ii?iii ■^^fy?J:i';.:':0\(:i;!% IS iiliiiiiiiii .NT irfc- '^yS; lll»l !m:-:j ^:;'-;^v ::-::-:j' ■ ■:; Mia-i::",.: ::::■::.■ - ■■- ■■■:■• '■■ifli ■'■"jr---^ ^-■■^ki> I,' K NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgie Islands VOLUME I^^^--Part 4 The Physiography of Porto Wicir^^-A. K. Lobeck NEW YORK: THE PHYSIOGRAPHY OF PORTO RICO By Armin K. Lobeck CONTENTS Page Introduction 302 Physiographic history 304 The oldland 307 Volcanic tuffs and shales 307 Granite. 309 Serpentine 309 Limestone 309 Structure 310 The first or higher peneplane 312 The peneplane 312 Monadnocks 314 The Luquillo Mountains 314 The Cordillera Central 315 The second or lower peneplane 315 Deposition of the Tertiary coastal plain 320 Uplift and dissection of the Tertiary coastal plain 327 Character of the uplift .327 Assumed faulting 328 Dissection of the coastal plains 329 The northern coastal plain 329 The belted character of the coastal plain 331 The inner lowland 331 Lares limestone cuesta 332 Cibao limestone lowland 333 Los Puertos limestone cuesta 333 Quebradillas limestone lowland 334 Special features ''»34 Isolated remnants of the Tertiary 337 The southern coastal plain 337 Eastern portion, Juana Diaz to Yauco 337 Western portion, Guanica to Cape Rojo 339 Sink holes and the question of assumed faulting 339 Vieques • -"'^^ Recent slight submergence •^^'^ General ^*^^ East coast ***^^ South coast ""'^^ West coast ^^"^^ North coast ^^^ Theoretical considerations 3^6 (301) 302 SCIENTIFIC SURVEY OF PORTO RICO Pago Amount and cause of submergence ; glacial control theory 34(j Submarine profiles east of Porto Rico ; origin of Cordilleras reefs 347 Recent changes 34 s^ Alluvial deposits 34^ Wave and wind work 354 Cape San Juan region 354 East coast 35^ South coast 35j^ West coast 3^^ North coast ^qq The San Juan formation 3^4 Summary 3^1* Probable very recent slight emergence 30(^ Brief notes on adjacent islands 374 Desecheo 374 Mona 372 Vieques 373 Culebra 374 Muertos 374 Acknowledgments 375 Bibliography 376 INTRODUCTION Porto Eico^ the easternmost of the larger Antilles^ is a rugged moun- tain mass trending east and west for a distance of over one hundred miles, flanked by uplifted limestone plateaus on its north and south sides. It lies well within the tropics^ in the latitude of 18 degrees north, and is subject to the influence of the steady northeast trades. The persistence and regularity of the trade winds can not fail to excite the notice of the northerner who is accustomed to more variable conditions. Their effect upon the island is pronounced and shows itself in the marked contrast in precipitation between the north and south sides. An elevation of about 2000 feet prevails throughout much of Porto Eico, so that the trades are forced to rise and precipitate their moisture over the northern two-thirds of the island (Fig. 1). Out of the brilliant sky dense cloud masses form with great rapidity over the uplands and several downpours may be ex- pected during the day throughout most of the year. But when the winds reach the lower lands of the southern coast they have not only lost a large part of their moisture, but in their downward journey they have been transformed into drying wdnds, with the result that this whole coastal area is almost barren and parts of it experience months, or even years, without rainfall. Irrigation is there essential for the cultivation of largo LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 303 i-rops, and in tlie southwestern corner of the island, where there occur the longest periods of drought, considerable areas are densely covered with cactus. Over the rainy windward portion of Porto Kico streams are nu- merous and always flowing. In the dry leeward portion, river courses are intermittent and coalescing alluvial fans fringe the shallow coast for many miles. The trade winds are also responsible for the strong waves of the north coast, which have thrown up beaches and bars and have developed a magnificent stretch of cliffs. This is in marked contrast with the south- ern coast, where there is almost no disturbance to the water and mangrove swamps abound. The shape of Porto Rico in general is that of a rectangle, with a long east-and-west dimension of 110 miles and a north-south width of 35 miles, giving the island an area three-fourths that of the state of Connecticut. INCHES RAINFALL PER VEAR Fig. 1. — Map showing the average annual rainfall in Porto Rico The population of Porto Rico is approximately half that of Manhattan Island, roughly between 1,200,000 and 1,250,000 people. It is, in fact, one of the most densely inhabited regions of the globe. Although there are some 65 to 70 villages on Porto Rico, the people are predominately rural and are scattered quite uniformly over the entire island, even throughout its most rugged portions. The largest three cities — San Juan, Ponce, and Mayaguez — have populations from 35,000 to 50,000. Many of the smaller villages contain less than 1000 people, but most of them average between 1000 and 2000. The towns are invariably built com- pactly around the public square or plaza which the cathedral fronts on the <'ast. The only extensive part of the island not settled is the area com- prised in the Luquillo National Forest, which covers the higher portions of the Luquillo Mountains. 30^ SCIENTIFIC SURVEY OF PORTO RICO Coffee, tobacco, sugar, and citrus fruits make up almost the entire agri- cultural output of Porto Eico. Coffee-raising is confined to the upland areas, tobacco is the important product of the Caguas and Cayey valleys in the east, sugar is the great crop on the extensive flood plains and coastal regions, and the raising of grape-fruit and oranges has been de- veloped in the hills along the north coast west of San Juan. In spite of its splendid possibilities the island is far from being self-sustaining in the matter of food, and the great mass of the natives depends almost en- tirely upon the rice, beans and salt fish shipped from the states. Compared with the conditions of living within the continental United States, even in the smaller and more remote towns, life in Porto Eico must seem at first to a visitor from the north very difficult, but to be fair one must take into account the vicissitudes through which the country has passed and realize what splendid headway is now being made in the direction of culture. On the other hand, no visitor, even while adjusting himself to the mode of life in Porto Eico, can fail to be charmed by the idyllic beauty of her scenery and the splendor of her setting imder the tropic skies. PHYSIOGEAPHIC HISTOEY The physiographic history of Porto Eico takes us back to an oldland composed of a complex mass of igneous rocks (Pig. 4). The rock types involve sedimentaries of volcanic origin and intrusive masses ranging in size from those showing only a few feet of outcrop to the largest one cover- ing almost one-eighth of the island. Under the influence of subaerial ero- sion, this oldland was reduced to a rather perfect peneplane except for two well-defined monadnock groups. The smaller of these groups now com- prises the Luquillo Mountains at the eastern end of the island. The larger one is known as the Cordillera Central and forms the main crest of the mountainous area of Porto Eico. It is probable that Porto Eico is essentially a block or horst bounded by faults, and it is likely that several of the larger elements comprising the oldland mass are due to faulting which antedated the first peneplanation. Ko direct evidence on this point is available. Uplift of this the first peneplane stimulated a new cycle of erosion. Over most of the island only a mature stage of dissection was reached in this new cycle, but along the north coast an east-west belt about ten miles wide was reduced to base-level, which for convenience may be known as the second or lower peneplane. A corresponding belt on the south side of the island was also worn down far less effectively, probably be- <^^ M@PH-B=S^ ^l^^m^lZ. jLuAOILlA t y Vv^ '^^^J'tANAri ^''— x« Jp"tow^ A''*~"V>^ v^*-..OE:sE:fHEo t. ^^^ti^^ ^^^v**\ y ^ \,o 'IsN ^oaaita/™ |^;n!51v,«ftoij/(A'' jwooRA&o^ ;» '^» Vrincon ''^^^v^^iv^'^— s^ / |o ■^Xl^ CORD7AL>0^ y , / GUAINA /-^^ ^~T"^^^^ '.-A J CULE8RA 1. ""^^^^^^-.^^ v^ y^T "';^ >.«ROV.sf\. , Y A^^" A TAjWlOoV f--. '■•."S^^^^s ^^§^^^°^^ •^^"^'W ^ 1^ i^GOAsN oLnas 3 I. J y' ) ? /^^jT.- 51 |*UTUADO \ [X/'^S^^^-^'v.^ f ) / \« ••.. ^ ^^bv 3 / J^OMf-RlO > -V^ \ .^t"^!^^^^^ NNu-. r ^^ BARRANQum y-^ V_ 5^y!/^J^te5(j_,\w '^'^^b^sW^-'^ 'T ----^-^™-s^ SVadjont^ <^ cioraS 7 ^I^!!iv<^*^^ / ' MONA 1. ..••5 ;^'r='*""----v „^m^ ..••' f \>^ *<». s / \^ ^eoNWi^^p^ / (~ / r-^e^ /k^^^^^ -C\x ■-" ^ V---^\^2ANGE_R M AM \S \ CAYE^ ^ ^^ { ^^s^ f^^'^'^^ ^^ ^ Jw' >^ENUe\a5 , , ^co'amo .„ \ J^^ VIEQUES 1. V ^ 1 /y^ ^^■ES \ lyPAriLLAS A^^*^" J^-TT^^^ J^UKf^'^^ ^^^=C;^^^^^n^ .--'^'''^^'~^\!N-<;::^^^^^r" —- T~— ^^ iy^^^SijJ^ ITINERARX - — CAPe ROJO^ ^MUFRT05 1. ■^r^>^ /""^"^ 5 )o 15 HU :. Fig. 2. — Sketch map of Porto Rico fihowing regions visited PiQ 3, — Map of Porto Rico showing the chief physiographic regions LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 305 cause of inadequate rainfall. Submergence of much of the oldland mass then permitted the deposition of a coastal plain upon both the Pig. 4. — llie main stages in the physiographic development of Porto Rico 1. The initial complex mountain mass of Porto Rico. 2. After the first peneplanation. Showing the monadnock group forming the Luquillo Mountains and the cordillera Central. 3. The peneplane has been uplifted and dissected and a second lower peneplane has been formed along the north side of the island. 4. The whole island has been lowered, its northern and southern flanks have been covered with coastal-plain deposits, and the mass as a whole has again been raised. 5. Dissection of the coastal plain has taken place. Distinct cuestas appear on both the north and south sides of the island. north and south sides of Porto Eico. That on the north side cov- ered most of the lower peneplane at the east end of the island and 306 SCIENTIFIC SURVEY OF PORTO RICO in the western part even extended so far inland as to rest upon the first or higher peneplane; that on the south side was deposited upon a very irregular oldland surface. Dissection of both the coastal plains was then initiated by uplift. The coastal plain on the north side acquired a well defined belted character, exhibiting two pronounced cuestas, each sur- mounted by the so-called haystack hills of the limestone country. These parallel east-west belts of hills alternate with other belts of lower country, which on account of the sharpness of dissection have a plateau-like aspect. The inner cuesta of the north coastal plain in places fronts a broad inner lowland from which the basal deposits of the coastal plain have been re- moved. The outer or northern cuesta usually looks down upon one of the plateau-like belts just mentioned. On the south side of Porto Rico a distinct cuesta was developed, but occasionally it is obscured by the irreg- ular topography of the oldland upon which it rests. After the dissection of the coastal plains the entire coast suffered a slight submergence, the result possibly of the melting of the continental ice sheets at the end of glacial time. The recent changes which have taken place since this submergence involve the deposition of alluviinn along stream courses and its dissection to form terraces, the building of extensive alluvial fans on the south coast, the silting up of bays, and the work of the waves and wind, which has been most pronounced on the north side of the island. Finally there is some evidence of a slight emergence at a very recent date. For ease of reference the physiographic history of Porto Rico may l)e divided into several events or stages which can be tabulated as follows : 1. The oldland, a complex mass of igneous rocks consisting of sedi- mentaries and intrusives subjected to subaerial erosion. 2. The first or higher peneplanation with monadnocks. 3. Uplift, mature dissection, and development of the second or lower peneplane on the north and south sides. 4. Partial submergence and deposition of coastal plains on the nortli and south coasts, to be known as the Tertiary coastal plains. 5. Uplift, dissection of coastal plains, and continued erosion of the oldland. 6. Slight drowning of entire coastline. 7. Recent changes involving : a. The deposition of alluvium in stream valleys, in drowned bays. and as alluvial fans on the south side of the island. 1). Wave and wind work. c. Slight emergence. LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 307 The discussion of these stages will follow the order of succession of events as given above, and in pursuing it the reader will do well to make frequent reference to the accompanying large map. THE OLDLAND The rocks of the oldland comprise what has been termed the Older Series by Dr. Berkey. While over large areas the erosion of the oldland mass has not been appreciably affected by variations in the character or the structure of the rocks, there are districts where quite the reverse is true. A brief consideration of the rock types and the simpler deforma- tions which have taken place in the older series will therefore be of dis- tinct help in appreciating the present topography. YoLCANic Tuffs and Shales As is to be expected in a region isolated from a continental mass, prac- tically none of the formations have a clastic derivation. Volcanic tuffs represent the most abundant of tlie rock types and they are usually massive in habit and made up of andesitic fragments. This implies a practical absence of quartz, a condition which is true also of the shales. These two rock types, together with the igneous intrusives of the andesite- diorite family, make up the bulk of the older series, and as they grade into and are frequently indistinguishable from each other it is practica- ble to draw a few generalizations regarding the effect of erosion upon them. Although it is usually true that weathering has gone to considerable depths, 20 to 30 feet being quite common, most streams are cutting into rocky beds. Berkey (1915, p. 34) has remarked upon the striking man- ner in Avhich badly decayed outcrops resist destruction or removal by ordinary weathering and erosion agents. He mentions three factors to account for this stability of the soil mantle : One is the clinging character of some of tlie vegetation which tends to hind the soil together; another is the small range of temperature variation, which reduces disintegration or disruption tendencies to a minimum; and still an- other is the low content of inert or refractory materials, such as quartz, in the rocks whose destruction has furnished the soils; all of which factors favor the making of a specially tenacious soil. It is probable that the last factor is by far the most important. In general, the hillside slopes throughout the older series are very steep, ^)0 degrees being exceedingly common and 40 to 45 degrees not rare, but this does not hold true in the granite areas where rolling topography and 30S SCIENTIFIC SURVEY OF PORTO RICO gentle slopes are ttie rule. In spite of the fact that the tenacious char acter of the soil, due to its high clay content, enables the vertical cuts ol trails to stand for long periods and prevents the washing of steep hill- sides, it is no uncommon thing to see slumping of the ground even on moderate slopes, and occasionally land slides of really great proportions. A large landslide may be seen a few miles north of Yauco on the trail to Maricao, where the mountain sides for the space of a thousand feet above the valley floor have given way. Another factor which may be of significance in developing steep slopes and the characteristic cuchUlo or knife-edge divides on the members of the older series is the torrential character of the rainfall. The average annual rainfall over the upland portion of Porto Kico is between 80 and 90 inches, or more than twice that for the vicinity Of New York. But, unlike the precipitation of middle latitudes, where the duration is to be measured in hours, and even days, and the amount in hundredths or tenths of an inch, the average duration of a shower in Porto Kico is ten or twelve minutes, and there are numerous instances of successive showers which totaled 10 inches rainfall in twelve hours, while amounts of 4 to 5 inches in twenty-four hours are of frequent occurrence. A record of 23 inches for twenty-three hours, as an example of an extended period oi heavy precipitation, and of 1 inch in nine minutes for a short period may suggest to the reader that important consequences must result from th(^ accumulation and run-off of so great a volume of water in so brief a period of time. Those portions of Porto Eico which have only moderate rainfall, but apparently the same general rock make-up as that of the mountainous rainy district, exhibit much smoother and more rounded slopes. Another phase of the situation, dependent upon the plasticity of the weathering product derived from the quartz-free volcanics, is the imper- viousness of the soil mantle. This encourages a large and exceedingly rapid run-off, as may be appreciated from the fact that many streams immediately rise 15 to 20 feet after heavy showers. In one case the Plata Kiver, twenty-five minutes after it began to rise, poured over the dam near Comerio in a sheet 15 feet or more in thickness throughout the entire 575 feet length of the dam, the flood continuing all day at 10 feet above the dam. The impervious character of the soil causes not only a rapid run-oif, but also an accumulation of water in all the little pockets and irregu- larities of the surface. In consequence the trails over the upland portioii of the island, except during the comparatively dry months of January and February, are often veritable sloughs of red mud, which is both unctuous and tenacious and exasperatingly slippery. LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 309 GRANITE Isext in size to the areas of tuffs, shales, and associated igneous intru- sives making up the bulk of the island are two or three areas underlain by large intrusions of granite. The largest of these covers most of the southeast corner of the island from Caguas to Yabucoa. In general, the granite is less resistant than the volcanic rocks and most of this region has been reduced to a more or less open lowland. At the north the limits of the lowland coincide rather closely with the extent of the granite, but the southern margin is less definite and much of the granite still retains the upland level. Throughout the granite area the hillsides are covered with large weathered blocks and boulders, and the soil is distinctly gran- ular and resembles a fine gravel. There is a marked contrast between the cleaner condition of the native huts and the tidier way of hving in this area, where the soil is porous, and the usual filthy condition wliicli prevails over most of the island where mud is such an important factor. A second granite mass, of unknown extent, is found in the region about Utuado, in the west central part of the island, but its effect upon the to- pography is not marked. The granite is massive and jointed, and in a minor way some control is exerted upon stream position. Even a master stream like the Arecibo Eiver follows joint planes for short distances. Pot-holes, a feature observed practically nowhere else in Porto Kico, occui' along this river, and their presence is ascribed to the substantial and massive character of the granite. SERPENTINE Besides the granite intrusions there are in the western part of the island one or two masses of serpentine of smaller areal extent. They resist ero- sion more readily than the adjacent rocks and appear as round, full-bodied forms topped by more or less level surfaces. They constitute the so-called mesas near Mayaguez and similar larger masses farther east. The weath- ering of the serpentine has produced a thick, red covering of limonitic soil and has given rise to the smooth-flowing outline of the crest of the mountains which is distinctly characteristic of the region north of San German, where the bulk of the range looks much like the back of a gigantic elephant. LIMESTONE The fourth group of rocks occurring in the older series exerts by far the most effective control over the erosional forms. These are the crys- talline limestones. Almost invariably they stand up as distinct ridges. 310 SCIENTIFIC .SURVEY OF PORTO RICO despite their high solubility and the frequent caverns to which this char- acter gives rise. The two most prominent of the limestone belts make ii[) the chain of hills known as the Cerros, running from Cabo Kojo to Yauco, and a parallel chain to the south extending from Boqueron Bay to Guanica. The relation between these two belts is discussed below under "Structure." Another limestone ridge runs from Juana Diaz eastward to Salinas^ the formation being known as the Coama tuff limestone. Other smaller belts occur in the interior^ as, for instance, the Aguas Buenas limestone ; but they are usually too insignificant in thickness ma- terially to affect the topography, although their ragged white outcrops may be a conspicuous feature of the landscape. STRUCTURE The usual trend of the members of the older series is in a west to east or a northwest-southeast direction (Fig. 3). Over large areas it is next to impossible to determine the strike at all, owing to the massiveiiess of the formations and their decomposed condition ; and even where the strike is to be recognized the yielding character of the rock, due to the advanced state of decay, makes it a negligible factor in controlling stream position. South of San Juan a definite alignment of features in a northwest- southeast direction reflects a greater degree of structural control than is commonly the case in the volcanic members of the older series. There is a high, continuous ridge which persists from near La Muda southeast- ward to the gap cut by the Eio Grande; and running parallel to the ridge on its south side is a distinct subsequent lowdand developed on weaker rock, not occupied throughout its length by any one stream. The road from San Juan to Caguas follows this narrow lowland for several miles east from La Muda. At the west end of the island the east-west trend of the Atalaya Eange is in sympathy with the strike of the formations, as may readily be seen at many places on its southern flank. In the region about Coamo Springs there is a well-defined parallelism in the series of ridges developed upon the Coamo limestone and its asso- ciated formations. This limestone, which has already been mentioned, strikes from the northwest to the southeast, has a pronounced dip to the southwest, and gives rise to the most marked feature in this stretch of the southern coast. The main ridge or hogback in its highest part, in the vicinity of Juana Diaz, has an elevation of almost a thousand feet higher than most of the hills of the coast region. Its uneven crest, interrupted by several deep water-gaps, descends more or less regularly until it pitches beneath the alluvial plains near Salinas, but large outliers are foun*l alone? the continuation of its strike at Central A'guirre. LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 311 ro ►^^ P J^ ^ 5 •//;\'77rif/ srin'i^:\' <>r i'>>in:o 'h'lr 1L: 1.1 llir .^^ -M Vil ^wir.l .1.1. ..r t!,.. liiiM |.:u. Til.. nnrtJi a.lul Hcillii llilll.:« ..| I ho ill 1l!i' ....nil Ijinh .,r i!i.> njiiirlm.' i^ kn.nvi, ;m>,..! hv .! -Ti.-:- ., ..I- lc<>^ imr;tll.-l .nii.l (.|..ii--af.Ml hills which aiv lh..r..ii.iil)ly sug-er-iix Apiiiihi.-liinn ri swiii.^s arniin.l toward tif south, ami the .lr<.w!H-,l aiilicliiial'viill.'y ivpraH.nto.l bv r'lu.iuen.n Ihiy. 'I'liK Fll.hST on 11 Kill HI? PKXKFLAXE 'I'm; ,Pi%xj"i',L.ANi': 1.;irt ut ! ihi. ivjiiarkii t> -rrikinir t(r /.oinjfK. 77/f; /'//v.v/fK//,' i/7/r (if /'o/r/v* i:n ill.. ..Mer ^.Ti.-. If tlu'iv uvr,. an .-ailiM^ nlaiuninh ' Ih.- |n>!H.i.li.n.- rf|.r.-,.|ii.M' In ih- |Mv.o„i mi,, mill ii^ii ^ ..r the Plnt,.-i iiiul r.ayairi Mitiiiiiuiis skviiiie jiih|"-i\^. nil.., un.l I he rumiHrai i... 314 SCIENTIFIC SURVEY OF PORTO RICO foot. The peneplane was best developed, or at least its remnants have been best preserved, in the central part of the island. All the way froiii the Plata Eiver region westward to Lares its fairly accordant skyliiK^ forms the controlling element in the topography. West of Lares the peneplane has been so extensively dissected that only isolated remnants and spurs are to be recognized, the Atalaya Eange forming the largest unit; and east of Caguas the prevailing level of the country is so much below the upland surface that it must be considered as a lower base-level formed at the time of the second peneplane. The same is true of the region about Cayey (Lobeck, 1919, p. 535). It is believed that the accordant level of the hill-tops on Vieques, hav- ing an elevation of about 600 feet, represents the upper peneplane, this assumption being based mainly on the fact that the Tertiary formations rest upon a lowland cut below that level. On Culebra there is no accord- ance of level, neither is the Tertiary represented to furnish a key to the situation, but because of similar orders of magnitude of the features to those of Vieques it is probably safe to assume that the tops of the hills roughly indicate the upper level (Fig. 40). MONADNOCKS THE LUQUILLO MOUNTAINS This range stands up as a prominent mass at the eastern end of Porto Hico. The upper peneplane flanks it only on its western side; above it the mountain summits rise over a thousand feet; elsewhere they drop in long radiating spurs to the coast or abruptly to the adjacent lowlands. Having an elevation of nearly 4000 feet, these mountains form one of the most imposing features of Porto Eico. The best explanation for the position of this monadnock mass is the presence of more resistant rock types, though in the field the rocks here appear to suffer weathering as readily as those over most of the adjacent upland. A puzzling feature is the fact that all sorts of rocks are repre- sented in the area, including tuffs, limestones, and even granite, whicli is so non-resistant in the tropics, on the southern flank of the range. Tlie possibility that the mass represents an upwarped part of the upper pene- plane can not readily be entertained because the peneplane remnants ap- pear to fringe it so nicely on the west and to form a narrow shelf on the south, and because the peneplane can not be recognized elsewhere about its base. It is quite possible that a better knowledge of the rocks involved would show that the higher areas are actually made up of more resistant types. LOBEGK, THE PHYSIOGRAPHY OF PORTO RICO 315 The Luquillo Mountains^ as a wliole^ are maturely and deeply dissected, Mild though at a distance they appear as a unit mass, a. traverse of the range shows them to be made up of several culminating peaks separated by gorge-like valleys 1000 to 2000 feet deep. El Yunque, ^'the anvil/'' is the highest of the peaks and has an altitude of close to 4000 feet. Al- though the Luquillo Mountains are forested, rock ledges and bold and imposing cliffs are not uncommon. The Luquillo National Forest covers most of the range and is the only extensive part of Porto Kico still pre- serving the virgin timber. Contrary to a general conception, this orig- inal tropical forest is not an impassable jungle, but is pleasingly open and park-like. Tree ferns abound, and occasionally one meets with flowers of great brilliancy, but there is almost no animal life. Even birds are rare. The sound of running water is always to be heard, so frequent are the streams. The higher parts of the range are almost continually drenched in a heavy mist, l^owhere else in Porto Rico may one walk for hours without seeing one of the thatched huts of the natives or w^ithout meeting people at every step. THE COKDILLERA CENTRAL Practically all of the other residuals rising above the peneplane level are concentrated in a chain of peaks along its southern margin, known ill the western part as the Cordillera Central and further to the east as tlie Sierra de Cayey. Many of these peaks, especially in the west, rise to the same or to an elevation even greater than those of the Luquillo Moun- tains. Viewed from the south, the whole range has the appearance of a ragged escarpment with long, irregular spurs trailing down between steep- walled and cirque-like valley heads. Just as the lower peneplane, to be described later, reached a far greater degree of perfection on the north coast than on the south, due presumably to the abundance of rain on the north side and its almost complete lack on the other, so it seems that the same cause may have been effective in the disposition of this chain of monadnocks along the southern edge of the higher peneplane. In the region south of Adjuntas a wide belt of country is involved in the Monad- iiock group, and here it is not always easy to say just what the elevation of the peneplane is ; but further east the belt becomes narrower, and in the Sierra de Cayey only one or two masses rise above the well-marked |)eneplane level. THE SECOND OR LOWER PENEPLANE From San Juan on a clear day the sharp break between the higher and lower peneplanes twelve miles to the southwest is a sufficiently marked 310 SCIENTIFIC SURVEY OF PORTO RICO feature to be easily noted. The escarpment was mentioned by Semmes in his report on the San Juan District. In the space of only a mile or two there is a descent of 1000 feet or more northward to the lower pene- plane. Although Semmes briefly entertains the idea that there has been simply a warping of one peneplane, all the evidence, both here and else- where, points to the later development of a lower base-level. The strong- est evidence is the fact that the lower level is to be recognized between the spurs coming down from the higher one, and that more or less extensive basins like the Caguas and the Cayey Valleys have been opened out upon softer rocks below the prevailing upland. A corresponding lower level on the southern side of the island has also been produced by erosion, but the stage of developmeiit is not so advanced as that which characterizes the lower peneplane on the north coast. The escarpment, which virtually marks the northern limit of what is known as the mountainous part of Porto Eico, is not to be seen west of Ciales. The coastal plain in the western half of the island laps so far over the oldland as to bury completely the lower peneplane if it was de- veloped there. Just south of Corozal the break between the two pene- planes is more marked and regular than anywhere else. Further .to the east the spurs from the upland level become more and more confusing until directly south of San Juan, in the region around La Muda, it is quite impossible to distinguish with any degree of satisfaction between the elements of the landscape. It is not possible to know just how far the Tertiary coastal plain for- merly extended over the lower peneplane on the north coast, but it is quite probable that much of the area was covered and that the inner low- land now developed represents a stripped belt. Were it not for the irreg- ular surface, both on the north and south sides of the island, upon which the Tertiary was laid down, it might be argued that wave planation was a factor in developing the lower peneplane. But the ruggedness of the buried surface, especially on the southern coast, favors the theory of sub- aerial erosion, although it is not unlikely that waves were effective during the transgression of the sea in which the coastal plain was deposited. Throughout the eastern half of the north coast region, from Morovis to the base of the Luquillo Mountains, the lower peneplane is a gently roll- ing surface with few interruptions rising above the general level. The streams are numerous, but the depth of dissection is rarely over 100 to 200 feet (Fig. 8). On the south coast conditions are quite analogous to those on the north side of the island, but the reduction toward a lower level did not advance far enough to justify the term peneplane over any extended area. A> fJ)Hf-:i'K, TIIH PHYSHKiRAPHY i)F PORTO Rl(. (jiLslj .>iig;uT'.lrtiii ji: Jill 18 i-x[>lrtiiH'il Ijy lln/(!uritr!ii?t ill pre- jiiiatiuii oil the north mul nvuili euast.^. Dii the iinrtli collet iJic ^xnin' uC le soulJi .gi(.ii a. few iiiih>< haek twnn t\\v itith w.\ level, tlicre is ill Yido \l\i-o is idle most extensive |)hysiogTa|)hie feature on the island ajul the same tinie the must inipesing. It would lie even innre imposing II \\in:v jicd; that the spurs and I'ootliill eounlry eover siieli a wide helt (1 hiiv(> sneh eonsiderabh; eleva,ti(.ns as partly to detroet Irorti iln' [trumi- lice (d' the main crent. In tlie eastxnii portion near (■iuayania tin? taei- the esearprnent is exeeedingly irregular and i.s hroki'u^ by gTeat eirqui»- :•• vadey liends, whuse walls attain a steepiJe.<8 not exeeeded elsewhere Porto llieo. Slopes ol' 40 degrees are eojnmon, and even 50 degrees is •(jiieidly noted. In its western half the upland margin has a Caiiiy iiiiite east-west trend and the foothill eountrv becomes narrowi;]-. 318 SCIENTIFIC SURVEY OF PORTO RICO We may say^ then, that it is this foothill country on the south side o! Porto Eico which corresponds with the lower peneplane on the north side. On the north side rejuvenation is evidenced by the incision of the streams 200 feet or so below the peneplane level; on the south side the effect of the uplift is not so apparent, because the new cycle was introduced while the work of the earlier cycle was still in its youthful stage, and the result was a continuous down-cutting. Locally, however, there is some sugges- tion of the development of a lowland before the advent of the new cycle. A study of a typical profile across the island and a comparison of the lower peneplane on the north and south coasts both above and below sea- level is helpful to an appreciation of its true character. Such a profile is shown in Fig. 9, which represents an approximately north-south section through San Juan. The essential features to be noted are : a. The higher peneplane forming the great upland portion of Porto Eico, fringed along its southern margin by the chain of monadnocks known as the Cordillera Central. I). The lower peneplane, well developed on the north side of the island, but represented on the south side by a belt of greater ruggedness. c. The pronounced break or escarpment dropping down from the upper to the lower peneplane on both the north and south sides of Porto Eico. d. The extension of the lower peneplane as a continental shelf beneath the Atlantic Ocean on the north side of the island and beneath the Carib- bean Sea on the south side. e. Eemnants of the Tertiary coastal plain resting upon the lower pene- plane of the north coast in the region of Bayamon. On the south coast alluvial deposits bury some of the irregularities of the lower peneplane. Further west along this same coast similar irregularities are buried under the cover of the Tertiary coastal plain. The progress of erosion during the second cycle is attested not only by the denudation of the north and south coastal areas, but also by basins in the interior of the island, notably two in the eastern part. The larger of these, which for convenience may be known as the Caguas Valley, is drained in its western part by the headwaters of the Eio Grande and in the east by several small streams flowing in the opposite direction. Its reduction to a lower level seems to be due entirely to the weakness of the underlying granite rock. The high remnant of the upper peneplane which forms an imposing barrier on the north side of the entire Caguas lowland has been trenched by a narrow valley which serves as an outlet for the Eio Grande. The southern margin of the lowland is more diih- cult to define because in many places the higher peneplane is not so well preserved. In general, the smooth and gently rolling surface of the lo^\ ATLANTIC OCEAN Bavamon i PLAIN ^ Naraniiio r PENEPLANE SAN JUAN TERT. COASTAL PLAIN HILLS HARBOR ALLUVIAL PLAINS ±^ LOWER PENEPLANE Barranquitas \- FEET -^2000 £ 1000 t 2S Aibonito CordilleraCentrai alluvial fans Aguirre Hills I I CARIBBEAN SEA Barca Islands ^ 2000 -i 1000 SEA^ leve l 5Q 3S 40 FiQ. 9. — North-south profile through Porto Rico Co^* ^rfiary remnants Fig. 10.— Diagram of the Guanica district The essential feature to be noted is the relation of the Tertiary coastal plain to the oldland rocks and the fact that the oldland upon which the coastal plain was deposited was a very hilly region. Two coastal-plain cuestas are distinctly shown. The inner lowland has been drowned to form Guanica harbor and the ends of the outer lowland have likewise been submerged, forming Salinas Cove and Pardas Hay. Occasional remnants of the coastal plain are to be found in the oldland region, notably near Guanica Central and south of Yauco The point A on the diagram marks the position from which Fig. 11 was taken. The point B marks the position of the observe Fig. 12. in /-: -4 Xr LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 319 jaiid has strong resemblance to the lower peneplane of the north coast, hut the abundance of weathered granite blocks which cover it give it quite a different aspect in detail. In the northwestern part of the Caguas low- land between Caguas and Gurabo the rolling country representing the lower base-level has been opened out by the Eio Grande and its tribu- taries, whose flood plains lie 200 to 300 feet below the surface of the lowland. Some hills of more than ordinary prominence have their bases l)uried in the alluvium. They trend in a general east-west direction in accordance with what seems to be the structure of the older rocks. The average elevation of the surface of the Caguas lowland is about 550 feet, which is over 1000 feet below the bordering upland. Whether the rather high and more or less open valley in which the town of Aguas Buenas stands is a correlative of this same lowland it is difficult to say, but it seems probable and indeed almost certain that all the basin-like depres- sions and occasional broad valleys, which break up the upland surface and which are quite common in the eastern half of Porto Kico, were formed during the same cycle of erosion marked by the development of the lower peneplane on the north coast. In the region around Cidra the high upland level has not been preserved, neither has a well-defined lower base-level been opened out, except possibly very locally. The second depression, known as the Cayey Valley, is much smaller and stands at a far greater elevation than the Caguas lowland, but no- where shows a well-defined lower peneplane. It is drained by the head- waters of the Plata Eiver, which doubtless have discovered more yielding formations in this part of their course than a few miles downstream, where the valley is narrow and gorge-like. Erosion at the western end of the island during and since the second period of base-leveling seems to have proceeded continuously. The result lias been simply the mature dissection of the higher upland. The demar- cation between the different cycles is not expressed by any well-marked lower peneplane such as has been described for the north coast. That some of the valleys are really the time equivalents of the lower peneplane is suggested by the fact that they are filled with deposits of the Tertiary coastal plain where it now laps over the spurs of the higher upland, thus showing that the valleys hold the same position relative to the upland as the more extensive lower peneplane does further to the east. In the case of such a stream as the Culebrinas Eiver, it is not possible to demonstrate exactly how much of the valley was cut before the deposition of the Tertiary coastal plain or how much afterward, for the reason that it is •hfficult to determine just how much of it was filled with the Tertiary «i "posits. The river instead of being a subsequent stream developed at 320 SCIENTIFIC SURVEY OF PORTO RICO the base of the Tertiary cuesta, as it appears to be, is in reality a stream which has resumed a former course as the limestone which buried its original valley was worn and dissolved away. Further mention of this will be made under the discussion of post-Tertiary erosion. The distinction between the two periods of base-leveling can hardly be made in the southwest corner of the island, but it is highly probable that erosion during the second cycle opened out two broad valleys, that of the Guanajibo Eiver and the Boqueron-Yauco anticlinal valley, almost to their present extent before the Tertiary period. A point in favor of this interpretation is the fact that the Tertiary in the Yauco-Guanica region filled the end of the broad Boqueron-Yauco anticlinal valley. One large hill of coastal plain deposits and many remnants in the form of basal gravels still remain. On Vieques the work of erosion during the second period of base-level- ing produced the lowland and the valleys on the east and south coasts, now partly covered with the Tertiary coastal plain deposits. On Culel)ra the development of the large valleys probably proceeded during this time. These valleys, like the ones on Vieques, have since been drowned. The largest, known as Great Harbor, was formerly a coaling station for the United States I^avy. In brief, the second period of base-leveling is represented on the north coast of Porto Rico, east of Morovis, by a well-developed peneplane. On the south side of the island the same time interval is represented in gen- eral by the development of a belt of rugged country; in the interior by the erosion of the Caguas and Cayey valleys; on the west end of the island by the mature dissection of the higher upland, and by two broad valleys in the southwest corner; on Vieques by the formation of broad valleys, since drowned, and of the lowland on the south coast; and on Culebra by the excavation of valleys, later drowned like those of Vieques. DEPOSITION OF THE TERTIAEY COASTAL PLAIK The character of the oldland surface upon which the coastal plain was laid down and tlie nature of the material deposited strongly merit careful consideration if a clear explanation is to be had of the features later to result from the dissection of the coastal plain. It is especially important in this case because Berkey (1915) believes there is a fault between the Tertiary coastal plain and the oldland on the south coast, whereas th<' present writer has come to the conclusion that all the evidence cited in favor of a fault can better be accounted for on the basis of an alterna tive interpretation. Three essential facts may be presented in this con iJJilEVli, THE I'lIYBlOGBAPHY (fP t'Oh'TO h'lCO ;)2 I ■li.ioii. Fnvi, the Teriin.rv ('oa8tal plain on buth the north and south .U'< of Porto Iiico as well as on Vieqiit'S was (Jepositctl in iiuiiiy plaeos ...n a (liversificl okLlan J a "No part uf Porto I?ico shows imym clearly the rehitioii h(d;ween llio •astrtl phndi and the oldland than tin:; region about (luajdea on tlic south ■e.>t. TIjc general i-oJation htdnroon the d'how that the Tertiarv has bcfii deposited upon a \erv hillv ;j22 SCIHNTJFW fUIiVEY OF I'ORTO JHfO (>h\\'Aw\, rcpivsciitiiig the dissoetcd eastward |)ilx;iiiiig end i»f the Koquei Y'aiico iiniielino. Immodiatelv south of Yaueo the liills arc of cryst.il limc'itfmo of the older scries ami these a.re uverlapped on tlie snutjiurtsi the Terliary eiK'Sta. Altliongh thcj are more or less imormpt(;; south as (iuajiiea. Here, just east of the town, smuo 'hcdded Yoh:a are as.sticiai.etl with IJu> Innestoiie. They strike Jictrtheiist-soidhwcsi I'astv^wi'st and arc presiunahly a eoniiiniatioii of tiie formations Iniviiyt tiary as w.'ll as aliijuthmt o<-currciu-es ui' thf hasal gra.'t^els proves that: valley had reached approximately its present deveh>pmei:it hcfore Ter tiiiKi. Tlus belied', that modern erosion changes are not to any cons ahle degree accoimtable for the present size and shape of the okler ^. hills, but that lliey are simply being iinbuTied of their Tertiary covi ahiindantly strengtliened bj a study of the district adjacent to Gu; Cciiitrah At (Jnimiea (^'entral ttie oldlan,d hills can lie seen act emerging from heueath the coastal plaiji. ajul on tlieir flanks tfierc remains a coating of chalk, attestin,g i;lie r. TopoLriajJiically the -'Itfci in t,li<.se i.tla-vs lia^ hwu th.- .i velopinejit oJ' more or ]e«s fxteiisivo Hat areas wlicro the rnftro soiuhlo ou- lvi-n.i; limi'Htoiics Imv-e hoeii rcrtiovtMl fSuch ft;atiLrt>8 are to he >(:ni 1 = iweou ]iavrti!ioii ami U'n> Piedras on Uu' iiortJi coast. In no oilier pHM • I\trfo Kico h.'Ls the Tertiary been iJepoHiled ufioii sricli an cvci! siirfiu yUvl of th.c inner l(>whin oC the Tertiary coastal ]jhmi upon whiel! t1ie ihit tra.ets a (le\'(>loped f(»ve'r a part of tlie iniUM- hnvlanrl adjacent t.j isohiled. reimmii eiJs witli ihe ^=lll•l It is frrr»bat>le that the coastal plain deposits m«ver nrelied e.nt.irol\' owr ttie oldland. The oceiirrenee of beds of small .^ravels, derived rHreeil) I'roin tbi> o.lder series at intervals throughout t!ie ceiastal jihiin dc|.^osiP^ and the abundant clay contejit of the coastal plain limestojies, niake^ ii sc'i^m liki'ly that a larg(> area of the oldland remained ctontinuously suhje.- to erosion. The belief that the coastal plain did not reach over the wlin'., ishind is also substantiated by tlie alirnpt termination of tlie eeMistal phii'^ deposii.? again>t (h,e lower slopes of the upland ma.ss, especially o,n xh' south side !,)f the island. LOB EC K, THE PHYSIOGRAPHY OF PORTO RICO 32; UPLIFT AND DISSECTION OF THE TEETIARY COASTAL PLAIN Character of the Uplift The uplift of the oldland which brought the Tertiary coastal plain above sealevel appears to have been greatest in the western part of the island in the region approximately between Lares and A^anco. This is indicated by the fact that near these two places^ on the north and south side of the island respectively^ the north coastal plain and the south coastal plain attain their highest elevations and at the same time reach farthest inland. On the north coast stratigraphic studies by Hubbard, as well as those by Semmes, show that the lowest coastal plain beds in the Lares district (the Lares limestone, lower Oligocenc) stand at an eleva- tion several hundred feet higher than the lowest beds further east in the vicinity of Corozal, although the latter are of a higher geological horizon (upper Oligocene). This indicates not only a greater uplift of the island in the Lares region than further east, but also suggests that the Lares area stood at a lower elevation at the beginning of Tertiary deposition. In other words, it appears that the Lares region was uplifted some 500 feet or so higher than the region around Corozal, some 35 miles eastward. A Avarping of this amount would give to originally level strata a slope of much less than a degree. The warping of the coastal plain, then, while having hardly any effect upon the attitude of the coastal plain beds, has lifted above sealevel a much greater extent of the coastal plain in the western part of the island and a progressively smaller area toward the east. The dip of the coastal plain beds is one or two degrees toward the north away from the oldland on the north side of the island. An incli- nation of this amount may readily have been the original inclination of the beds at the time of deposition. In fact, on the south side of the island beds having a dip of five to ten degrees apparently preserve tljeir original angle, for they are traversed diagonally by worm-tubes, still standing in the vertical position of growth. Therefore it is unsafe to cite this northward dip as due to the greater elevation of the oldland. It would appear more reasonable to say that the oldland and the coastal plain portions of the island were uplifted uniformly above the sea, but that the western part of the island suffered a greater elevation than the eastern. While a differential uplift appears to offer the best explanation, it nevertheless may well be argued that the coastal plain was not warped during uplift but that its various dips are the initial dips of deposition due to the fact that the plane upon which it was laid down was not a LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 329 land of the north coast. It is true that on the south side of the island the cuesta is not so continuous a feature^ but by no means is it entirely lack- ing^ and if it appears vague in places this is always due to the irregular oldland upon which it rests. Finally, Berkey notes the existence of •'crush zones, slight folding and drag'' in the strata of the coastal plain deposits near Pehuelas (K 10) on the Ponce-Pehuelas road, and the pro- nounced tilting of the coastal plain beds a mile southwest of Juana Diaz. xVll of these disturbances are extremely local and, in the writer's opinion, are due unquestionably to the collapse of the beds about sink-holes. These are very common features and exert a marked effect upon the topography in both localities cited. A more thorough discussion of this point is taken up in a subsequent part of this report. DlSSKCTlOX OF THE COASTAL PlAINS The dissection of the coastal plains lias resulted in a main cuesta and a series of more or less distinct secondary cuestas, with other features of a belted coastal plain, on both the north and south coasts; but the drain- age development departs from the normal owing to the overwhelming]: preponderance of limestone in the coastal deposits. An essential point to be kept in mind is the irregularity of the oldland surface upon Avhich the limestone was laid down. This affects not only the relative height of the cuesta in different places along its front, but it also exerts a control upon the position of underground river systems. By the collapse of overlying beds the subterranean channels eventually form the main drainage lines of the district. As the north and south coastal plains are isolated units, . it is believed that a clearer presentation of the main features can be made by treating each separately. THE NORTHERN COASTAL PLAIN Normally a coastal plain when first uplifted above the sea is drained by consequent streams of two types — initial and extended. Since it is tliought, for several reasons, that the coastal plain never spread much further upon the oldland than it does now near Lares, it is believed that the several master streams crossing to the coast originated as extended consequents. The streams that may be so designated are the Arecibo, the Manati, the Plata, the Bayamon, and the Kio Grande. All of these have their sources far back in the mountains, and those in the western part, where the coastal plain is highest, have cut deep, vertical-walled trenches Jieneath its surface (Fig. 14). But it is only an assumption to assert ihat these trench-like valleys were formed in the ordinary way by direct ;■!:;() SCIKXTJFIG survey of PORTO RICO .l(.wj>oiittijig. WJiat seems more probahJe i> tliai, tlioii-h at tlie time ui their origin iho?e sti'eiuiis flowecl upoji tlio surface, nn iiinlergroviiuJ .'-xs-^ tcm \r:irt soon dovejoijud along the bnrieil valleys (,i: the oldlao.l. Th.' ultimate eollapse of tiie hc'.is above the iiiid.crgrulro(l ill the well-fttnoed valleys now trciicliing tlio couhtal plain. Tlie Tanama Eivi>)\ a tributary to ihe Ar^-eilH., Hiill pre^^erves a mhivv^^^ rfiiieaii jiddfllf course and up]H'ars to follow a clmnnel t:lie comphde cob hipsi. of whoHc rouf luiH not yt>t hticii r.|feciei[. The main cvKb-nc*' in fa,vnr of \hv6 bisfory of ^;ilvanl deveh.pjiieni is tlio fact that tlio old pre^Tw-tiaiT vaH..y> (.r snrni' streams, a:^. b.r iuslaneo, the i!aiiiu.y Hiver vaHey ^a^t .if Liire->, wbioli w.-re buried by tbic Tertiary deposit^, appear to have b-.-en reoecu,)>iod by the present st:i\;ams. Tbi.s h to l»e noted where the strcanr- enter the cut'sta face and pass fitmi the stripped upper portion of tiH> valley into that part still flllod with limestane. Jt is inl'erred that tlu; underground portions of suajIi streams as the Tanama ami tlie Oimiiv rivers are following ttie old buried valleys. These streams are similar in origin to the "resurrected" streams r)f tlie soutliem states described bv ^FcCJee (1801). In the cases cited by McGee, liowever, the old, vallevs wv.ni eovei'i^d o\^er by only a shallow- deposit ami the streams iii)oii tie.; retreat of the ocean bad Jio diificulty in fin,ding their earlier channels. In tlto J'orto Eieo case the valleys were enniphdely fillcid, and it was only tlie inaliility of tbe filling material to withstand solution bv water seeping LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 33 1 iilong the buried channels which resulted in the resurrection of the orig- inal valleys. The Culebrinas Elver is another example of a resurrected stream. Its course appears to follow an old drainage Ime and cuts diag- onally across the coastal plain formations, thus truncating several cuestas. Its tributaries have stripped away all of the Tertiary on the southern side of its valley except the few isolated remnants in the Aguada region. Part of its broad valley, therefore, functions as an inner lowland, but the stream itself can not be classed as a subsequent stream, at least in refer- ence to the coastal plain. It was indeed originally subsequent if it is presumed that its course was determined by less resistant members of the oldland rocks. Whether there are any streams to be definitely classed as initial conse- (juents upon the coastal plain it is difficult to say, but presumably the (Jedros as well as the shorter streams along the coast are examples. As for large, well-defined examples of typical subsequent streams developed on the coastal plain, one must doubt if there are any. Two tributaries of the Arecibo, the Caguanitas and the Limon rivers, might appear to hold such a position, for they flow at the base of a cuesta. But their gorges are cut so deeply into the older rocks as to make it unlikely that all this cutting took place in post-Tertiary time, especially since the limestone cuesta has retreated hardly any from their valley sides. It is more prob- able that these are also resurrected streams which have succeeded in re- gaining their old courses by the removal of the limestone which buried them. The best and about the only examples of subsequent streams devel- oped upon the north coastal plain are the Guatemala and other minor tributaries to the Culebrinas between San Sebastian and Moca. These have been developed upon the Cibao limestone between the two cuestas, and have succeeded in opening out a large subsequent lowland, larger than any other lowland developed within the limits of the coastal plain. It is also believed that short east-west trending sections of the Guajataca Kiver are subsequent in origin. THE BELTED CPIARACTER OF THE COASTAL PLAIN The coastal plain on the north side of Porto Kico may be concisely de- scribed as a belfed coastal plain gradually increasing in width from the east, where it occurs only in patches, to its broad western end. Let us amplify this picture by a description of each of the successive belts. The inner lowland. — Normally the innermost belt of a coastal plain is Its inner lowland. On the north side of Porto Rico an inner lowland ex- Hmds eastward from Morovis. East of San Juan remnants of the coastal 332 SCIENTIFIC SURVEY OF PORTO RICO plain are almost entirely lacking, but the rolling coastal country all tlu way to Eio Grande and Luquillo is simply a continuation of the inner lowland further west. In reality this inner lowland is the stripped sur- face of the lower peneplane, which has already been described. It is therefore rather a part of the oldland than of the coastal plain. In places, howcTer, it apparently still preserves remnants of the lowest coastal plain deposits. This is the case between Bayamon and Eio Piedras. Here the usual rolling surface of the inner lowland gives way to rather flat ex- panses of country. When these distinctly flat areas are dissected by streams, as, for instance, in the case of the Bayamon Eiver, and further east as well, they have the aspect of alluvial terraces. The best preserved portions of -these basal beds appear to be those areas closest to the rem- nants of the cuesta. Between Ciales and Lares no inner lowland appears and the inner cuesta rests upon the spurs of the rugged oldland. West of Lares the open lowland drained by the Culebrinas Eiver system functions as an inner lowland. The Culebrinas Eiver flows toward the northwest and truncates the east-west cuestas and belts of the coastal plain. There- fore its valley is not a true inner lowland. Eather, as previously statcMl, the river appears to have resumed its course on the oldland, which was buried by the coastal plain — that is to say, it is resurrecting its buried channel. The fact that its course follows the general strike of the older rocks and that it has no relation whatever to the weaker beds of the coastal plain bears out this interpretation. The Lares limestone cuesta, — The southernmost or inner cuesta is de- veloped upon the Lares limestone, a massive formation which gives rise to exceedingly rugged, hiUy topography. The hills are close-set, 100 to 200 feet high, and dominate the cuesta from Lares to the region about Morovis, east of which point the main cuesta is somewhat less pronounced and more discontinuous. Where this belt exhibits its maximum width, in the Lares region, it is only about two miles wide and is the narrowest of the chief belts of the coastal plain. At Lares the cuesta face is a strik- ing topographic featiire, having in places an absolute elevation of about 1800 feet above sealevel. It stands 500 to 600 feet or more above thc' floors of the valleys at Lares, but where the cuesta rests upon the spurs of the oldland it has a face measuring only 100 to 200 feet in height. Throughout its extent the cuesta face cuts through the conical hills which happen to stand at its edge, and the resulting cliffs, exposing a cross- section of the almost horizontal white limestone, are perfectly vertical and usually bare of vegetation. From Lares to Ciales the cuesta has the same general aspect. Its crest-line is irregular and the white cliffs mark iog its front are conspicuous at long distances. Conical or irregular hill- LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 333 typify this belt. They appear to be the remnants of a continuous surface left after the collapse of underground cavities, brought about by the ready solubility of the massive limestone. The hills are always small in areal extent. The depressions between them are usually without drainage out- lets. Toward the north the hills of this belt become lower and gradually disappear where the Cibao limestone, which is soft and shaly, overlaps them; but the tendency to form conical and irregular hills can actually l)e seen in the sides of valleys cut below the rolling country of the Cibao limestone into the underlying Lares formation. The Cibao limestone lotvland. — The next belt to the north is technically a lowland. Actually it has the aspect of a rolling plateau. This lowland belt is three to four miles wide and may be traced from between San Sebastian and Moca eastward to Morovis. In its eastern portion it ap- pears to develop a series of minor cuestas, as may be seen in valley sides of the Manati Eiver and along the Morovis road. The general aspect of this lowland belt, however, is that of a plain, sometimes deeply incised ])y streams, but almost always devoid of the numerous haystack hills which characterize the belts to the north and to the south. The limestone (Cibao limestone) making up this belt has a tendency to develop flat ])enches or terraces along the walls of streams which have cut below the surface of the plain. Benches of this kind are due to differential erosion upon the members of this formation and are distinct features in the walls of the valley of the Manati Eiver. Two of these, an upper and a lower bench, the one standing a hundred feet or so above the other, are espe- cially striking. They slope perceptibly toward the north, where they dip below the next higher hill-forming beds. The western end of this low- land belt between San Sebastian and Moca has been reduced by the Guatemala and other minor tributaries of the Culebrinas to a decidedly lower level. The Los Ptiertos limestone cuesta. — The fourth belt consists of rugged and bold hills, limited on the south by a distinct cuesta face which looks down upon the lowland just described. It may be traced all the way from Moca to the Arecibo River. Further to the east the cuesta face becomes less defined, but the belt of hills persists. The formation responsible for this belt is the massive Los Puertos limestone. Its pure character has encouraged the development of caves and sink-holes. The depressions are much larger than those in the Lares belt and, as Hubbard has pointed out, frequently occur in a series trending toward the ocean, as if induced hj well-formed underground drainage lines. The hills are high and rugged and angular in outline. Their aspect is often that of the mesas and buttes of the southwestern United States, especially where they are 334 SCIENTIFIC SURVEY OF PORTO RICO a little set apart from their neighbors. In the midst of the hills the broad, fiat-floored depressions, sometimes several acres in extent, grass- covered, and walled in by high and ragged limestone cliffs, rich in the luxuriance of tropical vegetation, are picturesque in the extreme. The Quehradillas limestone lowland. — Next to the north and extendint^ to the coast is a broad lowland or plain, in large part undisseeted. Like the other lowland to the south, this one also has the aspect of a plateau because of the canyon-like nature of the stream courses which occasion- ally break its surface. It forms a belt five miles in width in the western part, becoming somewhat narrower and more diversified toward the east until the Arecibo Kiver is reached. East of this point the uninterrupted plain-like character entirely disappears and the region is made up of haystack hills and intervening flat expanses. Hubbard has assigned the term Quehradillas limestone to the strata responsible for this topographic belt. At the western end of the island this rolling belt extends along the coast from Aguadilla around to Camuy in a fine stretch of wave-cut cliffs approximating 200 feet in elevation. Throughout most of their extent, however, the cliffs are at the present time protected against wave attack by narrow beaches. Between Hatillo and Arecibo the hilly back country slopes gradually seaward and the wave-cut cliff is usually not present. In the western part of this lowland belt, as in the region between Agua- dilla and Isabella, shallow depressions or sinks, frequently holding small ponds, abound. Even when no water is visible, the fact that these depres- sions serve to collect water is attested by the presence of large trees grow- ing in them. Elsewhere the landscape is open and presents the aspect of a rolling plain. SPECIAL FEATURES OF THE NORTH COASTAL PLAIN To divide the eastern portion of the north coastal plain, in general that part east of the Manati Eiver, into belts such as have just been described is not expedient because the haystack hills become such prominent fea- tures throughout, whereas in the western part of the coastal plain the haystack hills were most numerous on the cuesta-forming belts and were infrequent on the intervening lowlands. In the eastern part of the coastal plain the entire country is characterized by the presence of the haystack or pepino hills. There are two elements in the landscape (Fig. 15), the hills and the flats. Locally the proportionate area involving each form varies from 100 per cent hills to 100 per cent flats. Practically all the features of the limestone district owe their presence and form either directly or indi- i.nta:cf\. iiii: PiiysionKAniv nr i'iiirr<'> uir rvv\\x lo ihr pivunliiti: \v\uWws l.nvnr.I siiKiprtrroiiiMl .Iniiiuip'. CaxtTiis -.mil >it!k^^lsul,.> lurni ..xi-n wliciv. Al l!rr enllnp«' ui flifir \v:ill> iiik...s pt.'H-,', jiii.l ;h iIh' n.,,r^ ..f adjoiniiii:; .•;n..rti> -i\i> way, s.-paraTi. sinks hrrMinc c.nii.Nir.l .-uk) iIm' ll<.or> vuihai uui mi.l Uw.omv fiat, h nni\ iia|)|.i'n thai lli.- lIo.>rs ..1* udjati'iil, .l.-prnssiniL- arr- m.i ar fiu> sain.- h-xv\ l.c(^ans.. 111.' iin.! l.v (ii,.iy. nr \v« imnv.'i.ly .!i-tnlHit<-d ,.ha!v Ih'.Is m ilu' liiia-i.,n... klrrk-v tliiH ..Kplain.s !h.- ilisrn.|»ui.-y HI levl he(ivi.«-ii adjai-.aii dcpn-.^ions jh well a< llir t.'i-^ hcnvoHi i!i.' hills. As Ih.f sinks wi«h-ii, iJu' .•ii.guhii->hap.Ml iiills n.niainin.ii isuIhKmI upon the llat plains liav- n rlr.s.. n.s,.nil»l;inri. iii form io th- i„,sm and liutt<'s nl" th.- Wfstr.rn Tnited Slah'S. Hut uluai thcv air- fiirthi-r iv^^ 33C SaiKNTlFJV SURVKY Of i'OUTO RICO j^ioiially eiilrivati'd J'ur pineapples. Tbe disei'iptioii ul' tbt; liiiystack hJIL- aiid the fifcoriipanying Hats just given fur tlie easlcni part of the eoa.^rai plain applit'S efpialJy t:y those belts in the western [,)art wliei'c tlie hn\^^^ stiickrs prevail An uniiHiiaJ feature and one of more than ordinary ie^^^ terest results v^lwra the waves of the Atlantic have pushed ba,ek tlu'liiiih elilfft near (^uehradiUas to siieli a point an to In-iiia; \hv cmsi next to one of the open, \h\\>.. A mariiuM-ut terraee is thereby Hirmilatt-d (Fig-, lb). The shipe of the U;rra,ee surfaee is toward the nortli and eoid'urms wiidi Urn bedding; the first tinn^ will immediately assume tliat it is doe to wave pla nation, when the hiiul stood at a lower level. But after having- heeoriie familitii: with the flat streicfies described above, and liaving not(>d tlieir charaeter- istic devcdopment througliout miicli of tlie area immediately to tlie south of QnebradiHas, he lias no difficulty in assigidng to tlie apparent wave hen(di a similar origin. If dilTerential erosion lias \mm the factor in lormiiig this terrace, similar terraces would be expected akaig; the sidi'^ of valleys cut in the Tertiary coastal plain. Such are to be seen m their f)est (le\'elopment along the Blanati Biver. and liave already Ijeen fleseribefb LOB EC K, THE PHYSIOGRAPHY OF PORTO RICO 337 ISOLATED REMNANTS OF THE TERTIARY East of San Juan the remnants of the coastal plain are small and in- frequent. There is a group of hills just east of the highway between San Juan and Eio Piedras^ and an east-west trending row of hills northeai-t of Carolina, entirely surrounded with alluvial deposits. These are con spicuous features in an otherwise monotonous landscape^ and their alti- tude, although inconsiderable^ suggests that the coastal plain formerly extended much further south over the flat rolling country between Caro- lina and Rio Grande. Whether a greater amount of material has been stripped from the oldland in this eastern end of the coastal plain than in the western part it is impossible to ascertain ; but it does not seem un- likely, inasmuch as tliere is a greater precipitation in the east. THE SOUTHERN COASTAL PLAIiN" Edslern portion, Juana Diaz to Yauco. — Evini more than on the north side of the island, the coastal plain on the south lias been cut into blocks l)y transecting streams. In the eastern part between Ponce and Juana Diaz the valleys of the streams crossing to the south are wider than the remnants of the coastal plain remaining between them. The coastal plain remnants stand up as isolated masses of hills, more or less flat-topped when seen from a distance, and with a gentle southerly slope. Their bases appear to be buried in the alluvium of the river flood plains. At close range the contrast between the steep cuesta front and the gentle l)ack-slope is not evident) but from a commanding viewpoint the land- scape resolves itself into the customary orderly elements of a maturely dissected coastal plain. There is good reason to believe that the Tertiary in the region between Ponce and Juana Diaz was laid down in a former bay in the oldland. The deposits filling the head of the bay have since been stripped away and their place partly taken with alluvium. The re- sulting flat-floored inner lowland looks like an estuary extending back into the foothills of the oldland (Fig. 17). The charactt^r of the deposits, which as noted by Berkey are more shaly in this region, is in accord with the assumption that the Tertiary beds were laid down in an estuary in the Juana Diaz region, an estuary which received the waters of the Jaca- guas River. The streams coming down from the oldland have swift and turbulent courses, with frequent falls and rapids over rock ledges, until they emerge from their comparatively narrow valleys and flow out upon the flood plains of the open valleys in the coastal plain province. Part of the city of Ponce is built on the southern slope of a block of Tertiary hills Iving between the Portugues and Canas rivers. On its '■iHXTinr si:i,'Vf:y of i>oin:t> inr iiofth side il.|> I. lurk ilispliivs a nx'll^^ h.T.. jiiinr.w.Hl to a xalU'x imi „„„■.■ ihati a' nni- wiilc. Ai nil,. jHHiif a, (lisfiiirt iK.t.'h iiihTni|>i< (he ni.-c u|* Hi,. Mi,>,-ia C..iiiiinmii:ai.inu wirh lhi> iiutrh uii ili,. har-k .^lop,. ..f thr .aH-via, i. a win.! iii.i: Vitlh.v. II. nv uikh-.-uju,'.! hy any sln-am. 11h< valh-v wa^ nr')iaiviil I; f'trin.'d h,v .an- of tlu- rxlciidi'd c.iii-c.ni.ail i:i I'hiin. That If was .HtaipiiMl l,v a suvam .•uiuui;: ih.wii fnan ihr uhUan; IS nllcstcl hv !h,. la.rt r.hat in ils hed w.'lh^a-'.nn.l.-d hi.uhlcrs -.f tjic n|,(.^ HH-k^ .K>(air. This wniih.l sutrirrst iliai fhr .Iu.tm.mi ..f ,-.ah-.i|ii.au ^tivain has hiH'-n pITccl...! h\ th.. >uhs,M,n(Mif draina-c n.-nipviii- tli.. hiiut lowlanal 'mm^mm / Ol-OLAND At the ina-sont time tht- Ihiur of |hi> iiuaa: htwlaiid Inas hn-n mt .lowii I'Hi r.'of hnl.Hv the Hour c.f tlir- ahanduiu'd valh-y r-rossmt;- tlm .nu'sla. Jn-I ..r th,> hlocks. whilp liavino^ a pau-ral Maithward slop,., is rxrrn.lin-b invuadar. Th*> mmsta hniwcai Vmuv ami Pniliudas h iHnally vnrv w.d! .hdinnd. lait, as |.r<-\ imislv d-aiicnsi rafrd. the irn-ada.rily ,d' idic .ddhind Mirlarp upon wliirh 111.. Tprtiary was hiid down i< .,<-.=ii^innallv arrunn!^. ahk: I'm- a niiltiiv .d' thn i'usloiiiarv fnafiirns. Th.' r.,ad rn.tu I'.aicn d. I'.aHndrts runs for a i-. Thn .ai.'Sta is iiittTnipt..d l»etwv(.n run.-," and Yau.M, h;. l.nlil'.rK, nir. I'll) sli)ly .!i>-ii»r.|. Ii' !> ini.Tr-iin- t- .Kill Ihiil ill II niiiH.r \\:\\ th.- coiisial jihiin ivmiuml ...iithwvsl <,f (;u:iiiiV:i .!i-^|.lii}^ a lM-lt,..l .-liarnctrr. Two .•ncstn> .....air, T\u: uvo ,',h!:. ,,-: ih.- -mall •;uiK*^,,u,.|it lowlainl mt.a-uaim-r Ih-Iw.m.h iho mitm- aiai inn.-r ..ai.-ia ;.ri. iH.w .InauH-.l fu Inrm l'aial:i> Pniv on |lu-<.a-l and Salinas ( ^u\^. ..n t!.,; urr-t iFi-. lit). An nhai ..r the fra-micntarv .-xt.-m: ^t\^ ih,- i-ua.sial phuii iii tli.. >..,|ii,.^ w(-l rnrnrr of IN.rto 1,'u-n mav !),:• -aincd rrnm lla.' map. All l.ni fiir ~;^'; !^, COA3-mL PLAIN ws;.lanv a ('li>f,in i>lan a r.'^iill <,r .In.wi Ilt,^•, l.ni now allachf.l lu it by ;i hcaiinrul MTi<-> of .•niiiavf iiio- b.-arla.:-. Si„l.-J,oh:< iijiil Ihn <,n,:slioii of iis^iniu'.l hiullhuj. Tlio .|n..siHm .• mMlm-^roiiiul .Irania^u ami llm sark,.v io major laiilrino;, So\,m: hiH'^ of oviil.anof havo l.r.ai mio,! bv pM-rk.^y as s(.r.„,m_ir f,, t'avor laiillin o.-iwoon tl,,. q-,,rtiarv ami oldrr >,'ri<'^ (ai tbi- muiIIi (i.a^l. Tla-y bava bo,, brndly i.roM>ntope j> snrniuur.tcil by Lnininmeky and eooieal hills, .suggesting in some degreij tlic hills oji ifie euesra fron,t at Lares. Deprensitais without cuitlets ar<: cuniinoin. 'J'here are also many large de])re&sions wdiiedi ser\e as eijie^ tijiueiis (Irainaux^ lines for insiH|neid, as well u-^ for eensequent si.reanis. dis aiHl sink lioh'S4 whlfli eliaj-iU'tfriKp toward the nionutnlnoiis ol(l!!ui !ie«l> may br- very uiirkeil when; ilie ilepre^sion i^ unJv a few xnnU acruss. The uHoal appea.rajiuc of tlie geiille h.T.- is i) tr. 10 iletn-ces tou'anl he uoiih, a dim-tioii nt.t etonpatihlc with the ulea that llie oldtT series Hul he.. II faulted up. TJie dip lu thin vn>v is tu\v;»r.l ili(> reiifr of a lai-ge lepre^.^iun. In tJie held il8 relation tu tho d,.pr..s mad. Ili^re tte.- Tertiary beds lit; in a liighly tilted position npon th(» oldei' series, as if dn.y liad'heen dragged into that sliape liy an upla-aval of the ..Ider ro.-k>. I'^n-haps the best way to aet|naint the readtn- uith the situation auil at die ,san)e tinn- present ttie writer's inferf)reration of tlte t'aels is to jrdor i'l tlic atreoiiipauyjng fliagrain. wliieh is drawn from >keteln's jnade in tlie e«dd (Fig. 21). The interpretation is coiuparativtdy shupli.. The eol- "iipsc of a eavern has taken plaee jnst at the point when- the Teaiaary ■vsrs upon one of t!ie lulls uf the older s.-ries. This is tu lie o.\pe(do< sxim: islands iiujjiediaT.'h' after siihiiicrgence. West of Ponce the sidyiiergenee alTecti'd not only xanc of the su,bseqiient valleys brd.ween tlie cuestas ot the coastal plain. I'Ui; also parts i)i the inner lowland. (Jiianiea Lake is apparently a urr.wned portion of a broad valley or lowland in iJie oldlarnl itself. !t has since \)wn cut (df from direct cominimieatioii witti the sea l)v the 'b'pijsitH of the Susna llivcr. Gnariica llarlxjr is a part of the drowue'l inner lowland (Fig. iir) and Tardas Bay, as well as Salinas Cove, near 'inanica. are ])arts rd' a tlrowned snbsetpient valley, (luayaiiilla Harbor ^loparently Inis a similar urigin. The region about Parguera on tbt' -"Utdiwest coast exhibits further evidence of drowning in the peninsulas =ou! islands of the oldland. Hie Morillos dc C'abo Rojo represent paj-ts 346 SCIENTIFIC SURVEY OF PORTO RICO of the coastal plain detached as a result of submergence, but now con- nected with the mainland to form a complex tombolo of rare beauty. West Coast The west coast of Porto Kico iB in many respects similar to the east. The three largest rivers — the Guanajibo, Blanco, and Culebrinas — emerge from the hills upon extensive flood plains, which are apparently alluvium- filled estuaries. Boqueron Bay still displays rather distinctly its drowned character. Whether the drowning affected the entire valley from Bo- queron to Guanica it is impossible to assert definitely, but it seems very likely that this was the case. Though most of the valley floor is now spread over with alluvium, it is very low and swampy, and the wells put down through the alluvium yield water of a distinctly saline character. The divide between Guanica Lake and the marshes draining westward to Boqueron Bay is low and in times of heavy rains covered with water, so that even now a very slight rise of sealevel would effect a junction between Guanica and Boqueron bays (Fig. 5). North of Boqueron Bay the west coast is only moderately irregular, but this is because the bays have to so large an extent been filled with alluvial deposits. KoRTH Coast Along the north coast of Porto Kico evidences of embayment are not striking. Theoretical Considerations AMOUNT AND CAUSE OF SUBMERGENCE) GLACIAL CONTROL Vaughan (1916) has made a study of the Virgin and Leeward Islands and St. Thomas, to the east of Porto Eico, as well as of other West In- dian islands, and concludes from an investigation of hydrographic charts that the recent submergence in this whole region has been approximately 20 fathoms. He notes also that "there is in the Virgin Islands and in Cuba clear evidence of a lowering of sealevel by about 20 fathoms, per- haps more, previous to resubmergence." He states in addition that in other places, as, for instance, off the north side of St. Thomas, the west side of Anguilla, the southeast coast of Antigua, and Mosquito Bank of! Nicaragua, the similarity of submarine profiles suggests that a lowering of sealevel preceded resubmergence. W^hat caused this lowering and sub- sequent rise of sealevel? To cite Vaughan again: ''As it affects a large area, it appears too widespread to be explained by local crustal move ments. The changes in position of strand line here noted are more rea^ LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 347 sonably explained by a lowering of sealevel due to the withdrawal of water in the Pleistocene ice epochs to form the great Continental glaciers and the raising of sealevel after each epoch through the melting of glaciers." The amount of drowning, 20 fathoms, proposed by Vaughan, agrees closely with Daly^s estimates (1915) of the rise of sealevel due to ice melting. SUBMARINE PROFILES EAST OF PORTO RICO ; ORIGIN OF CORDILLERAS REEFS A submarine profile just east of Porto Eico reveals a sea floor under Vieques Sound standing at a depth of 8 to 16 fathoms. Much of it is flat, but it appears to be made up of different levels or terraces. To the north of the Cordilleras Reefs and Culebra Island there is a second flat area or bench 5 to 10 miles in breadth. This stands approximately 30 fathoms below sealevel and is separated from the higher level to the south. Just described, by a distinct break or submarine escarpment. Upon this escarpment stands the east-west trending chain of islands known as the Cordilleras Reefs. Yaughan has expressed the opinion that the outer bench represents a surface reduced by marine planation during the last period of maximum glaciation when the sealevel was lowered some 20 fathoms and then submerged. The escarpment he considers a wave-cut cliff marking the southern edge of this bench. As for the apparent ter- races on the shallower flats to the south, Vaughan suggests that they may have been formed as a result of oscillations in water level during the different glacial epochs, or in places they may represent submarine ter- races being formed at the present time. The depth of the outer platform, as before stated, is approximately 30 fathoms. This depth below sealevel Daly (1915, p. 182) estimates as the present position of platforms cut and built by the waves during maximum glaciation. The essential line of reasoning as to how this present depth was brought about is as fol- lows : It is assumed that in weak materials open-ocean waves can quickly form a bench about 10 meters below low-water level, but that abrasion in greater depths is infinitely slower ; that in the course of 50,000 years the depth of the bench surface would probably not be increased to more than 20 meters or roughly 10 fathoms. It is also assumed, after careful esti- mates, that the maximum lowering of level in the tropical ocean during the Pleistocene was 30 fathoms below present sealevel. This amount added to the 10 fathoms cut by the waves would leave the bench surfaces now 40 fathoms below sealevel as a maximum. Variation in the strength of the wave attack in different localities and in the character of the ma- terial eroded, as well as in other factors, would account for the slight 348 SCIENTIFIC SURVEY OF PORTO RICO variation in the present depth of the benches* As a rule, a depth of 30 fathoms would be the average. No other theory has been proposed which so well accounts for the a})- proximat« accordance in level of submarine benches at a depth of 30 fathoms the world over. And there is apparently no other way to explain the escarpment marking the inner margin of the platform than to assume that it is the wave-cut cliff cut during the time the platform was being beveled. From the crest of this escarpment rise the Cordilleras Reefs. This line of reefs may be explained as a range of dunes capping the escarpment and formed of the material cut from the platform during its erosion in Pleistocene time. The summit of the ridge in its highest points rises some 50 to 60 feet above the escarpment on which it stands. The bed rock making up this platform is apparently Tertiary limestone, al- thougli no lime'fetone outcrops on the immediate mainland. There is a marked cross-bedded structure of the limestone material composing the islands, and in general aspect they suggest dune formation. If the glacial-control theory is accepted, then it must be assumed that the top of the escarpment stood slightly above sealevel during the Pleistocene. Dune formation was therefore possible. In the western part of Porto Rico dunes of similar height are now forming. The Cordilleras Reefs appear to be the eastern expression of the San Juan formation occurring on the mainland. RECENT CHANGES The changes which have been effected in Porto Rico since the period of drowning, though all going on simultaneously, can be most clearly treated as separate topics. They involve the following events or processes : The deposition of alluvium in stream valleys, in drowned bays, and as alluvial fans on the south side of the island. Wave and wind work. A probable slight emergence. Alluvial Deposits Extensive alluvial deposits form broad plains known as playas along the lower courses of most of the rivers of Porto Rico. These deposits, especially those on the east and west ends of the island, apparently fill estuaries caused by drowning. In the case of the Fa jar do, Naguabo, Blanco, Yabucoa, and Maunabo rivers on the cast coast and the Guana- jibo, Mayaguez, Blanco, and Culebrinas on the west coast, this relation- ship is distinct. Undoubtedly portions of these plains are flood plains LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 349 developed by the widening out of the lower courses of the rivers. The surface of the alluvial plains along the lower river courses grades into the surface of the terraces which occur further upstream. During excessive rains the rivers overflow much of the play a areas. There, old courses, meanders, and other usual features of flood plains are to be observed. In occasional instances the rivers bring down sufficient sediment to extend the flood plain forward into the sea as a delta. But this occurs only in protected places where the waves and currents along shore are weak. The Fajardo Eiver entering Vieques Sound appears to be a case in point. Along the north and along the south coast the alluvial plains extend as a broad strip parallel with the coast rather than as estuarine fillings. However, there is a distinct contrast in the aspect of the alluvial deposits of these two coasts. The north coastal stretches are low and swampy. The plains on the south side stand considerably above sealevel. Along the north coast the deposits are often typical flood plains with almost horizontal surfaces. The deposits on the south coast are more truly allu- vial fans with a pronounced seaward slope. Practically all of the low north coast is fringed with barrier reefs. The waves of the south coast, however, have thrown up no such reefs; there the alluvial plains pass imperceptibly into the mangrove swamps along the shore, which shallows out for long distances, and only in one or two places, as, for instance, south of Guayama, have the quiet waters of the Caribbean cut away the non-resisting alluvium to form low cliffs. Occasional observers have suggested that the low plains bordering the coast of Porto Eico, as also on other West Indies islands, represent a sea floor recently uplifted. It is true that in isolated places there is evidence of recent uplift in Porto Eico to an extent of 20 or 30 feet. On the other hand, it is rather certain that these coastal deposits are not of marine origin. Old stream channels filled with gravel, combined with angularity of the boulders, lack of assortment, and poor bedding, indicate that they are alluvial fans deposited by streams. Most of the streams flowing across the alluvial plains of the south coast usually carry very small volumes of water. ^^Owing to the infrequeney and small amount of the precipitation and the relatively porous character of the soil reducing the percentage of run-off, as well as the smallness of their catchment* basins, they discharge minimum volumes of but 50 to 100 second-feet'^ (Wilson, 1899). In time of flood, however, they attain maximum discharges of nearly 10,000 to 20,000 second-feet, almost equal to that of the maximum of the larger northward-flowing streams. In the drier season of the year, during the months of January and February, c ven the larger streams of the southern coast do not flow through on the -nrfiwv. Ti.rir 1m,iiI.1i-i->1 n-wii vhmvnoh aiv mark.-.! hciv aiit Hh'v How mi the Mirlac«. uf tli.' Jl.,.,.; plains (Fi.t;-. til). XoiH' i.r tla- depusiis aloiii^ Ih.- roast west of Puiicc can i... ,-la>si'd with th... alluvial fans just de-cril.fd. They an- tyiHcally flood plains liUin;:, tiroad valleys and tcrjiiinatiiyi!- appart'iitlv in .lelta d.^i-nsits. I'miiajH Uriel' iiieiiiiMH should !..■ made nf ih,. euarse wardi rnmi i!i,. hii ides m the suiitla.'isl and soiillnve^l, <-(.rners of tiir island. Tlii-^ .^mi-^ iil.> u>rv lu.-al featniT'S. alfh.ni.uii simdar dei.usit^ w.av oh>..rved ah-i In. nnrih.-iH maruin. uf the Me,.pierun^Yaiie,, Valley, n<-|,o>it:^ of li ,air. Hxi-epI ui) (he hmad plains near their iikhii tainni- well he'l.uv ih.- nrniinal -urfae.- of ih,- alh ijnii:ri{. Tin: niYsnnm \f'}iY of I'ojrra hh: :\:>\ !i.' ri-iiltin- \i'\-r\Wi-> nw oc.-asiuiially KHJ t"<-fi or >u al.ovr th.- strciiiii. !h' t ..f ill.. Phita Jii\.:r iiniy he rii*..! as a, sirikiiig ,.xanii.lr ( Fiu^ >l. .hist.if^ 111,. liiuuth oC ill,' o-ui-nv (iii-ar Iv l:] i.ii tin- r<.iii imI\ \i^il.'.|. TlH-ir ..li.vHliuii ixhAv scalrvcl is abmii ->!Ni U'v\. s,.i,u-'|imi .>! iiliov.' tlio siirtiu/.' ..r i!i<- nv.M-. Their surfaci' appan-iitly sI.,)h.s .Knvir^. ream iunl Ix'runies r.,iiliminti> with that of the alliuial ilcpusiis uf i!i.. .nil r..ast. In the alliivialH-,.ver...l Capuas hnvhiii l.nvhin-l Is olU e. f rearli of ;,! nres.-nlHlav llrH.t ooii~t have hroatl lioud phmr^ e l.'io lUaiioo. .• i>laiHl won T)m^ lerrare. mio hi-h ■^ main Ihi. of the i\n allip.ia lliiniaea iig Teh ,.rpi«in HUM \hx 1 |errae,..s. 11ir^ i^ 1 u. Some nf the he~| (Fur. ■;'<;). rif of the Fa ^^h.rmed lorra ahiiiarv |o li, Ml Ihe v.Mlth s 352 RCIENTIFIG BUIiVEY OF PORTO 1U€0 t'he isJarn;!, are loure readily ()l)servt'd than i.liose of innst:. of t.lie (>tli»:'i streams in Porto Eico. East of Coamo llic milita.ry road follows th'- Ouyon Iliver, a valley deeply filled with alluvium, into wliieh the stream is now incising hi^iAl'. Tlie, town of ('f>anio stands upon a distinct terra''i: and tlie roiid from Ck)amo to C'oamo Springs rniis on the top of a terrrici- At (^k)aiiio Sprino-s Ilritol several terraei'S may be seen. The lowest onc^ upon which ih(» hotel is built, stands 100 feet or so above the river. Tie main valUiV i>l' tie-! C'oamo Eiver above tlie t<.>vvn of Coamo is heavily hll<'tJ with a.rhi\'iuni. ]n cutting into tliis material the stream has in sevcrnl [ilaeos superposed itself upon hard rock ledges and this has resulted n^ l)ron.ouneed watfU'i'alls. The steep gradient attained by the alluvial deposits (ui the soutli sidt of Porto Eire jnay lie apj)reciated by the fact tiiat in the uppw' valley oi the Jueyes Kivcr. less than three miles back fi-om the coast, tlie surface ol the alhivinin which fills tlie snJisecpieirt valley behind the Cuaiim limesttuu. I^^^^^^K^ fe:f^^^^^^^^Hi| >.s-|. f,f nio Itknu ridge stands almost 5()(i feet abovt; sealcvd. 11ie slope seawanl is m^u eiently projiounced to be detech^d by ttie eye. tliougli it is ditlicnlt to rca- witli an ordinary clinometer, since it animmts to only one or two ilcgree%- The upper brandies of the Jiieyes Eiver liave cut down \'ery (h'ef)lv inb tin's alluvium, producing in places miniature canyons, wh.icli are disiine hindrauc(>s to cross^coun,tr\' tra\xd. Tlu> upper course of the rioiiiiajib^: b(dA?een San (Jernmn and Sabana Grande ami the Eio Bhincii east n Anasco provide examph^'S of terraces in the western part ut the island. Tlie smallest, but in some ways the most striking, of tlie alluvial «!• posits of Porto Kico are those clogging the shnrt, open valleys in th- soutlieast corner of tlie island between Maunalto and Yalnicoa. The-- valleys head back only a mile, or even less, from the coast auil are sc|};i rated by headlands subject to wa-ve attack. 'J'he surface of tlie alluviui: is exeeediuglv steep and is nnw only in the initial stage of dissectioe Along tlie shore these deposits are terminated liy hyw elitl's "iO feet oi* r- LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 353 in lieiglit;, which are usually fringed with a narrow beach. The alluvium is entirely the result of wash from the granite hills, whose slopes are cov- ered with a very deep residual soil, granular in texture, but so firmly held together by its lime content as to be extremely brittle and ringing like cast iron under the hammer. The walls of the streams in the alluvium, like the sides of the gullies in the granitic soil, maintain an almost vertical attitude. Two explanations may be offered for the development of terraces in Porto Eico, which, however, are not incompatible with each other. The first one suggests that the recent slight uplift of the island rejuvenated the streams and caused them to incise themselves below the flood deposits formerly laid down. The evidence of uplift, to be presented later, hardly justifies assumption of an uplift greater than 20 to 30 feet, and this is insufficient to account for the high terraces everywhere observed. Ac- cording to the second explanation, the streams of Porto Eico have changed in general from aggrading streams to degrading streams, without neces- sarily any change in the relative position of land and sea. In the streams of 'New England such a condition is recognized. During the end of glacial time the rivers were heavily charged with debris and accordingly filled their channels with alluvium. But when the ice-sheets ceased to provide material for the streams to carry, their preglacial vigor was re- gained and terraces were formed as they cut down into the alluvium. We do not know just what causes may have contributed to the earlier over- loading of the Porto Eico rivers. It is not unreasonable to think, how- ever, that during glacial time climatic conditions in Porto Eico were different from those of the present. Not that any glaciers existed there, for there is no evidence of this, but it is conceivable that a heavier rain- fall prevailed. Such a climatic condition is thought to have accounted for the ancient Lake Bonneville in Utah. In Porto Eico a prolonged period of heavy rains, though adding to the actual volume, would at the same time overload the streams with sediment. The rapid run-off makes almost every drop of rain that falls an active erosional agent, and because the effectiveness of the erosion is not in direct proportion to the run-off, but increases at a much greater rate, the tendency is therefore toward an overloaded condition of the streams below the steeper headwater portions. Increase of rainfall at the end of the glacial period would therefore add greatly to the load of the streams and cause aggrading of their courses where the slopes were modest. With the dying-out of these conditions ^iegrading would again come into play until the stream profile prior to glacial times was restored. Terraces thus formed would grade into the present flood plains of the lower river courses. :',5I i WXXV. AM) Wl\|. VVol.'K Wnv,. .ltni.-k ,.li I'nrto 1,'lrn hiis Ih-imi IHlfll lIH.n- .•liV.ilVr ul, fh.. n liiaii on tl.M- sciuh to r..rni tin- sn^-nllr.l Sun Junii tnrin:i on th.' north nuist. It is iM'lirvo.l tlinl tlu- u\mwm-iHv m ih.. ;siibniatiHv >ta,iii; of d.n. opinont. The hoadland^ havi- heoii mJ Kai'k, Lars h.-ivo Im-om h„i1i, non. striu.uiii. "M:niiy of tho Imv- liavp hrvu oiitirolv rUlcl wilh allovmn others roniaiii as o|H.ti liiu:.H.n^. A sk,.t,-li map oC the r.-iioii ( Fi.u\ "^I hiisod on a roiiuli fiold sk.;(rli. i> sorvieoahlo m indiratin.ir iho >aIionf IV lun-. Th.. Mi.'oessivo l.iiil.Iinij ot^ Lars noaivr the m.mlli^ uf tho bay^ : a- 1.. Inrtii a su,ve<>i..n of hm.um< i^ xsvll lmrs hiiw l.....n Iniill iic.-tr 11 ilii: |.i-(.)l luiira tliinl Inn- i> lo lH.;i. Ill Miunathv will, il ilsiiHaiiii. 'rh..rlil1iiiir. I...tw.-cii Wwni. 1- a n-nt! ill. I'nint Siii) Hi »•-:«'. Al M|ll ilv oIh,. I !itivain- an- Hna!L ul.,;ii lii^H.H^ still iviuaiii. :M(.st ..r ih,. !)(.;). •li.'> sliuw ! 11 LTi'^l I n- niinur feiifiiivs in \\\v torn! of inhil (lrllii> cit'li.T Iniill mil int.. the la-uon. as at Liiinullu ( V\i:. 'i^). nr alnHn.l hiiriiMi un-l.-r llu- laKT .l.>|H.si|. r.f alluvmin. Boa./h ri.|<:vs arc cimi Inn parallrl ri.l-is wv al.nvo th.' Ill 1 iliis r.-'yauii. .Iii>i ua-st cif liiHiiullu ai lr>;ii"'« •'• I'nniU.'l H.luT's shows'iliat th.. roast line lias a.lvanro.l s,,nvar.l. .hi^i otfshoiv tlu- vrafor i.< oxtiviticlv vhallow. At lo« ii.lf. nlfliou-ii iIh; ran-- in waf.-r !.>\c.| (lot's noi iiniount to inor.' than oiif or fnn fool, coiisi.h.rahl.. aivas .,[ tHial (hits aro .■x|)oh.,|. <-m-h as may !..■ s.-mi ju-l oaM of Point Hniharca-^ •Inros (Tm. •>!•). Wave aotinn upon tho hoadhiiMls s..rv.,s f. .nnplni-i/.- th- .>-tHu ..i" ioinis wliirh woll a,-u.|oiH..! in tiH^oIiH: :]-,G sUIEyTlFlC PURVEY OF I'OjrfCf KKJO jKHihrasl.Tn t-.tnir-r of l\n'iu IIwk llvre, as w Cnlchra, iiiul it^ uear^liv i.slaiids, ilie cliffs arc i'uatiwMl bv sliarp clcrts aloii_i;- tho proiniiieiit Joints wliirli, m .iix'iioral troii-l in ;i iinTili«i* \ :.' f.--^ ■ f- sr-r- ■ ■■ ■ ". .;'-.*■' " •^' «'■' ■■*«■ -' , tW.^^^^Wun, artimi almnj joint plan. r. .-SfiM J((o« /'^ .,i?lf iJiHHrh, THE l'11Vslfn;h' M'ilV (iF PnUJii H H ..r \An\n^ wliicli rcsnil. hiiv.- alri'.i.jy in-.«ii .!.^serilH.'il, Th-y coiisiJlule \h:' aiViit iiiMl.,Tuv,nur ^\av.> at^ tii.'k. OcriisK.i.allv lluwi]i<: ii|hiii riic llh- hviU'T j^lrcaiHr^ are able to iiiaiiiiain ai out ill fr..iit of llif bar. HrTVfs as a supplv i '•..me iu throw ii|> aiK.lber bcadi. < >craHo!iaLl v. wb.-ii tbi \ivmi is <'xcf"ssi\f.. or Xlm waves aiid (airreiHs a.ri,* imi; sunicienily \i_tror i.a rs jii^r il.'scrib...!. bm mir ihruiiiih ih.-m. Al al which, if 11 is carriei ■•b.-ii tbi' siitqilv .;.!' albi b.'lta. 1.1. i< i~ tij. ii'.MiFiC si:i:vi:v f>F I'Oirfo i.'/i hII.-v~ , ■■■;; M- T^a- \h\r,-n-. Mo^t oi /ji/.'/;r/i\ 777 /■; i'!i YSKfUh'.ifiiy of pt^rfi i:h 3G0 SCIENTIFIC SURVEY OF PORTO RICO WEST COAST The irregularity consequent upon the drowning of the west coast of Porto Eico has been destroyed by the development of beaches and the filling of the bays. Occa- sional lagoons still remain, as at Lake Joy- uda. The beaches customarily reveal their stages of development by parallel ridges, a feature well marked between Point Guana- jibo and Maya^ez. As a whole, the work of the waves along the west coast has pro- duced effects similar to those along the east coast, but cliffing of headlands has been much less pronounced. NORTH COAST The north coast from Aguadilla to Camuy is characterized by marine cliffs, averaging over 200 feet in height, cut in the limestone coastal plain. Considerable stretches of these cliffs, however, notably to the west of Isabela, are no longer subject to wave attack, owing to the protecting beaches built in front of them, in places to a width of one- quarter to one-third of a mile (Fig. 33). A protecting beach in front of a marine cliff is a normal feature in the process of wave ero- sion along a steep coast. It represents a temporary reversal of conditions from those favoring cutting to those favoring deposition and is quite analogous to similar alternations in the life of a stream. The extent to which the sea has cut back into the Tertiary coastal plain may be esti- mated by carrying the slope of the plain sur- face seaward until it meets sealevel. With a slope of 114 degrees, the distance is ap- proximately two miles. It must be remem- bered that the outer thin edge of the wedge was easily removed, and it may also be as- LOB EC K, THE PHYSIOGRAPHY OF PORTO RICO 361 sumed that cutting has been long active and was going on before the period of submergence. Fig. 34 provides two profiles from this part of the coast. The one through Isabela does not need any especial explana- tion beyond what has Just been said. The other profile;, through Quebra- dillas^ seems to be anomalous, in that the continuation of the slope of the coastal plain surface meets the sea even beyond the edge of the narrow continental shelf. Two interpretations are possible : first, the dip of the strata is steeper than the slope of the plain surface. If the normal dip of the beds instead of the slope of the plain surface be continued seaward from the cliff, assuming that this is approximately the top of the series, sealevel is reached less than a mile from shore and within the limits of the continental shelf; second, faulting may have taken place at the edge of the continental shelf, so that the Tertiary coastal plain was broken off during uplift. The apparent marine bench, sho^vn in the profile near Quebradillas, standing at an elevation of 150 to 200 feet, has already been explained as being due to differential erosion along bedding planes. East of Camuy most of the north coast is low and bordered with allu- vial deposits. Fringing the shore is a chain of sand dunes exhibiting all stages of consolidation and dissection and in part constituting the San Juan formation about to be described. These sand deposits frequently hold the position of barrier beaches, in which case they have behind them marshy lagoons or open bodies of water. This is the explanation of Lake Tortuguero near Manati, San Juan Harbor, and the smaller lakes of San Jose, Cangrejos, and Piiiones to the east. The prevailing drift of the water along the north coast is toward the west, as a result of the strong trade winds from the northeast. In the coves between the minor promontories formed by remnants of the San. Juan formation secondary eddy currents are developed. These affect the bars at the mouths of streams and serve to deflect the stream outlets to- war.d the east. In Fig. 35 several sketch maps illustrate this tendency in the case of the Bayamon and Cocal rivers and San Fernando Creek near San Juan, and of the Arecibo, Manati, and Cibuco rivers further west. In each instance the mouth of the stream is diverted eastward. THE SAN JUAN FORMATION This interesting formation occurs only along the north coast of Porto Rico, and is described by Berkey as a series of solidified or fossil sand dunes dating from the Pleistocene. At San Juan city, which is built upon the formation, its elevation is 100 feet above sealevel. Nowhere else does it rise to quite this height. Almost everywhere associated with 362 SCIENTIFIC SURVEY OF PORTO RICO the solidified dunes are other dunes of loose sand now forming. Even these unquestionably very recent dunes show tendency toward solidifica- tion. Therefore, it is difiicult to draw a sharp line between the two types of dunes and, in the opinion of the present writer, there has been an unin- terrupted process of dune formation. In a bold way the San Juan forma tion may be pictured as occupying the position of a broken barrier reef Fig. 35. — River mouths of the north coast Showing the effect of secondary eddy currents in diverting the rivers toward the east. along the north coast, continuing eastward in the Cordilleras Eeefs, and finding its easternmost expression in a small mass just off the northwest corner of Culebra Island. Behind the barrier there is usually a marshy lagoon or open body of water, but occasionally the dunes rest directly upon the mainland. The cross-bedded structure is almost everywhere a pronounced feature. In addition there are two or three other features of unique character which apparently demand special explanation. LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 363 The first of these exceptional features is to be noted in the cliff of the San Jnan formation immediately east of Arecibo. This cliff faces toward the south and overlooks the broad flood plain of the Arecibo Eiver. The cliff face shows the strikingly cross-bedded structure with steep dips characteristic of the San Juan formation; but it shows also a series of horizontal lines crossing the structure, but in no way interfering with it. This unique feature Berkey has suggested was induced by the gradual submergence of the sand dune after formation. He therefore associates it with the oscillation of the coast line believed to have occurred recently in the island's history. The cross-bedding is therefore to be considered the primary structure of the dunes and the horizontal lines a secondary matter. In addition to these horizontal consolidation lines, as they might be termed, there occurs a prominent horizontal layer of sand near the base of the cliff. This layer is unconsolidated and of course interrupts the cross-bedded structure which occurs above and below it. Inasmuch as neither of tRese two features — that is, neither the horizontal consoli- dation lines nor the interrupting horizontal layer of sand — is a feature to be noted elsewhere in the San Juan formation, it seems reasonable to believe that a local cause might furnish the necessary explanation. The xlrecibo Eiver, which flows not far from the base of the cliff, is a powerful stream and is subject to extreme oscillations of level. It is therefore sug- gested that the horizontal unconsolidated layer near the base of the ex- posed part of the formation represents a deposit of alluvium left by the stream upon a part of the sand dune which it had truncated in its en- deavor to break through to the coast during heavy floods. The upper cross-bedded part of the formation represents the dune material built upon the alluvium when the river had subsided or altered its course to a different channel. The alluvial layer may therefore be considered a part of the river's flood plain upon which later dune-building has taken place. The secondary structure or horizontal consolidation lines occurring above this sandy layer may have been induced by subsequent rises in the level of the river so as partly to submerge the more recently built dunes. This is a very probable event, for it is on record that the Arecibo Eiver in heavy floods deeply submerges its flood plain. Such an event is aided by the very narrow outlet to the sea which the river now has between the continuous dune formation on either side. During the heavy floods of 1896 much of the town of Arecibo was submerged, indicating a rise of 20 to 30 feet at that time. A rise of 30 or 40 feet would account for practically all the phenomena now observed. It is to be noted, too, that the secondary horizontal structure is to be observed only on the landward side of the dune in rather close proximity to the river and adjacent to its 364 SCIENTIFIC SURVEY OF PORTO RICO flood plain. The conclusion;, then, regarding the secondary horizontal structure is that it does not necessarily indicate a rise of sealevel. As an alternative explanation the rise of the river level in floods is offered, and it is shown that such an event is a probable occurrence. Another unique feature of the San Juan formation, and one which the writer has not seen described elsewhere, is the series of small terraces occurring near sealevel and having characteristics which may be briefly summarized as follows : 1. The terraces always occur within the range of present wave action. 2. In appearance the terraces resemble those of the well-known Mam- moth Hot Springs in Yellowstone Park. 3. There is no order whatever in the arrangement or size of the terraces. 4. The terraces have not been cut by the waves ; they have been formed by the precipitation of material around the rims of shallow basins as the water overflowed. This material usually has a pinkish hue and may l)e easily distinguished from the granular sandstone of the formation. 5. The terraces are practically horizontal. 6. The cross-bedding of the sandstone cuts across the terraces at all angles. 7. Where irregular masses or blocks have fallen into the water, they first assume a very jagged character, but as loose material is filled in and cemented into the crevices, and as the water pouring in and out precipi- tates lime around the edges, gradually the sharp, angular forms give way to a terraced aspect and the original disordered make-up becomes obscure. 8. Terraces of this type are developed not only in the San Juan forma- tion, but also at the base of the Tertiary limestone cliffs near the west end of the north coast (Fig. 36). The conclusion from these observations is that consolidation is now rapidly going on in the zone of water action. Other observations along the coast bear out this statement. For instance, it is to be noted that the modern beaches are occasionally firmly cemented like a pavement. The stone reefs of Brazil described by Branner (1904) is a well-known example of a modern beach now consolidated. Branner suggests that local lithification may be brought about in the following ways : 1. By carbonated rain water dissolving out the lime carbonate in the upper portions of calcareous sands and depositing it in the lower portions. 2. By the escape of carbon dioxide from the sea water when the surf ■breaks upon the beaches. 3. By the escape of carbon dioxide from sea water where it is warmed by the tropical sun. 4. By the submarine escape of carbon dioxide about volcanic vents. lJ>liEi:K, THE PJJYSiOGKAI'flY OF I'Vh'TO RH!0 ;;,;; '^'ii'' tlis[ thrv.v ..r thes,' .'xpiiiiuit mns wuul.l umlouhi,'dly apHlv to ili< -.■n>,,lH!ai,i..,i ni: tlip saiul doiies almig flH> jiurili cuiist niul tn the .J.'volni.. m.MH uf tl„. i,.Tni.-,.s .l.-crilMMl. In mldhion i,r. iIm-s,. .•xpbiiiati.)!!.,, il mnx Ih' jh.u-.I ili.-it .pray fruni i,[m-, wav.., is .■utitimially In/iiyu- l.|nw]i iii>iinr'., I'xr liiuHlr,..:!- ..f Icoi alon- jh.: ,,,,,1,1, ,:,,,st; ..f l'..i1u l{iro. h,, iliai tli,. ..nn^ \uirtl -i.h' of th.: .hint's i> k.'pt lur.isl. Tlic i^.mshiiH cvapur:,! ion of til.. ~i'.-i '.v:ifor himI ill., lii.enitioii r.f ,:a.rl,oii ili,.v!.l.' w.„il,l oiii-.-ura-v- iithilioa.^ II'. II. ..>p,.,-i:,llv HI, ih,. .urlac- .-f tlu; dunes. Uniirmlily it i-; tiial porlioi, "i 'I"' luhs b,..>n a ivecut .nn-'i- .i»viH,. of not iiiHiv ilian JO r,,..|. ^l,,>i, oi" the ovidiMiro Ins in ih.; w.si,.rn liuir of the i'(-i.M! aiul tlieiv i> pm.iirally m- eudMiicn in ili.. rnstrrn jmrr. Hunu' f.r Ih,' ,xvi.l<>iH-o pr.-<'iii(..!, in.li,-;ifit,u^ an uplifl, M^vmiT Hum tli.it ju.-^t nH'iil.ionr.l, J h.'li..\i' n, Ik> invni|.|. In tli.;- (nllowui^- pi^u^s nil .,1 Tli,. h1<.(HV illV . iiiHi ii.-ar Puuit Ji; uiJht isulatn.l l.H^alitifs on iiin mainhin.i. 'Tiu, |.:rrao..> or li.an on II.-mh'Ih.o Jslau.l and noar (^n•lH•ad^Ia^. <^ii!,yanni. I'.nnt, Jit Mavn-noz nil llir nniinland. DnH'olHM) Island la- M.na- twojvv m\U'> to (Jio xu->i of IN.n and i> lornind (aifnvl^ o|^ .,ld,n- xtm-s rnrk> whioli hav.. h,,ai (Fur. :•:). '\'\w lomi -yipliir^ i. nM'd h,van~f i!io rnmnM.n .- 368 SCIENTIFIC SURVEY OF PORTO RICO all the West Indies favors a movement of the land rather than one of the sea. Marine benches and terraces lack a uniformity of elevation in the various islands. The bench on Desecheo intermittently rims the whole island, but is preserved to a greater width and in much better perfection on the south coast. There it may be as much as 100 yards wide, cutting sharply across the upturned edges of the volcanic sediments and ending abruptly on its landward side against the steep hill slopes which are pre- sumably wave-cut cliffs. The platform dips seaward in all directions from the island and thus passes down gradually into the water. The surface of the platform is heavily coated with a thick deposit of consoli- dated beach material consisting of coral heads, boulders, gravel, and shells. The coral heads appear occasionally to be in position, as though still at- tached to the places where they grew. The evidence which this island presents surpasses that noted elsewhere and would indicate a minimum uplift of 20 feet. Mona Island is fifty miles to the southwest of Porto Eico. On its southern side is a platform several hundred yards wide in places. Its surface stands 10 feet or so above sealevel. The platform is composed almost entirely of consolidated beach material consisting of coral heads and modem shells. Its inner margin is made up largely of talus from the limestone cliffs, all thoroughly cemented. The platform is every- where made up of this heterogeneous material. Nowhere is it a rock platform. Its outer margin, in the zone of water action, shows terraces like those described for the San Juan formation. These terraces are not due to wave cutting, but to the deposition of material around the edges of shallow depressions. Along the southeast coast of Mona Island, a mile or two south of the lighthouse, remnants of the former beach may be seen adhering to the lower portion of large talus blocks standing in the water at the foot of the cliffs. The blocks are as large as a two-story or three- story building. The presence of beach material at their base indicates the extreme recency of uplift. The evidence on Mona Island favors a mini- mum uplift of five to ten feet. It is certain that the uplift here was less than that on Desecheo. Fringing the north side of Muertos Island, near Ponce, is a little ledge of fragmental beach material consisting of shells, coral heads, and lime- stone fragments firmly cemented together (Fig. 38). It is only a few yards wide and stands now some five or six feet above sealevel. The evi- dence here would suggest an uplift approximating that on Mona Island, but very much less than that on Desecheo. Near Point Jiguero, Hubbard reports finding consolidated sands and gravels with modern corals and gastropods resting upon the Tertiary at LOHKVK, lilt: PIIYSIUiillMMiY OF I'ORTO Rir<> ;;,;■.) an e:it'\:it!oii of 40 tn io jVct above sriilevvl A niili' NOiiih oi \\u< x^mw iriil.bard rc))uii> Hinihir grav«'k at :i5 I'eot alNive s.-iilev,'!. Xea,r Ma; Ii-rlit- house tlic surnc in vc>iigiii.>r fomul Haiul .miu.I gi'iivel^ witii .lunnern Ak-\\< ivf:ting Mil ta-uti<':i,|,.il rock.s uf the ol-U;!' ncries at an clevatM.n of si\ to !<•!! tVel:. 'riii> cvKieiirn wouhl appear to favor an uplift in lliis luealiiy ..f approKiniately -.I*) i'«'et. At other is.ol.'ited loe'aJities uplifual beach -nvA\vvv.\\ faxors an uj.hit ut 1?3 to :i:. hr-l. On Puint AguiJa, ftinoing the Riuthwost comer of rorio hMeo, J, ni.ov or !.-;> ooi!>oli(late(l layer of .Mu-lls ami beaeli materia 1^ rest> upon the 'Teruarv ai, 10 to lb b'et, above eateh » i< not goo.I evbhaiue of uplift. DiHV >nli5 in the .le-veb.pnauit of h..nebe> in the ^uek^. Near lkri:..innela uiobern shells ajal 1 '.iisolidatia! gravel> at 12 f<>ff aixo^e seahn-.b. foi- riplifte-b heach. inab«rial on t'orto Ilu-o lbe iro 3rO SCIENTIFIC SURVEY OF PORTO RICO pottery and other criteria^ such as black soil, indicative of former camp sites, will reveal the true origin. Such shell heaps were noted near Kin- con, Coamo Springs, and on Vieques. The splendid example on Desecheo Island of a marine bench uplifted 20 feet above sealevel has already been described. Near Quebradillas there is a pronounced terrace (Fig. 16) along the coast, having an eleva- tion of 180 feet above sealevel and a width of one-quarter to one-third of a mile. This terrace is developed upon the almost horizontal Tertiary limestone and it is strongly suggestive of wave planation. The question of its origin, however, has been discussed, and it was pointed out that the terrace is due to the differential erosion of the limestone along the bed- ding planes. In this region the surface of the coastal plain consists of groups of haystack hills separated by intervening fiat areas. It happens that at just this point the present waves have cut back to one of the *^^flats,^' and thus an uplifted marine-cut terrace is simulated. Part of the town of Guayama is built upon rock terraces, which ap parently stand slightly above the alluvium deposits of the playas or flood plains. Berkey (1915, p. 48) has interpreted these as due to marine cut- ting. A careful study of this locality, however, leads the present writer to the conclusion that their flat surfaces have been produced by the plan- ing action of the stream which deposited the alluvium, in this instance the Guamani Eiver. The reasons for this belief may be briefly summed up as follows : 1. The rock terraces lie in the mouth of the valley of the Guamani River and may be traced upstream between the valley walls. 2. The surface of the terraces stands approximately at the same eleva- tion (180 feet) as the surface of the highest part of the alluvial plains and the slope of the two is practically identical. Much of the alluvial plain has been recently cut away by the river. Those regions consisting of hard rock stand up now as terraces. 3. Parts of the terraces are covered with angular gravel exactly like the material composing the alluvial fans. 4. There is no cliffing of the mountain spurs back of the terraces. 5. Plat rock surfaces of similar character are to be observed elsewhere, as along the lower course of the Jueyes River, so intimately related with the alluvial deposits as to leave no doubt that they were formed at the same time the alluvium was laid down. 6. The rocks of the older series are so much decayed that in numerous places throughout Porto Rico they are being beveled across by the streams on their present flood plains. 7. Similar truncating and planation of strata by streams at the point LOBECK, THE PHYSIOGRAPHY OF PORTO RICO 371 where they emerge from a mountamous area is to be noted in other parts of the world. For instance^ near Cody, Wyoming, where the Shoshone Eiver emerges from its canyon, there is a splendid terrace whose surface bevels across dipping strata. In this case no agent other than the river could have been operative. Similar conditions are to be noted in the Ortiz Mountains of New Mexico. The conclusion regarding the Guayama rock terraces is that they are river-cut and do not represent an uplifted marine bench. A small terrace occurs at Point Jiguero, its inner margin standing some 80 feet above sealevel. Because it holds a position near the mouth of a stream, somewhat similar to those at Guayama, it may have been formed in a similar way. Moreover, it is developed upon almost hori- zontal beds of Tertiary limestone, and the strong tendency toward the development of flat areas by differential erosion would somewhat favor such an explanation. Its position along the coast gives it the aspect of a marine-cut bench, but consideration of other factors just mentioned favors considering it as due to stream origin. Near the Reform School, just south of Mayaguez, a narrow bench, standing 12 to 15 feet above sealevel, can apparently be ascribed to no origin other than that of wave planation. It truncates the older forma- tions, but a search did not reveal the presence of beach material. Most of the evidence cited would indicate an uplift less than 20 feet. Apparently the greatest uplift indicated is at Point Jiguero, where Hub- bard finds beach material as high as 35 to 40 feet above sealevel. The easternmost piece of evidence noted was on Muertos Island, 50 niiles to the east of Point Jiguero, where the uplift was considerably under 10 feet. The eastern end of Porto Eico and the islands to the east nowhero revealed any indication of recent change of sealevel. The westernmost evidence was on Mona Island, some 50 miles west of Point Jiguero, where a modest uplift of 10 feet or so was indicated. Nowhere are there displayed series of terraces similar to the splendid examples featuring the coast of Cuba. BRIEF NOTES ON xlDJACENT ISLANDS Desbcheo This small island, lying 12 miles to the west of Porto Rico, is almost circular in shape and has a diameter of about one mile. It is made up entirely of older series rocks, striking almost east-west, but crossed by joint systems which in general run transverse to the structure. The little harbor providing a refuge on the south coast appears to be developed m SCIENTIFIC SUIiVEY OP FORTO RICO along sueli a joint piano, to which is due also the vallcj in the Iiills abow. Around the base of the island is an uplifted wave-cut platform and beaeli, having its inner margin 2-0 feet above sealevel. Tliis lias l)eeii deseribetl in the preeedinw pages and is illustrated in Fig. 37. wlheh also shows a part of tbe small harbor and the vallcj above. Mo flom'ing streams are to be found on the ishmd. The extremely dry conditions favor an extensive growth of cactus, although a forest oi medium-size«l trees covers many of the slopiss. A l)rief init intercstiiig- account of tlie vegetation of the island bas boon given l)y Dr. Is. h. Britton (1914). MONA Mona Island, 50 miles to the southwest, is not within sight of Porto Kieo except from the liiglier sumniits, and it tlien appears as a km haze on the horizon. This island, as well as its little neighbor, Monita Island, is a remnant of the Tertiary, having an almost horizontal surface, which ends abruptly on all sides in cliffs 150 to 200 feet above the sea. Tliouglu in general, circular in shape, its outline is angular, a feature due to the jointing wliich divides the limestone into large blocks. The entire island is literallv lioneyconibed with caves (Pig, 39). These are developed in LOBECK, THE PHY8I00RAPHY OF PORTO RICO 373 one of the more soluble limestone layers outcropping about 75 feet above the sea and form one of the important sources of guano in the Porto Rico district. Along the southwest coast there appears good evidence of recent uplift in the narrow platform at the base of the cliffs. It stands 10 feet or so above sealevel, has a width of several hundred yards, and consists of consolidated beach material containing coral heads and modern shells. The inner margin of the platform is made up of talus from the limestone cliffs, thoroughly cemented. The outer margin, in the zone of wave activity, shows terraces at the water level like those described for the San Juan formation. Along the southeast coast, a mile or two south of the lighthouse, remnants of the former beach still appear adhering to the base of large talus blocks standing in the water at the foot of the cliffs. There is practically no soil on the island, and passage over the top of the plateau is arduous in the extreme on account of the sharp and Jagged surface of the limestone and the abundant growth of cactus. The only water is that from the rain which accumulates in the slight irregularities on the surface of the rock. Over this forbidding country there roam small herds of wild goats, cattle, and pigs which have escaped from do- mestication and have taken on characteristics in keeping with the harsh conditions of life to which they are now subjected. For instance, the pigs have apparently reverted to a type of boar. Prominent tusks, two or three inches long, were noted on one which had been captured. The animal is so vicious that the natives always carry their long, sharp machetes for self-protection. Besides the lighthouse, a small temporar camp is located on the island for the workmen who are employed in re moving the guano from the caves. These people supply themselves with fresh meat by hunting, a wild goat or two being the usual result of their efforts. Vieques The narrow channel between Vieques and the eastern coast of Porto Rico is noAvhere more than 60 feet in depth and in general is a great deal less. This island, like Culebra, was presumably separated from the main- land as a result of submergence. It is low-lying and is made up mainly of the oldland rocks. These are flanked on the east end and south side by remnants of the Tertiary coastal plain. A well-formed cuesta has been developed, and the subsequent drowning of the island has permitted the sea to enter the inner lowland at several places along the south coast. Some facts regarding the Tertiary coastal plain have already been given. The accompanying diagram (Fig. 40) is a sketch drawn looking south from near Culebra Island and conveys a fair idea of the general aspect of le 'V 374 1 a SCIENTIFIC SURVEY OF PORTO RICO Vieques. Among the minor features of interest are the alhivial deposits clogging the valleys on both the north and south sides, resembling those near Yeguas Point on the southeast corner of Porto Kico. At Playa Grande a graceful and picturesque beach has been thrown, across one of the drowned bays which has not yet been filled with alluvium. The low headlands throughout the coast are being cliffed by the waves. In general the island is dry, as evidenced by the com- mon occurrence of cactus, although it is not at all bar- ren. The raising of cattle is the most important indus- try in the hilly parts, but the broad inner lowland on the south side, as well as extensive tracts on the north, are devoted to the production of sugar-cane. CULEBRA Culebra and its numerous adjacent cays is represented wholly by the oldland rocks, which are moderately folded, although no prevailing trend in the structure was noted. The salient features of the topography are controlled by erosion along strong joint systems, the major one running in a northwest-southeast direction and a minor one almost transverse thereto. Great Har- bor and Flamingo Bay are valleys developed along the line of the major system and later drowned. This island exhibits the customary features due to drowning. Deep bays indent its coast and numerous islets lie offshore. The head of Flamingo Bay has been cut off from the sea by a beautiful beach, and owing to the small size and intermittent character of the contributary streams the lagoon is but slowly being filled with alluvium. There is now only a small settlement on this island, which was formerly an important coaling station for the United States Kavy. Dairying and the raising of cattle appear to be the only industries. ISTo sugar-cane is grown here. MUERTOS The Caha de Muertos (Fig. 41), or the "box for the dead,'^ is a coffin-shaped remnant of the Tertiary coastal plain lying offshore from Ponce. An islet at its w^est end is made up of steeply tilted rocks of the older series, ijiHFjix. 'Jill-: I'll \'si(nii,' ii'iiv Of I'uirn) imuj ;j;:, .hnrpiv f.lHi.eil nrrns,- hy a |.l;iflor.m r.\or whitli al lui.- ii)u.> llio l\-rliiirv- Hc.ls iiii.lniilit.pdly I'Al.'iHl.'.l.' Ali.i!- til." M.u!h i..ii.' Km to rion I'c.l, iJiirk. Ap.paiviPlv thi* pri>-.-niv ..r ih.' iii-.r.' ivsisiant "M rork- al \w ImM. has pn-v.aito.l tin' .ifstni, rrinpal witli wliai app.'ars in In> an nplifi.'.i 'u.-a.-h ..f caar nlalai.-d Imu'stuiu. rra.ii:na'iii>. slir-lls, an-l .-ura! la-ads. innv -tan.ling li\( n ^ix (vi-i ab-.v.. scalcvaJ (Fig. ;]S). At'iv'X()\viJ-:i) pn.bl.aii iMaaipa'd a i/aaan.! ..f jiu- months, from S,.f.|1iidifs .,r I'orp. IIj.'., .;ariaed^ .m hv tho Ad-w Y..rk Arad.auy .,f S.i.aieas. Id. Dr. ('. V, Berkoy J ajii p.Ts.Hially "aaidddd for ila. kiudly aid aod laiik whi.-h lio sho\v<'d iii tJa,- iiiuk>riid inv hisiiv to acknowkalge ibio aid alfotaka! 'by (kna-rnor Vaa-"r in ilic innitor ..C irans- pt.ilation and of (kdon.-l SJuniion. (dnot of tha insular Jkjlir.y in pro Haiiiiiir nu- wilb tifstiinoriials to bis varams otliiaTS, 376 SCIENTIFIC SURVEY OF PORTO RICO Finally, I am more than pleased to include the name of Mr. Fernando Oliver, of Anasco, whose perseverance and pluck during three months of arduous work in the capacity of interpreter and assistant are deserving of mention. BIBLIOGRAPHY Bbbkey, C. p. 1915. Geological reconnoissance of Porto Rico. Ann. N. Y. Acad. Sci., XXVI. The most modern and detailed description of the geology and physical features of Porto Rico. Branner, J. C. 1904. The stone reefs of Brazil. Bull. Mus. Comp. Zool., XLIV, pp. 1-2Guayama ) , 189 quartz (Ponce), 269; (San Juan), 85 Dissection, mature, 41 Dolomites, origin of (Coamo-Guayama), 159 Drainage (Coamo-Guayama), 122 (Ponce), 236, 238 Dune area (San Juan), 42 Economic geology (Ponce), 290 Emergence (Ponce), 290 Erosion, Cretaceous (Ponce), 288 Tertiary (Ponce), 289 Expeditions, 24 Explanation of Geologic map (Coamo- Guayama), 228 (Ponce), 232 (San Juan), 106 Faulting (Ponce), 265 (San Juan), 96 assumed, 328, 339 Felspathic andesites (Coamo - (Juayama), 171 Flows, early (Coamo-Guayama), 170 Folding (Ponce), 263 (San Juan), 98 Fossils, Cretaceous (Ponce), 285 Post-Tertiary (Ponce), 283 Tertiary (Ponce), 283 Fossil diatoms (Coamo-Guayama), l;H, 194 Fossil fauna (Coamo-Guayama), 102 (Ponce), 240, 282 (381) 382 SCIENTIFIC SURVEY OF PORTO RICO t'ossil foraminifera (Coamo - Ouayama), 132. 193, 194 (San Juan)» 74, 75 Fossil moUusca (Coamo-Giiayama), 196 (San Juan), 58 Fossil radlolarlans (Coamo - Guayama), 134, 193 Garnet rock (Ponce), 280 Geography (Coamo-Guayama), 115 Geology, economic (Ponce), 290 general (Coamo-Giiayama), 124 historical (Ponce), 287 Geologic features, 15 history, 23: (Ponce), 287 Geologic map, explanation of (Coamo- Guayama), 228 (Ponce), 232 (San Juan), 106 Geological reconnaissance, 14 Glacial control theory, 346 Gold (San Juan), 102 Granites (Coamo-Guayama), 185 (San Juan). 82 Granodiorites (Coamo-Guayama), 186 (San Juan), 85 Guano (San Juan), 106 Guayama Series (Coamo-Guayama). 142 Haystack topography. 45 Hemiaster her key i, n, sp., 283 Hills (Coamo-Gunyama), 118 Historical geology (Ponce). 287 Historical summary (Coamo-Guayama). 198 Hodge, Edwin T.. Geology of the Coamo- Guayama District. 111-228 Hornblende andesite (Coamo-Guayama), 189 (Ponce), 271 Hornblende-augite andesite (Coamo-Guay- ama), 191 Hornblende-augite diorite (Coamo-Guay- ama), 189 Hornblende diorite (Coamo-Guayama), 187 Hornblende granite (Coamo-Guayama), 185 Hydrothermal effects (Coamo-Guayama), 202 Igneous rocks (Coamo-(»uayamaK 128, 197 (Ponce), 261, 269 (San Juan). 76-94 Interior mountainous region (San Juan), 48 Introduction to the Geology of the San Juan District. 36 Intrusion (Ponce), 288 Intrusive rocks (Coamo-Guayama), 186 Intrusives. early (Coamo-Guayama), 170 Islands, adjacent, 371 Jasper-bed (San Juan). 66 l^atites, quartz (San Juan), 83 (Trachy-andesites) (San Juan), 87 Lavas (San Juan), 76 Lead (San Juan), 105 Lime (Coamo-Guayama), 224 sand (San Juan), 52 Limestone, 309 Aguas Buenas (San Juan), 64 at K 29 (San Juan), 65 belt, Tertiary (San Juan). 44, 47 (Coamo-Guayama), 134, 147, 149, 153. 15.5, 161 Coamo Springs Series (Coamo-Guay- ama), 153 Corozal, 18; (San Juan), 62 cuesta. Lares, 332 Los Puertos, 333 garnetiferous (Ponce), 280 Guayabal (Ponce), 256 La Muda, 18; (San Juan), 64 lowland, Cibao, 333 Quebradillas, 333 San German (Ponce), 253 structural (Coamo-Guayama), 224 -tuff. Coamo, 18: (Ponce), 255 Limonite (Ponce), 292 liObeck, Armin K., Physiography of Porto Kico, 301-379 Lowland, Cibao limestone, 333 inner, 331 (Ponce). 242 Quebradillas limestone, 333 Luqulllo Mountains, 314 Manganese (Ponce), 290 (San Juan), 105 Magnetite (Ponce), 292 Meanders, incised (San Juan), 49 Metamorphism (San ,Tuan), 98 Metallic minerals (Coamo-Guayama), 220 ^linerals. metallic (Coamo-Guayama). 220 occurrences (San Juan), 102 resources (Coamo-Guayama), 219 Mineralization (Coamo-Guayama), 209 Mitchell, Graham John. Geology of the Ponce District, 229-300 Mona Island, 372 Monzonites (San Juan), HH quartz (San Juan), 84 Mountains (Coamo-Guayama). 119 complex (Ponce), 234 Luqulllo, 314 relation of streams to trend of (Coamo-Guayama ) . 122 Muertos Island. 374 Natural history survey of Porto Rico, 37 Oil shales and petroleum (Coamo-Guay- ama), 222 Oldland, 307 complex mountainous (San Juan), 38 INDEX TO VOLUME 1 383 Older series, 17 (San Juan), 60 Olivine andesites (Coamo-Giiayama), 1T(> basalts (Coamo-Giiayama), 170 (San Juan), 93 Olivine-free basalts (Coamo - Giiayama), 179 Palaeontology (Ponce), 282 Peneplanation (San Juan), 40 Peneplane (San Juan), 50 first or higlier, 312 second or lower, 315 I'etrographic range, 18 Petrology (Ponce), 260 Petroleum (Coamo-Guayania), 222 (Ponce), 297 I*hysiographic features (Ponce), 245 history, 304: (San Juan), 35 Pliysiography ((^oamo-Guayama District), 116 of tlie Island. 38 (Ponce District), 233 (San Juan District), 41 Placers (San Juan), 102 Platinum (San Juan), 104 Playa plains (San Juan), 52 Playas (Ponce), 244 (San Juan), 43 Pleistocene (?) deposits (San Juan), 53 Ponce formation (Ponce), 258 Porphyrite. augite (Ponce), 275 Post-Tertiary fossils (Ponce), 283 Problems of the Survey, 24 I'rofiles. submarine, 347 I'yroclastics (Ponce), 275 ( Coamo-Guayama ) , 170 Quartz diorite (Ponce), 260 (San Juan). 85 latites and rhyo-andesites (San Juan), 83 mines, auriferous (San .Tuan), 103 monzonites (San Juan), 84 Quaternary deposits (Ponce). 260 Quebradillas limestone lowland, 333 liadiolarians ((^oamo-Guayama), 134, 103 Recent changes, 348 deposits (San Juan), 52 submergence. 343 submergence and uplift, 41 Peeds, Chester A., New Base Map of I'orto Rico, 30 Reefs, Cordilleras, origin of. 347 Relation of streams to trend of valleys and mountains (Coamo-Guayama), 122 of topography to rocks and rock structure (Ponce), 235, 238 Relief features, 22 Rhyolites (Coamo-Guayama), 170 (San Juan), 82 Rhyo-andesites (San Juan), 83 Rio Descalabrados Series (Coamo-Guay- ama), 161 Rio Jueyes Series (Coamo-Gua.vama), 140 Rio de la Plata Series (Coamo-Guavaraa), 130 Road metal (Coamo-Guayama), 222 (Ponce), 297 (San Juan), 105 Rocks, anamorphic (l*once), 262 basis for attempted division of, 120 batholithic (Coamo-Guayama), 184 Cretaceous (Ponce), 263 deposition. Cretaceous (Ponce). 288 epidote (Ponce), 281 formation, 15-22; (San Juan), 51 garnet (Ponce), 280 hydrothermically altered and miner- alized (Coamo-Guayama), 120 intrusive (Coamo-Guayama). 1S6 katamorphic (I*once), 262, 281 metamorphic (Ponce). 280 mineralized (Coamo-Guayama), 120 sedimentary ( Coamo-Guayama ) , 126 ; (Ponce), 277 structure, relation of topography to (Ponce), 235, 238 structure, general (Coamo-Guavama), 125 physical character (Coamo-Guavama), 124 Routes of travel (Ponce), 232 Salt (Coamo-Guayama), 223 (Ponce), 205 San Juan District, description of, 37 synopsis, 35 San Juan Formation. 361 (Ponce), 260, 270, 200 (San Juan), 53 Sandstones (San Juan), 60 Santa Isabel Series (Coamo-Guayama), 166 Sea Cliffs (Coamo-Guayama >, 110 Sedimentation, Tertiary (Ponce), 280 Sediments, Tertiary (Ponce), 258 Serpentine, 300 (Ponce), 281 Semmes, Douglas R., Geology of the San Juan District, 33-110 Shale (Coamo-Guayama), 128, 133, 143, 151, 162. 222 (I'once), 248, 251, 277 (San Juan), 67 Esenada (Ponce), 252 Oil (Coamo-(iruayama), 222 Peuuelas (Ponce), 251 Rio Yauco (Ponce), 240 Sierra de Cayey Series (Coamo-CJuayama), 137 Sills (San Juan), 76 Sink holes (Ponce), 246 and the question of assumed faulting, 339 384 SCIENTIFIC SURVEY OF PORTO RICO Slumping (Ponce), 245 Springs, Coamo Thermal (Coamo-Guay- ama), 200 and wells, occurrence of (Coamo- Guayama), 210 Stocks and bosses (San Juan), 79 Stone, building (Ponce), 296 (San Juan), 106 Stratigrapblcal summary (San Juan), 99 Stream capture (San Juan), 50 terraces, entrenched (Coamo - Guay- ama), 123 Streams, coastal (Coamo-Guayama), t2'^ relation to trend of valleys and moun- tains (Coamo-Guayama), 122 Structures, minor (San Juan), 98 Structural features (San Juan), 95 limestone (Coamo-Guayama), 224 Submarine profiles, 347 Submergence, very recent, 360 recent, 343; (San Juan), 41 with terrace-cutting (Ponce), 290 Syenites (San Juan), 86 Teri-ace-cutting (Ponce), 290 Terraces and sea cliffs (Coamo-Guayama). 119 entrenched, stream (Coamo-Guayama) 123 (Ponce), 237, 239 (San Juan), 46 Tertiary coastal plain (Ponce), 237 fossils (Ponce), 283 limestone belt (San Juan), 44 limestone series (San Juan), 55 isolated remnants of, 337 Thermal springs (Coamo-Guayama), 224 Coamo (Coamo-Guayama), 209 Topography, relation to rocks and rock structure (Ponce), 235, 238 Trachy-andesites (Ponce), 271 (San Juan), 87 Trachytes (San Juan), 86 Tuffs (Coamo-Guayama), 126, 131, 135, 145, 151, 156 (Ponce), 246, 275 (San Juan), 71 volcanic, 307 Tuff-limestone, Coamo (Ponce), 255 TurriteVa halensis, new variety (Ponce), 285 Uplift, character of, 327 and dissection, coastal plain, 327 recent (San Juan), 41 Unconformities (San Juan), 95 Valleys, interior (Coamo-Guayama), 122 (Ponce), 242 relation of streams to trend of (Coa- mo-Guayama), 122 * Vegetation and climate (Ponce), 232 Vieques Island, 343, 373 Vitrophyrs (San Juan), 81 Volcanic breccias (Coamo-Guayama), 127 tuffs and shales, 307 Vulcanism (Coamo-Guayama), 202 Wave and wind work, 354 Wells and springs, occurrence of ^ Coamo- Guayama), 210 Younger series, 16 (San Juan), 52 ^ . 4 THE UNIVERSITY OF MICHIGAN DATE DUE > ^ ** ' . .^ ,t UNIVERSITY OF MICHIGAN 3 9015 03458 5607 BOUND OCT -■ ^ iy43 UN'. . ■ ir (.iiCH. 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