES FOEMATION The term Lares Formation was first proposed by the writer (see Table 3) for the massive limestone overlying the San Sebastian shale, and most conspicuous north of the town of Lares, where it forms the cuesta and an extensive area of haystack or pepino hills. The distribution and bound- aries of the formation were definitely shown in a paper presented before the Few York Academy of Sciences in 1917. In a paper on the Porto Eican Tertiary, Dr. Maury (1919) uses the term Lares limestone for beds overlying the San Sebastian shale, but no distribution or strati- graphic limits are given. The chief horizon marker of the formation, according to Maury, is a large cerite shell {Campanile (Portoricia) larica Maury). This shell was not found by the writer in any of the numerous localities studied, but a very similar shell {Cerithium {Campa- nile) collazum Hubbard) was found in abundance in the San Sebastian shale, and sparingly in the lower part of the Lares formation. It should be stated in this connection that these shells are poorly preserved, and that the above two designations may represent the same species. In the type locality, north of Lares, the Lares formation is a massive, SciEJ^TiFic Survey of Porto Rico and the Virgin Islands Volume II, Part 1, Plate II EZI Ezr EZT EZI rzi E^g fmmental wMe kt Ostrea aniiguensis mra 1st fragments in soft * i ^ , . ,, Tsetr/x Upicat QuebradiUas fcum- Fqranosa, Metis trivitaria Qfthaulat fiortericoensis ' Cjittierea berkt^,(cmi5 catemtis, m/ctTiofi/ij'lia sp. soft^ cklkt with ht CBmloimnk ~^ ht.covqhn. witiMAoukx corals "^corafs alternate Juni aJid soft 6tds hard, red -pi/ik,semi<nist(injiie. Tmsshe red "^^"^"^ lo<:a]ij fragmntal red ht Hith vmsroui , smil shell frvgwents mtftm Mtcateriteus retf ht *•!?/ wrrridf/r catvrwus M. with mtnf stalactites. » fossi/s mmrous ttkmids ^ n-cien sp. •^te • ^ '• • vW;;^ti,f^%$t"tVd red horizon at Agitadilh -massite carerncus. Ixall] fraqmental, witA sM/ fmqmevts, hard, phik^ utith Tnuih vein cakite - Bttknofihjllia and comIs of trinitaria and it -Met massive pink - red caterncus, Se nhCf-ptcll- "f - nrthnu Tn i avd Tnanf si red ir^staliine ht soft,»,h:tc,chcn.f 1st -ccrah of ynassne cofonia] type soft, triable, podularyellow ^, U v6 fossils ' safl , cream -colored 1st nn neten sp and Teredo ixnssa, rpn(E n uft,jelhn- tc white weotAerf* Ist^iocaHf hard, sem-cry^alii^ pink to redJ and with occasional coTKlomeritic beds —^ Teredo incras'sata and Lam fauna mined mith some I upper San Sthasiien Species pebble bed - pebbles of Older Series rocks base not exposed j creciTihcohrvdM -fkctev s^H f- ^(.frmUe^rrvam-coiorfdl^ •- wftfj Smaster sp and Cstrea . antiguensis yelloK ht with anjular 1st. f-crystallirre /onJk^^f' mh foramir.ifera mkite to mottled red ^<t/i smIl fragments of mdluscm hard,pink endn^ite kt. semicrystallirKj cciernous pink am wme reef 1st -no hard iMtk arJ%hi\e*W'»itb ^ corah HE?- -i^ red reef ht - semi -crystalline with much seccndari reJcak'lt and ara^mte in MrjeidaJmssrs red crjstalline ht white chalky conglomeritic lit with molluscs and corals moliuscan launa soft chilly reef M. with byanchina corals ixkmathcn rat soft white shaly 1st -neathei nitttkd red - miluscan fau pebble bed- fra<imental oyster shflis -Cstrfa spfjoU bed^nlih pSisfiedpc-oiM-^ 'Ihitk •fij:3-'><vi^ sandii calcareous shale vtith S>S&£A fra-jments of marl rB^S'lSl red sandy c/au red sand If cky .San Sebastian shale not exposed MG iLMim T FIGURES 1 TO 6 CORRESPOND WITH FIGURES I TO 6 ON PLATE I ^ S^H^P EIIZI red ht and red 1/ mesand g thm,irre<jukrbeds cf g Ksicuhr, crfstaUine red ht n^lternate redend -.Mte 1st '^ x»K/Jf irreaukr and Hmesm ]j nassive, pin/i reef At. - ceierrtous , rvfossih CjycijTi white co/or, medium Ksra, Jmcnstailim treat/wrs teabuffccicr aifernaie hardarfdsoft beds soft w/th kcal hard beds with abundant Pecten she/Is ft'irdoini', and wliite-von ccIoTe abundant, - /ock weathers reddish -contaifis seme ccrcls ' Salanopofl/ia beds xto 10 feet tfiick Chione urcodwcrdi s/i^iitiy caifernous rznzT 5^Q rZT izr I~E - - , - - -S, pinfi afid w/iite 1st.- small molluscs red cri'stallirie 1st itith tein,^ iakit.e apd ncdu/a,r coral 'heads '^rtliTlT>&saLu,iSni fihite chalky ht narl with Corns, N.^/ica and Tnarf nith knsos or pockets of lit r- f'ecton.Cardium, TUrritelia , Butla ria arqillaceous Ist-Cpterkds marl with beds of white marl vnith Oyster lieds white chalk with small mdiuscs '' %'^cn^?^iis weathered sh -friable calcareous bbcky fracture, -smallpekcypods coral zove - marl mth large Twssive corals - Ostrea Ftcten zone ofpementia and mrritella cysltrb-jd ioft w/iite, meothered ht cgkarecus sh. and marl Tcssiliferous fmy sh and maff >lap sh- sandy le calcareous mth thin seams of lignite blue-qray to qrten sh ^ra<i sh red , sandy sh pebble beds, each about 1 foot thick 24 'k'^-liA'^ hrotrn sandy sh mth ptitL fj-ifijfai ccngkmeralt -13 ft exposed I—- f j ^ji^j, „^,^ cipescd beds WllG 3 JPILAT m ^^s CO rza 53 ^EJ=^ S^Z^ ClZl :S^; zzr izr riE rrr S^ rTTT^ rzE Ezniii g^ s^ rzzizrr eES: r*&sWM hmesand and chalk - femmiwfera- massive reef ht 1st. conglomerate ^itropods and corals hard, pink, cotennis,- veimttcaldte *""§^» noHuican skelts Metis trintar,a, dilaruphyriia sp. plnli to nkte ht.-unfoiSiliferoai same (osnk an at sta. SZ(a) bmnchiiiftjipe corals, reiemUnt those in the Lares 1st ' caferrtom reef Iit. bed, / to ,0 feet thick Fiden sp , Strcmbui sp. soft, nhite, nodular 1st - fossils viarl with bedi ot Ostrea ip- 7r:olhscttn fauna -chiefIf pekci)pods(<tOf>l soft. ^Iiite, chaihj, nedylar Osti^a. fx ipcc.fS),iini>, iP , and numeniis n.t tK [© 4 Cpiif coral zffnt - skihlit m/h -- bridge -Lares read , ara'd-qrecii marl- mamve coiui 'top it is! fdHs jra^-qrccn marl «- top led. lolh^ - 10 ft marl bed ,_ CTener.tia darieriazone ca^wonacec Hi lmh mitn i» •;.,.' seems, fcr.v kaves, f-rcsli titer carbonaceous shale gra^ marl nith Ostrea ^ ' colfai, btse net exposed E^ K^ ra m JZL rnH _.xi: X-i ir^ =c^ XZI tathtred lit un^hmeratt pink to wiite ht tilth large Orthaulat stielh mmsin white chalk a.'terfiittiiif Kith tttrdfink be<i% -unhiUliletcia except for sniill pekcypoi thin bedded ht hard, cavernous beds I foot thick (average) thin bedded ht chalkf fviite ht with niimiltnt Echinclatnpas sp ariii nolhscan skells, cfi.'eriv fictei and inlerntl mads of ^strepods Orkitoid foraminilera hafi SoH,m4iiive,m/iite *eds aiteriutii^ with thin beds of cciiutiia made up chiefIf of Often and Ostrea caarneus, iitii piiik cakite fiilinf picten and Strqmtus abundtnt ka'i, covernous wMe or pmk iicds alternatino mth salt ee/fta/ish ist untiur.lnii feiji-like misses cf v^kite corals - massive colonial tipe- 2 feel in diameter nodular white Ist wm numerous branckinf type corals '''o/heroir""* Teredo incrassata - crtistcceans red hmesand sendst, carbonaceous sfi seems sh. wdh intercalated 1st fossiliferous sanist 5S Ballaria ^favosa rTicHkd jelloM and vtMe Tfidish Hfllon - «!!<(« - MiwrnoKi »i,t>i vnmscin fauna nd U conqkm. - nulluKan ftunt altfrnate -Imd^fink anj ult.feHc* WHiJiVf tt irrfplttr hedS i'tDW' c:ruls - tetf buUirt^ t,pi:'$ hard cawrmu! - viitis larae Orthaulat shills ye/Zow uverncus Ist - mlliscs ,ilV'S- I'ihijrlhw caternous utlhu calartou, 'e^creaa-cohrei reef Ist f;hiKidloxe-ib;;tidant [durnhmpal icds Kuh ctuvdant ficten tktili wA/f» chclk^ Ist mth Ostreain •xtremely cavernous reef Ist i !— zone of k/oklt weathered ht marMinqpelMof submergence of Rio camuj - massive ht with brancHiru type corals wm (B white Ist -weatners ycllowhcm wry fossiliferous - Corals, Notica, Cstrea, shell froipnefts fcraminifera qrmi-blue-ma^'^'^'''"^' red undgray chj/s HUBBARD, GEOLOGY OF THE LARES DISTRICT 43 cavernous^ white limestone^ with scattered portions of thin bedded, hard, or chalky limestone. As a whole, the Lares limestone is of shell and foraminiferal origin, although many massive, reef-like portions are made np in large part of corals. The coral limestone facies is so intimately mixed with the well bedded portions that it is not possible to estimate in what proportion the formation is of coral origin. However, the limestone should be classed as a reef formation. It shows the lateral gradation from stratified to massive structure, characteristic of reef formations in general. The massive portions are either largely of coral origin or else made up chiefly of foraminifera {Lepidocyclina and others) and micro- scopic shell fragments. In most localities, this massive rock is hard, compact, and partially crystalline, and extremely cavernous. The thin bedded portions are chiefly chalky and soft, or alternate hard and soft layers, many of which are yellowish from limonite stains. These strata are not true chalk, but finely ground limestone, practically a rock flour, derived probably from the adjacent reef structures. Most of the mol- luscan shells occur in these well stratified portions of the formation. So far the description applies to the eastern part of the district. From the Eio Guajataca westward there is a change in facies. The white lime- stone, with its massive exposures and pepino hills, grades into a softer, more argillaceous limestone. The topographic expression of this differ- ence in character of the rock is brought out in the geologic map. In the Hato arriba section, west of San Sebastian, the formation consists of alternating beds of white chalky limestone, argillaceous limestone, and marls with abundant oyster shells, (0. virginica). Still farther west, in the vicinity of Moca, the San Sebastian shale is missing, and the Lares formation is the basal member. It might easily be mistaken for the San Sebastian shale in this locality, because of its lithologic similarity, as shown in the columnar section (Plate II). The presence of typical Lares fossils, such as Cardium cinderellce alternatum and Pecten grabauh and the absence of nearly all of the most characteristic San Sebastian species, is sufficient evidence to show that these basal beds are equivalent to the Lares limestone of the eastern part of the district. South of Aguadilla, the formation is largely buried by the recent playa deposits of the Eio Culebrinas. However, the Lares formation occurs southwest of the Eio Culebrinas in a narrow coastal belt extending to Pt. Jiguero. Through- out this area the rock is a limestone, grading from soft white or yellowish chalky material to a hard, semi-crystalline limestone, predominantly white, but locally red in color. At the base of the formation, a small thickness of shale or gravel beds is present in most of the localities. At 44 SCIENTIFIC SURVEY OF PORTO RICO Pt. Jiguero, however, the limestone . apparently rests directly upon the upturned Cretaceous beds, though the actual contact is not exposed to view. The contact with the San Sebastian shale has already been described as conformable. Similarly the contact with the overlying Cibao lime- stone is a conformable, somewhat gradational contact, though very defi- nitely marked topographically. There is no evidence of a discordance of dip, of an erosion interval, or of a faunal hiatus between the Lares for- mation and Ciboa limestone. The same was shown to be true of the contact with the San Sebastian shale. The position of the Lares as basal formation in the west shows a progressive overlap during a gradual sub- mergence of the oldland. N"o other interpretation of the above data is possible. THE CIBAO LIMESTONE This name is taken from the barrio of Cibao, north of Lares, where the formation is best developed. In the type locality (barrio of Cibao), the Cibao is essentially a soft, white, chalky limestone with an abundant but poorly preserved molluscan fauna. Interbedded with this white chalky limestone are 1. Beds of marl with abundant oyster shells (0. sella'formis porto- ricoensis) 2. Beds of hard pink or white limestone with LepidocycUna and Orbit olites. The predominant softness of the material explains the rolling prairie lowland developed on this belt. The high content of argillaceous ma- terial in the rock is well shown by the extensive covering of red and black residual clay soils. From its maximum thickness in the barrio of Cibao, the formation thins westward, and south of Aguadilla it is only 300 feet thick. The character of the rock is different in this locality, the white chalky facies containing a greater abundance of hard intercalated beds, many of which are red in color. E"o evidence was found of an unconformity, discon- formity or faunal hiatus between the Cibao and the overlying formation. It is not possible to distinguish zones w^ithin the Cibao limestone which continue laterally throughout the formation. A white chalky zone, strati- graphically near the top, contains an abundance of an echinoid (Echino- lanipus resembling E. aldrichi). This echinoid zone can be traced from the Rio Camuy westward for at least six miles. IIVBBARIK (JFJyUKiY OF THE LAKHS DISTKICT tii:e H).s ih:eri:os limkstoxe The uaiiie is taken IVrtm tiie barrio of Loh Puertos, jiortli of Li: vliere the >-\ iii-:;^"' I '^^ - :— is m ^^'.>*^h:'M^% Fio. Irt.^^^^^^L-/.^- P((«-f'>«,- f/wes^-ne. wfl.s.s/re reef fmw I'Isposed in rei'ilcal clitf, tn-uiiy 150 feet IUkIi, and in wliieli tliere Is i The rock is prerioiiiiiuiiitlv a series of massive, reef-hke hoih, alternat- ing with thill betldecl etialky st^rata. It is a reef formation essentially 46 SCIEXTIFW SUBVEY OF PORTO RICO like the Lares limestone, but with pepino hills developed on a greater scale with greater relief. Excellent exposures of this formation may be seen in the canyons of the Eio Camuy and Eio Gnajataea (Fig. 19). The massive reef-like portions are hard, semi-crystalline, and very cav- ernous. The prevailing color is pink. Fossils are not well preserved and are chiefly small forms, including foraminifera {OrhitoUtes and Lepi- docyclina). The thin bedded portions of the formation are soft chalky strata^ alternating with harder strata. The soft layers are in many places stained with limonite, giving a varied coloring to the canyon walls. These thin strata are in some localities intercalated with other soft chalky beds containing angular fragments of hard reef-type limestone. The chalky beds are iisnally deeply weathered and contain very few fossils preserved well enough to recognize. Corals occur in the massive zones, but apparently for only a fraction of one percent of the entire formation. Foraminifera and molluscs are the chief contributors. The Los Puertos limestone is entirely conformable with the overlying Quebradillas limestone, and shows no faunal hiatus. JSTo zones, either fossil or lithologic, can be estabHshed in the Los Puertos limestone. Ex- cept for the variations above noted, the formation is lithologically a unit, just as it is a physiographic unit, with its distinctive belt of pepino hills. The columnar sections (Plate II) illustrate the differences found. In the west coast section, the formation contains a great deal of the hard, red, semi-crystalline limestone which is characteristic of portions of all the limestone formations of the Arecibo group in this part of the district. It will be noted (Plates I and II) that the Los Puertos limestone, while variable in thickness, does not thin to the west, as do the underlying formations. THE QUEBEADILLAS LIMESTOiN^E As shown in Table 3, the name was first proposed by Berkey. It is named after the town of Quebradillas on iha north coast, where the best exposures of the formation are to be found. The type fossil locality is near Quebradillas at the mouth of the Eio Quajataca (Fig. 20). This formation consists for the most part of a hard cavernous reef limestone which has a flinty appearance. It is made up chiefly of minute shell fragments (molluscan and foraminiferal) and may be in part chemically deposted lime carbonate. A few corals occur, but these are not of the reef building type. In places, the formation is well bedded, with the hard flinty limestone alternating with soft white chalky limestone or with layers of limestone breccia. Fossils occur in all of the beds but HPHBAHn, UfJOLOdY OF Till': LARES DJi^TRICT 47 are iicvor foiiiid abiiiulaiitlv (!XC(»pt in Hmall areas or "|}(>ckeiH."^ These "por'kot.s*' are uiimoriraK, njid do jiot all ecmtaiii the mmc S|)(!ei(>s. .Some of liiem are extremely crowded willi iiiolhiseaii shells. Almost invariably the shell rtnietnres have heeu dissolved away, leaving only the exfeiMial and internal molds. Tliis eliaraeteristie, together witli the general aspect of the rock, is so tyijieal of tlie Qnebradillas liiuestoiie that it affords a relialile naean^s oi recognising the furiiiation in tlie field. A tyjdeal exposure of the hard fossilifei'oiis beds is shown in figure 20, which also ilhistrates the massive character. Other phases of tlie fomia- '. QuebraiUllm Imv.rtMie at the li/j.e fon-sll Inealitp d chM: entrance of Ameiicai) rainHnid tunnel. Month Oiiajntaca. ivest of OuebmdiJlas. tioii of le»eal extent o<'eiir at Isabela and on Pt. Borincpien. The sea, cliffs at IsiilMda are made up in large part (jf a thin l>ed(led red limestone, ali.ernating wdtli thin he<ls of indiirjitfid red lime sand, the entires exj»osiirc totalling iriu fed thick. The rock is largely fragmeiital in origin, rela- tively free from argilhiccons ma,tter, like all the rest of the formation, and abnost inifossiliferoiis. On Pt. Pnvrinfjiien, i.n the sea cliffs norl beast of the Uglithonse, is a UO-fiiot exposiirc; (d* a w^hite to yellowish elialky limestone in which, are interbedded vast iimnhers of large oyster shelLs (0. antiffiienKis). well shown in iigiire 21. This zone of 0. antignetms can be traced a,s far east as l<af)ela. and apjiarently occurs in the San, Jiain, District. The Qiiehrailillns limostuiu' is cojiroriiiiilile upon tiii^' nndtMiyii),! PiiertOH liiin'stoiie aiul the buiUH.hiry hetwccu the twd i.^ somewhat trary. Thf QiU'bradilla.s is iinieh more iinit'orm in tiiifkiic.ss t\\i\ l<nvor fomiatiuiis (see sociioiis, I'latos \ and 11). It is {\xv ii,o>t ^mM\(] Tertiary formfrtion in Porto Kico, and it jnarks the final dt tion of a iransgreBsiiig sea at th.e imviml of maxinniin Hiibinergeiift'. Qiiel)radiJla.« lime.stoiie rests uj:h)11 Oetaeeous roe-kn in the San diian trJet find on Yieques Islarnl and shows elear evi<lenee of a prouTf HUBBARD, GEOLOGY OF THE LARE^ DISTRICT 49 Like the lower formations, the Quel)ra(li]las limestone is not readily divisible into zones or horizons, as sm inspection of the columnar sections (Plate II) will show. Lithologically, tliere can be no sulidivision. From a study of the distribution of the fossils, howe\'er, two zones oE ratlier doubtful validit}^ may be distinguished : 1. Upper Zone, including uppermost 2()() to 300 feet, characterized by extremely abundant molluscan sliells (Bowden fauna) and 0. antiguensis. 2. Lower Zone, marked by relative scarcity of fossils and presence of Oi^J)ifolites mixed with Bowden fossils. >SrMMAirY OF FOHMATIONS Table 4 (page 50) gives briefly the general make-up of the Tertiary of the Lares District, with a few of the important features l)y which the different formations are recognized. STRrCTUKE It has already been pointed out that the Tertiary formations are a structural unit. There are lU) breaks anywliere in the series. Altliough there is no discordance of dip denoting an unconformity, it will l)e no- ticed from the dips recorded on the cross-sec^tions (Plate I, Figs. 3-6) that those of the San Sebastian shale are somewhat erratic and in most cases at higlier angles than tliose of the overlying formations. The sec- tions further show that there is, on the whole, a gradual decrease in dip as one passes from the base to the top of the series. Thus, while the average dip of all formations is 4°, the San Sebastian averages over ()° and the Quebradillas between 2° and 3°. The erratic dips recorded in the San Sebastian shale are due in most cases to cross-bed diiig wliich cannot always be distinguished as such in the field. Frequently abnormal dips are noted in the limestone formations, l)ut these were almost in- variably found to be due to slumping resulting from underground solu- tion. Such abnormal dips are in every case very local in extent. From the San Juan District west to Lares, the normal dip of the beds is to tlie north, but from Lares to the west coast it is to tlie northwest. The nearer the west coast the more westerly the direction of dip. The signifi- cance of this will be takn up later in discussing the Tertiary In'story. Except for the gentle warping and local slumping ahove noted, no faults, folds or other structural features w^ere observed anywhere in the Tertiary fonnations of the district. There are no dikes or other evi- dences of igneous action in any of tlie Tertiary rocks, hence all volcanic activity had died out before middle Oligocene time. HO sH CO Pi <3 H -d P5 P5g 03 CC WO OMH s" g^ 55 low s or m . ^ y^ w «5 s?i. J3 w Pu 0) -^ ^ X J^'S '^ 2 I- r t^ o " • ;-( !> ^ 02 0)0 -M •^^ i ft '^^ ^^ a E o^.S '^ ® ^ i^ g a §So c3 ^ ^ .1:5 •^ O ^ K' xll a^Ij A (^ cd 2 :C' O) cS .9 c ^ oJ ^ S O) c3 5J -t^ .!m J» « c rC3 P hm rc! ^ OJ OJ ;-( fl <D a; ^ o ft a; 5C . ks ;^ r^ TO . g '^ a; ^ <^ ;S ^ *S ^ o ft'-^ bJD M 3 'r^ ^ p:3 S. ^ >* ^ t "S^ ;^IS 0) 0^oo P 5/} OP "lU §0 53 5^ 'TS c>S 4~> ^ ft fIJ a> sa :-i 5 1 •^ m ^ 55 m Ci'^ 0) g .^ 0) 3o o ^ h:3 go S'^ SO X5 ;=J P>i a; a? §2 ^ 5::^ Oi cd O O) *-i g =j P Cj cj ^ C) ^ a; ^r ft-^r^^ c3 t2 ti].S fl-^ ^ a; n3 S 0, ^ Qi ?^ <^ .i^ '« g •^ 5^ •-n. s s ^ ;s ;^ ?*! ^^ S? ^ ?= 8' C3 ^ •r^ o p 2 Oi 0) S.-S ^ ^ fl § =^ cd S O h5 'HO anoHO oeioanv aA HUBBARD, GEOLOGY OF THE LARES DISTRICT 51 Age and Correlation The various speculations and conclusions regarding the age of the north coast Tertiary formations has been shown in Table 3. The diver- sity of opinion is explained by the fact that not until the summer of 1919 has a detailed study of the fossils been made. A large collection of fossils made by Dr. C. A. Eeeds in 1914 was described by Maury (1919), and her conclusions regarding the age of the formations are given in Table 3. During the survey of the Lares District in 1916, the writer made a very large collection of Tertiary fossils, all of which were recorded as accurately as possible as to locality and stratigraphic position. These fossils were studied during the summer of 1919, and the final conclusions regarding . the age of the formations based on this study are shown in Table 3. It will be noted that the writer's conclusions agree with those of Dr. Maury with the exception of the Miocene or Oligocene age of the Quebradillas limestone. Dr. Semmes and the writer recognized in 1917 that the Quebradillas limestone is approximately equivalent in age to the Bowden marl of Jamaica. The remaining point at issue is whether the Bowden is Upper Oligocene or Lower Miocene. Evidence will be pre- sented to show that it is Upper Oligocene. About nine-tenths of all the fossils collected are molluscs.^ Of these, 140 species or varieties have been described or recorded by the writer (Hubbard, 1921). A study of these fossils by horizons brings out some interesting results. In figure 22, the stratigraphic range of the fauna of each formation is shown graphically, the ordinate, or height of the curve at each formational horizon from left to right, denoting the total number of species of the fauna appearing in that horizon. Thus the curve of the Los Puertos fauna shows that a total of 17 species were recorded from the Los Puertos limestone. Of these, only 1 was present in the San Sebastian shale, and about 13 survived in the Upper Quebradillas limestone. These faunal curves illustrate the following facts 1. All the faunas are transitional. There is no hiatus anywhere in the series. This corroborates the stratigraphic evidence. 2. The faunas of the San Sebastian shale. Lares formation, and Que- bradillas limestone are somewhat distinctive. The faunas of the other two formations are mixed or transitional. 3. The San Sebastian fauna is almost wholly distinct from the Que- bradillas fauna, and had almost died out in this region at the close of Middle Oligocene (Cibao) time. 3 Wherever the term "fauna" is used in the present paper it applies to the moUuscan element, except where noted otherwise. 52 ^ClEyTIFlC t^URVEY OF I'ORTO RICO 4. The Lares fauna eoiitaiiis three elements: {a) Speeies limited to the Lares formation, about 40 pereeiit of the total, {h) Speeies left over from the San KSebastian. (c) A few speeies typieal of the Quebradillas, markiiior the be^i^imiino^ of the invasion of the Bowden fauna. Pig. 22. Graphic representation of the range of the Tertiary faunas The figures at the left show the number of species. 5. The Cibao fauna has more in eommon with tlie Lares than with the Los Puertos, and hence the Cibao limestone should be grouped with the two lower formations in the Middle Oligocene. 6. The Los Puertos fauna is most like the Quebradillas fauna, and properly belongs in the Upper Oligocene. It marks a further increase of the invading Bowden species. HUBBARD, aKOLOGY OF THE LARE^ DISTRICT 53 7. Ill general, tlie invasion of tlie region by new species was a more sudden process than the decline of the species already established. The only exception to this is the early appearance and gradual increase in number of a portion of the Bowden (or Quebradillas) fauna. The ap- pearance of the bulk of the Bowden fauna, however, was relatively sud- den. The term Bowden fauna, as used here, includes that characteristic group of molluscan types common to the Bowden marl of Jamaica and the Aphera-Sconsia formations of Santo Domingo. 8. It should be pointed out that a further collection and determination of species from these formations would undoubtedly alter the curves given above. (3wing to the poor state of preservation of fossils in the Cibao and Los Puertos limestones, only a small percentage of their faunas could be determined and recorded. Further work in the Los Puertos limestone will probably increase the number of known Quebradillas (or Bowden) species in that formation, and show that the appearance of the Quebra- dillas fauna was not so sudden as the curve indicates. 9. Tlie decline of the San Sebastian fauna was due to the change in bionomic conditions. ]\Iany of the San Sebastian species arc brackish water forms, none of which survived the (^hauge to pure marine condi- tions, but some of which survived through Lares time. The latter are found chiefly in tlie western part of the district, where the Lares is to a large extent a brackish water or near shore deposit. TRE SAX SEBABTTAX FAUNA The following is a list of the molluscs found in the San Sebastian sliale and described by the writer (Hubbard, 1921). Those forms wliich do not occur above the San Sebastian shale are indicated by an asterisk: "^ Pecten (Pccten) larcseusc ii. sp. * Feeten (Chlamijs) coJlazoenms ii. sp. ^ Peeten {Chlamys) porioricoohns n. sp. Pc'ctcii (Chlaniifs) portoricoensiH var. retivulatu. Pecten {(Jhlamifs) portorlafcnms var. grand i"^. * Pccten {Plagiocteniirni) ra belli n. sp. * Amusium papyraccum Gabb. * Amusium mauryi n. sp. * AmuKium (Propcamusium) hoUicki Maury. ^^po 1idy I u s b os tryehi t es G u pj )y l^pondyJus giimanomacon B. and P. Ostrea collazica Maury. "^ Area dariensis B. and P. * Area sp. iiidet. (u. sp.). Area {^eapliarea) eollaziea Maury. 54 SCIENTIFIC SURVEY OF PORTO RICO * Glyeimeris collnsocnsis n. sp. * Lucina collazoensis n. sp. * Phacoidcs (Pseudomiltha) laresensis ii. si). * dementia dariena Conrad. Pitana {Hyphantosoma) carhasea Guppy. * Solen (Plectosolen) collazoensis. Corhula collazica Maury. Teredo incrassaia Gabb. Dentalium sp. indet. "^ Neritina {chipolana var.?) collazoensis ii. sp. * Crucibulum {Dispotaea) collazum n. sp. * Natica (Ampulinaf) collazoensis n. sp. * Epitonium (Cirsotrema) collazoensis n. sp. Turritella tornata var. portoricoensis ii. var. * TurHtella planigyrata Guppy. * Turritella initchelli n. sp. * Petaloco^whusf collazoensis n. sp. Cerithium {Campanile) collazum n. sp. * Cypraca sancti-sehastiani Maury. * Cassis sp. indet. * TurMnella cliipolana var. precursor n. var. Bullaria paupercula Sowerby This list brings out the following important facts : 1. A large part of the fauna consists of new species or new varieties. 2. There are many brackish water and near shore forms. Among these are Neritina, Ampulina, Cerithium (Campanile) and a large number of pelecypods of fresh or brackish water type, most of which are specifically indeterminate and not listed above. The San Sebastian fauna as a whole is unique among the Antillean Tertiary faunas so far described. It has, however, certain elements show- ing relationship with other faunas. The fauna has a very strong resemblance to the Gatun (Upper Oligo- cene). In most cases, however, the species are similar, but not identical. It seems probable that a portion of the Gatun fauna was derived from the San Sebastian fauna. The following is a list of the San Sebastian species with their Gatun analogues : San Sebastian. Gatun. 1. Olycimens collazoensis n. sp. Glycimesis gatunensis Toula. 2. Lucina collazoensis n. sp. Lucina sp., an undescribed shell in the Kemp collection, at Columbia University. 'i. Natica {Ampulina) collazoensis Natica sp. indet. B. and P. n. sp. 4. Turritella planigyrata Guppy. Turritella gatunensis Conr. HUBBARD, GEOLOGY OF THE LARES DISTRICT 55 5. Turritella mitchelU n. sp. Tiirritella altilira Conr. 6. Spondylus gumanomocon B. and P. Spondylus scotti B. and P. 7. Afnusiuni mauryi n. sp. Amusium luna B. and P. 8. Area daricnsis B. and P. Area dariensis B. and P. 9. dementia dariena Conrad. Clementia dariena Conr. There are many forms showing a general Flint Eiver aspect. Many of the indeterminate fossils found in the San Sebastian^ and not listed or described in the present report^ show that the fauna resembles the Flint Eiver Oligocene fauna more closely than the species described would indicate. The two most notable Flint Eiver types are San Sebastian. Flint River, Ga. 1. Turritella mitehelU n. sp. Turitella halensis Dall. 2. Cerithium {Campanile) eolla^^um Cerithimn halensis Dall. n. sp. There is a fairly large representation of Bowden and Aphera-Sconsia species, which migrated from Porto Eico before Bowden time, since they do not occur in the Quebradillas fauna. These are San Sebastian Shale. Other Occurrences. 1. Amusium papyraeeum Gabb. St. Domingo, Bowden. 2. Spondylus hostryehites Guppy. St. Domingo, Bowden, Tampa, Chl- pola, Anguilla. 3. Spondylus gumanomoeon B. and P. St. Domingo. 4. Pitaria earbasea Guppy. St. Domingo, Bowden. 5. Turritella tornata var. St. Domingo (type), Cumana. 0. Turritella planigyrata Guppy. St. Domingo, Caroni Ser., Gatun. Chipolan. 1. Turhinella ehipolana var. proeursor n. var. 2. Neritina {Chipolana var.?) eollazoensis n. sp. The variety of T. chipolana is more primitive than the Chipolan shell, and hence supports the evidence that tlie San Sebastian shale is older than the Chipola formation. The Eocene aspect is only superficial, and there are actually few fossils closely related to American Eocene species. Solen (Plectosohn) colla- zoensis is strongly suggestive of Eocene species, such as S. plagiaulax Cossmann, 8. laversinensis Lef. and Wat., and S. obliquus Sowerby, all from the Paris Basin. Corhula coUazica Maury and Ostrea collazica Maury (especially the small variety with regular, divaricate ribs) might be considered to have an Eocene aspect. Ceritliium {Campanile) colla- zum resembles some of the large Cerite shells from the Paris Basin Eo- 56 SCIENTIFIC SURVEY OF PORTO RICO eeiie, but is really niucli closer to the Flint Iliver Oligoeene forms, as |)reviously pointed out. The possible European connections of Solen col/azoensis and Cerifhuun coUaziim have been referred to. One of the most interesting shells found in the San Sebastian shale is a Cirsotrema, described as Epiioniuin {Cir- sotrema) coUazoensis n. sp. Apparently, its nearest relatives are found in the Paris Basin. It is closest to C. ^iil)spinosum (Grat.) of the Aqui- tanian and ('. bourgeoisi de Boury of the Helvetian. In Europe, the genus Cirsoirema ranges from the Eocene to the Pliocene, but those of the peculiar type represented by the above-named species are limited to the later Tertiary, and are particularly characteristic of the Aquitanian and Helvetian of the Paris Basin region. In the discussion of the stratigraphy, it was noted that the San Sebas- tian shale is divisible into three zones 1. Upper or Coral Zone (Zone C of Vaughan), 150'-200\ 2. Middle or Clementia dariena Zone, 50'. 3. Lower Zone, lignitic clays and gravels. The corals are confined to the Upper Zone, and many of these same corals are found in the lower part of the Lares limestone. Vaughan (1919) lists the following species from the Pepino Formation, giving the other occurrences of each species 1. Afitrocoenia pcirioriccnMS Vaughan, Antigua, and Canal Zone. 2. OrhiceUa costata (Duncan), Antigua, Anguilla, Canal Zone. o. AntiguaHttea celiulosa (Duncan), xVntigua, Florida, Georgia, etc. 4. Meandra portoru crisis Vaughan. 5. Lcptoscris portoriccnsis Vaughan. n. I'ironastrca anguiJlensis Vaughan, Anguilla. 7. Side rastrca c<mferta (Duncan), Antigua, Canal Zone, Anguilla. 8. Cyathomurpha antigiiensis (Duncan), Antigua, Cuha, Mexico. 0. Cj/dtJiomorpha tenuis (Duncan), xVntigua, Cuba. 10. Diploastrca vrassolamellata (Duncan), Antigua, Cuba, (jJeorgia. 11. .1 strcopora portoricensis Vaughan. 12. Goniopota portoricensis Vaughan, Antigua. These corals are from the collections made by K. T. Hill, and a few collected by the writer. They are probably all from Zone C or Upper San Sebastian shale, with possibly a few from the basal Lares limestone. Vaughan points out that 8 of the 12 species occur in the Middle Oligo- eene of Antigua, and he concludes that this portion of the Pepino For- mation is of Middle Oligoeene age. At Collazo, near the base of the San Sebastian shale, occurs a small ecliinoid Scutella resembling S. fnwsissippjiensis Twitchell of the Clai- HUBBARD, GEOLOGY OF THE LARES DISTRICT 57 borne, but apparently a new species. AVhile it resembles the Eocene spe- cies superficially, it also resembles some of the small ScuteUas from the Pacific coast Miocene, and hence does not indicate an early Tertiary age for the basal portions of the San Sebastian shale. The following foraminifera were found in the San Sebastian shale : 1. OrJdtolifcs cf. americana Cushman, Zones 2 and 8. 2. Polystomclla sp., Zone 3. 3. Leindocyclma (Heveral species), Zones 2 and 3. Of these, the first is by far the most abundant, and is found in all of the Tertiary formations of tlie Lares District. It is very similar in size and structure to 0. americana Cushman, from tha Culebra and Em- perador formations of the Canal Zone. Lepidocyclina is also abundant. On the whole, these foraminifera indicate tlie Oligocene age of the San Sebastian shale, but they will require further study before anything more definite can be said. Conclusion. The corals listed by Vaughan are undoubtedly conclusive evidence of the Antiguan, or Middle Oligocene age of the San Sebastian shale. The evidence of the other kinds of fossils is not so conclusive, but does not disagree with the evidence of tlie corals. Tlie molluscan fauna, while somewhat mixed in its aspect, shows that the San Sebastian shale cannot be as old as Eocene. It is more nearly similar to the recognized Upper Oligocene than to the Lower Oligocene faunas. THE LARES FAUNA The following is a list of the molluscan fossils found in the Lares formation :* Pecten (Chlamjjs) portoricoensis n. sp. Peetcii (Ckkwii/s) portoricoensis var. reticulatis. Pecten (Chlamijs) portoricoensis var. grandis. "^Pecten (Chlami/s) grahaui n. sp. '^Pecten {Chlamijs) gralxiui yhy. aguddensis. Pecten {Chlamys) grahaui var. haioensis. Pecten {ChlcDriys) grahaui var. guagahensis. * Pecten (Aequipeeten) lohecki n. sp. * Pecten {Plagiocteniuni) cercadica Maury. Ostrea haitensis Sowerby. Ostrea seUaeformis var. portoncoensis n. var. Ostrea collazica Maury. * Ostrea virginica, Gnielin. Ostrea cahohasensis var. portoricana n. var. ^ Index of the Lares formation is indicated by an asterisk. 58 SCIENTIFIC SURVEY OF PORTO RICO * Lithophaga nigra d'Orbigny. Area yaquensisf Maury. Area (Scapharca) collasica Maury. * Venericardia scabricostataf Guppy. . Phacoides {Miliha) riocancnsis Maury. Cardium (Laevicardiiim) cf. serratum Linne. Curdium {Trachycardiiim) muricoides n. sp. * Cardium {Trachycardium) cvyiderelUe var. alternatum n. var. Pitaria {Hyphantosoma) carbasea Guppy. Chione woodwardi Guppy. CJiione hendersoni Dall. * Semele sp. indet. Cyathodonta reedsi Maury. Corhula coUazica Maury. Teredo incrassata Gabb. Xetiophora conehyliophora Born. Turritella tornata var. portoricoensis n. var. Cerithium {Campanile) collazum n. sp. * Xancus n. sp. ? * Mitra symmetrica Gabb. . Bullaria granosa Sowerby. Ill addition to these species^ many indeterminate fossil?^ chiefly gastro- pods, were collected. Some of these are apparently limited to the Lares formation. The presence of some of the Antiguan corals in the basal Lares limestone indicated that the formation is Middle Oligocene in age. Other fossils indicating Oligocene age are 1. Agassizia sp., a small species resembling A, conradi (Bouve), Oli- gocene of Bainbridge, Georgia. It is apparently limited to the basal portion of the Lares limestone. 2. Orbitoid foraminifera, including OrUtolites cf. americana (Culebra and Emperador), and Lepidocyclina cf. mantelli, a large flat form. The same species occurs abundantly in the Juana Diaz shale and basal Ponce formation of the south coast of Porto Eico, and is listed by Mitchell (1922) as Orhitoides maritelli. THE CIBAO FAUNA The following list represents probably not more than a third of the molluscan species in this formation. JSTearly all of the material is poorly preserved and largely indeterminate Pecten (Envola) reliquus var. portoricoensis n. var. Peeten (Chlamys) portoricoensis var. grandis. Pecten (Chamys) grahaui var. hatoensis. Pecten iChlamys) grahaui var. guayahensis. Spondylus hostrychites Guppy. HUBBARD, GEOLOGY OF THE LARES DISTRICT 59 Spondylus gumanomocon Brown and Pilsbry. Ostrea haitensis Sowerby. Ostrea haitensis^ var.? Ostrea sellaeformis var. portoricoensis n. var. Area sp. Maury. Cardium cf. serratum Lirme. Pitaria (Ilyphantosoma) Carhasea Guppy. Ghione Jiendersoni Dall. Teredo incrassata Gabb. Hipponyx poriorieoensis n. sp. This list illustrates clearly the mixed or transitional character of the molluscan fauna. The only distinctive index fossil of the Cibao limestone is an echinoid, Echinolampas sp., which is very similar to E. aldrichi Twitchell of the Vicksburg Oligocene. This fossil is found! in great abundance in the upper portion of the formation, and the zone in which it occurs can be traced from the Kio Camuy westward for a distance of 5 or 6 miles. The basis for including the Cibao limestone in the Middle Oligocene has already been given in discussing the faunal curve (Fig. 22). THE LOS PUERTOS FAUNA A comparison of the following list of the Los Puertos fossils with the list of the Quebradillas species will show the close relationship of the two faunas Ostrea sellaeformis var. portoricoensis n. var. Glyeimeris portoricoensis n. sp. Lucina cf. clirysostoma (Meuschen) I*hilippi. Phacoides {Miltha) riocanensis Maury. Cardium muricoides n. sp. Chione woodA^ardi Guppy. Chione hendersoni Dall. Metis trinitaria Dall. Teredo incrassata Gabb. Xenophora conchyliophora Born. Hipponyx portoricoensis n. sp. Orthaulax portoricoensis n. sp. The poor state of preservation, as previously noted, is the reason for the small number of species listed. Future work should increase the total of Bowden types in this formation, but the above list is sufficient to determine the age. Orhitolites cf. americana is the most abiindant fossil throughout the formation and in the lower part of the Quebradillas lime- stone. ^ Index of the Cibao Hmestone. 60 SCIENTIFIC SURVEY OF PORTO RICO THE QUEBRADILLAS FAUNA The following list includes all the species identified and described, but probably represents less than half of the entire molluscan fauna of this formation. Those forms which are an index of the Quebradillas lime- stone are designated by an asterisk * Atrina rabelU ii. sp. Pectcn {Euvola) reliqtius var. porforicoensls ii. var. * Feeten {Nodipccten) nodosus IJiine. * Feeten iChlamys) hodgii n. sp. * Fecten {Flagiocteniumf) honnquenense li. sp. * Ostrea antiguefisis Brown. * Leda peUella Dall. Ai'ca yaquensiH Maury. * Area {Seapharea) ef. donaeia Dall. * Barbatia retieulata (imelin. * Barbatia ef. bonaezyi Gabl). Crlyeimeris portorieoensis n, sp. * Chama involuta Guppy. * Chama portoricana u. sp. Lueina ef. chrysostoma (Meuscb.) Pliilii)pi. Fhaeoides (MiliJia) rioeanensis Maury. * Fhaeoides (Miltha) sp. indet. * Fhaeoides {Miltha) sp. iudet. * Fhaeoides (Lueinisea) ealhounensis Dall. * Divarieella prevarieata Guppy. * Codakia magnoliana var. borinquenense ii. var. Cardium ef. serratiim Liune. * Cardium (Trigonoeardia) sambaieuni var. portorieoensis n. var. "^Cardium {Trigonoeardia) n. sp.? aff. C. sambaieuni Maury. * Cardium {Trigonoeardia) haitense var. eereadieum Maury. * Cardium (Trigonoeardia) haitense var. areeiboense, ii. var. * Cytherea {Cytherea) berkeyi n. sp. Chione tvoodwardi Guppy. * Tellina strophoidea n. sp. * Tellina portorieoensis ii. sp. * Tellina (Seissnla) grabaui n. sp. * Tellina aff. T. (Angulns) atossa Dall. Metis trinitaria Dall. * Fsammosolen saneti-dominiei Maury. Cyathodonta reedsi Maury. Teredo inerassata Gabb. * CalUostoma portorieoen^ns u. sp. * Turbo fettkii ii. sp. * Liotia (Arene) coronata var. portorieoenis ii. var. Xenophora conehyliophora Born. Hipponyx portorieoensis ii. sp. HUBBARD, GEOLOGY OF THE LAREH Dim'RlCT 61 * Calj/pfniea cf. ceniraUH Conrad. * CruciJnilum aurivnla var. portoriroensis n. var. * Crucihiihim (luricnlaf var. '^ Natica canrena? (Liiiiie) Moercli. * Turritclla portoricoensis ii. sp. * TurritcJld bcrJrCj/i n. .sp. * J*etalo('onchus dofnim/cnsis Sowerby. * PprdtnideUa portoricocnsis ii. s\). * Bittimn sp. iiidet. * Cerithiirm portoricocnsis ii. s]). * Ccrithium qnebradilk thsis ii. sp. * Modulus 7nodulus var. hasilcns Giippy. * l:<1romJ)U8 proxunns? Sowerhy. * t^tromhuH Mfronsf Sowerhy. * Orthaular gahhif Dall. Orthanhix portoricocnsis ii. sp. * Cf/praea spurcoides? Gab)). * Malea ('(innira (iuppy. * Stromhina portaricana n. sp. "^ J'Jios costat us (Jahh. * Phos elvgans var. portoricocnsis ii. ^'ar. ^ Murcx {PhjiUonotns) cornnrcctns (iii])i)y. "^ Alcctrion guraJnnsis var. portoricocnsis n. var. ^ Alcctrlon gurahcnsis var. varicuni ii. var. * Fusus hcnclcni Sowerby. * TurJyinclla chipoJana var. arccihocnsc n. var. * Xancus valid us Sower) )y. * Mitra hcnckcni Sowerl)y. * OlivclJa nufticoidcs var. poiioricociisis ii. var. * Olivclla porioricocnsis n. sp. * Canccllaria lacvcsccns Guppy. * Turris alhida var. haitcnsis Sowerby. * Turris alhida var. ef. virgo Lamarck. * Drillia consors var. portoricocnsis ii. var. * Drillia grahaui ii. sp. * Drillia portoricocnsis n. sp. * Drillia scmmcsi ii. sp. * Cf/thara ef. donga ta (labl). * Tcrchra que brad illcnsis ii. sp. * Conus catena tus Sowerby. * Conus ef. warginatus Sowerby. Bullaria paupcrcula Sowerby. * Bullaria portoricocnsis n. sp. Bullaria granosa Sowerby. The most striking feature of this list is the large representation of Bowden and Aphera-Seonsia species. The approximate age equivalence of the Quebradillas limestone with Bowden horizons in the West Indies is too evident to require further discussion. 62 SCIENTIFIC SURVEY OF PORTO RICO O OQ < t km CQ ir5 lO 1 CO 1 tH I O I t- 1 1 1 rH 1 rH 1 r- - 1 cr ^ 1 ^ 1 \ 1 ^ j ^^ 1 \<r, 1 r- 1 -^ 1 T- 1 O^ 1 01 1 < 5 ItH 1 tH 1 01 1 CO 1 C i 1 1 1 T-1 1 tH 1 O \ \ \ \ \ I rq I CO j Osl 1 rH 1 Ci I o xn H O OS Ah CO tH lO • tH 1 ^ 1 • 1 (M 1 • 1 • 1 • 1 !M 1 .1 . 1 < -^ ItH 1 • 1 tH 1 C-t) 1 Tt^ 1 iM 1 . 1 -1 • 1 rM 1 (M , /I \ \ \ \ S \ \ \ \ I o CQ tH 1 tH 1 • 1 T}H 1 CC 1 CI 1 CI 1 ; 1 iO 1 CO I • I . 1 < TtH iC1 1 A 1 ' 1 CO 1 -ii 1 1 1 tH 1 • 1 <M 1 • 1 • 1 -~ CQ tH 1 r- • 1 lO 1 CO 1 I- ' -i^ 1 • ! CO 1 00 1 ^ 1 < 1 CO 1 lO 1 QC) 1 01 1 I ^' 1 1 * 1 * ' £5 am CQ } o j CO j »r { t- 1 oc < ( t > 1 1 1 1 1 1 t 1 I CO 11- 1 • 1 (M 1 (M 1 CO 1 01 1 • 1 • 1 Ol 1 rS 1 . 1 ao a= a ii Bm o +^^ II 1.1 ; <J S 53 « a "'a :; o t a -1 2 -a - O) ? ill pi * a c c/a 1 1 JC cdOQ J3O 02 1 * I a 1 ^ \^ 1 ^ 1 ^ 1 ^ 1 H" I =^ <= j j J 1 a i l 1 5 1 ^ 'i 1 ! ^ j -^ I o 1 s; r ^ i 2 1 ^ E 1 ^ 1 ."^ 1 *a ! c 1 e i £ ! ^ 1 0- 1 ^ 1 <^ 1 ^ ! ^ 1 Xf 1 K i g i 3c 1 a i 1 1 fC 1 ."^ 1 *^ ' 1 c: a. D rC 1 C \ i 1 5 ! c 1 H 1 0- 1 c 1 u 1 ^ 1 c - 1 s 1 §fc i S i ^ 1 ^ ' 1 i 1 rH 1 "^ 1 1 1 ^ i fc 1 c 1 c: 1 E 1 t: 1 ;c ' *;- 1 e- 1 ^~' . 1 ^ ! c 1 ^ 1 ^ il HUBBARD, GEOLOGY OF THE LARES DISTRICT 63 o o PG S cdH P^ US O 3 o +J o cu ^ 9 'O fi 12; o 64 SCIENTIFIC SURVEY OF PORTO RICO Foraminifera are abundant and include Orbitolites cf. americana Cush- man (lower Quebradillas) and Polystomellaf sp. (upper Quebradillas). In the uppermost horizon and fragments of Scutella sp. and Cidaris sp. and a Schizasier resembling 8. scherzeri Gabb, a species from the Sapote, Costa Eica and the Emperador limestone, Canal Zone. It also resembles S, floridanus Clark, of the Vicksburg Oligocene. Corals are not common in the Quebradillas, and none of the chief reef-building types occur. Semmes (1919) illustrates a Fungid coral, indeterminate, from the Quebradillas limestone of the San Juan District. This fossil occurs in the Lares District, but is not abundant. The commonest coral in the Quebradillas limestone (and equivalent horizons on the south coast and on Vieques Island) is a Balanophyllia. There seem to be two species (or varieties), each resembling species found in the Bowden or equivalent formations. Another abundant coral is a Stylophora like S. affinis Duncan. SUMMARY OF THE FAUNAS The molluscs, which make up about nine-tenths of the fauna, show that the uppermost Tertiary formation of the Lares District is of Bowden age. The basal member is shown by the corals, and less certainly by the molluscs, to be of Antiguan or Middle Oligocene age. There are only two distinct faunas, the San Sebastian-Lares and the Quebradillas. The others are mixtures of San Sebastian-Lares and Quebradillas (Bowden) types. Table 5 (pages 62-63) is based upon the molluscan species above de- scribed^ and shows the relative numerical distribution of the Lares Dis- trict fossils in other localities and horizons of the West Indies and else- where. It shows the close relationship of the Quebradillas fauna with the Aphera-Sconsia faunas of Santo Domingo. Another feature brought out is the effect of geographical position upon the similarity of the faunas. Thus formations of the same age as the Quebradillas limestone, such as the Gatun formation and the Chipola-Oak Grove series, but situ- ated at considerable distance from Porto Eico, have few identical species in common, but a considerable number of similar species or varieties. CORRELATION WITH THE SOUTH COAST The question of the correlation of the north and south coast Tertiary formations has been awaiting the collection and comparison of fossils. « In the case of the two south coast formations (Juana Diaz and Ponce) the data include other types of fossils besides molluscs. HUBBARD, GEOLOGY OF THE LARES DISTRICT 65 Berkey made no attempt at a correlation because the fossils then collected (1914) did not warrant it. Semmes (1919, p. 59) pointed out that the Ponce formation is ^^in part practically equivalent to the Arecibo forma- tion of the northern coast/^ but he did not make any subdivisions of the south coast series. Maury (1919, p. 214), after a study of fossils col- lected by Eeeds, made a correlation of the north and south coast Tertiary (Table 6). This correlation was advanced by Maury as a tentative one and is the first attempt of a detailed character. However, the evidence at hand seems to warrant certain departures from this correlation, as shown by the following considerations Table 6 North Coast South CoavSt 1 Middle Miocene .2 I o 1< o .2X 'p. 02 Quebradillas limestone with Bowden fauna and Metis trinitaria. Lower Miocene Upper Oligocene Aguadilla limestone with Orthaulax aguadillensis. Ponce chalky beds with Ostrea cahobasensis. Lares limestone with Campanile {Portori- cia) larlca. Middle Oligocene 0) MS o Rio Collazo shales with dementia ra- helli. Guanica shaly lime- stone with Ostrea antiguensis. 1. Mitchell (1922) shows that the beds at Guanica are stratigraphic- ally ^t or near the top of the Ponce formation. In speaking of the Ponce chalky beds, Berkey (1915, p. 14) says: ^'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/^ From this, it seems evident that 66 SCIENTIFIC SURVEY OF PORTO RICO the Guanica beds overlie the Ponce chalky heds^ and are at the top of the column, not at the bottom as Maury stated. 2. The Guanica beds (together with the upper part of the Ponce beds to the east) carry a typical Quebradillas (Bowden) faima^ as will be shown later. Osirea antigiiensis which Maury lists from the Guanica beds is one of the best index fossils of the Quebradillas limestone. Eegarding the Juana Diaz, the basal shale member of the south coast group^ Maury (1919, p. 215) says: ^'The Juana Diaz shales furnished very few molluscan shells. . . . The evidence at hand is too scanty for any definite stratigrapliic conclusion regarding these beds.^^ As shown by Table o, and as will appear later, the Juana Diaz shale carries several of the best index fossils of the San Sebastian shale, among which dementia dariena (= C rabelli of Maury) is the most significant. From a careful comparison of south coast fossils collected by Berkey, Lobeck, and Mitchell, with material from the Lares District, the correla- tion table (7) is proposed and is believed to be essentially correct. A more detailed comparison is impossible because the south coast series has not been definitely subdivided in the field. Table 7 North Coast South Coast Pleistocene San Juan formation _ __ - Diseonformity. San Juan formation _ _ Disconformitv Quebradillas limestone Guanica beds and Upper Upper Oligoeene Los Puertos limestone Ponce limestone. Cibao limestone Lower Ponce limestone Middle Oligoeene Lares formation ___?_ San Sebastian sliale Unconformity Juana Diaz sbale _ _ _Unconformitv Upper Cretaceous Older Series Older Series HUBBARD, GEOLOGY OF THE LARES DISTRICT 67 The essential points in the basis of correlation have been referred to, but the following lists of fossils from the south coast formations are offered as further evidence. The localities recorded are taken from notes bj Berkey, Lobeck, and Mitchell : FOSSILS FROM THE JUANA DIAZ SHALE Name of Fossil TuriteUa halensis Dall Tiiritclla halensis var. alpha Mitchell Scalaf sp. Localities Remarks Juana Diaz, Yauco Juana Diaz Closely related to T. coUa- zocnsis of the San Sebas- tian shale. Juana Diaz An internal mold. A simi- lar or identical fossil occurs in the upper San Sebastian shale. Pccten (Chlamys) por- toricoensis n. sp. Near Juaua Diaz Index of San Sebastian and Lares formations. Clementia d a r i en a Conrad {=C. rabelli Maury) Juana Diaz Index of San Sebastian shale. Solen {Plectosolen coll- azoensis n. sp. Juana Diaz, Yauco. Index of San Sebastian i shale. • Natica sp. indet. Juana Diaz internal mold. Similar molds are found in the San Sebastian shale and range through the Cibao limestone. Cypraxi sancti-se'bas- tiani Maury Juana Diaz Index of San Sebastian shale. Pecten rahelli n. sp. Juana Diaz, Yauco and K. 25, Ponce- xVdjuntas road. Index of San Sebastian shale. Stromhus sp. indet. Yauco Resembles some internal molds from the San Se- bastian shale. Vencricardia cf. scahri- costata Guppy Juana Diaz, Yauco Occurs in Lares formation. 68 SCIENTIFIC SURVEY OF PORTO RICO FOSSILS FROM THE JUANA DIAZ SHALE—Continued Name of Fossil Localities Remarks Teredo incrassata Gabb. Ranges throughout the series, north and south coasts and Vieques Is- land. Of no value as an index fossil. LepidocycUnaf sp. Juana Diaz, Yauco. A large foraminifer, ap- parently the same as the one occurring abundantly in the Upper San Sebas- tian shale, Lares forma- tion, and Cibao limestone. FOSSILS OF THE PONCE LIMESTONE Name of Fossil Localities Remarks OrMlolites cf. ameri- ccma Cushman Ponce, Guanica Quebradillas limestone. Balanophyllia sp. Widespread, along coast. Extremely abundant in both the Quebradillas and the Upper Ponce limestones. Stylophora sp. (like ^. afflnis Duncan) Culebrinas Pt., and along coast. Characteristic of Quebra- dillas and Upper Ponce limestones. Btrombus proximus Sowerby Mona Island Quebradillas limestone. BuUaria cf. paupercula Sowerby Mona Island Quebradillas limestone. Turritella cf. gatunen- sis Conrad Mona Island Apparently related to T. portorieoensis of the Que- bradillas limestone. Cardium cf. muricoides n. sp. Near Guanica Ranges from Lares forma- tion to Los Puertos lime- stone. Cardium cf. lingualeonis Guppy Near (iuanica Bowden, Jamaica. HUBBARD, GEOLOGY OF THE LARES DISTRICT 69 FOSSILS OF THE PONCE LIMESTONE—Continued Name of Fossil Localities Remarks Cardium haitense var. cercadicum Maury Ponce Quebradillas limestone. Cardium haitense var. areeihoense n. var. Ponce, Culebrinas Pt. Quebradillas limestone. Corhula sp. indet. A small species, not found in any of the north coast formations. CMone woodwardi Guppy Culebrinas Pt., Ponce, Guanica, Rio Yauco 2y2 miles S. E. of Yauco. Abundant in Quebradillas limestone. Olycimeris cf. portori- coensis n. sp. Guanica May be the Quebradillas species. Lucina cf. chryostoma (Meusch.) Phil. Vieques Is., Culebrinas Pt., Ponce, N. W. of Ponce, near Guan- ica, K. 2.4, Ponce- Adjuntas road. The most characteristic fos- sil of the Ponce forma- tion. It occurs in the Quebradillas and Los Puertos limestones. Phacoidcs {Lucinisva ) calho unensis Dall Near Ponce Quebradillas limestone. Cytherca ( Gytherea ) herkcyi n. sj). Ponce, West of Ponce, N. E. of Ponce. Quebradillas limestone. Teredo incrassata Gabb. Vieques, Ponce Quebradillas limestone. No index value. Ostrea antigucnsis Brown Guanica? Quebradillas limestone. Re- ported by Maury from the Guanica shaly limestone. Terehra cirrus Dall Near Guanica Found in the lower Alphera formation of Santo Do- mingo. Tcrchra cf. quehradil- lensis n. sp. Ponce Resembles the Quebradillas species and may be the same. 70 SCIENTIFIC SURVEY OF PORTO RICO FOSSILS OF THE PONCE LIMESTONE—Continued Name of Fossil Localities Remarks A rca ( Scap liarca ) cf riocancnsis Maury I*once Upper Aphera of Santo Domingo. Not found on the north coast of Porto Rico. Bullaria granosa? Sowerby Cuiebrinas Pt. Quel)radillas limestone. Olivella muticoides var. portoricoensis n. var. K. 75.2, Ponce-Penue- las Road, Aguila Pt. Quebradillas limestone. Leda cf. pcUella Dall K. 75.2, Ponce-Peiiue- las Road Resembles this species, which is very typical of the Quebradillas lime- stone. Metis trinitaria Dall Cuiebrinas Pt. Quebradillas limestone. Ap- parently rare on the south coast. Ostrea cahohasensis var. portoricana n. var. Guanica Boring Sponge? Cuiebrinas Pt. The casts of the burrows of some organism like CMonc. These are so widespread and charac- teristic of the Quebradil- las limestone that they may be considered as in- dex of this horizon. Their burrows are chiefly in the large gastropod shells. It is believed that the last list includes fossils from all horizons of the limestone overlying the Juana Diaz shale, and hence ranging from Ijares to Quebradillas in age. It is inadvisable with the data at hand to attempt listing the fossils from the limestone as Upper or Lower Ponce. Never- theless, it is evident that most of the more typical Quebradillas species occur near the coast and therefore in the upper part of the Ponce lime- stone. HUBBARD, GEOLOGY OF THE LARES DISTRICT 71 COMPARISON WITH IMPORTANT ANTILLEAN LOCALITII-S Antifjua. The Island of Antigua, in the Lesser iintilles, southeast of Porto Eico, is considered the type locality of the Middle Oligocene (Antiguan) of the Caribbean region. The island consists of an igneous basement njion which rests a series of tuffs with interbedded marine strata, and finally an uppermost limestone series known as the Antigua formation, and considered by Vaughan as the type section of the Middle Oligocene. The evidence given by Vaughan (1919, p. 259) is based npon the fossil corals which he finds chiefly in a GO-foot fossil reef at or near the base of the Antigua formation. The evidence of tliese corals shows that the x'\ntigua formation is equivalent to the lower mem])ers of the Tertiary series in the Lares District of Porto Eico. On the basis of the €orals, Vaughan puts the entire iintigua formation in the Middle Oligo- cene, and estimates its thickness at 350 feet. These conclusions appar- ently do not agree with evidence furiiished by others who have studied the geology of this island. Of a list of 10 molluscan species from the Antigua formation, collected and described by Brown (1913, p. 598), 6 are identical with, and 3 are similar or related to species occurring in Antillean formations of Bowdcn age, while another species is character- istic of the Anguillan, or so called Upper Oligocene. The thickness of tlie Antigua formation is estimated by Spencer (1901) as '^at least many hundred feet," and by Brown as "^^upwards of 1500 feet at least.'' It forms a belt about 5 miles in width, and Vaughan estimates the seaward dip to range from 10° to 15°. Taking the average at 10°, the thickness must be several times greater than Vaughan estimated, and lience more nearly comparable with the north coast series of Porto Eico. Consider- ation of the above conflicting evidence seems to show that more than one formation is represented in the Antiguan formation, and detailed strati- graphic work will have to be done before the Antigua formation can be regarded as the type section of the Middle Oligocene in the West Indies. Santo Domingo. The highest Tertiary formations are the Aphera- Sconsia formations described by Maury (1917). These formations are approximately equivalent to the Bowden marl of Jamaica and to the Quebradillas limestone of Porto Eico. Correlation of the Orthaulax Zone of Santo Domingo and the pre-Quebradillas formations of Porto Eico cannot be made with certainty because no detailed stratigraphic work has been done in the older Santo Domingo formations. Haiti. In Haiti, the Maissade beds (uppermost horizon) correspond with the Quebradillas limestone. The underlying formation (Las Cahobes) is probably equivalent to the San Sebastian shale, and overlying 72 SCIENTIFIC SURVEY OF PORTO RICO limestones below the Quebradillas, as indicated by its stratigraphic posi- tion. Among the fossils listed by Jones (1918, p. 738) from this forma- tion are Turritella planigyrata Guppy. Turritella tornata Guppy. Vener^wardia scal)ricostata Guppy, which in the Porto Eico section are limited to the San Sebastian and Lares formations. Other localities. It is quite likely that future work will greatly in- crease the known number of localities in which the uppermost portion of the Tertiary is of Bowden age. The small key of Sombrero, 140 miles east of Porto Eico, is built ap of a white limestone of Bowden (or Que- bradillas) age. The following is a partial list of the fossils:^ 8trombus proximus Sowerby. Xenopliora conchyliiphora Born. Bullaria granosa Sowerby. Tellina cf. strophoidea n. sp. Cijathodonta cf. reedsi Maury. Cardium haitense Sowerby. Cardium cf. samhaicum Maury. Chione woodwardi Guppy. In addition to these, the casts of the Sponge (?) burrows, character- istic of the Quebradillas limestone, occur abundantly in the Sombrero limestone. The proximity of Sombrero to Anguilla, Vaughan's type locality of the Upper Oligocene, suggests the need of detailed strati- graphic work in this vicinity. The conclusions regarding the correlation of the Porto Eican with some of the other Antillean localities is given in the accompanying table (8). This table is based on those by Vaughan (1919^, p. 595), Maury, and Jones, with minor changes. The chief departure from the authori- ties above named is the placing of the Bowden formation and its equiv- alents in the Upper Oligocene. OLIGOCENE OR MIOCENE The question of the Miocene or Oligocene age of the formations equiv- alent or approximately equivalent to the Bowden marl of Jamaica is one on which authorities do not agree. Within the last few years Yaughan, Maury and others have maintained that the age of these formations is ' Determinations by the writer from material in Paleontological Museum, Columbia University. Table 8 PROPOSED CORRELATION OF ANTILLEAN TERTIARY FORMATIONS Age Porto Rico, North Coast PoBTo Rico, South Coast Santo Domingo Haiti Jamaica Cuba Panama Sombrero Anguilla Antigua Florida AND Georgia Maryland, Vir- ginia, North Carolina, and South Carolina Pliocene Caloosahatchie marl Jacksonville formation Waccamaw marl, etc. Duplin County marl, etc. Upper Miocene liower Miocene Shoal River marl Oak Grove Chipola Upper Oligocene Quebradillas limestone 800 feet Los Puertos limestone lOOO feet Ponce limestone 3000 to 4000 feet ? Juana Diaz shale 3000 to 4000 feet Sconsia forma- tion Aphera forma- tion "Orthaulax in- ornatus zone" Maissade beds 1000 feet Las Cahobes beds 6500 feet Bowden marl La Cruz marl Baracoa marl Miscellaneous limestones Guantanamo coral lime- stone Gatun formation Emperador limestone Sombrero limestone Anguilla limestone Antigua formation (undivided) Thickness more than 1500 feet Chief coral zone at base Middle Oligocene Cibao limestone 250-1000 feet Lares formation 350-1200 feet San Sebastian shale 700 feet 9 Tliomonde beds 1500 feet 9 Culebra formation Tampa Flint River Lower Oligocene Limestone series 8000 feet 7 Montpelier limestone ? • Limestone ? Tonosi ? Tuffs with ma- rine beds Seaforth limestone ? Igneous basement Vicksburg Group Upper Eocene Cambridge formation Richmond formation St. Bartholomew limestone Ocala limestone HUBBARD, GEOLOGY OF THE LARES DISTRICT 73 Lower Miocene. On the other hand, Dall and some other authorities still take the opposite view and place them in the Upper Oligocene. The writer believes that the position taken by Dall is correct. The evidence favoring the Miocene age of the Bowden is given by Vaughan and need not be reviewed here. The evidence favoring the Oligocene age may be summarized as follows 1. The genus Orthaulaa) has until recently been considered an index of Oligocene age. It is now regarded by Cook and Yaughan as indicative of either Oligocene or Lower Miocene. This change in the status of Orthaulax is not due to any new discoveries of that genus in horizons higher than it has previously been known to occur, but is due to the placing of previously recognized Upper Oligocene formations in the Mio- cene nomenclature by Vaughan and others. The genus Orthaulax is one of the most abundant fossils in the Quebradillas limestone, as well as in the Los Puertos limestone. In the former horizon, there are at least two, and very likely as many as four species of Orthaulax present. According to Vaughan the Quebradillas limestone is Lower Miocene in age.^ If this view be accepted, then we must admit that the Lower Miocene fauna of Porto Eico is largely Oligocene in its general aspect. Thus one of the strongest arguments of the Miocene advocates is weakened, namely, that the Chipola fauna (and presumably its time equivalents) in its aspect "looks forward to the later Tertiary and Eecent, rather than backward" (see Vaughan, 1919a, p. 573). It is true that Orthaulax has been noted as an exception to this statement, but every new discovery of abundant Orthaulax in strata of Bowden-Chipola age tends to increase the signifi- cance of this exception, and the Quebradillas fauna with its myriads of Orthaulax shells mixed with the host of Bowden-Aphera-Sconsia types furnishes one more difficulty in the way of any argument based upon the above quotation. 2. The presence of Ostrea antiguensis in great numbers at the top of the Quebradillas limestone. Maury regards this fossil as so excellent an index of the Oligocene that its presence in the Guanica shaly limestone is sufficient evidence for correlating this formation with the Antiguan. 3. The gradual appearance of some of the Quebradillas species, start- ing with the Lares formation (Antiguan). 4. The absence of any unconformity or marked faunal hiatus between the Quebradillas limestone and the lower limestones. The entire series is a structural unit. 5. Erosion interval and faunal break occurs in Porto Eico, as else- where, at the end of the Bowden time. ^ Personal communication. 74 SCIENTTFJG SURVEY OF PORTO RICO 6. Large representation in the San Sebastian shale of Gatun, Chipola, and Bowden types shows relationship with the Bowden horizon. 7. Two species in the San Sebastian shale (Glycimeris collazoensis and TurUnella chipolana prectirsor) apparently have closely related deriva- tives, showing slightly greater specialization in the Quebradillas lime- stone. 8. The marked difference of aspect between the San Sebastian and Quebradillas faunas is due chiefly to change in bionomic conditions. Thus the San Sebastian fauna, typically a lagoon or brackish water facies, was forced to migrate because of changing conditions. It reap- peared in Bowden time in the Canal Zone (Gatun fauna), with consid- erable changes in species, but with the same general aspect it had in San Sebastian time. The San Sebastian and Gatun faunas have in common such characteristic species as Clementm dariena and Area dariensis. 9. Inspection of the fossils listed from the Canal Zone shows that the Gatun formation has many species in common wdth the underlying lime- stones. Most of the change in faunal aspect may be explained by change of conditions of deposition. In Antigua, Bowden molluscan fossils are apparently so intimately mixed with Oligocene corals that no Upper Antiguan or Bowden horizon w^as differentiated th3re by Vaughan. In Florida, the Alum Bluff series is so closely associated with the underlying Tampa and Chattahoochee horizons that authorities are still not in agree- ment regarding the existence of a physical and faunal break, even after considerable stratigraphic work has been done. All are agreed, however, that there is a break at the top of the Alum Bluff group (= end of Bowden time). 10. In the San Sebastian shale is found species of Cirsotrema which is hardly distinguishable from certain species in the Helvetian (Miocene) of Europe. Species of Campanile and Plectosolen in the San Sebastan shale have allied forms in the European Eocene. Facts such as these suggest caution in attempting to decide the age of an American fauna by comparing it, in its general aspect, with European faunas. The only infallible criterion for correlation with European or other distant sections is the recognition of a world wide crustal movement or change of sea level. If the elevation of the South Atlantic-Antillean region at the close of the Bowden-Alum Bluff time can be definitely correlated with movement in other parts of the world, a definite and logical division line can be drawn between the Oligocene and the Miocene. In the American-Antillean Province this line of division is distinct, as Ball (1898^ p. 329) long ago pointed out. The evidence obtained in HUBBARD, GEOLOGY OF THE LARES DISTRICT 75 studying the sections in the Lares District supports the early conclusions of Dall rather than the recent conclusions of Vaughan. Tertiahy Histoky The entire group of Tertiary formations of Porto Eico were deposited during a continuous period of gradual submergence^ which began in Middle Oligocene time. The formations are therefore all conformable with one another, and show a progressive overlap over the Older Series rocks. The relationship is brought out in the ideal section (Fig. 23). With the initiation of submergence^ the sea encroached in the valleys of the old land surface, forming embayments, as for example at San Sebas- West Lares District San J (JAN District East Vieques Is. 5 + _____^=..=^ 3 — —"^5 ^^4-:^ ^ Fig. 23. Ideal section along the north coast at the time of maximum siihmerfjcncc (late Oligocene) Showing transsTessiA''e overlap of tho Tertiary formations on tlie old land surface. The San Sebastian shale is confined to the Lares-San Sebastian embayment. The San Sebastian shale and basal shale facies of the higher horizons are shown by the broken lines. The formations are numbered consecutively: (1) San Sebastian shale, (2) Lares formation. (3) Cibao limestone, (4) Los I'uertos limestone, (5) Quebradillas limestone. tian and Juana Diaz, in which the chief deposits of basal shales accumu- lated, under an alternation of brackish, freshwater and marine condi- tions. At San Sebastian coral reefs formed across the mouth of the embayment. The large '^heads^^ or colonies of corals, broken from this reef by the surf, are found imbedded in the marls of the San Sebastian formation, exposed along the Lares Eoad. In connection witli the theories of origin of coral reefs, it is interesting to note that several hundred feet of shale, in part marine, were deposited before the actual reef itself was formed. Comparison of sections at Collazo and Lares would seem to show that the Upper shales and coral marls of Collazo grade into coral-reef limestone when traced toward Lares. This would indicate that the green coral-bearing marls at Collazo were formed behind an off-shore or fringing reef. 7g SCIENTIFIC SURVEY OF PORTO RICO With continued submergence corals ceased to be the dominant reef- building organisms, molluscs and foraminifera taking their place. Throughout the entire series, the deposition kept pace with subsidence, as shown by the fact that shallow water organisms predominate in all the formations. In the eastern part of the Lares District, there was apparently frequent emergence, with oxidation of the newly deposited strata. The frequent occurrence of red limestone at various horizons in the eastern part of the district may be evidence of this. The chief evidence, however, is to be found in the thinning of the Cibao limestone and Lares formation toward the west, indicating near shore deposition, with a peninsula in the longitude of Moca. The submergence was prob- ably accompanied by a seaward down-warping movement illustrated by the fact that the Quebradillas limestond dips at lower angles than the lower formations. This statement is true even where the Quebradillas limestone is the basal formation, as in the San Juan District. MAGNITUDE OF THE SUBMERGENCE One of the unsettled problems has been the extent to which the island was sugmerged in Upper Oligocene time. Lobeck concludes (1922) that the island was not entirely submerged, as shown by 1. Presence of gravel and clay at various horizons in the Tertiary beds. 2. Abrupt termination of Tertiary beds against the upland slopes of the old land surface. The evidence found in the Lares District may be summarized as follows 1. The faunal difference between the lower formations in the Lares District and the lower formations of the south coast is considerable, and indicates a land mass quite extensive in an east-west direction, with no connecting passages close at hand. 2. The Bowden faunas of the Quebradillas and IJpper Ponce show much greater similarity than do the older faunas, but still show greater difference than the present beach faunas of the north and south coasts. The extreme abundance of certain shells (like Metis trinitaria) in the Quebradillas limestone, and their rare occurrence in the Ponce limestone is very significant. 3. The great thickness of the Tertiary series (nearly 4000 feet) would seem to show that the island was completely submerged. It has been show^n, however, that the formations near the old (Oligocene) shore line are only a fraction of their maximum or off-shore thickness. The beds were undoubtedly deposited with a slight initial dip to seaward, and HUBBARD, GEOLOGY OF THE LARES DISTRICT 77 hence are of the nature of fore-set beds. The maximum accumulation of limestone was in an outward direction, rather than upward. This may be one reason why the enormous thicknesses recorded for Oligocene reef limestones like those of Haiti and Porto Rico are not comparable with measurements of beds the same age deposited in Florida and elsewhere on the continental shelf. 4. A earful search was made for outliers of Tertiary rock in the higher portions of interior mountains. The only evidence found was a single houlder of Tertiary limestone (probably Lares) in the channel of the Eio Guayaba, two miles southeast of Aguada. However, this does not show that the Tertiary beds covered the Cordillera Central to the south. From the above evidence, it may be stated that Porto Eico was not •completely submerged in Upper Oligocene time. THE UPLIFT The uplift was differential. It amounted to at least 1300 feet in the vicinity of Lares, and less than 100 feet in the extreme eastern part of the island. This differential uplift resulted in the truncation of the w^est end of the island, chiefly by warping. The northwest dip of the Tertiary strata along the west coast has already been referred to in dis- cussing the structure. It is probable that there was some faulting in connection with this truncation of the island, and that the zone of fault- ing lies somewhere to the west of the island, where faulting is now taking place. This was shown by the 1918 earthquake. PHYSIOGRAPHY There are three major physiographic provinces represented in the Lares District: 1. The Complex Mountainous Oldland. 2. The Elevated Coastal Plain. 3. The Playa Plains. These are the most important physiographic units of the island, and have been described by Hill, Berkey, Lobeck, and the authors of the geological reports on the different districts. The geologic map brings out the contrasts of relief and topographic characteristics of the three provinces as they occur in the Lares District. The Complex Mountainous Oldland comprises the central mountain chain; the core or backbone of the island. In the Lares District it is the mountainous area south of the Lares Road; that is, the area making 78 . SCIENTIFIC SURVEY OF PORTO RICO up the southern half of the district. The maximum elevation {2i)0(f feet) is in the southeast corner of this area, and the relief here is 500 to 700 feet. The Coastal Plain comprises the area north of the Lares Road. It is a plateau in a youthful or submature stage of dissection. From a maxi- mum elevation of about 1500 feet (east of Lares )^ the plateau surface slopes very gradually to the north coast, termiiuiting there in sea cliffs 50 to 100 feet high. The rocks are Oligocene limestones with some basal shale beds, the entire series lying nearly horizontal, or with a slight dip to the north and northwest. These Oligocene formations overlap the mountainous oldland as far as the Lares Eoad, where they culminate in a more or less distinct cuesta, in places more than 300 feet high. This cuesta marks the boundary between the Coastal Plain and Mountainous Oldland provinces, and its position can readily be seen on the geologic map. The Playa Plains are the nearly flat alluvial plains at or near sea level, occurring along the coast at the mouths of the rivers. They are especially large on the west coast of the Lares District^ as for example, the Culebrinas and Ahasco Playas. Smaller ones occur on the north coast of the district, as for example, the Guajataca and Camuy Playas. The Playa Plains (locally known as Playas) are of comparatively recent origin, and are found only along the coast. Some occur adjacent to the mountainous oldland ; others adjacent to the coastal plain. The Complex Mountainous Oldland This province in the Lares District, includes practically all the area south of tlie Lares Eoad (see map). The relief is very considerable throughout the greater part, especially in the southeast corner of the district. The central mountain range of Porto Rico, known as the Cor- dillera Central, can be traced westward across the island to the vicinity of Adjuntas. West of Adjuntas it divides into a southern range and a northern range. The southern range extends along the southern border of the Lares District, through Maricao and Consumo, to Maya- guez. The northern range passes south of Lares, where it is indistinct, to Atalaya Peak, north of Afiasco. Northwest of Point Jiguero, it cou- tinues as a distinct submerged range, of which the highest summit forms Desecheo Island, 15 miles from the west coast at Rincon. The Mountainous Oldland in the Lares District is drained by two master streams, the Rio iViiasco-Rio Blanco, and the Rio Culebrinas. These two rivers flow west and northwest through the area to the west HUBBARD, GEOLOGY OF THE LARES DISTRICT 79 coast^ following in a general way^ the rock structure. The Eio Anasco- Eio Blanco system forms the parting valley between the north and south branches of the Cordillera Central. The Eio Culebrinas forms the part- ing valley (or inner lowland) between the oldland on the south, and the overlapping Tertiary formations of the coastal plain on the north. The Mountainous Oldland is maturely dissected. All of the streams are in the stage of youth, and are characterized by narrow, steep-sided valleys, and numerous falls and rapids. As compared with the playas, or lowlands of the coast, the climate of these interior mountains is cool and humid. Eains are of almost daily occurrence, but are typically of short duration. The moimtain slopes, almost everywhere developed on clay or laterite, are exceedingly steep, and as a rule, are covered with forest trees. All of the trees are second growth, utilized as shade for coffee. Thus what appears as wild forest land, is in reality highly cul- tivated coffee land. Travel in this mountainous area is very difficult. Automobile roads are rare, and the native cart roads (caminos) are often impassable after showers. Grades of 30° or more are commonly met with in travelling these roads, and in the areas of greatest relief, coffee, bananas, and all other products must be transported by pack animals. Some of the richest coffee districts of Porto Eico are in the most inaccessible parts of the mountains. THE PENEPLANE SURFACE The summits of the Cordillera Central mark the remnants of a formerly continuous surface of moderate relief (Fig. 24). This old surface is the upper peneplane described by Lobeck. Above the pene- plane surface rise a few scattered monadnocks of quartz diorite, or other relatively resistant rock (Fig. 1). The peneplane has been maturely dissected in the Lares District, and is not a striking feature except where viewed from certain points of advantage. In the southeast corner of the district, where it has an elevation of 1700 to 1900 feet, it slopes gradually to the west. Near the west coast it is preserved only on por- tions of the Atalaya Eange north of Anasco, and possibly on the Mesa at Mayaguez. In central and eastern Porto Eico, Lobeck has distinguished a lower peneplane, marking a second erosion cycle. No traces of this lower peneplane are found in the Lares District, probably because it is buried here by the overlapping Tertiary formations. The upper peneplane was formed some time after the close of the Cretaceous Period and before the beginning of the Oligocene Period. This is proven by the fact that late Cretaceous formations make up a gl) SCfE.XTIFie SSUJIVKY OF POKTO RJVO portion of tbo kMed Ohh^r Scrit'H rock? on which tlie poiiephiiu' k (levelopw:!, and arc the jinm^m fornuitioiis known to exkt in the Older Beries.' That ilu! peneplaLiatioii ttKjk plaeo before the Oligoeeiie Pi>ri<HJ ir^ shown by the fact that tlic earliest marine formation.^ doposite-l on the ptmepIajHMJ area are of middle Oligoeene age. M previously nr.Uid, Lobeek has shown the e.xisionet* of two penoplanes in Forto Jiieo. Jiotli of tbew most have been^ made during tlie interval Iw'twecm llie close of the Cretaceous PericKJ and the Ix'ginniiig of midrlle Oligoeene time. Ilie most probable date for the upper peneplane h the Palooe<nie Ferin,! ; for tlio lower peneplane, the Eocene Period. Tlw. fornuUi<)M ol* the view lookltii .^fiiricii<> Kiwd. lower pen(?plane seems to liave Iteon interrupted^ l)y an, uplilt, initiating a ihir<l eyel{> uf erosion, and resulting in the dissection of b'oth ptna- planes. This is sliown by tlie fact that the basal formations of the Tertiary Series lie upon an extremely irregular surface, in, some ])laces filling old valleys in the oldlaud snrraee. From their locutiou, these hurit'd valleys seem to Inive been cut in tin; lower peneplan,e. Coarse gra\i:!ls occiipyin,g one of tlicse buried valksys near MotMi, ami immediately underlying tlic basal uurrine Cligoeene fornmtions in this locality, p<sint to tb.o' existence of youtb,rnl streams in this .region Just preceding the in,vasion of the sea in middle Oligoeene time. Thus the ttdrd erosion cA^ele belomjs to th,c late Flocnvne or early Oligoeeiu^ .Periods, or possildy HUBBARD, GEOLOGY OF THE LARES DISTRICT gl both. This cycle had apparently reached a stage of late youth or early maturity before it was interrupted by tlie middle Oligocene submergence. The following are the important physiographic events which took place during the time interval between the folding of the Cretaceous formations of Porto Rico and the deposition of the Tertiary coastal plain strata: 1. First erosion cycle, ending in the formation of the upper pene- plane. (Late Cretaceous to end of the Paleocene.) 2. Uplift, with dissection of the upper peneplane, and the formation of a second or lower peneplane. (Eocene.) 3. Uplift, with dissection of the lower peneplane and further dissec- tion of the upper peneplane. (Late Eocene tcf early Oligocene.) 4. Subsidence, resulting in partial submergence of the island, and interruption of the third erosion cycle. (Middle Oligocene to early Miocene.) The geological dates given in this outline are to be considered as the probable approximate dates of these events. The date of the close of the third erosion cycle, however, is based on good strati graphic evideiice. At the close of the Oligocene Period, tbe entire island was uplifted, the amount of vertical movement being differential, but reaching a nuiximum of 1500 feet in the eastern part of the Lares District. I'hus erosion of the present cycle was initiated in early Miocene tinu^, and has coutinued to the present. Those portions of the interior uu)Ujitains which were not subnu^rged in Oligocene time liave been subjected to erosion since the close of the Cretaceous Period. Other ])ortions nearer the coast have been stripped of some of tlieir mantle of Oligocene strata during the present cycle. This is true of most of the area drained by the Pio Culebrinas and its tributaries. Drainage Features All the streams of the mountainous oldland are in the stage of youth. Falls and rapids occur in abundance in streams of all sizes. The ouly outcrops of fresh, unweathered rock in the oldland area are found in the stream channels. This, in a region where rock decay is extremely rapid, is a good indication that the rivers are still actively deepening their valleys. Eiver terraces occur in many places along the Pio Ahasco, Rio Blanco, and Rio Culebrinas valleys. None of theme are continuous, and they occur at all elevations above river level, from 10 to 100 feet. Most of them are built of river gravel; some are rock cut. Li most places they 82 SCIENTIFW SURVEY OF PORTO RICO are obscured by forest trees which cover the valley walls^ and their true nature is therefore not evident. A special feature of the drainage of the oldland area is the presence of hanging valleys where small tributaries enter the valleys of trunk streams, such as the Eio Blanco. In the upper Rio Blanco valley in the southeast part of the Lares District, hanging valleys are very numerous. The small streams enter the deep, narrow valley of the main stream with rapids and in many cases falls of considerable height. The same feature may be seen in the upper Rio Culebrinas valley, and,, in fact, seems to be the rule wherever small tributaries enter a trunk stream of large volume. Any attempt at interpretation of the origin of the streams of the mountainous oldland is made difficult because the relationship of stream to rock structure is in most places not apparent, and the structure, where not obscured by soil and vegetation, is as a rule so complex that a long and careful study would be required to unravel it. This state- ment applies especially to the southeastern part of the Lares District. A glance at the map will show that in the western part of the oldland area, the main streams follow the strike of the Cretaceous formations more or less closely. Such streams are developed in belts of the less resistant rocks, especially tuffs and shales, and should be classified as subsequent streams. Examples of these are the Rio Casey, Rio Cafias, Rio Santiago, Rio Grande, Rio Culebrinas (in part), and Rio Blanco (in part). The Rio Aiiasco-Rio Blanco is a stream of complex origin, and being the largest stream in the district, deserves further description. The upper portion, known as the Rio Blanco, follows the structure where it leaves the areas of massive igneous rock and enters belts of stratified rock. In these portions, it may be regarded as subsequent. The lower portion, known as the Rio Aiiasco, cuts across the strike of the forma- tions without regard to the relative resistance of the different types of rock. That part of the mountainous oldland drained by the Rio Aiiasco was probably submerged during the Oligocene Period. If so, the de- posits formed at that time have been removed by post-Oligocene erosion, and the area is to be classed as a pseudo-oldland. This theory is sup- ported by the fact the nearest outliers of Tertiary strata (at Pt. Jiguero) are of middle Oligocene age, indicating that some 2000 feet of upper Oligocene strata have been removed by erosion. If this portion of the area is indeed a pseudo-oldland, then the Rio Aiiasco is probably a super- imposed stream. Additional evidence favoring this view may be obtained from a study of two other streams flowing to the west coast. These HUBBARD, GEOLOGY OF THE LARES DISTRICT §3 are Calvaehe Creek and Pueblo Creek, both southeast of llincon. The map shows that the upper portions of these streams follow the belts of weak rock, and are thus adjusted to the structure. In each case, the stream disregards the rock structure in the lower part of its course, exactly as the Eio Ahasco does. As a whole, the mountainous oldland area in the Lares District pre- sents two contrasting types of drainage pattern (see map). In the eastern part, the pattern is dendritic ; in the western part, trellis. The explanation of this is apparent. In the eastern half, the country rock is largely igneous intrusive bodies and massive tuffs and agglomerates. In the western half, stratified rocks and interbedded lava flows and sills predominate, and the beds are nearly everywhere folded and tilted at high angles. The influence of hard and soft beds on the drainage lines is very marked. The Elevated Coastal Plain This province includes the area between the Lares Eoad and the north coast, and a narrow strip along the west coast from Aguadilla to Point Jiguero. The Coastal Plain is developed on a belt of nearly horizontal Oligocene strata, mostly limestone, which overlap the old- land. It has a topography wholly distinct from the topography of the complex mountainous oldland on the south. Because of its elevated position and nearly horizontal strata, the coastal plain belt may be classed as a plateau. It is marked off from 'the oldland by a more or less distinct cuesta, which faces south and overlooks an inner-lowland. The inner4owland is developed on the Older Series rocks, and in the Lares District, owes its existence to the erosive action of the Rio Cule- brinas and its tributaries. From the summit of the cuesta, which marks the highest elevation of the Tertiary coastal plain strata, the plateau surface slopes seaward (north and northwest) at an average angle of less than 1°. This plateau surface is in a youthful or submature stage of dissection. Only two master consequent streams cross the belt, from the oldland area to the north coast. These are the Rio Guajataca and the Rio Camuy. They have cut narrow canyons in the limestone to a maximum depth of more than 300 feet. River erosion, however, has not been the only destructive agency at work in this limestone area. Surface solution and underground solution have been exceedingly active, and have produced a peculiar type of Karst topography, characterized by sink holes and conical mounds or hills of limestone known as pepinos (Spanish for cucumbers), or haystack hills. This sink-hole pepino hill topography S4 .vC/f^lV/'iF/C HUMTEY OF FOIfTO EfVO k m niggfd t'h<U travel across it is ponsibk' only by a. few I'avorahle routes. Thus, wliiie tbc jyiateaii is in a yontlifol ste^yv of disi^er'tion. as far as stream erosion gues, tlie topograj>hy in certain belts is as rough a,s tliat of a. maturely <lis.se(-tc'(l country, because of the a(;tivity of uiulcr- grouud soluriou. Th(; pepino liilk are the uiost inten»stiug and uiriqiu; feai.ure of th,e eoaslal plain. Jllustratioos o1' theju are shown in Jigures 28 aiui 'ii), and their distribiitiou in cast-west belts across tlu; plateau surface is brought out on the geologic map. Tliey .have been sliowu on this map by hiiehures, because to contour theui would be an cud less task, oord:our iid^unul of 100 hot would fail to bring out the extreuudy i topograpliy ol; tliese belts of liills. As may be seen from the ilJustrations, these hills are roughly ( or inoujul-shaped. Jji size, they range fr()m small mo'uiuls l(»ss t!' feet high, to liiJIs at least ;IO0 fe«t liigh. i:b,ey are, wluu-e best dev( cdosely crowded, the intervening spaces being o('<;upied by sink-Iu elongate or irregular pattern. In these; belts of p>cflno hills, oi often not find level spaces large euougti to pitch a tent on. All and cart roads iiuiander around the steep sides of tlie hills, avoid ii onieal .loped, dcs of w can trails nc tlie MVBBAEIK (SEOLOaV OF THE LAMES IJliSTKICT So Hin.k^li(>l(jH. TJie traiL< are (luiig(!r()ii.< io tra.vcl aStoT eliower.<. when the rcsidiLul clay .^oil is iruule extremely sli])}M'ry. Soil, however, h. not thick or widch|tre;el in these areas, the oiiteroj).s and talus ot* wliiti- liiiis'stoue eovering luiieli (if the Kuiiace. Ill vimviiig these hills from a disiafict', ur from the siiinmii of oin- of tlieii), it h noticeable tliat the 8nmmiis (rf all the hii^iier hills have a lair aeeordaiiee in Jevelj presumably mui'kiiig the plateau snrt'aee ii> it was before being cut up into tliis rough topography l>y inidcrgroirnd Hnlnti<iii. (hi the plateau surfaee, adjaeeut to tlie belrs^ of pepifio hills tliere are belts of relatively smooth, urujissectt'd eoinrtry. Typieal views of tliis type of topography arc; shown in figure 2T. The prairie belts are covered witli black or red residual clay soils, excellent tor growijig sugai" cane and tobacco. Wlu>re not cultivated, they nmke good, grazing land. Idicy are naturally grass-covered, anel the trees afe wirlely seattcjred, and consist cfii(>ny of the Koyal Palm. The smooth or rolling country is broken here and there by canyfms, siuk-holes, and low cuestas which mark tiie outcrop o( relatively resistani; limestone tieds dipping li" or 4° to the north. The principal cuesta occurs aloug the Lares lload, overlooking the 3(3 SCIENTIFIC SURVEY OF PORTO RICO inner-lowland, as already noted. It marks the southern limit of the Tertiary coastal plain strata which overlap the complex mountainous oldland. Figures 25 and 26 show the variations in the topography of this cuesta. Where the basal shale beds of the Tertiary Series are capped by resistant limestone, the cuesta is prominent and not maturely dis- sected. Where only Tertiary shale is present, however, the cuesta is so thoroughly dissected by the tributaries of the Kio Culebrinas that it is not a prominent topographic feature. The inner-lowland is developed throughout on shales, tuffs, and other rocks of the Older Series, which in general, are more easily eroded than the Tertiary limestone forming the summit of most of the cuesta. At some points along the cuesta, the basal shale of the Tertiary Series is absent, and the higher horizons of limestone lie directly on half buried spurs of the rugged oldland surface. At such places, there is no inner-lowland, and the cuesta is low and indis- tinct. Other cuestas, of minor size and importance, occur in various parts of the plateau surface north of the main cuesta. They occur where "resistant reef limestones alternate with soft, chalky or argillaceous beds. The regional dip is north or northwest at low angles; thus all of these cuestas face south or southeast. THE COASTAL PLAIN BELTS The belted character of the coastal plain has been referred to in describing the belts of pepino hills, and it is brought out on the geologic map. Each belt owes its topographic characteristics to the rock forma- tion on which it is developed, hence the boundaries of the different belts are almost the same as the boundaries of the different Tertiary forma- tions. K description of the Tertiary formations has already been given and the topographic expression of each was summarized in Table 4. 1'he belts, named in stratigraphic order, from the lowest to the highest, are 1. The Lares Pepino Belt. The belt of pepino hills developed in the Lares limestone, on the plateau surface north of the cuesta in the eastern half of the Lares District. 2. The Cibao Prairie Belt. Adjacent to, and north of the Lares Pepino Belt. Developed on the Cibao limestone. 3. The Los Puertos Pepino Belt. North of the Cibao Belt, and developed on the Los Puertos limestone. 4. The Quebradillas Plateau Belt. Developed on the Quebradillas limestone. Extends from the Los Puertos Pepino Belt to the north coast. IIUBBAIilK (JEOLOaY OF THE LARES DlHTh'lCT S7 To ()l)iaiii a clear eoiieeptioii of the relation.<liip of thci^e boltn, and tlie obvious reusoii for the belted ehaxaeter of tlie coastal plain, the readtu- is referred to tlie geologie cross-sectious (Plate I), showing the north- tfontli profiles across these topographic belts. The Lm-es Pcpino Belt. Tlie bent development of thJs l)elt i? the area ni>rth of tlie town, ot liares. From, the Guajataea Eiver eastwar<l be- yond tlie east border of the Lares District, the })lateau surface jiwt biH:k of the cuesta is cluiraeterizcd by high and densely crowded pepino bills. These Idlls are formed in the pure white ami pink T.ares limestone, both of the nnissire and tlnndji'dded types. West of the Guajataea lliver, the Lures fornnition grades from pure limestone into soft, chalky and argil- laceous limestones, and finally into slniles and gravel be^ls. It is tlu'rc- fore signitlcant to note that the pepino hills do not exten.d very far west of the Ginijataea Kiver in the Lares Belt. Thfi Cilmo Prairiti BeM. Tliis helfc is a relatively flat, rolling, grass- covered prairie, situated between the two l)elts of pepino h'M^ (see njai)), and at a slightly lower elevation tlian the adjacent pepino hill country on eitlKiT side. "The Cibao limestone is clrielly a soft, cluilky roek^^wntli intercalated beds of hard limestone, winch form low cuestas. Figur<! 21 shows typical views of the topography of this belt. ,SH ^f'lEXTIFlC SFRVEY OF f'OKTO KIVO The Lijs Fufiios I'l'inno Belt. This belt is essentially like the Lares J^M'lt, but ihe liep'uio hills are larger, and the to|)o<i;Taphy cerresptmdiiig'ly .more riig<ied and difficult to traverse. The Lets l^iiertos Belt is inueli more eoiitiriiioits eiisf aiid west ttuoi is ilie l«ires Belt. The striking eont.rast bi'twetsii this iiepitio liill topography and th(^ Cibao |.»rrtirie tcrp(»grfi]»by is shown in figures 28 and W. The QvehrcuMllas Plakmi Bell. ThJs belt is for t\w, most part a plateau in a yuuthfiil stage of dissection, sloping gradiially seaward, and terii'iiriated bv sea cliffs along most of the coast line. The surface is reniarkat)ly flnl, especially in the northwest corner of the dii-^trict. .Abo\'e ibis surface rise a few hills of the pepino tYf)(>, grouped iriHHrn- larly, or in long, ridgc^like cluiins some of whi<;li are quite cemtimious east and west. 8i,nk holes are eoimoon, but not as large or numerous as those of the Cibao prairie. The coastal plain is traver.-ed by two master consequent streams, t'ae Ki(i (luajataca and liio Camay, each of which has its headwaters in tlu; (Vklland area to the south. With the exce})tioii of these two streams, which cut deep canyons (Fig. 30) tli rough tlie limestone belt, about HUBBARD. (iEOhOGY OF THE L/IJIHS DrSTEIOT 89 niiio-tonths of hII tlic drainage is KobtCTriiiieHii. Kvcii fiic Camuy takes lu a subterraiieari eourBe in ernsHing the Cibrto l)clt (Fig. Ml). In jiiaii}' plac(K<, partJciilarly in the (1l)a(i priiirie, Ihe low riijnl)le of iiiider^ gTOund rivets' can bc^' Iieard: in otlier places tJicy are seen in t\w l.olloms of llie larg-C! sink4u:des, where tlie water eoiiu'S briefiv to view, boiling a> tiiuiigh in Bomt giant eaiildroii. After a t-ontiiiuouB 2-1 tionrs ol' rain, it^ w'.rn iioUhI tliat tbere wag i)raetically lut surface rim-olf on tbe t^Xibao prairi(», not even the smallest rivnlet bein,g in eviden(re. The rain waiter .bad e.^t:apiHl almost immediately into tlie undergToiind ebiinmds. It is possible thai the JJio (Inajataea and JJio l^anuiy were at one Tin»e biriridy subterranean, in tbeir coursi's tlinnigli tbe limestones, and tbat the fH-esent canyons liave Iseen profhieed in jnaiiy phices Ity tnuing of tlie surfaee. Tbe lollowing consideratitrns snpport this theory; 1. Most of the present drainage line? arc siibterrantian. 2. Sinkdiok'H are evithaitly forming at tbe present tim(> iibove these subterranean ebiinnels. and given time enongh, should resnlt in tbo e<>in- plete caving in of the rock overlying such dnuniels. 3. Lar^c unisses nt lini(»stone, nniny (»f them nnire than KKi feet in diameter, oeeur in the Eio Cainuy canyon, nnn'c or hi'ss obstrueting tbe stream. Tliey are espe(-ially nnmennis in tbat part of the canyon adjacent to tlie snijterranean route o[ llie river. They imlieate tbat at <)() SCII-:M'JFI€ f(UR]'EY OF PORTO RICO one tiine, a larger portit^ni of the Rio C'amuy was subtcrraiioan, and that the |}reseiit siil)t(>iTaiiean course is io process of heiiig de^t^()yed. Tlie future drainage hi'story of the eoastal plain will very probably be an opening up of underground water courses by caving, until surface drainage of i\w area is (sventnally established. OJilGI-N OK Tl' EPJNTO HILLS The tof)Ogra.pliy of these hills lias been described, but a discnjssioii of ilieir origin has been left for tlie last, b(.'eansc it is a siibjeet (dosely con- nected with the draining of the coastal plain. It is believed that the foHowing U'bservntions are of critical iniijortaiice in formulating any theory of tlic origin ot this type of toijography: 1. 1lie C'ihao and (|uebra(lillas limestone belts do not have a typieal development of pcprtio hills. Likewise, these hills do not occur in the liarcs formation in the western part of the Lares District. In aiudjzing these associations, it is evident tliat (pertain tyi^es of formt»ti(ins are not favoral)k' to their development. These types are («) shales, marls, argillaceous or impure limestones, (h) eoniinuoos linie.<tone strata of the hard, fine-grained, flinty type, cliaracteristic of the Qnebradillas fornurtion,. HUBBARD, (JEOIAXtY OF THE LABES DIHT'ltlCT 9i; 2. In the most arid portion of the I'onstal pkiu, the northwest corner, there are no pepino hJJ.ls. Thi.« k illustrated by the even plateaiitf back of Point BoriiMintm. 'riie more arid the climate, the less the aincnini of siirl'aee sol'itir^o wbirh ha- iaki^n nlnrn. ^M.; :;1. //'. The iiv«r ll<>«s tliv->ij:;b a .lo-p >-i!u;,..ii wall, and there takes to a stibterranean clii belt. Tlic "bUie trnle" Is seen at the estn in tlie foregrownd. in passing tlirough the filiao limestim«» •iglit. nearly bJocked by the timber jam 3. The pepino liills are not individual reefs, since tticiv are not limited to the massive reef type of lime«totie. Some of them are developed in 5,2 f^ClKXTIFIiJ HVRVKY OF PORTO HIOO well stratifuMl lunostoijc, mul m meh vmc?, llie mnw strata mm Ih> traeocl ill detail from one hill to another (¥ip:. '.I'i). 4. Fr.pino liiJIs tofjograpby is best tiovelupwl where the lirrn'siono i^ the mo.-^t I'iiveriious ; that is, in those ^nlle^te]lert which have .-utferecl liu inaximuDi amount of ?^ohitioii bv gj-niUHl watiT. The hills are not invari^ alily fuiirxl, liowever, in tlris type ol' limestone formation. 5. In. many loealities the hills sliow a north^Houth linear .uTouping- with intervoning lanes of sink holes (Fig'. 3;j). Ttiis arrangement sink hnlt.'s ean bo |>rodn<'od in no other way than by caving along m^h^ i.erranoaii drainage lines. b. C'arel'nl observation sliows that the hills have a tendeney to he steep- est on the side toward the west. This feature is so noticeable in plaees as to «(ivo a sawtooth effect to tbc sky line (Fig. :tt). Further observa- tion shows that tlris asynnnetry has no rcdation to skimping or tilting, no reflation to difference in structure or eonrposition of ttic ro(;k, and no relalieni to vegetation covering the hills. Ttie only apparent explanation left is that of ditferential weatbei'ing am! solntion. Tin;- daily showers u<-eur usmilly in. ihe aftenn.on, when the sun. has been shining on the bill^ rr«.)in the west, antl he.u(!e while the rock on the west is at a higher jniBBARD, Gf-JOLOGY OF THE LARES DISTINCT m toniitcratirre tlum that on the east <»r .shaded, slides. This iiu;roasc>;s tlw rate ot solution, and results in a more cavernous stnieture hidng devel- oped cm the west siden. Suxdi dilferential surfaee >ohitioJ) would he notjeealde only mdiere th,e proe(;ss is extreiiielj raj)id. From the ahove ohservation«, it is (Mdieve*! that the iM'plNO hills are the hvproduct ot; exten.iive iiiiderground sulntiun and extremely ra|)id Hurfaee solution, eombiiied with the other neeessary faetors, lithologie and <-li- njatie, above dcseribed. T.'nder these eondiiions. pi-fyino hills might he <le\"eloped anywlif^re in tlu:' tropical 2oin\ : loul-iilij n<,)ih fn TiiK Playa Pi.ain.s Tlie term Flaya h^ u.-^ed in the West Indies to deiu.te tlu' Jhii oi- gently s1oi)irig alluna] plaiJis whi(,'h oe-enr along lh,e i-oasb< of nearly all (he large islands at the mouths of rivers. Siieli playas are in nurnv cases sfx-cral ndles wide, but none is elevated more tiniii a few bn't above sea level. In the liares District, large pbiyas oeeu,r at the nnuith of the Hio Afuiseo and at the mouth of the Kio (hilehrin,as. There are- many smaller om?s on the nortli and west coasts. The playas, or playa [jlairis dilTer ehielly in size and outline, anxl a deseri|)tion of the largest (the .\nasco Pbivat will include all the features to he tnuml in ajiy of the others. f>4 SVlEyTIFlC SUIf'VEY OF PORTO RICO 'riic xiilosco Plavii occupies the mouth of a drowned TaJlev, and is- hoimdod on its inland sides hj the luuiintaiiis or Irilis of the oldlaiul. In gToiiud plaij, tlie playa i.'^ faji-shapedj wliich is fiic form of a typical (hdta or alluvial fan. TJio surface in most places is srnool;li and apparently hn'eJ, but lia]id-lc\-cJ measurements show a gradual slope to eeawaixl froju tlie interior portion, where the (ilcvation is SO to 30 feet above high tide level. Tlie eoastal margin is juarked by a broad sandy beach, sand dunes back of the beach, and a zone of coeoarmt palms back of the ridge of ihnie sajuh Then; is a narrow zoue back of the zone of pahus where the- (liUa-Isa'behi Komi in toregnmrn}. pepino hills of the Los Piiertos lirne^ • illustrates Use "saw-tooth" skyline, the steeper si(l«s land stands at or slightly below sea level, with small patches of salt marsh, hut no large <tr coid;inuou8 lagoon. The remainder of the playa is planted in sugar caoe. The town of Ailasco is located in the intcirior part of the playa, at an elevation of about 80 feet aliove sea level. The Bio Ailasco, leaving the narrow flood plain between the mountains, oast of Anasco, takes a meandering course across the playa to the sea. In the interior part of the playa, the river is intrenelied 10 to 15 feet below the playa surface, exposing and actively eroding the layers of unconsolidated m,aterial of which the playa is hiiilt. Numerous gullies have been erodcnl into adjac-ent parte? of the playa surface by small inter- mittent tributaries of the main stream. HUBBARD, GEOLOGY OF THE LARES DISTRICT 95 The lower (seaward) portions of Anasco Playa are said to be flooded by the river during periods of very high water, but this is of rare occur- rence. On the north coast, however, small playas of intermittent streams, as, for example, the playa of Los Cedros, are flooded more frequently and more completely. This is because the river water is ponded behind an unbroken ridge of dune sand at the outer margin of the playa. The playas, while making up but a small part of Porto Eico, are very impor- tant economically, since they are the best sugar cane districts of the island, and the sites of nearly all the large sea-port towns. FExVTUEES OF THE COAST LINE Terraces Terraces were first noted by Berkey (1915) and classed as coastal (marine) and river. The marine terraces are prominent features in many places along the coast line of the Lares District and on Desecheo Island. They are more or less local in extent and occur at elevations ranging from 2 or 3 feet above high tide to about 200 feet. Most of the lower terraces carry ^^fossiF' beaches or sands and gravels of undoubted marine origin. The higher terraces, however, rarely show exposures of gravels, and where such gravels occur, they are not always recognizable as marine deposits. Along tlie automobile road west of Camuy are extensive ex- posures of conglomerate, composed entirely of fragments of Oligocene limestones, highly weathered and disintegrated. Similar deposits occur elsewhere along the north coast on the higher terraces. No marine Pleis- tocene or Eecent fossils are found in these conglomerates, but their ma- rine origin is suggested by the fact that they are never found except near the coast at altitudes up to about 200 feet. On the coast of the Ponce District, Mitchell (1922) has found coastal terraces carrying undoubted marine deposits. Pleistocene to Eecent in age, and ranging in altitude from 10 to 200 feet. In comparing the terrace elevations listed by Mitchell with those recorded in the Lares District, it was found that the elevations covering the stretch of coast from Camuy to Juana Diaz fall into four rather distinct groups, indicating as many stages in the Pleis- tocene-Eecent uplift of this portion of the island. These stages may be described as follows 1. The Isabela Stage. Named from the well-preserved terrace (175 feet) at Isabela on the north coast. Other well marked terraces of this stage were found near Camuy (125 feet), Quebradillas (160 feet), Pt. Jiguero (165 feet), and by Mitchell near Guanica (150 feet), Yauco R(,UKNTIFH' i^UKVMY OF I'OHTO HIVO (2<>0 feet), ? (UJO I'eet), i t,erniees on \ Tlie averauv Pleistocene. ith of Yuiiw) (150 feet), southwest ol' Oniiyaiiilla Harbor 1 w(>st of Police (U30 to 180 feet). These are the liigliest ieh (letMjsits of uiuloiibted iiuiriue origin have Ijeeii foioid. evatioji of tlie group is probably ehjsc to ir,t» footj* The (J bv intchcll show tlu:it the ati'e k not ohk^r tliiiii 2. Tlie Ciijjo Rojo Stage. Named from the well luarkerl tcrrat-e f'oinnl by ^Ptehell ois C'abo Hojo (75 feet). Other turracH"* of this groirp wwe "Tlio fJiscrciwnc.v in eh-vatlon of terraces .eroupefl togptlicr as Ix'iMtisinc to tlip same- sdiev. may be I'xulnlned in one or more of tfiw followkiK wn.vs : :i.' Errors in aiiproid retullngs, by which the cle-s^arions were «lf.teriuliKM]. 2. Errors of Jiidss.iein in selt'ctiiig the lop of the tei'race for moasureuioiit:. Many of these terrsiees are not level, but slope 1<> seaward. iMirtiierniore, they have hv-en cxtT-n^ sivelv eroded by wave and river action. Thus tJie eroded remnant of tlie outer marjrin mislit Rive a iniich lower aneroid reading than wonUl a remnant i»t; the inland border of the same terrace. 3. Loeal warping, affecting tlie terraces along tlie coast, 4. Tlsc terraces grouped under any single stage, as under the Isahela Stage of npllfr, may not tieeessarily huve been exactly eonteinporaneous lu orisln. Tims during the time interval in winch all iJie terraces of the Isahela Stage were formed, there may have been a series of mimn- nplifts. total ins 50 t:eor. HUBBARD, (JEOIJMIY OF THE LARE8 DISTBtOT 97 found iMsar Qiicbradillas (:>0 to 100 feet), l:»t. Jiguero (45 to 70 feet), and by ACitchell ^outJiweKt uf Mayti^iicz, at iho Icform Behool (50 feet), Enseiiada (75 foot), Guanica (50 foct), i-iiiaiiicij Light House (50 feet), Ft. Broa (1)5 to 100 ftsct), and soutliwo^t of (liuiyaiiilla Harbor (00 feet). The.ic are ttie id^ghost ttirract't carryiiiir the typical indurated duiio sands and beatdi cong-loriiorates known an tlie San .Tuan forniatiori (Fig. 35). 3. The Upper Doseolieo 8ta.ge. Xamcd from rlie ('X('(dlent rock terrafro with coiisolidatod l)ea.cl) gravel (eloYatiof), 20 to 25 feet) preserved in many places on Descfdieo Island (Fig. 36). Terraocs of ihi's stage on Porto liico were found at the old Port of (Juebradillas hetweeii (Jamny I iHllllKl on. twent-r-flve feet) and rftDHMiiit.'ii of the the liigli tidet o and Qiiehradillas (JO U) t5 feet), near lsal)ela (20 feet), Pt. diguero (10 to 30 feet), and by Miteludl at Pt. Aguila (25 feet), and Guanica Light Hxnise (10 feel;). ^lany of these terraces,^ as on Desetdieo Island and at Pt. Jiguero .Light House, are (;arved in liighl}^ tilted Glder S(>ries rocks, and their mariru; origin is indisf)u,tal)le. •t. The Lower Desechco Stage. Kained from the well defined h:)Wt;'r roek terraces (elevation, 3 U) 5 feet) on, Deseeheo Island (,Fig. ;jr), Lvi- dence on the Porto Bico coast consists chiefly in tlie exj)osures of con- solidated beach sand and gravel a few leet above liigli tide levtd at various points along the nxirth and west (;oasts of the Lares .District. On the beacli at Camuy, these consolidated beach deposits are made up in part 98 SCIENTIFIC SURVEY OF PORTO RICO of calcareous alga^^ marine worms^ and corals^ apparently still in situ, but slightly above high tide level. On the lower portions of the terrace at the old Port of Quebradillas, many corals were found in position of growth at elevations of from 3 to 6 feet above high tide level. Similar occurrences were found at higher elevations in other localities, but no- where offering such unmistakable evidence of recent uplift as here. The Sa^" Juax PoRMATioisr The name San Juan Formation was first applied by Berkey to the con- solidated dune sands of Pleistocene or Recent .age found at many locali- ties along the north coast, notably at San Juan. The term has since been extended to include the firmly consolidated elevated beach gravels, such as those on Desecheo Island. Both types are found along the coast in the Lares District. As these deposits have an important bearing upon the question of Pleistocene and Recent changes in the coast line, a somewhat detailed description of them will be required. THE CONSOLIDATED DUNE SANDS These deposits occur in three different forms 1. As a solid core, anchoring the modern unconsolidated dune ridges. This can be seen at a number of places, notably at Camuy, and at the mouth of Los Cedras canyon. Consolidation of the grains seems to com- mence around the roots of palm trees (Fig. 38). These dune ridges are so thoroughly anchored by the solid core and fixed along the rear slope by cocoanut palms that migration is apparently impossible. 2. As promontories, reefs, or small islands a short distance from the shore. Most of these have the form of spits. A glance at any map of Porto Rico will show the great number of these spits along the north coast, all solidified, all elevated well above sea level, and all pointing toward the west in the direction the littoral currents move. Excellent illustrations in the Lares District are Pehon Pt. near Camuy, Sardina Pt. near Isabela, and Jacinto Pt. near Los Cedras canyon. Fossil palm roots (molds) are found in the upper portions of these outcrops, but never occur below 4 or 5 feet above sea level. 3. Capping the elevated beach gravels. The best illustration of this type is found at Pt. Jiguero Light House. In this occurrence, molds of cocoanut palms are well preserved (Fig. 39). In general, consolidated dune sands may be distinguished from con- solidated beach sand by the following criteria: IIVBBdlll), GEOLOiJY OF THE LAUKS DimiiKJT Oij 1. The (liino .sands have crus.s-beddiiig (jipping f-Jiaractori.stieallv as high as :i()% while tlio clips cf the bciich saiul he<hling are very inueli le.^s. 2. Tlie consolidated dime sands iitn'er eontfiiji large peljbh^s. The only fossiJ.s are small sliells wliich eould be eas^ily moved by wind. 3. The consolidated dune sands contain caleareons mohjs of palm roots (Fig. 38). 4. The consolidated dnne sands contain relHtively few fossils; the con- solidated beach sands ha\x» cxniasional strata nui<h> up cd' little else but fossil shells. Jt is very impurtant to distinguish, between dime and bcsacli sand in i mW^^"t::j^-X^i:..-.^. Terrace of t.lie Lower DeseelH-n SSI ">f the T'pypr Deseclie necessarily show evidence of nplift. since the process of solidification (cementing wdtli CaCO,/) takes place at»o\-e and l>elow sea level. Till-: COXSOLIDATI-:!) BMICII DJ-M'OSITS These consist of both sajids and gravels, in. all stages of cementation occnrring from below sea level to elcva,tions 1'ar above tlic rcacli of the waves. The elevated gravels are generally strati fietl and vei-y fossil iter- ons. Thc'y differ m no r(>spect from the gravels on the presejif: l)eael)cs. The yame is J.rue of the i>levated and present day Iteaeli san<ls. Both sands and gravels are nnule up in part of material derived from the 100 SCIENTIFIC SURVEY OF PORTO RICO Cretaceous rocks, mixed with material from the Oligocene limestones and Pleistocene or Eecent shell fragments. The rock is in some localities so firmly cemented that it might readily be mistaken for basal sandstone or conglomerate of the Tertiary series. On Desecheo Island, guano is the chief cementing material, but in other localities it is calcium carbonate. EVIDENCE OF UPLIFT Lobeck (1922) has advanced the theory that many of these elevated gravels may have been deposited by storm waves, and hence are not evi- dence of uplift of the coast. There are two considerations which make this theory doubtful 1. At the old Port of Quebradillas, on the marine bench previously described, there stands an old stone structure (Fig. 40) in which the roof, cross-beams, and other wooden parts have long since turned to dust and disappeared. This old building is at least 100 years old.^^ It stands upon the high inward portion of the elevated marine bench, at an eleva- tion of about 12 feet above the sea level. Firmly cemented to the floor of the surrounding rock terrace are gravels, fossiliferous sandstone, and large coral heads (msandra) above the reach of the average waves. Within the old stone building, however, none of these materials are found. Although the open doorway faces the sea, only wind blown sand has found entrance. Furthermore, the building fails to show any injuries which might have been caused by hurricane waves. 2. The beach gravels and fossiliferous beach sands are in many places too well stratified and too well assorted to be the work of hurricane waves. They show no differences from the present beach deposits at sea level and below sea leveh THE AGE OP THE SAN JUAN FORMATION It has been pointed out that there are very few exposures of San Juan formations in the Lares District on terraces higher than the Upper Desecheo Stage. Mitchell (1922) has shown that the highest fossiliferous gravels and the highest outcrops of typical San Juan formation (Cabo Eojo Stage) contain fossils of Pleistocene or Eecent age, in any case not older than Pleistocene. In the Lares District, the writer made a collec- tion of more than loOO specimens from the modern beach, the San Juan formation of the Lower Desecheo Stage, and of the Upper Desecheo Stage, representing a total of 25 localities on the coast of the Lares Dis- 10 Personal communication from Sefior Ramon Cordova, Sub Commissioner of Agricul- ture and Labor, Porto Rico. 'HrHHARTK (iHOLOar OF TJIE LARES DhSTh'ICT JOl triet mnl I)e>e(;lu'o Island. Of tlris nuiteriiil, 103 npeeies and varieties were iflentificd., conipriHiiig" moHiiHcs, c-erals, echinoderms, and enistacf^a. The data thus gatliererl niav bo snrnmarized as follcws: mem, JmiMki. t palm JoilietJdc'd in the roek. 1. Xiiuiher of genera jdenlifie<l in modern beaches was 5^>, and 47 in San Jnaii formation. 2. A'unibor of speeies and varieties idcntifietl in modern Ijeaehijs was !>2. and 50 in San Jiiaii formation. a. Xiind.ier of molliisctan sp(H;ie.s not reeordiH] by Dall (1901) is in San Juan formation and 2 in modern beaebes. 103 t^ClHKTIFlC SURVEY OF PORTO RICO 4. ^X iiiTiber of «|MKi'es or varic'ticH in Haii Jiinii forinatiuii of the I'ppt'i* Desi'clux) Staj^v whitth wiAre not I'oiuh] on tho riio^Juni bcrtclios was 11. 5. Xiirnl>crs of species or varieties in SaJi fjuaii formHtion of tho fjinver Disscelioo Stage which were not i'ouiKi on the ;n,UHlern l)eachi;ti wan 4^. G, NmntNT of inolluscaii species in the iiiuileni heacrht'K which Hceiii to nhow nligiit mutational changes w'lien coinpared with s|)eeiinej).s of the same speejes from tlie San Juan foi'matioii is Ci. T. Other faunal diifereuees wh,ieh CHOOot be recorded quantitatively. (n) Tlie marked difference in faunal content of certain ehn-ated beacli gravels and the modem or present beach gravels innnediafely adjacent. (b) Some of the most abundant molliiscan species loiind on the nioicm beaches are rare in the San Juan formation, and vke vemi. Tin's dilTnrence is more prononn(!d for the Upper Desecheo Stage ihaii for tho Ijower. {(•) (/crtain corals, particularly Mmmdm eUi'osa (EH. and Sol.) and B-Ia'andm lahyrirdhiformu (Linn)^^^ arc extremely abimdarit in tlie San .Jiian formation ami in situ on tlie (Jevated. benclies; bnt on the adjacent snbsea flats, where Pariies sp., Sidermlrma nid'ums. (J:'alla?). and « Bc'termuiatiuns of c HUBBARD, GEOLOGY OF THE LARES DISTRICT 103 Agaricia sp. grow in great abiindance, Mwandra is conspicuous by its absence. CONCLUSIONS The faunal differences above noted are too numerous and persistent to be accidental. From a careful study of the fossils and from the various data described above, the following statements seem highly probable : 1. The deposits of the Upper Desecheo Stage are distinctly older than those of the Lower Desecheo Stage, and both are older than the present beach deposits. The higher gravels, therefore, cannot be attributed to hurricane waves, without ignoring certain facts of critical importance. 2. The following ages may be assigned to the elevated beach deposits of the different stages of the uplift (a) Isabela Stage—Early Pleistocene. (h) Cabo Eojo Stage—Late Pleistocene. (c) Upper Desecheo Stage—Post Pleistocene. (d) Lower Desecheo Stage—Eecent (Historic?). The Plata Deposits The Playa Plains have already been described. The material under- lying these plains is for the most part fine-grained alhivium, de})osited in horizontal and very regular, extensive strata. Typical exposures are to be seen in many places along the lower courses of the JUo Culebriuas and Ilio Ahasco, where these streams have intrenched tbeir channels, exposing in some places (as due sonth of Ahasco) at least 15 feet of strata. Except for a few pebble beds here and there, the material is very fine grained, consisting of carbonaceous chiy, silt, and sandv loam. The fossils are chiefly fresh water or land nu)lhiscs and plant remains, but occasionally one may fiiid beds carrying marine shells (Slro tubus, Area, Mytilus, and others) and brackish water forms (chiefly Nerifirm). Near the coastal margins, the number of intercalated marine beds is greater. Marine and land molluscs are found mixed in the same strata. Old fossil beaches (San Juan formation) are found almost buried by Playa deposits near Camuy, at Columbus Monument, and at other })laces, demonstrating that the Playas have been extended withi]i recent time. This feature might conceivably be the result of recent uplift, but it cannot be used as one of the criteria. No marine terraces or deposits were found around the interior borders of the larger Playas of the west coast in the Lares District. The smaller Playas (Los Cedras and Guajataca in particular) show evidences of marine action (caves) in the limestone cliffs which form their inland boundaries ,i^ig. 41). HUBBARD, GEOLOGY OF THE LARES DISTRICT 105 A. Plistocene. Formation of the highest terraces during the inter- glacial epochs, and the submarine benches during the glacial epochs. B. Post-Pleistoeene. 1. Eise of sea level and formation of the present playas. 2. A series of uplifts, affecting the western part of Porto Eico, initiated possibly in early Pleistocene time, totaling at least 200 feet, and possibly continuing at the present time. In the earthquake of 1918, which wrecked so many towns on the west coast of Porto Eico, the tremors travelled from west to east (Cordova, 1918). The probable source was somewhere in the submarine banks not far from the west coast. This earthquake is of special significance because it may indicate that the movements which elevated the western end of the island in Eecent time, are still going on. However, further evidence is needed to settle this question. MIXEEAL EESOUECES lliON No iron ore in commercial quantity occurs in the Lares District. Several small patches of limonite soil Avith limoite concretions, derived from serpentine, are found south of Aguada. The locations are shown on the geologic map. This ore is like that of the Mesa at Mayaguez and is of the same origin. From samples obtained in" the area south of Aguada, Fettke (1918, p. 673) determines the content of iron as 18.76 percent, which shows that the ore is too low grade to be used, even if it occurred in large enough quantity. Copper Copper is found at a locality 21/^ miles south of Aguada in the area of volcanic flows. The southern portion of this volcanic area, shown on the geologic map, is largely made up of a dark scoriaceous, vesicular, or amygdaloidal augite andesite. It doubtless represents a succession of surface flows, but these are too irregular to distinguish in the expo- sures. Locally the amygdaloidal cavities are filled with calcite, amor- phous silica, and zeolites, but throughout the greater portion, the cavities, where present, are empty. In a small portion of this volcanic area near its southern limit, and covering only a few acres, is the mineralized zone in which the copper occurs. The approximate location of the test pits and adit where it is mined is shown on the map. The rock at this particular location is 100 ^SCIENTIFIV SUKVKY OF PORTO KIVO eonsiiierably faulted, and the aiuygdaloiriai eaYilios are entirely tilled, priiK'i pally with cak-ite, aiul some zeolites. The etipper eecurs in ami adjacent to the crunh ZAXiies, wliieh are lU'ver nnieh more than 1 Foot wide, and a\"era,u;e oidy 2 or 3 inclies. 'Flic richest vein, whieh has hdH-n folhnvcd J'or a ,<h(>rt disiauee in^ the ailit, averages r^i/^ iiiebeg thick, strikes north 25"^' west, and dips 58 '" to ihc northeast. Otlier veins eiieounlercd in the exeavation range I'rtrni i/o to 2 inetuis thick, strike east and wes^t, and dip at angles ot from ?0'' to DO". The adit has penetrated the side of the small hill for a distaiiee of only 27 feet and View 1 hM It? KO Kea cuvi.' »<. H tJl C'MJras y an iritrri to a ileia h t> two erlg<^ at i w i la.va dune rMgp, kIi.h HNl <Jiii'i!ig r The diiiH* f jKl n has a eross-seeti(jii of 9 by 6 feet. Besides the (uipper-bearing veins, many minor veiulehs weri' euconnt;ered, carryi-iig ealeite ami stilbite. Tlw, ore emiHists (d' native copper, chaleoeitc, malachite, tetrahedrite, and ayarrite, of which the first is hy far the most ahiiiidant. It occurs in small irregxilar masses in the veins, and with, the nnihiebite and ebal- cotute, forms a matrix for the crushed fragments of wall roek. Aziirite is frequently found enenisting the otlier copper minerals, hut never in any great quiintity. Occasional amygdaloidal cavities and blow holes adjacent to the mineralized erusli zones are filled with native copper, nVUHARJ), (ffjOIA)aY OF THE LAIiHS DlHTIilCT 107 iihually iiiterruixod witli iiiirior airioiiiits oi' the oilier copper minerals. Ojie ui tlvose large nuggets, almost cjilirelv of iiativo coppur, u sliown in Fig. 12. This prcipcrtv;, covering 160 acres, h owned by Sefior Antonio San- eliez and ,ju(jge Luis Vadis of Agnada, Seilor Poali of (Jatano and l>r. dinieirez oC Agnadilla. 'JMiese gentlemen Inivo rented adjoining land dor exploration. In addilJon to the small adit, wlrieh represent.s^^he ehiid' excavation on llu! property; several lest pit-? have been sunk at various Fio. 42. Nugget of iiatin in Ific localhm stmth of Aff found. Altogether, alK/ut five Ions of ore have becjn ship|ied to Agnada. All work has been done by hand labor, no niaelvinery of any kind having been used. In the opinion ol' the writer, this prospcid will not warrant the distalla- tiun of ebiborato maeliiiKsry or expendituR! of large sums of luoiHyy for developing and exploitation. While tlus ore is rich and the veln.-i nuruerons, the area involved seems to be very limited in extent. How- ever, tlie property will doubtless yiehl eoiisiderable profit if worked on a small scale with sinrple methods, u.'^ing hand bdjor and tra^isportatioii by oxcarts. TJic topograj)liy between this property and tluj town ol' Agnada on the Anunlcan railroad, is not excessively hillj, and the dis- ta,iicc not great. Improvements in ttic present cart, road could 1)0 nnide 108 SCIENTIFIC SURVEY OF PORTO RICO at small expense. It is to be further recommended that more test pits be dug in this area, most of which is covered by clay soil. Kaolin Kaolin of high grade of purity occurs at many places in the Older Series, but especially along the road south of Lares^ near the Eio Blanco (Fig. 2). For the present, however, with the scarcity of fuel, distance from railroad, and lack of any nearby market, the material will remain of potential value only. Brick Clay Clay is the most common type soil in the district, especially in the moimtainous portions developed on the Older Series rocks. Most of these clays would require mixing with sand before using for brick manu- facture. Considerable red clay and black clay loam is developed in the Tertiary limestone area north of Lares, particularly along the belt of Cibao limestone. Generally this material is too high in organic matter to be of any value, and otherwise it is altogether inaccessible. Along the belt of Quebradillas limestone, particularly over the plateau west and southwest of Isabela are many areas of red sandy clay which may some time prove valuable for brick. Some brick has been made at Lares, and doubtless at other localities from time to time, chiefly for local use. The only place where brick is now being made is in and around Mayaguez, where, according to the reports, there are three plants, all of smaU size. One of these was visited by the writer and the following notes regarding it may be of interest: The material used is taken from the small flood plain of the Yaguez Eiver, close to the plant. The section exposed in the excavations shows a dark red clay overlain by 6 feet of lighter colored reddish brown clay w4th a capping of 1 to 4 feet of river gravel. The plant contains three compound ovens, a mixing pit, and drying racks with wooden molds. The mixer consists of a circular pit in which is set a center-post. On this post as pivot is mounted an horizontal shaft, counterweighted at one end and at the other hitched to a pair of oxen, who operate on a circular path surrounding the pit. • A large cart wheel, mounted on the horizontal shaft ploughs through the clay and water in the pit and serves as the mixing device. This wheel revolves on the beam, and is regulated in position from center to periphery of the pit by a system of ratchet and worm gears operated from the end of the shaft. In the pit, with the clay and water, is mixed sand from the river channel to the extent of one cartload per 1000 bricks produced, or for HUBBARD, GEOLOGY OF THE LARES DISTRICT 109 a single filling of the pit^ 12 cartloads of red clay (known locally as the "black clay^" because of its relatively dark color), 12 cartloads of the reddish brown clay, and 6 loads of sand, with water to give proper con- sistency. It takes 1 day to fill the pit, 2 days for the mixing, and then the material is dumped by hand into the molds (wooden frames laid on planks), and left to dry. This takes 8 days in dry weather. Frames are then removed, and the bricks loaded into the ovens, requiring two days to fill. Fires are built during the third day, starting with wood charcoal, and then adding coke (one-inch size). Two cartloads of coke are said to be required for every 15,000 bricks. The bricks are baked for 24 hours. The shrinkage of the sun-dried brick was found to be high, between 20 and 30 per cent by volume, with further very slight shrinkage after firing. The price obtained (in 1916) in the local market was $10 per thousand. The quality of the brick is very poor, owing to the poor material used and lack of proper mixing. The apparent success of this enterprise, with its more or less primitive methods, would seem to war- rant further development of the brick industry, using more up-to-date methods, and taking a full advantage of the more valuable clays and other materials occurring abundantly in this portion of the island. Lime Pure limestone for burning purposes may be obtained at so many easily accessible points on the American Railroad along the north and west coasts from Camuy to Aguadilla that no specific localities need be men- tioned. The material as a whole, however, will not be found as good for this purpose as the limestone farther east from Camuy to San Juan, and on the south coast near Ponce. Building Stone No first-class building stone is to be found among the Cretaceous rocks of the Lares District. The lime-shales are too thin-bedded and fractured to obtain slabs of proper size. The igneous rocks are of little value owing to the weathered condition, difficulties in quarrying, and high content of ferro-magnesian minerals and oxides of iron. The best build- ing stones are to be found in the Tertiary limestones. At various places along the north coast in the Quebradillas limestone, hard, flinty beds alternate with chalky layers. These hard strata can be easily quarried into large blocks. In many places, however, the rock will be found too massive and riddled with solution cavities to be of much value. 110 SCIENTIFIC SURVEY OF PORTO RICO KoAD Metal Eoad metal of first-class type is being quarried in many parts of the Lares District, chiefly along the antomobile roads where they traverse rocks of the proper type. On the Lares Eoad, andesite porphyries are used in the neighborhood of Lares. N^ear San Sebastian, the large coral heads, imbedded in the soft marl of the San Sebastian shale horizon are accessible in cliffs along the road ; they are easily quarried, and make an excellent and durable material. Farther west, around Aguadilla, the massive Los Puertos, and underlying limestones are quarried exten- sively. Xorthwest from Ahasco, the lime shales are quarried along the road at several places. These hard beds are easily broken to proper size and the material is of lasting quality. On the Mayaguez-Las Marias Eoad the various andesitic rocks are employed, and at a few places the black pyritic shale, which is quarried near Consumo. This shale is hard and fresh, and apparently a good material, but the high content of pyrite, and the depth to which this shale is characteristically weathered, makes it doubtful if it will last any length of time as road ballast. In general, the Tertiary limestones produce the smoothest and best road beds of any rock to be found in the district. Where igneous rocks are used, a cover- ing of this limestone would produce ideal conditions. Lignite At Lares, San Sebastian, west of Moca, and many other points along the Lares Eoad, the basal shales (San Sebastian) carry lenses of lignite of no great extent or thickness. They are seldom over six inches thick at a maximum, and average much less. They are extremely high in marcasite, and hence more or less completely oxidized where exposed at the surface. These coal lenses are valueless except occasionally for local use. An enterprising Lares blacksmith has dug out considerable quanti- ties for use in his forge. The frequency with which these lignite lenses are exposed in the San Sebastian shale along the Lares Eoad has led many of the local residents to believe that coal must occur in commercial quantity. This false impression has spread to other parts of the island, where people will often refer to the coal of the Lares-San Sebastian locality. Such lenses of this material as occur would not repay exploita- tion unless they were continuous over considerable distance. That they are not continuous can be seen from observation, and from a considera- tion of the conditions of origin of the San Sebastian shale in which they occur. HUBBARD, GEOLOGY OF THE LARES DISTRICT HI Guano Bat guano, taken from caves in the Tertiary limestone area, is used to some extent for fertilizer by local farmers. Bird guano is found on the Desecheo Island, chiefly as a cement matrix in the elevated beach gravels (San Juan formation). However, it has not accumulated as abundantly as on Mona Island, where it is now being quarried and shipped to Porto Rico. Oil 1^0 oil shales or surface indications of oil w^ere found in the Lares Dis- trict. The San Sebastian shale is the only formation observed in which hydrocarbons might originate, but lignite seems to be the only carbona- ceous material present. In view of this, there is very little to warrant an exploration for oil in this part of the island. Nevertheless, the San Sebastian formation is not essentially different from formations in Trin- idad which carry bituminous material, and therefore the possibility of oil in the Tertiary series of the Lares District must not be overlooked. The most promising location for test drilling is the area north of tlie Tertiary cuesta between the meridians of CoUazo and San Sebastian, since it is in this area that the San Sebastian shale attains its maximum ttrickness, and greatest development of carbonaceous clays. Summary The chief assets of the Lares District are the varied soils, and the agricultural products. Important studies in this field are being made at the Agricultural Experiment Station at Mayaguez. Of mineral re- sources, clays, road metal, and natural fertilizers will prove to be the most important. Copper is the only metallic mineral which shows possi- bility of commercial importance. Tlie reported presence of coal beds proved to be thin seams of lignite in the San Sebastian shale, and are of no value. iN^o oil shales Avere found, and there is no good evidence that oil exists in any of the rocks of the district. BIBLIOGRAPHY Alexander, W. A. 1902. Porto Rico, its climate and resources. Bull. Am. Geog. Soc, XXXIV, p. 401. Allen, J. A. 1916. An extinct octodont from the island of Porto Rico, W. I. Ann. N. Y. Acad. Sci., XXVII, pp. 17-22. 112 SCIENTIFIC SURVEY OF PORTO RICO Anthony, H. E. 1916. Prelimiiiury report on fossil mammals from Porto Rico. Ami. N. Y. Acad. Sci., XXVII, pp. l<K5-203. Berkey, C. p. 1915. Geological reeoniioissance of Porto Rico. Ann. N. Y. Acad. Sci., XXVI, pp. 1-70. 1919. Introduction to the geology of Porto Rico. Scientific Survey of Porto Rico and the Virgin Islands, I, part 1, i)p. 11-29. Brixton, N. L. 1919. History of the survey. Scientific Survey of Porto Rico and the Virgin Ishmds, I, i)t. 1, pp. 1-10. Brown, A. P. 1913. Notes on the geology of the island of xVntigua. Proc. Acad. Nat. Sci. Phila., LXV, pp. 598-610. Cleve, p. T. 1871. Geology of the northern West Indies. Kungl. Sven. Vet. Akad. Handl., IX, No. 12. 1883. Outline of the geology of the northeastern West India Islands. Ann. N. Y. Acad. Sci., II, pp. 185-192. Cordova, Ramon G. 1918. Earthquakes. Bull. Dept. Agric. and Labor, San Juan, P. R., No. G. Crampton, H. E. 1910. I»orto Rico. Am. Mus. Journ., XVI, Jan. Dale, W. H. 1898. A table of the North American Tertiary horizons, correlated with one another and with those of western Europe, ^vith annotations. 18th Ann. Rept. J]. S. Geol. Surv., part 2, pp. 323-348. Dale, W. IP, and Simpson, C. T. 1901. The mollusca of Porto Rico. U. S. Fish. Comm. Bull, for 1900, vol. 1, pp. 351-524. Dinwiddie, W. 1899. Physical features of the island (Puerto Rico). Harper's Weekly, XLIII, p. 248, IMarch 11. 1899a. The great eaves of Puerto Rico. Harper's Weekly, XLIII, p. 293, March 25. Domenecii, M. K. 1890. Mineral resources of Porto Rico. Mines and Minerals, XIX, pp. 529-532. Dorse V, C, W. 1902. Soil survey from Arecibo to Ponce, Porto Rico. Report of Bureau of Soils, U. S. Dept. Agric, pp. 793-839. Falconer, J. D. 1902. Evolution of the Antilles. Scot. Geog. Mag., XVIII, pp. 309-370. Fettke, C. R., and IlLn'.BARD, B. 1918. 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Scientific Survey of Porto Rico and the Virgin Islands, I, part 4. Matthew, W. D. 1916. New sirenian from tlie Tertiary of Porto Rico. Aim. N. Y. Acad. Sci., XVII, pp. 23-29. Maury, C. J. 1917. Santo Domingo, type sections and fossils. Bull. Amer. Paleont., V, Nos. 29 and 30. 1919. On the correlation of Porto Rican Tertiary formations with other antillean and mainland horizons. Amer. Jour. Sci., XLVIII, pp. 209-215. 1920. Tertiary mollusca from Porto Rico. Scientific Survey of Porto Rico and the Virgin Islands, III, part 1. 114 SCIENTIFIC BURV^Y OF PORTO RICO Mitchell, O. J. 1922. Geology of the Ponce District, Porto Rico. Scientific Survey of Porto Rico and the Virgin Ishmds, I, part 3. Newberry, J. S. 1882. Geology of the West Indies. Trans. N. Y. Acjid. Sci., I, pp. 23-24. NiTZE, H. C. B. 1899. Investigation of some mineral resources of I*orto Rico. 20tli Ann. liei)t. U. S. Geol. Surv., part il pp. 779-878. Reeds, C. A. 19K). Report of progress. Ann. N. Y. Acad. Sci., XXVI, pp. 435-438. 1917. Report of progress. Ann. N. Y. Acad. Sci., XXVII, pp. 280-282. 1919. New base map of Porto Rico. Scientific Survey of I*orto Rico and the Virgin Islands, I, part 1, pp. 30-31. Semmes, I). R. 1910. Report of progress. Ann. N. Y. Acad. Sci., XXVI, pp. 433-434. 1917. Report of progress. Ann. N. Y. xicad. Sci., XXVII, pp. 279-280. 1919. Geology of tlie San Juan District, l*orto Rico. Scientific Survey of Porto Rico and the Virgin Islands, I, part 1, pp. 33-100. Stevpirs, W. 1895. Zur Kentniss Puerto Ricos. Mittheil. Geog. Gesell., Hamburg, Heft. 11. pp. 217-230, pi. 4. Spencer, J. W, 1903. On tlie geological relationship of the volcanoes of the AVest Indies. Victoria Inst. Jour. Trans., XXXV, pp. 189-207. 1904. Tlie Windward Islands of the West Indies. Trans. Canadian Insti- tute, \'II, pp. 351-371. Vaigiian, T. Waylani). 1902. Earliest T(»rtiary coral reefs in Antilles and United States (Ab- stract). Science, XV, March 28. 190G. Some littoral and sublittoral physiographic features of the A^irgin Islands and northern lA^eward Islands, and their bearing on the coral reef problem. Jour. Wash. Acad. Sci., VI, No. 3, pp. 53-00. 1919. Fossil corals from Central America, Cuba, and Porto Rico, with an account of thc^ American Tertiary, I*leistocene, and Recent coral reefs. Bull. U. S. Natl. Mus., No. 103, pp. 189-524. Ii919(/. The biologic cliaracter and geologii* corrcdations of the sedimen- tary formations of Panama in their relation to the geologic his- tory of Central America and the West Indies. Bull. IT. S. Natl. Mus., No. 103, pp. 547-612. Weather Rej^ort (Annual). 1914. Climatologlcal data, Porto Rico Section U. S. Dept. of Agric, Weather Bureau. W^harton, W. J. L. 1894. The physical condition of the ocean. Geog. Jour., IV, Sept., p. 255. Willis, Bailey. 1912. Index to the stratigraphy of North America. Professional Paper 71, U. S. Geol. Survey, pp. 13, 342, 348, 794, 801-803, 887. HUBBARD, GEOLOGY OF THE LARES DISTRICT 115 Wilson, H. M. 1899. Water resources in Porto Rico. Water Supply Paper No. 32, U. S. Geol. Survey. 1899a. The engineering development of Porto Rico. Eng. Mag., XVII, July, pp. 602-G21. 1900. Porto Rico, its topography and aspects. Bull. Amer. Geog. Soc, No. 32, pp. 220-238. Anonymous. 1778. North America and West India Gazetteer. London (6), XXIV, p. 216. 1898. Porto Rico, its natural history and products. Scien. Amer. Suppl., XLVI, July 30. 1916. Overcrow^ded Porto Rico. Geog. Review, March. 4-; f '^: =^3V 'SI?^^C *" ->'* ' ; •'', K 4'^; ;4^^ ;-; *.' • \ V Sf :«. DO NOT C :'UL,ATE ^1^^^^^**^^ vM •jfinfifA': r * ;::;: ihf 11 nf .ll- ;| %j !f||| H:i,dbbai:-^:i:i. Z: 2!; [iirNt.) fiJOB ljMiviFS"!T)N MEi-ACi LriMF'S'ii Q^NE. kHES fOHMfikTlOfi .;:; :S:.:; :::> I;::' i:.:): J\ C' 'T i A *-. ^i -C iJ; A i itr ^a i;:.: MMi:F;;:hi:i--T m .::«T :;. -it 4 J ;|, :::.....:?:;! i I!;..:.., i; ij^JW'Tf ¥ ::....? 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