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9//JT (J I fc^ I DtC 9 1924 NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands VOLUME II—Part 2 The Geology of the Humacao District, Porto Rico— Charles K Fetike NEW YORK: Published by the Academy 1924 f NEW YORK ACADEMY OF SCIENCES SCIENTIFIC SURVEY OF Porto Rico and the Virgin Islands VOLUME II—Part 2 The Geology of the Humacao District, Porto Rico- Charles R. Fettke NEW YORK: Published by thk Academy 1924 THE GEOLOGY OF THE HUMACAO DISTRICT, PORTO RICO By Charles R. Fettke CONTENTS Page Introduction 118 Location of the district 118 Field work and acknowledgments ll{) Previous work 120 Physiograpliy 120 Upland area 121 The upper peneplain 125 Monadnocks 127 The lower peneplain 127 Caguas basin 128 Coastal area 129 Promontories 130 Playas 130 Drainage 133 General geology 134 Formations present 134 Older series .' 137 Tuffs and volcanic breccias 137 Andesites 142 Felsite 147 Limestones 148 Collores limestone 149 Tuffaceous limestones and calcareous tuffs 150 Shales 151 Sandstones 152 Intrusives in the older series 152 Intrusive sheets and sills 152 Stocks, bosses, and batholiths 153 Juncos gabbro 153 San Lorenzo quartz diorite 153 Yabucoa granite 159 Patillas quartz monzonite 161 Dikes 163 Siliceous dikes 1^3 Granite and granite pegmatite dikes 164 Aplite dikes 165 Granite and rhyolite porphyry dikes 165 Rhyolite 167 Dikes of intermediate and basic composition 167 (117) 118 SCIENTIFIC SURVEY OF PORTO RICO Page Latite and monzonite dikes 167 Dacite and quartz diorite dilves 168 Andesite and diorite dilves 168 Diabase dikes 170 Contact metamorphism 170 Hornblende schists and associated rocks 170 Metamorphosed limestone 176 Quartz diorite gneiss 178 Folding 179 Faulting 179 Alluvial deposits 179 Historical geology summary 181 Economic geology 184 Magnetite deposits 184 Road metal 194 Limestone 195 Brick clay 195 Kaolin , 195 Thermal springs 195 Bibliography 197 INTEODUCTIOlsr Location of the District The district described in this report covers an area of approximately 400 square miles in the extreme southeastern portion of Porto Eico. It has been arbitrarily bounded on the north by the parallel of 18° 15' north latitude and on the west by the meridian of 65° 5'' west longitude. The Caribbean Sea lies to the south and east. The district has been named after the town of Humacao, the capital of the Department of Humacao^ the southern half of which is included within the area de- scribed . Important towns along the coast, starting at the southwestern end, are Arroyo, Patillas, Maunabo, Yabucoa, Humacao, and Naguabo. Interior towns in the northern portion of the area are Caguas, San Lorenzo, Juncos, and Las Piedras. The interior portion of the district is a rugged mountainous area in which only small villages are located. Caguas, in the northwestern corner, is connected with San Juan, the capital of Porto Rico, by an electric and steam railroad. In 1916 constru^ction work -was under way to similarly connect Humacao with Caguas. The only other railroad lines in the district have been built along the playas and interior valleys to furnish transportation facilities from the sugar- cane fields to the centrals or sugar mills. These are of only light con- FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 119 structioii and are not connected with one another. Good roads connect the coastal towns and those of the northern portion of the district, hut the interior is only accessible by native trails which have to be traversed on horseback or afoot. Field Work and Acknowledgments The geological survey of the Humacao district was made during the summer of 1916 under the auspices of the Kew York xVcademy of Sciences and the Insular Government of Porto Eico. Field work on the area was commenced on Jul}^ 8 and completed on Septeml)er 3. A map of the Island of Porto Kico, compiled by the Bureau of Public \¥orks of the Porto Eican Department of the Interior under the direc- tion of John A. Wilson, Commissioner of the Interior, and dated May 13, 1912, upon which the locations of the principal streams, roads, rail- roads, towns, and villages are shown, was available as a base. The charts of the United States Coast and Geodetic Survey, which accurately show the configuration of the coast^ were also used. In the southwestern corner of the district, the Arroyo-Patillas sheet of the Porto Rico Irrigation Service, drawn to a scale of , covers a small area in great detail. 12,000' ^ The various towns along the coast and in the northern portion of the district were used as headquarters as the work progressed. Pace and compass traverses were made along all of the roads and also along the greater portion of the coast, as well as along many of the trails which follow the principal valleys into the interior. Two north and south traverses were run across the eastern extension of the Sierra de Cayey in the western part of the district. On account of the limited time avail- able and the fact that much of the work was done during the rainy season, heavy tropical showers being encountered nearly every day, no attempt was made to sketch the topography. Seven hundred ninety-six rock specimens were collected during the course of the investigation, from 394 of which thin sections were later cut and examined under the microscope. All the data gathered have been assembled on a map drawn 1 to a scale of . 62,500 The office and petrographic work in connection with the preparation of this report has been carried on in the geological and mineraloglcal laboratories of the Carnegie Institute of Technology at Pittsburgh, Penn- sylvania. Its completion has been unduly delayed, first, by the entrance of the United States into the World War which attracted the writer's 120 SCIENTIFIC SURVEY OF PORTO RICO attention to other imperative calls, and later, by an investigation started at an earlier date for the Pennsylvania Bureau of Topographic and Geological Surve}^, for whose completion funds became available. The present investigation was undertaken at the suggestion of Pro- fessor Charles P. Berkey, of Columbia University, who furnished the writer with much valuable information derived from his own experience in making a reconnaissance survey of the Avhole island. Dr. E. T. Hodge, who had made a geological survey of the Coama-Guayama district, which adjoins the Humacao area on the west, during the previous summer, also gave helpful advice. The writer is also indebted to Colonel George E. Shanton, Chief of the Insular Police. The hearty co-operation of the insular police of the various towns of the district, obtained through his letter of introduction, did much toward facilitating the work. Eduardo Martinez, of Comerio Falls, acted as assistant and interpreter during the entire field season. His faithful and intelligent service was an important factor in enabling the field work to be brought to a successful completion within the limited time available. Previous Work Berkey's (1915) report on the general geology of Porto Eico, based upon a reconnaissance survey made by him and C. N. Fenner in 1914, was found an invaluable aid in carrying on detailed work in the Humacao area, and frequent reference to it was made in the field. Other articles dealing with the geology of Porto Eico, published prior to this, make only casual reference to the geology of the southeastern portion of the island and give very little information of a specific nature. A list of such articles is included in the bibliography attached to Berkey's paper. Since 1916, detailed reports on the geology of the San Juan (Semmes, 1919), Coamo-Guayama (Hodge, 1920), Ponce (Mitchell, 1922), and Lares (Hubbard, 1923) Districts and the Physiography of Porto Eico (Lobeck, 1922) have been published by the Kew York Academy of Sciences. The information contained in these has been very helpful in interpreting some of the data obtained on the geology of the Humacao district. PHYSIOGEAPHY The Humacao district, taken as a whole, has a mountainous aspect. It is a region of rugged relief. A considerable portion of the west cen- tral part is occupied by the eastern end of the Sierra de Cayey, some of wdiose peaks reach an elevation of 3000 feet. To the north lie the FHTTJ\h\ (U-:iHJ)<n- (iF THE IILWLICAO IHHTMICT ni Lii(|uillo moiiiitairis whidi rise to still greater elevations. Tlieir south- em Bloper- an* iiu-liHlcd in part within the buumlaries of the district. The Tertiarv (-oastal plain, which is proiiiinetitly developed along the northern an<l southern coasts of the iskiHl. is iibscut in tiie llomaeao (iistrict. till! coast line l»ein,i:' charaeterizerl inrteacl bv bold headlands sef)a rated by broad alluvial rrllcd valleys. In llio northwestern part oC the (lisiTiet tin: Caguas valley, -a broad interior lowland, forms a. rather hi r i k i ug ttvpcigraph ic feature. I'i'LAXn iVl.'KA The uplauil area, whieli eompri.ses tiie maJDi' portion oC the Huoiaeao district, is nriderluin largelv bv a comiilex mass of folded volcanic tiUTs, Fi«. ].- Mride between FntUhu nnd Dvl Mnimw drahmge mj«temis breccias, an<l lava flows intruded lyv large bodi(»s of quartz dioritc and granite. It has reached a matun; stap:e of erosion and lienee exliihits an extremeh- rugged aspect. The Inmud tropical climate Inis l>rought about the developmeut of a tine textured topograpliy. Weathering nearly cvery- wduu-e has proceeded to consid'erable depths so tliat outcrops of tfie bed^ rock fomiatituis are scarce even in the extremely uiouotaiuoug portiims or tlH^» disti'ittt. except where the streams have eui througli the soil mantle, rroruineut talus slopes, so cluiracferistie of mouutainoiis regions wliere nujre rig(u'ous diuiates i)revail, are praetieally absent. Figure 1 shows the appearaiu-e of the ridge in the Sierra de Cayey wdd,cli ttmns tfie divide between tfie PatiUas and Del Flspiru^ draiu,ag<'S. Only in a few places along it do any outcrops of tfie underlying fornudi(»ns appear. 122 ,s('!EXTIFJ€ 8('in'MY OF PORTO HH'O (liemical dei-omposition Mather tliiui inccJiaiiicHl disinro^ixmiion, has lioeii tlio prevailing process of weathering. Rofk< likf> quiirtz diorite frtM|ueiitly sbow etelied surfaces, where the soil has been removed, .^iinihir to those oi"«li]iari;ly associated with the outcrops of more ^ohilde rocks such as limestone. Outcrops of quartz diorite affected in this luanni'jr appear in tig'unis 2, 3, and 4. As may be seen in fiofiire 2. the tiixuriaiit tropical Yegctation^ probably plays au imporliiut role in, llic devclupjuent (d' thcs<' wcatlicred surfaces widcli are so unusual for quartz cJioritc. \"cry tenaeiou,s (daytjy soils arc formi'd. Hteep slopes, ranginii' from ;>(> to do degrees with cuchillo or knife^ed,ge divides, are cbaraetcristic of —Oiiterop of qtnirtz dh tli(» mature topography de\-eloped, aiid the striking feature is tluit the soil clings to tliem ajul frequently tliey are under cultivation. Figure 5. taken near tlie headwatx:»rs of the Turaho lliver wdiere the l)edrock is largely an aiidesitie tuff, is typical <jf this kind of topography. Native huts with little ])atcbes of cultivated ground artvund them can be seen at numerous places ahuig tlie steep hillsides. Figure (i shows a trail in the quartz diorite area in the region of the lugenio valley, which starter!, origi,iially on tlie surface of the soil majitlc overlying tlie quartz diorite hn,t lias j„iow been worn down n,early t« liedrock f>y tlie surface run-o,tf wldcli flows along the depression started by tlie traffic over it. The steep walls on either side are very characteristic of the manner in which the residual soils of Forto tiico stand uji, Bcrkcy (191,5, p. :ii) b(d,ieves tliat the three ,factors tluit are largely FETTKK, CEiUJMlY Of THE HUMAiWd IUf<TUI("r |23 Fluted tpiftftz rcHpoiisiljIe for tlio remarkaljle stubiJity of the i^oil inniitlo iire tlie eliiig-- iiig charaeter of miicli of ihe vegetation whieli teiid.< tu bind the soil together, the small range oJ' tein])eri;itnre variation whiefi rediiee.i ine- ehauieal (Ikiutegration or disruption tondeneies t'> a niiiiiininn, and ifie low (X)nt<>iit of hwrX or refractory materials, snoli as (jnartz, in tlie rock;^ ^^fc'lfttf&^Rfcin Fi<;. A.^^^-DetaU of (lutiHti hHuwh in figure 3 SCJEXTIFIV ISUEVEY OF IHilfTO RH'O wliuHC <l(•^;^rvK•tu>ll hrts furni^ihwl the soi'l^. Lt^hei-k (1!»22, ]>. /.OS) ihi,|iks that aiujtlier factor is ilie 9\\mi cliiratioii ix\\{\ 1i>rromi}il clinriu-tcr of tJu' riiiiifall whifh n-sults in ati execcHliiigiy hcavv and rapid nui-ojr ovi;r the surface of t'he iiiiperviuiis soil. Ill iny opiidoji, tlic ehariieter of the weatlu'rijig process iiiuler condi- tions such MS prevail in^ an extreiiiel_Y Jiunud ti'opieal climate, whieli Unii^-. :|iH.i)i n ciKii!ir:!l i!cMUnpositio!i r;il!h-i- Shan a mt tv ni-'-hiinicni diHn!c;M';nu^j: .:i ll,c *xp.is>.| vuik> iUid. u- a i-csuh. i>n>din-rv a ^crv FhTTKI'I, (IBOLOiiY Of THE IICMAVAO UJiSTRli'l' ia5 the behavior noted. Thi.> t^oil k not >iihjet*t to the action o1* frost which i.« mivh a potent factor in bringing rtbout shmiping in uokltn- climates. Its impervious imtnre vamei^ mueli of the rain whieli falk upon it to rmi off rea<Hly over lU mrinv.e. As » result, true spriugB are of compara^ lively rare oreurrenee in tlie distriet. The abnenee of quartz from many ol tlie rocksarrulonbtetlly is also an important: factor, although the author ohserve«l many stoop slopes under eullivatinn and saw soil-eovered knife- ed^ge ridges in the quartz diorite ari'H. l)iitero])S oi: the l>edrock forina^ irm Uidiic, a imrtUtii nf Jahiivun Hfunr tiojis along the divides, howev(»r, are of much more common occurKince in the region underlain by this rock. Tlic drainage system develfjped upoJi tlie upland area is of tlie typical dondritio type. In general it may l)e said that the character of the under]yin4' formations and tbcir structure bias had very little influence upon^ the directions iji which tlie streams lurw flow. Much of the area underlain by cpjartz diorite has been ^ worn to a somewhat lower level tlian those portions of the district in whieli in,du,rated volcanic tuffs and breccias and aJidesites constitute the bedrock. On tlie other liand, Panduras Kidge. sliowii in figure 1', wliic-h etuistitutcs one of the most imposing physiographic features along the soutliern coast, is underlain by (pnirtz diofite. TITK i:ri'KR FEX-EPLAIX Over mucb of the western portion of the u|)land area the c-m-u crest line, formed by the genera! aceordaiu-e of tlie snmmit levels of the in- numerable rido-es, is a particularly striking feature. Bucks, including SCfEX'llFie HVh'VEY OF POUTO RH'O jiuiiii-ulfcd ujii:tiiiJA: luilri iiiui bi:t:i-cja>, aJidu^ik-c. ijLUjrLz diuriu;;-, aihi irKtii;<oiiit:fs are beveled alike at this level. Apparently this region was at one time redneed to a,, peneplain whieb. has since Ixseii eltn'ated and niittnrel.T disseeted. The area over whieh this ohl erosion surfax-e (uui be recognized comprises that portion of tlie upland of the lluimieao clistriet f'xte:iidinrt- I'rom its 'western border to a line east of the Del Kspino Eiv^r, rnnning a|»|>roximate]y from San Lorenzo to Yabueoa, On the north it is bounded h}- flie (Jaguas lowhuid and on the south it extends nearly to Pic. 0. ilattirelv Mssected upper peneplain in t'Mmty of Mulns barrio FF.TTKE, ah'OlJ'HJY OF THE HVMJflAO IHHTMICT 12T llif eoast. The cvfrn crest line prodnecMl hv thf erosion rt'Jiiiirtiits of thif> old ])»;'iiepJaiii t-au he Heoii in fiji,iir(;' S irbicli was taken liM)kin<x eastward across the headwaters of the Del Espino River and in figure !>, taken in the vieinitv of Hulas barrio. In tiie eentral portion erf the area, this pejiephiiu level lias an eh:'vatiou of approixiniately 2(100 feet. , Xo distinet traees of it \xer(' reeognJzeil along the ,soiith.ern Hlo|)es oC tlie Lucpiillo niountairis within the limits of \h(^ Hnmaeao rlistrict. The peneplain (ieserihed al)Ove is iiiHloiibtedly the one wliieh Berkey (1015, ]). 50) recognlzetl in his reconnaissance study <d; Porto J»ieo in 15)14. Lobeek (1922, pp. :J12-:;I20) has referred to it as the first or higher penej)hd]i to distingiiisli it fi'om a second or lower one wliieli was develo|)ed over |)orrions of the island and which is also represented in the Hrtniapao di-trift. MOXADXOCKS Along the divide between tlie Del Espjiio ami l.a llata Bivto-s on^ the north and the streams that tk)W southward into the C'arilthean Sea, pco'- tions of the Sierra de (^'aycy stand ont as nionadiujeks ahovc tlie level of the n])per peneplain. Some ef tlie higher peaks rise nearly IdlK) feet above the peneplain level. The Luqiiillo niooiitains, whidi cxtenil irdn the northeastern jiortion (,»f the district, have a sinnhir relationship to this <)ld erosion snrlaee. xriE LOWKR i'i':xi:r;L.iLv A Itnver erosion siirfaec also njarked by concordant summit levels eeui'S between the (fuavanes and lliiniaeao drainaee areas. The rela- 138 saiKNTiFJC suHrnr of p(H{TO iuvo tionsliip of tliis lower piMieplaiii tx> the upper one can be seen in figure 10, wliieli is: a view taken about 4 uiiles northwest of Yaliueoa lookiiiij uortliwest across tlie lower erosiou s^urface toward tlie upper which foririf^ the crest of tlie esear])meut neeii in tlie haekg-roaiHi Tliis Io\Yer }>enc- plain lies alwnt lOOO feet below the upper. ('A<h;as basix An extensive lowland, jrraetieally snrrou ruled on three sidet? by h\<ih hills whose crests rise to tlie level of tlie upper peneplain, has been d^'- veloped by (\rosioji in the iiortliwestern portiioi of the I'Tuniaeao distri<'t. Its avera^^e elevation above sea-level is aliout 550 feet. On, the iiortli it extends beyond the limits of the district. On the west and sontli it is Ijounded by an uplan<l w'hose summit represents t.h,e level of the upper peneplain. On the S(uitheast a low divide sejjarates it from tlu' lower peneplain area previously described. Figure 11 is a pliotograph tak(n,i in the western portion of the lowland along the C'aguas-Aguas Buemis road. The upland area oji the scnith can be seen ii> the background. The Cagnas basin is drained by the Turabo, De Espiuo, and Ciurabo Rivers wliicli mute to form the Grande de Loiza. The latter stream leaves the basin at its northwest corner where ir has cut a valley into thn upland area. The (jrande de Loiza and its tributaries have developed l)road flood piaijis ahmg tlieir courses which lie from ?M) to :iOO feet below the level of the lo^vland. Lol)ei'k (H)3S, p. Ml 8) belit-ves tlui4. the Caguas lowland was developed at the same time as tlie lower peneplain. It stamls at a scmiewluit lower VF.TTKH, (JHOLO'fjy OF THE HUMAVAO Dh^TRICT *#. .^, |^^:^'J ^;i^i( FUi. 12. -irwr. IM f/ic nrinihi «f Qiiebnula llomla fni.tit levi'], liowever, than the surface of tlii.s pwiepkiii rt.< <'U>vol()|)e<l in the eastern |)(irt;i<m of the islaml between, the Unayanen and Tlninaeno Jiivcrs, Co.\STAL AiiEA The coast line of that pintinn of I'urto Jiu-o inehnled witliiii the limits o! the Ilmnacao ilif^lriet is one produced by dnnvnj'iig in whicli the Hubinerg-ed valleys have l)een filled with alhiviiini. Wave eronion upon ttie Jieadhinds and the filling of the drowned valleys, partieulaTly the latter, lutve to a eonsidi-ralde extent obliterated the mon- i)ronouneed ^'^-tr-1;''^-sS- ?. ' . #** ".. il l:.*-.H;%:i- v.,s-il»rft:- 1;: ::;:. SCtENTIFIV SUKV'EY OF PORTO RHjO irrognliii'itie?i m tJiat, with the p<)S^^il)h' oxec'ptioii of Eiisonada llniidrt. Ihoro are no «-oo(l harbors. liiOMONTOKfE.'^ At mirnermiR phn'e^ along the s;iiithi;'ast coast of Port(» Rico, lH.«<llaiHls made up of the ohicr tormatioiiH of the iiphind area or i«i,M!Ooii!s rocks intrij.^ive in them extend to tlie s^ea wliere they ternunate in wave-eut cliffs. Amongst tlie most imiiressive of tliese is !Pandn,ras Ridge between ^rannaho and Yalnieoa. Other •- prominent ones oc(:nr siiutheast of -f I'atillas and southwest of Man- ^ iml>o, also het-ween Yabneoa anxl i Candeh'ro |)hiyas, ('andtdero and -< irunja<'ao playas. ajul soidheast of '~ Xa,i;nabu. J In ijhu-es the headlands are I iiotclietl hy wlnit are |n-ot»abIy rem- :?, luuit- of marine terraces. Two I such terraces in tlic vicinity of '^ Qnehra<la llojuJa Roint are shown :i in figur<; 12. Tlie npper of these i is approximately 280 feet ahoA-e '" sea-Un-el, whih? the htwer one oc- enrs at an elevation of 1CK) feet. Fignre 13 shows anotlier of these terraces in^ the vidinity of Na- guaho Rhiya, which is of mueh^ more reci-Jit origin and stands only a short dJstanct; alcove sea- level. The hroatl alluvial plains slop- ing gently toward tlic sea that ceeur along the lower courses of all the major streams al(U)g the southeastern coast are locally known as playas. For the most FKTTKi:, CEOLOdY OF THE MUMACAO DIHTlllCT pnrt, tliev ri-proKoiit est4iaries fofinecl bv drowiiiiig tliiit luivo been filled with alliiviuiu brought down by tbe streaius. Inland they often g-ratle into typifjil J1<hhI plain?* dev<dopod by the widening out of tlie river valli-y:^. Fio-nro M is a panorama view of tlie I'atillas pbiya, while fe^*t "m^ 'm^:^& FffENTIFlP PURVEY OF POKT(f RfPO llotida and Yabueoa. Pointn. In thn former, the ii|>{ier |)ene[>laiii level is readily recjogiiizaljle along the crests of ilie liills which, bor- <ler the Irroad valh:\v. One of the oiitstHiiding topographic features along the southern c:oast is the abruptness with which the moun- tains rise to this h'vel In the ease of the Yalujcoa phiya, there is an al)sejice of a pron,oimced I'ave-cut I'ljll ah.Hiw- the shore line. This is cfiaraeteristie of most ol th,e phiyas. The sandy beach marking the playa margin is usually lK)r<lere<l by a zone of eocoaniit |)ahns. l»ack of wdiieh frequently ocmr small areas of salt marsh, an,d beyond which in turn lie fertile fields phmtcd in sugar cane. Xot all of the lylayas are of the cstuarine type. Along tlie south Coast some, sucli as the one upon whicli the town of Arroyo is built and the one developed in the vi- (•inity of Viento Point shown, in figure 1(i, are marginal and more in the nature of alluvial fans often possessing a i^rououu seaward slope. "\Vhik'> in most cases tlie i)laYas •jiri' not ternnnated by any pro- nount-eil cliff along their sea- ward margins, one striking ex- ception was noted. Figure 17 sliows a cliff ,^5 feet high cut by the waves into the nnconstilidated playa deposits a]<mg the coast just beyond the western limits of. the Hunnica«,> disti'i('t. The uppei' :> feet of the alluvial dei)Osit, as nuiv FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 133 be seen in figure 18^ are extremely fossiliferoiis. Abundant shells of many recent species of gastropods^ pelecypods, as well as coral remains in every way identical with those accumulating along the present beach, are present. Indian pottery fragments occur in the lower portion of this fossiliferous horizon, which are associated with the shells in such a man- ner as to lead one to the conclusion that they were deposited contempo- raneously. The great variety of genera and species and the lateral extent of the shell bed seem to indicate that the deposit does not represent an Indian shell heap, but rather that there has been along this portion of the coast an uplift of from 35 to 30 feet since the region first became inhabitated by the early Indians. Draixage The northwestern one-third of the Humacao district is drained by the Gurabo, Valenciano^ Del Espino, and Turabo Kivers, tributaries of the Grande de Loiza which enters the Atlantic Ocean along the north coast east of San Juan. Some of the headwater tributaries of the La Plata, which enters the Atlantic west of San Juan, also rise within the western limits of the district. The rest of the area is drained by streams flowing southward and eastward into the Caribbean Sea. One of the outstanding features of the drainage is the unsymmetrical position of the divide between the streams which flow northward into the Atlantic and those that flow southward and southeast\yard into the Caribbean. As an example, the Del Espino Kiver, which rises on the northern slopes of the Sierra de Cayey, and its continuation, the Grande de Loiza Eiver, have a total lengtli of 36 miles, while the Patillas Eiver which rises on the southern slopes of the same range opposite it has a length of only 8 miles. Berkey (1915, pp. 39-41) and Mitchell (1922, pp. 266-269) have traced a prominent fault of late Tertiary age along the southern margin of the island extending from the vicinity of Juana Diaz to a point 5 miles east of Point Melones on the west coast. Accord- ing to Mitchell, the displacement in the vicinity of Juana Diaz is at least 900 feet and probably considerably more. The presence of this fault and the unsymmetrical arrangement of the drainage, as well as the comparatively abrupt manner in which the island is terminated at both the east and west ends, has led Berkey to conclude that Porto Eico is probably bounded roughly on three sides by faults (the east, west and south) and is a block tilted gently to the north. Tbis would exphxin the drainage phenomena noted. Lobek (1922, pp. 304-305), on the other hand, believes that the greater length of the streams flowing nortliward is due to the fact that the precipitation is much greater on that side of X34 SCIENTIFIC SURVEY OF PORTO RICO the island. He attributes the phenomena along the south coast^ ascribed by Berke}^ and Mitchell to indicate faulting, to be the result of the sag- ging and collapse of cavern roofs in the Tertiary limestones. If the unsymmetrical drainage is due entirely to differences in rainfall, how- ever, it is difficult to explain why the lofty Luquillo mountain group, with very short streams flowing northward to the sea, should be located in the extreme northeastern portion of the island where the rainfall is heaviest. The character of the rocks that underlie this area does not appear to be essentially different from those of the rest of the island. Stream terraces occur along most of the m-ajor valleys of the Humacao district. Particularly good ones may be seen along the Gurabo, Valen- ciano, Del Espino, and Turabo Elvers. In most instances these can be explained by the fact that the rivers have changed from aggrading to degrading streams within comparatively recent times and they do not necessaril}^ indicate that an uplift has taken place. GENERAL GEOLOGY The interpretation of the bedrock geology of a large portion of the Humacao district is difficult on account of the thick mantle of residual soil which covers most of it. In the lowlands, outcrops are extremely scarce; while in the mountainous areas, where they are more plentiful, they are frequently so badly decomposed as to make the determination of the exact nature of the original rock almost impossible. Pormational boundary lines can, therefore, in many instances be traced only ap- proximately. The best exposures are found along stream beds as shown in figure 19, along the cliffs which the waves have cut back into the headlands, along road cuts, and occasionally at or near the crests of the ridges. Often residual boulders in the thick mantle of soil furnish the only clues available in regard to the character of the underlying rock over large areas. The formations have been folded and invaded by numerous in- trusives, but strike and dip measurements can rarely be obtained on account of the massive character of most of the rocks involved and the scarcity of outcrops. Formations Present The oldest formations exposed in the Humacao district belong to what Berkey has called the Older Series. Berkey (1915, p. 10) considers the whole series to be of Cretaceous age, using the term to include both the Upper Cretaceous and Lower or Comanchean.^ Later workers in other ^ Op. cit., p. 58. FETTKI'I, UEOJAHJY OF TH!-J JIUMAAJAO DIKl'MH'T 135 parLs «tf Portxt Hico {-oDfur with him pretty well in tliis correlatioji.^ Se-iiiHies eonc'liuTos from foratniiiifcTal reiriains foiiruJ in the upper por- tion of the older series in the San Juan distriet, northwest iif tlie Hnma- i'-i 2 Bemmes, I». It. <}»>oI<>gy of the San .luan district. Scientific Siirvfy of Port© Mm and the Virgin Islands, Xew York Aciulemj of Science. Vol. I. Part 1. pp. T2-?S. Hodge. E. T. Geology of the Coatno-Guayamu distiict. Idem., I'ol. I, Part i, pp. Mltcliell, ih .T. Geology of the Ponce dlslrlct. Idem., Yol. I, I'art 3, pp. SSi-iSf.- Hiibbsircl. Bt-la. The geology of the l4ires district. Idem., Vol. II, Psirt 1, pp. S<-tS. 136 SCIENTIFIC SURVEY OF PORTO RICO cao area, that some beds of Eocene age may also be present. In the Coamo-Gnayama district which adjoins the Humacao on the west, Hodge has come to a similar conckision, basing his belief on a specimen of Venericardia alticosta and large numbers of foraminifera, radiolarians, and diatoms which he fonnd in its uppermost beds. He thinks that the lower portion is of Comanchean age. Mitchell and Hubbard, on the other hand, working in the western part of the island, found no evidence of Eocene sediments in the older series. All fossils discovered by them point toward an Upper Cretaceous age for the whole series. In the Humacao district, unfortunately the writer w^as not able to find any fossils that would throw further light upon this problem. The older series of southeastern Porto Kico are made up predominat- ingly of volcanic rocks. They consist chiefly of tuffs and andesites, the former predominating in the western half of the district and the latter in the eastern. The tuffs are largely andesitic in composition, but latite varieties are also present. In texture, some of them are sufficiently coarse to be classed as volcanic breccias. Others contain sufficient amounts of cal- careous material to be classified as calcareous tuffs and these sometimes grade into tuffaceous limestone. The andesites show a rather wide range in texture and mineralogical composition and include some dacites and latites. More siliceous felsites are also present over limited areas. Augite and hornblende andesites are of most frequent occurrence and a porphyritic texture is usually de- veloped in them. Both extrusive and intrusive types are undoubtedly represented but it is difficult to differentiate these in the field. In many instances, a parallel alignment of the plagioclase laths of the ground- mass, indicating a flowage structure, is revealed in them under the microscope. Sandstones, true shales, and limestones make up only a very minor portion of the older series in the Humacao district. On account of the scarcity of outcrops, the rapid alternation from one type of rock to another, both vertically and horizontally, and the com- plexity of structure, it was not found possible to subdivide the older series in the Huitiacao district into a number of distinct formations, such as has been attempted in some of the other districts. Much more detailed work, than was possible in the liriiited time at the writer's dis- posal, will have to be done to make such a subdivision Justifiable. The older series in southeastern Porto Eico has been cut by a great number of intrusive bodies of igneous rock. One of these, a quartz diorite batholith, underlies the major portion of the Humacao district. FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 137 Smaller bodies in the form of stocks^, bosses^, intrusive sheets, and dikes, ranging in texture from granitoid to felsitic and in mineral composition from gabbro and augite andesite to granite and rhyolite, are also present. In many instances the areas covered by their outcrops are too small and too numerous to show on the geologic map. All of these intrusives are older than the oldest of the Tertiary beds that overlie the older series unconformably along the north and south coasts of the island as they are nowhere found to intersect them. The Tertiary limestones and shales, that are so extensively developed along the western portion of the north and south coasts of the island and which according to Hubbard (1923, p. 66) are of middle and upper Oligocene age, are absent from the Humacao district. The recent allu- vial deposits of the coastal playas and the flood plains and terraces of the interior valleys are the only representatives of Berkey's (1915, p. 10) "Younger Series" within the limits of the Humacao district. OLDER SERIES Tuffs and volcanic breccias—The formations of the older series in the Humacao district consist primarily of volcanic ashes, tuffs, and breccias of andesitic composition, tuffs being the most common. In the western half of the district the major portion is made up of materials of this character, while in the eastern part andesites predominate. As a rule, the fragmental igneous rocks are thoroughly indurated so that in many instances they resist weathering even better than some of the associated massive types. Over much of the area igneous rocks, also largely of andesitic compo- sition, are so intimately associated with the tuffs that separate mapping is impossible. On the geologic map, therefore, those areas in which the former predominate are indicated as being underlain by andesite; while those portions where the andesites constitute only minor portions of the formation are shown as tuff. It was not found possible to map the vol- canic ashes, tuffs, and breccias separately on account of their intimate association with one another. Gradations from one type to the other, both vertically and horizontally, are of too frequent occurrence. In the southwestern portion of the Humacao district in the area drained by the Llaunel Eiver, beds of consolidated ash alternate with tuff. Occasionally volcanic breccias are also present. A thin section cut from a greenish gray specimen of the latter type occurring near the head of the west fork proved to consist largely of fragments of andesite. These have a felsitic texture and consist of an interlocking mass of plagioclase laths with occasional phenocrysts of augite. Some have a trachytic type of groundmass developed in them. Chlorite and epidote 1:I8 SCIKNTlF/t' SL'h'VEY OF PORTft Uli'i) are the t;lut>f secondary minerals present;. The iiuilrix i-uii>isr.< largely of seeoriclary (|imrtz mid eiilorite. Microseopie exjmiiiiatioii of n speeimeii (jl)taiiied near the head of the east fork showed it to be a bitite tiiff. Oraiiis of orthnehise, pkgioelase, and aogite eaii l)e ideutilied in it. AJjuiulaut epidote has developed from an«-ite and feld>i)5ir, and hirgv iliuuitities of nceonchiry quartz as well a,s a littU* serieite are i)reHent in the matrix. The trdl's of tiie Patillas Hiver drainage north of th,(' reser- voir are also thoroughly indurated and much altered, (irains of plagio^ idase and ortli<x-Iase, however, can still he recognized umier tlu^ mienv seope. Epidot«, cldoritc, and serieite are ahundantly devekjped, as well as large amounts of seeondary quartz. .\ dark gro(!]iisli gray volcanic Ijreecia., eoiitaining fragments ii|) to 11.4 centimeters in diameter from the northern slopes of the Sierra de C'ayey.. was also found to l)e of tlie aiuiesitio type. Tlie fragments are felsitic; in tc'xture and consist largely of plagioclase latlis in more or less parallel alignment. Small amounts of needle-like amphihole are als<j prcssent. The ande8it(! is an. aiuygdaloidal varietv, the amygdules consisting of seeondary (|uartz and occasionally epidote;. Besides the fragments of aiulesite, a few fel<lsf)ar crystals occur in the imitrix wddch is composeel largely of secondary qnartz anrl epidote. Thoroughly indurated fijie-grained volcanic ashes, alteriuiting witli coarser tuffs, form very prominent outcrops along the upper portion of the Turabo valley, as shown in figure 20. Some t)f tliese are almost i<lentical in. appearance with the Swedhsh halleflinta. lender tlie mi<iro- FF/tTKE. (iHOUidV OF THE HVMACAO IH^riilVT j:|{j scope, they «re sewi tu consist of minote grai]ii4 of quartz, ortlioelase, and plagiocla^e togx'thcr with, a little pale grcwi aiirphihole, probahly «1ov(4- opeil through inetamorphisoi. The eoarst^r tuffs are made up largely of anclcsite fragments with feyitic textui'e and tra<:'liytie Htriicture, Occa- sional plagioclase pheiiocrysts af^pear in them. Soiih' fragments that are oompos(!d of a lliic mosaic of quartz grains are also present. 'Jlu? fragments I'ange up to 2 millimeters in diamcd'.er. The matrix consists largely of setiondary (piartz and epidotc. Figure '2'i is a photomicro- graph, of the ash and figure 32 of the tuff. Andesite tuifs are well exposed in cuts and rpuirries at a number of places alon,g tlie nulitary road running southwest i)et\vccn Caguas an,d Cayey. At K. -10.9 such a tulf crops out akmg Quehradillas creek. It coid;aiiis: fragments up to ;^?t-l> millimeters in diameter. Under the microscope they are seen, t« consist of andesites, varying in t(»xtnrc and ,number of phenocrysts as well as in t,l,ie degree of alteration which tliey srjKXTfFie BVh'VEY or porto rico Iiave uiMlfrgoBp, Host ut tliem contain plagioelase pheiioerysts wlricli in iHoiiie instance:^ have been largMily replac-t'd 'by .^ecoiidarv <jnai'tz. Ocea- fji(oial liornbU'iiflc phciioeryats are also present in tliein. Xuuierons needles of ]mk green ampin bole lja\'e de^'elope^l in ail of tliein. Sec- ondary quartz has been abniidauily dej^iji^ited lictween the fragments. A short distanee boyinnl I\. 44 a qiiarr}' i\)r road rocrk lias been opened iji a coar.<e tuff euu taiuing fragments of aiidesite up to 5 niillimeterg in diameter, n;'-^'' '^'' v.-hi'd! ''hnw --onio rnn!v!iinj\ M S'-r'V'crVHf. exannnation Fin, 22. Pfi If riimb'f i-fllle.!/ reveals a felsitic t.i>xtnre and traehytie strnctnre in them with occasional ptienocrvsts of plagioelase. i\fueli chlorite ami a little epidotc ami sericite liave developed as alteration prodncts. Fieyoiwl Iv. 40 \'ery uiiieli similar tuffs are exposed. The fragmen,ts of an<lesite range in diameter np to 7 niillinieters. In. textnre and structure they vary somewhat. Some contain nnmerons plagioelase phenocrTsts imbedded in a ground- mass of feldspar laths. Considerable epidotc and caleite have developed as seetmdarv minerals in them. Otln.'rs have a felsitic texture and a FHTTKE, fJEOLOay OF THE UVMACAO DIBTRIVT 141 tjpical traeliytie striictirre. All are amygclaloidal. elilorite usiuiHt filling the original vesicular openings, llueli e|ii(lotc and caleite occur lietwecn the fragments. Good exposnres of tutf ami volcanic breccias may ako be j^eeii along tlie C'aguas-Juncos ruad, jtartieiilarly in^ tlie vicinity of (Tiirabo, which lies jnst to the north of the lluniacao ilistriet. A tliiri section cut from a speeinn^u olifaijied southwest of K. ti cou>isis vl minuTon.s grains of plagioelase, aiigite, and in'owu^ horuhleiule imbedded in a line matrix. ph <>{ hitili; fiiff Situtliirc miith f,f T()m:.o tltc ifetwait .V<( ]<]pidote has partially replacetl the fehlspar grains. Between Iv. 13 ami K. W, a small cpiarry has heeu opened in a tulfaeixnis limestone inter- bedded witli tJici aude.sitic tulfs. Altout 30 j'eet of the i'onner arc exposed. Soutliwest of the magnetite deposit, stmth of Torres, a'ttdii bed of latite tuf! occurs associated with daciies aud aiidesites, a ])hotoonero- grapli of which is shown in figure 23, It is composed of raggnni and 142 SCIENTIFIC SURVEY OF PORTO RICO rounded grains of plagioclase^ orthoclase, and a little augite imbedded in a fine matrix in which some pale green amphibole and chlorite caji be distinguished. A little magnetite is also present. In the eastern portion of the district the greatest area of predomi- natingly tuffaceous rocks lies to the north of the Humacao playa. Some of these consist of consolidated ash beds. Numerous grains of plagio- clase are usually present, some representing broken fragments while others are complete crystals. Small amounts of orthoclase and quartz also occur in them. In many instances the feldspar has been more or less completely altered to epidote, chlorite, calcite and sericite. The matrix, as a rule, is extremely fine and consists of sericite, kaolinite, and calcite—the latter often being very abundant. Coarser fragmental volcanic rocks are interbedded with the ashes. The fragments in almost every instance consist of andesitic material. Thoroughly indurated volcanic ashes of the hdlleflinta type, resembling those along the upper portion of the Turabo valley, also occur along the Santiago valley, north of Naguabo. Plagioclase grains and minute frag- ments of well-rounded andesite can be recognized in them under the microscope. Epidote and needle-like crystals of amphibole can be dis- tinguished in the fine matrix. Where the tuffs come in contact w4th the major intrusive of south- eastern Porto Rico, the San Lorenzo quartz diorite batholith, they are usually metamorphosed into hornblende schist. Similar changes have also taken place in them in the vicinity of the magnetite deposits of the Juncos-Torres region. These are described under contact metamorphism. Andesites.—Andesites rank next to the tuffs in order of abundance in the older series. They show a rather wide range in texture and miner- alogical composition and include some dacites and latites. Augite and hornblende varieties are of most frequent occurrence, but some that are largely fekl spathic in composition are also present. A porphyritic tex- ture is commonly developed in them. Both extrusive and intrusive types are undoubtedly represented but it is difficult to differentiate these in the field. In many instances a parallel alignment of the plagioclase laths of the groundmass, indicating a flowage structure, is revealed in them under the microscope. Andesites are almost everywhere associated with the tuffs, but on the geologic map an attempt has been made to show only the larger areas. Andesites crop out along the headwaters of the west fork of the Patillas Eiver along the southern slopes of the Sierra de Cayey. These range in texture from felsitic to porphyritic, the former type predomi- nating. When phenocrysts are present they consist of plagioclase. Feld- FKTTKH, ai':(flJH!Y OF THE JIL'.UACAO DWTHICT J 43 spar laths iii'<- the chi<'f constituents of the inatrix and m«Miietite is the moi^t important accessory mineral. Clilorite, sericiie, and imditc arc the pri,n('i|)al secondary minerals along witli occasional nests or hunclies of cpidote. Here and there tJie anxlesites are cut by vcinlets ol" secondary (juartz. Some pyritc has been jji trod, need. An andesite (sf the honi- hlcndic va.riety occurs on the ojtposite slopes of the Sierra de C'aycy, with an area of tuff intcrvcnin*;-. Plagioclase and a light green horTd>lende oecnr as phcnocrysls in it. The matrix consists chiefly <d* plagioclase laihs with minor amoujits of magnetite and exhibits a typitnl trachytie structure. East of tlie bridge at K. 4L ahrng th,e road southwest of Caguas, a hornblende andesite porphyry crops along the bed ul a small stream. A photoniicrograpli of tliis rock appears in figure 24. TJie [ilienocTysts consist cduefly of plagioclase, now almost ctniiplctcly altered to scrieite and chlorite. Occasional er^^-^tals of bluisli green, horidilende, in part altered to chlorite, are also prescjit. The ground mass is made up of 144 SCIENTIFIC SURVEY OF PORTO RICO plagioclase laths and hornblende in more or less parallel alignment. Magnetite and a little quartz are the chief accessory constituents. About 850 feet southwest of K. 41 a trachyte occurs on the south side of a small stock of quartz monzonite that intervenes. It contains occasional pheno- crysts of orthoclase. Plagioclase and quartz are only sparingly present. The groundmass consists mostly of orthoclase feldspar, together with a little plagioclase and occasional flakes of biotite. Numerous spherulites of orthoclase occur in it. Between K. 41 and K. 42, a short distance up Quebradillas Creek, a quarry for road rock has been opened in a horn- blende latite. Phenocrysts of orthoclase and plagioclase are prominent, the latter in part altered to epidote. Crystals of pale green hornblende, almost completely altered to chlorite, are also present. The matrix is made up of innumerable small feldspar laths. Epidote, chlorite, and a little calcite occur as secondary minerals. A short distance further up the creek a fine-grained rock with only occasional phenocrysts of ortho- clase and plagioclase and with some quartz besides feldspar and occa- sional biotite in the matrix crops out. On up Quebradillas Creek out- crops of andesites, dacites, and latites may be seen interstratified with tuffs and volcanic breccias. Andesites and andesite porphyries occur along the road between Caguas and San Lorenzo. A thin section of a specimen obtained a short distance west of K. 5 contains numerous plagioclase phenocrysts which show maximum extinction angles of 21 degrees at right angles to the albite lamellae, and are, therefore, i^robably andesine. The groundmass consists principally of feldspar laths arranged in trachytic fashion. A little chlorite and occasional nests of epidote occur in it. iVndesites predominate in the older series in the northeastern portion of the Humacao district. Many different types are present representing feldspathic, hornblende, and augite varieties. Some have felsitic tex- tures while others are porphyritic. In many instances they show under the microscope a parallel alignment of the plagioclase laths of the groundmass, indicating a flowage structure. Ad augite andesite comprises the footwall of the magnetite deposit east of Juncos at its west end. It possesses a felsitic texture with a ten- dency for the many feldspar laths of the groundmass to be in parallel alignment. A few phenocrysts of plagioclase of an andesine or acid labradorite variety and a little augite are present. The groundmass con- sists chiefly of plagioclase. Titanite is an accessory mineral. Pale green amphibole aggregates occur along small fractures as well as in places once occupied by augite phenocrysts. The plagioclase phenocrysts are clouded by alteration products. Beyond the andesite tuff, adjoining the FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 145 magnetite deposit on the northwest, there is an occurrence of hornblende andesite porphyry. Augite phenocrysts with reaction rims of hornblende are prominent. The groundmass is made up predominantly of a brown- ish green hornblende with only minor amounts of plagioclase. It is coarse in texture. A little magnetite and apatite are present. North of the calcareous andesite tuff that overlies the second magnetite deposit southeast of Juncos an augite andesite with felsitic texture crops out. Under the microscope a few large phenocrysts of augite can still be recognized. Others have been altered into aggregates of epidote or brownish green biotite and secondary quartz. The groundmass is com- posed of plagioclase laths in more or less parallel alignment, biotite, and augite. The limestone, with which the third magnetite deposit is asso- ciated, is also overlain by an andesite. Southwest of this deposit, be- tween it and the quartz diorite batholith, a hornblende andesite porphyry occurs. Plagioclase and pale green hornblende are present in it as promi- nent phenocrysts. The groundmass consists of plagioclase, hornblende, and magnetite. The lath-like crystals of the former show a slight ten- dency toward parallel alignment. A feldspathic andesite comprises the hanging wall of the fourth mag- netite deposit southeast of Juncos. In thin section, it is seen to possess a felsitic texture and distinct flowage structure brought out by the parallel alignment of the plagioclase laths. A few phenocrysts of plagio- clase and a little light green hornblende derived possibly from augite are present. Occasional aggregates of epidote also occur. The matrix consists predominantly of plagioclase laths. Skeleton crystals of iron oxide, possibly magnetite, are prominent. A little hornblende is also present. South of the magnetite deposit at the head of the Anton Euiz valley prominent outcrops of an augite andesite porphyry occur over a considerable area. The phenocrysts in it consist of pale green augite and plagioclase. The matrix is composed of plagioclase laths, showing some tendency toward parallel alignment and magnetite. A little pale green hornblende and epidote have developed in it as alteration products from the augite. Andesites and dacites occupy tlie 1)elt between the above magnetite deposit and the one south of Torres. A hornblende andesite crops out on the south side of the Gurabo valley in the vicinity of Torres. This contains a few phenocrysts of greenish brown hornblende imbedded in a groundmass of plagioclase, hornblende, and magnetite. A faint flowage structure is noticeable. Small amounts of epidote and calcite have formed as alteration products from horn- blende and a little sericite from the feldspar. An augite andesite and a latite occur between a bed of limestone and 146 ^"^CIENTrFIC PURVEY OF PORTO RICO the outcrop of magnetite south of Torres. Whether these are extrusive or iiitrnsive sheets could not be determined from their field relations. Tlie andesite wliich immediately overlies the limestone, in thin section, shows a parallel alignment of its plagioclase laths indicating a flowage structure. Tlie latite contains phenocrvsts of plagioclase and orthoclase in a groundmass consisting chiefly of plagioclase laths and a little horn- blende. Small amounts of epidote occur as alteration products in the groundmass and also along fractures. (Alts along the road up Pena Pobre valley expose feldspathic andesites at several places. One of these has plagioclase latlis, small amounts of hornblende, and a little magnetite imbedded in a partly glassy matrix. The rock^ in thin section, shows a pronounced flowage structure, due to the parallel aligiiment of the feldspar crystals. The rock contains nu- merous small fragments of a pre-existing andesite imbedded in it. The latter contain feldspaper phenocrysts in a somewhat finer textured groundmass than the main rock. Very similar andesite may be seen along the Blanco valle}^ above its junction with the Pena Pobre. Here also in one instance, small fragments of pre-existing andesite were ob- served in a feldspathic augite andesite. Hornblende and feldspathic andesites are the pi'edominating rocks along the Santiago valley. At El Morillo Point a feldspathic andesite forms the low hill rising above the play a level. It contains occasional phenocrysts of andesine feldspar and hornblende in a groundmass of plagioclase laths and horn- blende. The former are in more or less parallel alignment. Similar andesites occur along the Humacao-Xaguabo road to the north. Along the shore southeast of Naguabo Playa, andesite porphyries form the rather prominent clifl's that have been cut back by the waves. An augite variety contains prominent phenocrysts of augite and labradorite in a ground nuiss of feldspar. Uralite has developed along the borders and cleavage cracks of the augite and epidote occurs along numerous minute fractures that intersect the rock in all directions. It shows only a faint flowage structure. A feldspathic hornblende variety contains nu- merous phenocrysts of plagioclase and a few of bluish green hornblende in a groundmass made up chiefly of minute feldspar laths. Both the plagioclase phenocrysts and laths are in parallel alignment. Some epi- dote has developed from the feldspar and epidote and secondary amphi- bole occur along minute fractures in the rock. Andesites and andesite porphyries crop out at a number of places along the road leadiiig northeast from N"aguabo Playa. At K. (V2.2 a quarry for road rock has been opened in andesite. It lias a felsitic texture and a flowage structure. The groundmass contains not only occasional FETTKE, GEOLOGY OF THE HUMACAG DISTRICT 14; phenocrysts of plagioelase but also small fragments of pre-existing andesite. Felsite.—At several widely separated localities in the Humacao dis- trict small areas of dense usually light-colored felsitic rocks, more siliceous than the andesites described, occur in the older series. Their exact rela- tionship to the other members of the series could not be determined. They may re^Dresent intrusive bodies or possibly they are lava flows interstratified with the tuffs and andesites. The more prominent of these occurrences are show^n under the heading of felsites on the geological map. They crop out on both sides of the Patillas valley south of Patillas and along the coast west of Naguabo Playa^ as well as at several places along the road leading northeastward from the latter point. Along the west bank of the Patillas River where the road from Patillas to Arroyo crosses it, a quarry for road material has been opened in a rock of this character. It possesses an extremely fine felsitic texture and light color. Fractures intersect it in all directions and it is cut by several dikes of rhyolite porphyry showing prominent phenocrysts of quartz. Under tlie microscope^ occasional small phenocrysts of orthoclase are noticeable in a fine groundmass composed chiefly of orthoclase and quartz. A little plagioelase is present as a minor accessory. I'he feldspar is cloudy from alteration. This rock apparently is a rhyolite. South of this locality along the west side of the Patillas valley a number of out- crops of very similar rock may be seen. Microscopic examination shows them to range in composition from rhyolite, through trachyte and latite to dacite. On the east side of the road leading southeastward from Patillas, about half w^ay to the coast, the contact between a dacite and rhyolite, in many respects similar to those noted above, can be seen in a small quarry. The dacite which overlies the rhyolite has a felsitic texture with a few small phenocrysts of plagioelase imbedded in a groundmass composed essen- tially of plagioelase and quartz. Minor amounts of magnetite occur as an accessory constituent. A little sericite and chlorite are present in the groundmass as secondary minerals. The rhyolite is considerably more brecciated. It also has a fine felsitic texture but contains no phenocrysts. Under the microscope, it is seen to consist of an allotriomorphic inter- growth of quartz and orthoclose. The orthoclase in part has been con- verted into sericite by alteration. AVest of Xaguabo Playa along the coast a fine-grained dense dacite forms the cliffs. Under the microscope it is seen to consist of plagioelase and quartz. Chlorite, calcite, and epidote have been developed in it by alteration. Along the road to the northeast of i^aguabo Playa between ]4,,S HI'IENTIFIV tiVIlVEY OF /'OKI'O lUf'O K. OT ajitl K. 1)8, a liglit (jolorcd, dense, badly fraetured rrn-k, very simi- lar to that w«'f,t til' rrttillas, crops out. A few seattHred pheiiocryBts oit'irr in a grouiidmass composed csseulially of orHioela^c and quartz. Titairite, fjlagioclase, and lioriiblende are accessory constituents. A pliol:o- mierograpli of tlnH rock is sliowii in figure 25. Between K. (iO and K. (51 a very similar rock has been exposed in a small quarry for road in;ilcrial It contaijis a few small pbeiiocrysts of orthoelase in a <4'i"«>"iidjimss con- nisting of an allotrioniorphie intergrowth of orthoelase and quartz. Orth, Flagioelast!. biotite, nuignetite, and apatite are pre>eiit m luiuor ac- cessories. Linie.HtoHe.i.-—True liiiiestones art; of comparatively raie ui-i-uj-reiice in the older scries of the Ihiinacao district. Those that were encountered during tbe course of the ])resi'nt investigation, had recryslallizcd to snob an^ extent that all traces of organic remains were oblitc-ratcd. Even in, these limestones, in sojne instances, small amounts of tull'aecoUH uui- FETTKE, (JFjOLOGY OF THE HUMACAO DISTRICT 149 terial can be detected. In manv cases the tuffaceoiis material predomi- nates so that the rock becomes a calcareous tiifr, which may in turn grade into an ordinary andesitic tuff. In still other instances, andesitic tuffs have been so completely altered to carbonate that they may readily be mistaken for true limestone. Collores limestone.—The most conspicuous outcrops of limestone ob- served in the Humacao district occur along two belts ; one of which be- gins 11/^ miles east of Juncos and extends S. 53° E. for a distance of about 6 miles. It has been called the Collores limestone on account of its typical development in the barrio of Collores de Piedras. The dip is toward the northeast and ranges from 32 to 41 degrees. The second Ijelt was traced for only a short distance. It lies about 7/10 of a mile nortli- east of the southeast end of the first and apparently dips toward the soutliwest, although the character of its outcrop is sucli as to make it impossible to determine this positively. This makes it appear that a syncline is present and that the two belts may be formed by outcrops of the same bed. xVlong the southwest belt, the best development of the limestone ap- pears in the barrio of Collores de Piedras. The limestone has a strike of X. 50° W. and dips 32° toward the northea(St. Prominent ledges of gray and white crystalline limestone, in some instances 40 feet thick, crop out. The total thickness is estimated to be 250 feet. A tlrin section of the crystalline gray limestone, a pliotomicrograpli of whicli is sliown in figure 2G, reveals an interlocking nu)saic of calcite crystals of medium texture with only occasional minute grains of iron oxide anu)ngst tluMu and a little chlorite. Qualitative analyses indicate a verv ])ure limestone. There is very little residue after treatment with cold dilute hydrocliloric acid and only slight amounts of iron and alumina and a trace of mag- nesium carbonate are present. In the vicinity of the magiu^tite deposit, whicli occurs in its upper portion, the limestone has been converted into an aggregate of calcite, garnet, pyroxene, chlorite, quartz, magnetite, and specular hematite. These clianges in the limestone are referred to again under contact metamorphism. At the northwest end of the belt, just southeast of the Juncos mag- netite deposit, the limestone has been completely metamorphosed into an aggregate of garnet, pyroxene, amphibole, epidote, clilorite, and talc. Only minor amonnts of calcite are still present in it. x\t the southeast end, 1500 feet northwest of the most southeasterly of the magnetite de- posits shown on the geological map, prominent ledges of white crystalline limestone can also be seen. Some garnet rock is associated with them. Along the northeast belt, outcrops of limestone were ol)served at only 150 .SCIEXTIF/C ^I'RVEV OF PORTO h'ICO one laertlity iiboiit 9/Hl of a mile south oC Torres, it is a dark gray, fine grained variety which litis not been nietrtinorphosecl to the extent that the linie^^tojie of the first belt has. Jn thin^ seetion, it is seen to consist prineipaJly of ealeite, hot occasional grains of (itiartz, angiie. and l^lagioclfisc are also present, Tbe lalter nndonbtedJy represent tntTaceoirs Mnitcrial lieposited .sijnnJtaiieoiisly with the ejileareons coostitiients, A few small grains of pyrite are also present. Figure "il' shows a photf)- niicrogra|)li of tliis limestone. Southeast of tliis limestone ahmg the strike, a pixnninent ledge of metamorphosed roek was noticed at one place, wtiieh, under tlie jnicroseope, was foiunl to consist ahnost entirely of an aggregate of garnet grains. A Jittic pale green amphihole and deep brown Iriotite occur betweciu tlie former. This rock apparently rep- resents metamorphosed portions of the limestone. TntTacerms limeston,e.s am] calcareous tiilts.— Sontlieast of I\. 12, along the Jnneos-Caguas roail northwest of the town of dnncos, a small h'HTTKK, ilFJUJtCY OF THE Hl'MAFAtt DlHTUIVT 1.31 qiiiiriT ha,s been opened in a tufrawous limestone iiiterl)e<lcle<l with eal- (•rti'eons tullB. The furnuition.s rtrike X. 42''' E. ami dip 73"' toward the sontlicttst. About :iO feet of limestone mv. expose*! in the (|iiaiTy. The upper beds consist ot a light g-reo rush -gray fine-grained frHgnneutal roek wtiieh, under the mieroseope, is seen to be made np chiefly of eaknte, serieite. chlorite, and kaolinite. i)eea<iorial remnants of [dagioehise can 'ho detected in it. This roek may be classified as a tuffaeeoirs limestone or perliaps Ijetter. a ealeareous ash. Below it oeeurs a sojnewhat coarser Qtz. greenJsh-gray hreceiated tufl*. Caleite has rephieed ahnost the entire matrix of the tuff, but original structures can still l»e detected in it. Chlorite now oeciipies the space wdiere fel(ls])ar graijis were originally present. Only oceasiomil remnaid:s of plagioelase crystals can be seen. Sinrihir calcareous tnlfs were observt^d in the tn,ft' areas souAli of the Anton Rniy. vallcsv near its low'er end and north of Arroyo and Patillas. Shales.—Sljales are of practically negligible isuportanee in the older series of the Humacao district. All of thosti examined bv the writer 152 SCIENTIFIC SURVEY OF PORTO RICO contained so much material that is clearly of volcanic origin and were so intimately associated with tuft's that no attempt was made to map them separately. They are included with the latter on the geological map. Sandstones.—Sandstones are of only minor importance. An occur- rence was observed at the east end of the Patillas dam. The rock has a mottled dark brown color and^ under the microscope, is seen to be made up chiefly of quartz grains reaching a maximum diameter of .32 milli- meter and an occasional fragment of some pre-existing rock. The bond between the grains consists of chlorite, hematite, and sericite. The out- crop is not extensive and probably represents a small lens interstratified with the volcanic tuffs and breccias which crop out at other places near by. At the dam, the contact between it and a monzonite porphyry is exposed. Along the upper portion of the west fork of the Patillas River, promi- nent ledges of medium-grained quartzite crop out along the creek bed. Microscopic examination " reveals quartz to be the chief constituent but orthoclase and plagioclase are also present. A little chlorite, calcite, and sericite occur between the grains of the former minerals. INTRUSIVE8 IN THE OLDER SERIES Xumerous intrusives, ranging in composition from augite andesite and gabbro on the one hand and to rhyolite and granite on the other, occur in association with the older series of the Humacao district. Some of these are present as intrusive sheets or sills varying from a few inches to several hundred feet in thickness, while others are in the form of stocks and bosses, ranging from a small fraction of a mile to several miles in largest horizontal dimension. One of the latter type has a suffi- ciently large area! extent to be called a batholith. Innumerable dikes of widely different mineralogical composition cut across, not only the forma- tions of the older series itself, but many of the igneous rocks intrusive in it as well. As has already been pointed out, all of these manifesta- tions of igneous activity antedate the middle Oligocene. INTRUSIVE SHEETS AND SILLS The numerous inirusive sheets and sills^ that are interstratified with the tuffs, lavas and sedimentary rocks of the older series, are, for the most part, composed of augite, hornblende, and feldspathic andesites and andesite porphyries. Somewhat more siliceous varieties, such as dacite and latite, are of only minor importance. They range from a few inches to several hundred feet and perhaps more in thickness. In the field, on account of the limited occurrence and extent of outcrops, it is FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 153 very difficult and often impossible to distinguish them from extrusive types of similar mineralogical composition with which they may be as- sociated and they have, therefore, not been mapped separately. It is believed that these igneous masses were intruded into the older series contemporaneously with their deposition or at least very shortly there- after while the beds were still in a nearly horizontal position and before the orogenic movement set in that produced the present folded structure. STOCKS, BOSSES, AND BATIIOLITHS The stocks, bosses, and batholiths of granitoid igneous rocks that have invaded the older series of the Humacao district are clearly of later origin than ihe intrusive sheets, and sills just described, as they intersect the latter as well as the tuffs, lavas, and sediments with which they are associated. They, therefore, belong to a somewhat later period of igneous activity, probably contemporaneous with the period of folding and uplift or else closely following it. The more important types represented in this group are gabbro, quartz diorite, granite, and quartz monzonite—the gabbro being the oldest and the quartz monzonite the youngest. Juncos (jabhro.—A small intrusive mass of gabbro apparently in the form of a stock, occurring 1^ miles northeast of Juncos, is the oldest of the second group of intrusives. Its outcrop covers an area of probably not more than 4000 feet in maximum and 2000 feet in minimum dimen- sions. The rock has a medium granitoid texture and .under the micro- scope is found to consist chiefly of plagioclase of the labradorite variety and augite. A little greenish-brown hornblende occasionally occurs as reaction rims around the augite. Titanite is the chief accessory con- stituent. The Juncos locality is the only place where a rock of this type was observed. San Lorenzo quartz diorite,—The major intrusive of the Humacao district consists of a medium to moderately coarse granitoid rock possess- ing the mineralogical composition of a quartz diorite. The largest out- crop underlies a rudely rectangular block approximately 15 miles long and 12 miles wide, which extends from Juncos on the north to Maunabo on the south and from the mouth of the Humacao Eiver on the east to the headquarters of the Del Espino on the west. Exposed over a con- tinuous area of over 180 square miles, it is evidently of sufficient size to be called a batholith (Daly, 1914) rather than a stock and will, there- fore, be designated hereafter as the San Lorenzo batholith, since typical exposures occur in the vicinity of that town, particularly along the high- way leading eastward toward Las Piedras and along the Del Espino valley to the south. The bed of the latter stream is strewn with great 151 Hi'lEXTU-'IC HFRVEY OF I'OUro Rid lRnil<lf»rs of tlie {|iiHrt/. diorite on some of which apjH'jir criide desigiij^. as shown ill ligiirt' 28. <rarve<l upon tliem by the aborigiiuil ijilinbitmits ol' l*ort.o Bico. Huge roiunhHl iiiouoliths, such as are hIiuwu in figure 21>, form eonspieuous outcrops in the quartz diorite area. The highway lead- ing across i'nnduTas liidge from Mauriabo to Yahucoii nho aifonk exeei- e)it «)pportiinities to examine the quartz diorite at eloi^e range. A similar (juarty. diorite crops out abng the (luralio valley uortht'ast of Junc'os and als<i along the upper portions of the lihmeo and l^'ena Puhre valleys in the nortbern portion of the Ilunuu.'ao district, ''J''lH^-*e areas are a part of a large stock which underlies a considerable pindJcrn of \hr f.sNiuillo Rnnp- ;ii!-l uIihIi e^ ni,.!uuhh.,iiy r::hn>.-:.^d is! dm uini; '^^:AJ, (l<']}th with the main Saii Lorenzo batholith. Another similar but some- what smaller stock occurs in the vicinity of (Jaguas. ^!ucli smaller satel- litie stocks of from '^4 to % of a mile in diameter were ol)served along tlie cast fork of the Lbrunel Kiver, along the military road about '4 jniies soulliwest of Caguas, and about % mile east of tlu' main hathtilitb, east of Las I'iedras. I'luter tlie ml(!rosco|ie, the San Lorenzo quartz diorite is seen to he composed principally of plagioclase of an andesiue or acid lal>raflorite variet}', quartz, and a dark brownish -green liomble.nde. Orthoclase, hio- tite. occasionally augite, magnetite, tilanite, and apatite are the chief accessory constituents. The rock, as a rule, possesses a motleratcly coarse granitoid texture. This phase is by far the predtunimiting type, but sometimes (piart'K drops out almost entirely so that the rock he<M)mes a Fh'TTKh\ (IFJiiJXiY (H-' THH HtW/AALiO nJSTh'If'T 15 diorite; iji a few otlier ijistrtiirt'S orthoelase takes^ tlie place of plagioekse procJocdrig a limiibleiMlo g-rnnite: and in rare eanes aiigite replaces liorii- bleiide m that the roek k a >r«l»l)n). Oeeasioiml'ly, in the case of tlie sinaUer satcllitiji stock!>, the texture m porphyritic. Figure -10 shows a ]jhotoiiiicrograpli of th.e (piartz dioritc from a spcei- men obtained from an mitf/rop .south of the Ban LorenKo-Las Fiedras roiu] al)out B^X. miles !>iout;lieast of tlie former town. Playioehi.se, quai'tx, and liornhleiid(! art; the iIhhh' chief constitnent.s present. The phigioehisu shows maAinium extijietioji angles of 25 degrees in seetiou!^ at right angles to the allute lamelhn and frequeiitlv exhibits pronoiun^ed zonal growtli. The liornblenth; is marl<erlly pleuchroic, ranging from yellowish browji throngli greenish brown to deep brownish green. Ortluxdase. l)n:)lite, jnagiielite, titanite, anil apatite are th.e chief accessory const it^ iients. The order of erystalli'/.ation of thi; various ndnerais appears to have been apatite, nnigiietite. titanite, iun-nblejnk^ plagiorlase, ortJiocbisc, and (puirtz. A little sericite has develoi)ed from the phigio<'lase aiul small amounts of chlorite and ei)idote fr(uii tln> biotitc. An augite rliorite phase of the 8an Lm-enzo ipiartz diorite occurs ah>ng the nortliern slopes of J^^mduras Ridgt! abcuit 2 ndles t^ist of 'Yabucoa. Figure 31 shows a jdiotoinicrograjdi of this rock. Hornhhmde, plagio- clase, and augite are the principal constituents. The hornblende is in- tensely pleoehroic, ranging from yellowish brown through greenisli brown to brownish gri'cn. Tbe plagiochnse sliows maximum extinction angh's of 35 degrees in sections at. right angles to tlie albite bunelbn. The augite freepiently is enveloped l>y reaction rims of hornblemje. Basal 150 ,srlEXTIFI€ Sl'RVEY OF PORTO KIPO f^wtiouH of fiiiirite .sniTimiHletl by liorJihJt?ii(lt' may be seen iii tbiii heetioii, eadi witli lifi typical prisniiitic cleavage (levelope<l. liitcrgrowtbs of aogite and bjsrnblende arc also present. Plagioelii.se appears to have crystallized before hornbleude, inclusions of the former occurring in the latter. The latter al.so i'recpieutly tills the space between, plagh)cbii^(! crystals, IMagnctiic, orthoelase, titanite, anil apatite are the principal aceessorv const itiients. South of Ilnmacao, 11/^ miles, a diorite phase develcjped. At tltis f»oint tiie chief constituents arc plagioclasc, :fre<piently sliowiiig zonal gro'^v'th with niaximum extinction, angles o,f 2() degrei>s in sections at right angles to the albite lamella?, and an, intensely ])leoebroic brownish green liornithi-mle. Quartz is only a Jiiinor ejuistitueuf.. Ttic other ac- cessory minerals are orthodase, biotite, magnetite, apatite, and titanite. A little sericite has developed from tlie fchlspar and chlorite and epidote from the hi<4.ite. One ,mile south of Humacao a gahbro phase occurs. Plagioelase ami aogite are its main c,-o,rnpoi]ents. The plagioclasc shows FHTTKE. (lEOJjmY OF THE II VMACAO IHBTRIOT 157 jiiHX 1111 mil I'xriiietioji angles itf -iSV, iU»«;ret'H in seeliuD.s sit right angle? t(> the albite himcliii'. The augiti; is a pale gTeenish-gniv iHiii-phwchroic variotj. Jt sometimes contains iiiternal growths of dark brownish green hoi-jibleinlc! in paraUel orientation with, one another, and is neeasionall}- sHrre»iinde<l \}x narrow rims of the .^anic mineral. Magnetite and ortho- elase a,re tlie otlier accessory minerals besides hornljlenxh^ The orcjcr of <;rv8tHliizHtion appears to have been magnetite, angite, and plagioeJase, Firj. 31. ^--^^Pl*. Irs caM <jf Yalimm aJiliough <ieeasio]ial ijudiisions (»f small plagioelasc crystals in tlie augitc indicate that some of the former antedates the aiigite. Alojig the headwaters of tlie Ingenio Iliver. in the Yicintty of Teja Piedras barrio, the (piartz diorite passes into a hornblende granite. W ieroclijn:*, orthoidase, cpiartz, lioriihh»:nde. and hiotitc are its eliief eon- stitnei)t,s. (:rrap]iic intergrowtiis of rpiartz and ortlniclase occur in it. Plagioelasc is of only minor im}KirtaiH'e. The other ancessory minerals are magnetite, titanite, and afjatite. A little chlorite and cpidote have develo|)ed from, the biotite. 158 t^CLENT/FlC .sr'KVNY OF I'OliTd RK.'O AH of the above varieties are merely tliffereit phaneH of tlie Sail Lorenzo fmirrfi rliorit-*. Nnwlirro is t1u'r«» eviflftu'c that- they were in- truded as distinct iiiiiis. (hi ihr <A\n'v hnsid. i nsn-il i-n^sl .lusiiges from one tjpe to the other may frequently be seen. They represtnit segrega- tion or clilTerentiation phenomena that took place iji the main mass alter .it had been intruded in its present position. Tilt Luqnillo stock con,sists of a quartz diorite almost identical in Mineral composition and texture with tl:ie main .San Jjorenzo mass. It FETTKE, OEOLOdY OF THE HUMACAO DISTRICT 159 is particularly well exposed at the Falls of the Blanco River, shown in figure 32. Plagioclase^ quartz, hornblende, and biotite are its principal constituents, while orthoclase, magnetite, apatite and titanite occur as accessory minerals. The plagioclase shows maximum extinction angles of 301/2 degrees in sections at right angles to the albite lamellae. Zonal banding is common in it. The hornblende exhibits pronounced pleo- chroism, ranging from yellowish-brown through greenish-brown to brownish-green. Biotite is not always an essential constituent. Along the north side of the Pena Pobre valley it is practically absent in places. The Caguas stock is composed of a grano-diorite phase of the San Lorenzo quartz diorite. Plagioclase, quartz, hornblende, and orthoclase are its essential constituents. The plagioclase possesses maximum ex- tinction angles of 24 degrees in sections at right angles to the albite lamellae and frequently exhibits zonal growth. Biotite, largely altered to chlorite, magnetite, apatite and titanite, are the principal accessory minerals. The San Lorenzo batholith has produced pronoimced contact meta- morphic changes in the tuffs of the older series in many places where it has come in contact with them. In some instances, all traces of original structures have been obliterated with their conversion into hornblende schists. These changes are further discussed under contact metamor- phism. The fact that the San Lorenzo quartz diorite is a younger in- trusive than the Juncos gabbro is clearly shown northeast of Jimcos where small tongues of quartz diorite, which are part of the Luquillo stock, project into the gabbro along its contact with the latter. Yahiicoa granite.—Around the borders of the San Lorenzo batholith and frequently in contact with it, there are a nimiber of small stocks of coarse granitoid, frequently almost pegmatitic granite, which will here- after be designated as the Yabucoa granite on account of the excellent exposures that may be seen in the vicinity of Yabucoa Point. The largest outcrop, which includes the above exposures and which covers an area roughly 4 miles long and li/j miles wide, occurs along the coast between the Yabucoa and Candelero playas. Other localities along the south coast where the granite is exposed are Point Yeguas, Cape Mala Pascua, and Patillas Landing. In the interior the only occurrence observed lies about 1 mile northeast of Las Piedras in the form of a strip 14 of a mile wide and 2 miles long adjacent to the San Lorenzo batholith. The Yabucoa granite possesses a very coarse granitoid, in some in- stances almost pegmatitic texture. Under the microscope, the two chief constituents are found to be orthoclase and quartz, the former sometimes being light pink in color. Only minor amounts of plagioclase, rarely ifio 8viEjriF/r ,^'/7i»r/-:r of porto mico showiji*!" maximimi e.\tiiii'tioii aiigics gri'ater than J,1 dewTees in sections at ri<i-lit iingie^ to the albite laiiii'lk'. and a littlf hiotiU' mvum m it. Hornblende is; pnietienlly alxsent. Othi>r aci'e<sory miiierals are apatito, inapiotite. Hiid zireon^. A thhi ssfctioii of th«* gTaniti:' from tlie vieioity of Yal)ueua Playa cuii- sists chietly of orrho(;hiHe and qmirfz in ali<>trio)nor|)hic inforgrowth.s. Its aeeessory eniistitnents are [dagioelaHe. dark In'ouii hiotilf. ma^jiictite, and a]:nitite. Figure 3»i represents a photomicrograph of tln^ granit<=. In the vicinity of FraiU» Point tlie granite still has a very mueJj siinilar oompo.sition. A little zireon^ appears iji sections of t!ie rock from this locality. At I'atiUas Landing a mierographie intergrowth of orthoda^? a.nd quartz is a [)roininent feature in the roc-lv' which was nut noted else- wJiere. The coHrHe, aliriost pegmatitic Yabueoa granite 1 mile Jiortti- east oi" Las Pieilras consists largely of light pink ortlioelase and (piartz. }*lagioelase and hiotite are its cliief a('cessory <M)iistiriU'nts. The strin:- tiire on the whole is allotriimiorphie. ]!ktierr)perthitic intergrowths of FETTKE, OEOLOay OF THE HUMACAG DISTRICT IGl orthoclase and plagioclase of minor importance are noticeal)le in some of the sections of this rock. In the vicinity of Yal)ucoa Plava the contact hetween the Yabucoa granite and the San Lorenzo batholith is exposed. The fact tliat the Yabucoa oTanite represents a later intrusion is plainly evident, as it ramifies the quartz diorite in all directions in the form of dikes. The latter has also, in part, been converted into gneiss in the vicinity of the contact. These metamorphic effects are described ill more detail under contact metamorphism. Patillas quartz monzonife.—A third group of stocks intersecting the ohler series of the Humacao district is made up of granitoid rocks of tlie mineralogical composition of quartz monzonite. Such stocks are par- ticularly well developed along the Patillas valley and the name Patillas quartz monzonite will, therefore, l^e applied to this tvpe of intrusive hereafter. Similar stocks also occur along tlie Llaurel valley nortli of Arroyo, along the military road southwest of Caguas, along tlie Tural)o valley, along the road leading from Cagilas to Juncos, along the contact between the San Lorenzo batliolith and the older series 3 miles east of Juncos, and in the vicinity of N^aguabo. The areas covered by their out- crops are usually small, in most instances being less than ^,/> mile in diameter. The lar(>"est of these intrusive bodies, that in tlie vicinity of the Patillas reservoir, however, is nearly 4 miles long and ha^ a maximum width of about 2 miles. Tn most instances plagioclase, orthnclase, hornblende, and quartz are the main constituents of the Patillas quartz monzonit'\ while maunetite, apatite, and titanite are the principal accessory minerals. Wliile the texture is usually granitoid, it is considerably finer tlian that develo])ed in the San T orenzo quartz diorite and Yabucoa izranite. Occasionally alon^T the borders of the larger stocks or throughout in tlr^ case of some of the smaller ones it is porphyritic. The Patillas quartz monzonite at the southeastern end of the stock, which occurs along the Patillas valley in the vicinity of tlu^ Patillas reservoir, is a fine granitoid dark greenish-grav rock in wliich feldspar, hornblende, and a little quartz are visible. In thin section the chief constituents are found to l)e orthoclase, pla^^ioclase, green pleochroic hornblende, and quartz. ^Maii'iietite is an abundant accessory mineral. Tlie feldspar is cloudy from tlie development of kaolinite and sericite. The hornblende has altered in part to epidote and chlorit^. Along the southern margins of the stock the texture is porphyritic. Phenocrysts ju^edominate over groundmass. They consist chiefly of plairioclase with maximum extinction angles of IS degrees in sections at rigbt angles to 1(32 SCIENTIFIC SURVEY OF PORTO RICO the albite laniell^e, indicating andesine, a ferromagnesian mineral now complete!}^ altered to epidote and chlorite, and a little orthoclase. The gronndmass consists of an allotriomorphic intergrowth of plagioclase, orthoclase, and quartz. Micrographic intergrowths of quartz and feld- spar are common. Considerable sericite has developed from the feldspar. At the northeastern end of the Patillas reservoir the rock possesses a fine granitoid texture. Plagioclase, ortlioclase, augite frequently sur- rounded by rims of green hornblende^ and quartz in part in micrographic intergrowth with orthoclase^, are the principal constituents. Magnetite in abundance and apatite are the accessory minerals. Considerable sericite and kaolinite as well as some chlorite have developed from the feldspar by alteration. The augite and hornblende have also in part been altered to chlorite and epidote. A small stock of the Patillas quartz monzonite on the west side of the Llaunel valley along the playa border consists of a fine granitoid gray rock in which feldspar, hornblende, and quartz are likewise visible. Under the microscope, plagioclase, orthoclase, quartz, and pale green hornblende are seen to be the important constituents. Almost colorless cores of augite surrounded by pale green hornblende are also present. The quartz occurs in part as micrographic intergrowths with the ortho- clase. Magnetite in abundance and apatite occur as accessory minerals. Some kaolinite and a little sericite have developed from the feldspar. The hornblende has partially altered to chlorite and epidote. The two small intrusive masses of Patillas quartz monzonite occurring 3 miles east of Juncos along the contact between the San Lorenzo batho- lith and the older series possess a porphyritic texture with phenocrysts in excess. The predominating minerals are plagioclase, orthoclase, and a dark greenish-brown hornblende. The groundmass is made up largely of feldspar and quartz occurring in part as micrographic intergrowths. Magnetite, apatite, and titanite are the principal accessory constituents. Abundant sericite has developed from feldspar. Small amounts of chlorite and epidote also occur in the rock as alteration products. The other areas of Patillas quartz monzonite, shown on the geologic map, are very similar in texture and mineralogical composition to those described. The relationship of the Patillas quartz monzonite to the Yabucoa granite is shown along the coast, 2 miles southeast of Patillas where their contact is exposed, as shown in figure 34. The rock at the left is the quartz monzonite and that at the right the granite. The point of the hammer rests upon the contact. The quartz monzonite 15 feet from the contact possesses a fine granitoid texture. Orthoclase, plagioclase, augite, FHTTKf], ilKOLOdY (JF THH HUMAVAff D/h'TIf/Cf and quartz are its inain C'onstitiunits. 8i>iTie of tlie ([oartz occurs in iiiicrograjiliic iiitergrowth with tlie orthoclase. A little green hornblende is prt'sent in the ftirni of reaction rims around the aogite. Magnetite, a|)atite, and tiititito an; acjcesaory minerals. Sericite has dev(!loped from ' felds^iinr, <,:Iilorite from augite, and a little eoidote occurs along minute fractnres in tln^ r<M'k. Approaching tlie con i.act tlie t.cxture becomes fiiicr and liner until at tlie eontiict it is felsitic and the rock beeonies a quarti!; hitite. The intrusion of the quartz monwnite app<!ars to haw liad. no eii'eet upon thc» I'ahueoa granite, whirh is clearly the older rot-k. DIKES Dikes of iguenus rocks, cutting not only the older «) the major intrusives in it, are proiniiicjit geologic ries itself hut ;»atures in the Hnnuicau district. They range in thJekTiess Ironi a fraction of an inch op tn 55 iVet and perliaps more in some instances. On account of their great alnoidance and the small thickness of -many (d' them., jh) attempt has been made to sliow them upon t.h(! geologic map. Vw pu.rp()ses of description, thev can he divided into two types, namelv, tliose of a siliceous characti'r and those of intermediate to basic composition. The major period of dike formation appears to have commencerl soon after tlie intru.'iioji of tlie Han Lorenzo bath<jlith and continued until shortly after the appearan<'e of the Patillas quartz monzonite. S'ilix'fMUH dikes.—The siliceous dikes consist of granite, granite peg- matite, aplitc, granite and rhyolite porphyry, and rhyolite. These are 164 HVIhlNTlFia 8Ull¥KY OF PORTO RIVO tiiidoubtedly eloselv related to the Yabucoa granite an<l tlie tivtit two are probably jdentieal witb it. 'rbey were found cutting oidy tbe Ban Lo- renzo quartz diorite. Tbe aplitos and granite an^d rbyolite |)orpliyries, on the otlier band, oeeur not only in the San Lorenzo «piartz diorite but are also intrusive in the Yabueoa granite itself, and, in one instance, an* apJite dike was foiuid eotting a quartz mouzoiiite porphyry that Ih prob- ably tbe e([uivalent of the i^atilbis. Granite ami <ji anile pegnmtUe dikes.— In the vieiiiity ot' Quebrarbi Honda Point, tbe San Lorenzo (pjartz diorite is cut by a dike of granite 1 foot 10 inches wide that splits as shown in figure -lo. The texture is granitoid and the injportant mineral constituents are ortbtH-lase. iriiiiro- cline, (piartz, and plagioelasc of the albite variety, showing ni:ixinimn extinction angles of 13% degress in sections at right ang hunella:'. Small amenjiits of biotite and a very Httle n,ni and zircon^ are accessory nunerals. This rock is undtiu with the Yabiieoa granJl(\ a stock of wliieh may be seen south abjng tbe eoasi. Pegmatite (hkes cut tlie San Lorenzo batholith at a ui .along its Ixirders. Such, a dike, I foot in thickness, nia the Juncos-Saii Lorenzo road about 4/5 of a mile south Orthochise in crystals up to 4 centimeters in length, and are its chief i-onstitu,ents, l)ut some quartz and biotite a It resemhh'S rather closely some of the coarser pliases i granite and is thought, therefore, to be identical with, it. he albite .patite. id entical (nid feet of places en along • J'nn(-os. agiotdase l> resent. Y abucoa FETTKE, UEOLOCY OF TEE HEIIAXliO DIBTMICT Apliie cH/i'fts'.—ApJite dikes are of mucli more freiiuent oceiirreuee thai) the pegmatite varieties aiir! are not eoiiliiiecl to the Ban Loreiizu quartz diorite areas but also oeeiir in the YaI)iieoa granite, and in one place along the military road southwest of (^'agnas in tlie viein,ity tif K. 41, a 12-foot dike of tliis type eutsi a quartz moiizonite p«)rphyry that has been ecsrreiatcd witi) the Patilla^s. Figure o(» sliows a niuriher of aplite dikes eiitting the Yahueoa granite in^ thi^ vieinitj of Yahucoa Point. The majority of these dikes do not ex{;eed 1 foot in tliick:ncs.s and nsiially tliey are eonKiderably ill inner, lliey always possess a rather fine granitoid iextnre and are composed j»rim;i pally of otlio- elasc and quartz. Phigiochise, bio- tite, muHCovite, and nuigrietite are only very minor constitncnts. frinnHi'- imd rhyoUte porphyrij ilikf'S.—The granite and rhyolite porpli\i'y dikes are tlie most; aiinii- dant and important of the silieeous group. 'J'he two differ from one another onli^ in tln^ relative al)un- dau(;e of tlie phenocrysts present in them and the texture of tlieir gronndmasses. In the former phe- nocrysts predominate, while in the latter tin:' grcmndmass Is in excess. (»ranlte and rhyt)]ite porphyry dikes are rather widely distributed throughout the Humacao district. They eut the formations of the ohh-r series as well as the San Lorenzo (pnirtz diorite ami the yahn<'(ia granite, but were nowhere found iuterseetiog tlie Patillas quartz umnzonite. It is probable, thtsrefore, that they fotlowed closely after the intrusion of the former and before tln^ appesiranee of the latter. The granite and rhyolite porphyries ean tie studied to good yilvajilago in the vicinity of Yabueoa Point where their intrusive relaliou to the Yahucoa granite is well shown. The dikes of this locality consist of a liglitgray coarse i>or])hyritic rock in which, abundant euhedral as well as anhedral crystals of both orthoelase and <|uartz and oeeasional flakes of biotite are visibh^;. The phenocrysts of orthoelase fre(pn:«ntlv reach lengths H5a SClEXTIFiC PURVEY' OF I'ORTO RfVO of 2% and diameters of i% eentiiiieters. Tliey eoiisi^^t of simple crystals boiiJided I'y a prip]ii and basal and (•liiHH)i]uu-ojds, Carlsbad twin!:* with the same foriiiH dt'vylopod on tbcni may also be seen. TLie eidiedral crystals of (jnartz sometimes reach lengths of 2 eentimelers and diameters of 1.2. 'Fliey eoiisist of <-ombiinitions of the hexag'onal prism with the positive and negative rliombohedrons, t)ne instanee was jmted where two crystals occurred in parallel growth in the form known as scepter 'w/ritiih fif rhiinUl€ P'trphpi'n cutlintf l^ati Lot the ririiiitij of gnrhniiln Hoiflii PninI qnartz. Wliile iiumer(His plienocrysts (tf ciiiartz arc; enhedral, the nni- jority are rounded at lea.st at tire t-orners. j^liotite is not a etnispicuous^ eoustitucnt. ()ligoela«e, magnetite, titanite. and zircon are minor acces- sories- The gronndnirtss consists of a fine niosHie of ortlioclasc ami quartz. A little chlorite and epidotc Inivc developed from the biotite and serieite from the feldspar. A ])liot(nnicrograph of the rhyolite porphyry from a dike 3(1 feet thick eutting the San Lorenzo qnartz diorite in the vicinity of (^nelirada Honda FETTKE, GEOLOGY OF THE HUMACAO DISTRICT 167 Point is shown in figure 37. It possesses a coarse pornhyritic texture. The phenocrj'Sts consist of euhedral and anhedral orthoclase and quartz. The orthoclase occurs in simple crystals and as Carlsbad twins. Oliogo- clase and minute flakes of biotite are accessory constituents. The latter indicate a fiowage structure by their parallel alignment around the hirger phenocrysts. The groundmass is cryptocrystalline. A little sericite has developed from the feldspar and chlorite and epidote from the biotite. Several dikes of rhyolite porphyry, cutting a felsite, may l)e seen in a quarry for road rock ak)ng the west bank of the Patillas Eiver where the road from Patillas to Arroyo crosses it. The phenocrysts consist prin- cipally of orthoclase and quartz but the rock is particularly exceptional in that occasional small trapezohedrons of garnet up to 1 millimeter in diameter occur in it. Sometimes the garnets appear in the ground- mass but often they are present as inclusions in the orthoclase. Occa- sional plagioclase phenocrysts may also be seen. The groundmass con- sists of a fine allotriomorphic intergrow^th of orthoclase and (juartz. Sericite in abuiulance has developed from the feldspar. Bhyolite.—About 14 i^^^l^ "^^P ^^^^ Quebradillas valley from the military road southw^est of Caguas, a dike of rhyolite about 10 feet tliick occurs in the rocks of the older series. In thin section ouly occasional small phenocrysts of orthoclase are visilrle. The groundmass. which consists largely of orthoclase and quartz, is extremely tine. A little epidote and chlorite are present in it. Dikes of intermediate and htmc com position.—The dikes of this group range in mineralogical composition from latites and monzonites on tlie one hand to augite and andesites and diabase on the other, and ])ossess textures ranging from felsitic through porphyritic to granitoid, the felsitic and porphyritic types being the more common. They are encoun- tered most frequently in the older series, but are also abundant in tlie marginal portions of the San I^orenzo batholith and are occasionally present in the Yabucoa granite and Patillas quartz monzonite. While some of these dikes may be older thau the San Lorenzo quartz diorite, it is believed that the majority followed shortly after the intrusion of that rock. Latite and nionzonite dikes.—jyikes of the mineralogical composition of latite and monzonite wTre found cutting the older series, the San Lorenzo quartz diorite, and the Yabucoa granite. A latite dike about 20 feet thick cuts the volcanic breccias of the older series along the w^est fork of the Llaunel Eiver. IMacroscopically it is a dark gray almost black felsitic rock. I'nder the microscope a few phenocrysts of plagioclase and orthoclase may be ol)served. :Magnetite 168 SCIENTIFIC SURVEY OF PORTO RICO occurs as an accessory constituent. The groundmass is composed largely of minute interlocking feldspar crystals and a little augite. Southeast of Naguabo Playa^ latite and monzonite porphyries are present amongst the numerous dikes that cut the andesitic rocks of the older series in that vicinity. A thin section from a latite porphyry dike only 11/2 inches wide contains numerous phenocrysts of plagioclase, ortho- clase, and hornblende in an extremely fine crypto-crystalline groundmass. Biotite, quartz, and magnetite are accessory constituents. Flowage is indicated by the parallel alignment of the phenocrysts, particularly the smaller ones, x^ear by, a thicker dike of monzonite porphyry, with pheno- crysts predominating over groundmass, has almost an identical min- eralogical composition. In the vicinity of Quebrada Honda Point, a quartz monzonite porphyry dike only 8 inches wide cuts the San Lorenzo quartz diorite. Plagioclase of the andesine variety, orthoclase, microcline, quartz, and biotite are its prominent constituents. Magnetite, apatite and titanite occur in it as accessory minerals. Along the coast near Patillas Landing, a dike of latite porphyry, 20 feet thick, cuts the Yabucoa granite. The prenocrysts consist of ortho- clase and plagioclase of the oligoclase variety: The groundmass shows a spherulitic structure. Apatite is present as an accessory mineral. Chlorite, sericite, and epidote occur as alteration products. Da cite and quartz diorite dilces.—Dacite and quartz diorite dikes are not very abundant. Along the coast west of Kaguabo Playa, a dacite dike, 12 feet thick, intersects a felsite belonging to* the older series. In thin section, the rock shows a felsitic texture. Plagioclase and quartz can be identified in it. Chlorite, calcite, and epidote are present as alteration products. At one place along the Guayanes valley, a dike of quartz diorite porphyry, 3 feet thick, w^as found cutting the San Lorenzo quartz diorite. Labradorite, quartz, hornblende, and biotite are its predominating min- erals. Orthoclase, a little magnetite, and apatite occur as accessory constituents. Andesite and diorite dikes.—Dikes of andesitic composition are the ones most frequently encountered in the Humacao district. Hornblende and augite varieties are about equally abundant. They are mostly con- fined to the older series and the San Lorenzo quartz diorite, but instances were also noted where augite andesites cut the Yabucoa granite and the Patillas quartz monzonite. In texture they range from felsitic to porphyritic. A hornblende andesite porphyry with very prominent large crystals FETTKE, GEOLOGY OF THE HUMAGAO DISTRICT 169 of hornblende cuts the older series near the crest of the Sierra de Cayey at the head of the west fork of the Patillas Eiver. In thin section^ phenocrysts of greenish-brown hornblende and lesser amounts of plagio- clase are prominent. Magnetite is present as a minor accessory. The groundmass consists principally of plagioclase and hornblende. Some epidote has developed as an alteration product from the hornblende. Along the coast in the vicinity of Naguabo Playa, a hornblende ande- site dike, 3 feet thick, cuts andesitic rocks belonging to the older series. Its texture is felsitic and under the microscope the structure is seen to be diabasic, but bluish-green hornblende containing inclusions of biotite occupies the space between the plagioclase laths instead of augite. In the vicinity of Quebrada Honda Point, a felsitic hornblende andesite dike, 2 feet thick, cuts the San Lorenzo quartz diorite. A greenish-brown pleochroic hornblende and plagioclase are its chief constituents. Mag- netite is abundant. An occasional grain of augite and a little titanite are also present. A flowage structure is indicated by the parallel align- ment of the amphibole crystals. A 55-foot dike of augite andesite intersects the volcanic rocks of the older series along the Llaunel Kiver a short distance below the forks. It is a dark gray, almost black porphyritic rock in which phenocrysts of plagioclase and augite are visible. In thin section, plagioclase pheno- crysts of the labradorite variety are seen to predominate but pale brown- ish-green non-pleochroic crystals of augite are also prorninent. The groundmass contains abundant microlites of plagioclase. A little mag- netite is also present. Small amounts of sericite have developed from plagioclase and chlorite and calcite from augite. Near the northeastern end of the Patillas reservoir, a felsitic dike of augite andesite, 15 feet wide, cuts the San Lorenzo quartz diorite. Its chief constituents are plagioclase and augite. A little magnetite is also present. Some calcite, chlorite, and epidote have developed by altera- tion from the plagioclase. Along the coast not far from Patillas Landing, a 15-foot dike of augite andesite cuts the Yabucoa granite. It is felsitic in texture and plagioclase and augite are its main components. Magnetite occurs as a minor constituent. Some chlorite is present as an alteration product. On the point which projects into the Patillas reservoir at the east end of the dam, a dike of augite andesite cuts the Patillas quartz mon- zonite. It is a felsitic dark gray, almost black rock. Under the micro- scope, a few phenocrysts of labradorite felspar and augite are visible. The groundmass is made up largely of feldspar laths with augite inter- 170 SCIENTIFIC SURVEY OF PORTO RICO spersed between. Abundant magnetite also occurs in it. A little chlorite associated with augite and calcite are pr^ent as alteration products. A diorite porphyry dike, 2 feet thick, cuts the San Lorenzo quartz diorite in the vicinity of Quebrada Honda Point, as shown in figure 38. It consists chiefly of labradorite feldspar and hornblende. Orthoclase, magnetite, titanite, and apatite are minor accessory constituents. A little epidote has developed by alteration from the plagioclase. D'mhase dikes.—Only two occurrences of diabase dikes were noted in the Humacao district, in both of which the diabase cuts Patillas quartz monzonite stocks in the Llaunel valley region. The diabase ranges from fine granitoid to porphyritic in texture. Labradorite feldspar, frequently zonal, and a pale green non-pleochroic augite are its chief constituents, the latter filling the space between the feldspar laths. Magnetite and highly pleochroic brown biotite are the accessory minerals. CoxTxVCT Metamorphism Contact metamorphic plienomena in the older series are confined ])rin- cipally to the immediate vicinity of the San Lorenzo batholith. Tlie metamorphism has resulted largely in the conversion of the tuffs and associated andesites of the former into schists and gneisses, cliiefly of a hornblendic variety. This has been brought about by a recrystallization of the original constituents of the former and the development of ^ foliated structure in the resulting rock. Apparently, no new material has been added from the outside. The pressure exerted ])y the invading magma upon its w^alls as it was making room for itself appears to have been responsible for the development of the foliated structure. The schistosity everywhere is parallel to the contact. The conversion of portions of the Collores limestone into an aggregate of lime iron silicates is also believed to have been the result of the intru- sion of the quartz diorite magma of the San Lorenzo batholith, but in this case emanations from the magma itself reacted with the calcium carbonate of the limestone to produce the metamorphic minerals. In the vicinity of Yabucoa Playa, the intrusion of the Yabucoa granite has converted the San Lorenzo quartz diorite into a gneiss just as the invasion of the latter converted the tuffs and andesites into scliists. HORNBLENDE SCHISTS AND ASSOCIATED ROCKS The Maunabo Eiver, along a considerable portion of its course, fol- lows rather closely the contact of the San Lorenzo quartz diorite with the tuffs of the older series and the effects of this intrusive upon the FHTTKtL (lEOLOar OP THE HVMACAO lHHTh'li'T latter cau, therefore, lie .<tiulie'l to g'ood {iclvaiitagx* at WMmy places its course. Ont;ero]>K of iiietHinoi'pliie rock appear about SU. m\\v< the town of MciiiiialM) ami eoiitimie at intervals for Jienrlv o niileH mil up HtroHin the imi Oulei'ops eiieoiinteri>(l jhk.h'ss h, dark ^niy color and line erystalline texture. The foliated ntrurture is imi very iiotieoahlo to the oakwl cje. Under the iiuVroHi'<»pe, the ruck \< nvn t<» crjiisisf of on iiiterloekino- luosair of au^u'ilt'. orthoelaso and a littic oiiartz. .Small 172 l-<CiENT/FIV 8UHVMY OF POHTO MICO grains of HuigDetho are lairly abiiiulam arid a Jittlc niiisecnjte is also ])re!*eiit. The <iiig'jtf is a pale brown ish-greeii iioii-pleochroic variety full of imiiisions of oilhoelase hiuI nuaj'tz as well as magnetite. Often con- siderable patches of individual aiigitc grains heliave as single nnits op- tieall}-. A slight indication of a foliated stnictnre may he ohsemtsd, due to the greater eonceiitration^ of aiigite grains along certain bands and their parallel orientation. A photomicrograph of this pyroxeini schist is showji in tigiirc d!>. A little further upstream plagioehise and a little Fig. 30. I'lioti Orth liornhlemlc also appear in the schist. About a mile above the first lo- eality, typical hornblende 'sctdsts make their appearance. TJiese also possess a tine to medium crystalling texture and show a slightly foliated strnetnj'e dne to rfie more or less parallel alignment oi the liornbleiide gi'aijis. In tldn seid'on, the ro(;k is foimd to consist of a mosa.ic of horn- blende, plagictelase. orthoclase, ami (piartz gj'ains. Sume magnetite is also present. "TIjc h<rrnl)]ende is a deep green, highly pleocliroie variety. Compared with tlie aiigite grains of the pyroxene schists it is rela- FETTh'E, OEOUMjy OF THE UrMAVAO iJhSTRIET ViS lhx-]y free from liielusioiis. Oecasioiwl larger gvalm of orthoclase ajitl plagioclaHt! with ragge,] hmkn aim oeeur in t\w mek. i:iiere was en- (•ouJit;cretl a finu grairioil <|uart2ite 214 .mile.s above the first loeality. Tlris tiHjsists proH-ij)uriy ot y fine jnonaie of: quartz wifli only oeeaj«iorjal grains id ortlioelase ami plagioclase. Now m\d then a ehister ut' pxromne grams ma.v also be oh.servcd in it. In, palehes elilcu'itc and soricite oecur iH'twern the iiiuirtz m-ainH. t t I t I I g mm. Fi(i. 40. PhuitMlvrMiftiph of hunhlciulr M-hi.si fmm^ Tuim Point in tlio vicinity of Tmui Poijit, sontlieast of Afaunabo, a small area of Inimblende schist an<l associated ijuartzite is exposetJ abmg ihv. coast. The liornblende schist possesses an extnynely line crystalline texture ion] a pronounced foliated, strmd^iire due to the concentration of the hor,ii- blende in hands l)etveen tlie Cfuartz grains. The rock crKUsists principally of hornblc!nde and quartz, the former lu'lng .relatively free from inclu- sions as arc tin- quartz grains. A little ortlHiclase showing Carlsbad twin],ii,ug an<l plagiocksc oxhibitiug albite twinning are also present. Pyrite and .magnetite in little grains a]«o oecnr in ttie rock. ,A photo- micrograph of this schist is sliown in figure -P). The quartzite associated :i : i SSClHXTfl'H' Hlli'VHV OF PORTO UK' with it is iui t'xtrt'iiifly lim-u-raimMl \arit»ty, a pliutoHiicrograidi of wiiich appoars in ih^wrv 41. It cuJisists of ;i fine musaic oi jiTrtimlMr qiiyrt/ in which a liftll- hornhle.idc in piiraHd aligiiiiifiit nirtv ho ohsprvcd. Soiti.- iHa.yiii'tit!' ,ii-?'ain8 arc also prosont, ^Icraincrplioso.l tiilfs also appear at the headwrttin's of tin' (;iiayjiiH'>. Hivcr and alou,^ the divide hctwccn that stream and Uh' Did I'^spin-. hhvcr. A dark .ya-ay line (awstallinc spctdnieii from iho hitter l..cnlii> I'xdiihitiim- a (h'stinetlv roliahnl striudiire wa^^ iunnd under the niiernsrup,. ii|. ed' alternate Imnds uf aimate. feld.spar and and lu.rnhhaide. The au-ite is nearly e<dnrle<.< i„ id. in see! ion. Consaha-ahle (tatehes of it- unnm hehave optieally a^ sin,u'le ynits. The h.^Tlan- cidored hand.^ eonnist ]>rincipally oi: pla.^lu(dase. urthoehi.M.. and rpjartz. Th.- hornldemle is a dark hrowninh-ea-een^ j.leeehmie variety. While its graJiis are al.Mi in more or less parallel alieannent alun-- eertain hands, patches of them do lint behavi. opti<-ally as units when viewed helween crossed nie<ds. Oeeasional hiru'er rag.a^d crystal^ <d* aiie-jre full ol mae-neiip. fi-:tti\h. i;Enijj(iY <n- the ih\\ia«\\o ihstuivt \]~^ inrlusioiis iiiul liaviiig 1ionil>h>ii(|(. <;it.vt.l<.|K-<l iilonu- their iK.rdtTs, as well nK ii! their iiilfiiors, ar«' tilsu pn'seiil. 'We^i- iiirtv represeiii jui.uitr .•rystulH of liic (rniium] tut! frum wl-jidi tlu' schist has Iteeii .leriwd. ' .Alojiu; liu' l'ii<>;iirt.--8an Lnreiizo yivm] lu.rtliwest of Sais J.uTenzo, in the vi<-inity of K. 8, rhe ineiiriijorpliit: hell is rtpproxiiiiatelv <JO0O iVet wi<le. 'Mil' metijmorpJios.-d tuff still eoiitaiiis oriuma] rrt«rjiXM! and eIou<ly «'rystals of pladuelase luxl nrth<M-his,'. Thr recryslrtllize.l' portions cunsist uf n mosaic of green pleuchrnic lioriiitlctHlc, pla.i.-i«rlase, orthudrtsc, and • piartz. Tlic latter feldspars arc dear. A speeijne]] of liornhlcnde schist e.lttained -i nnles sciutlnvest of .hnieus. ill tlie vit-initv of the San f.oren/u ipiartz diorite conta<-t. contains fairlv nimiercniH lar-X' ragged crystals of pl«gio<-ht-^i.. chuided hy the deveh.p- rnent of sorieite in ilienu imhcdded in a rccrvstallizcd matrix composed largelv (d* dark hluish-green In.rnldcndc and .^iiiaDer annnyit>: ..f plagiu- ela«%"qnarte. and possihlv orthcchise. The foliated stru.'tnre is dvie to the mniUvl alignm<mt of 'the h<n-nhlendc <.rystals ah.ng their long axes. 176 SCIENTIFIC SURVEY OF PORTO RICO A narrow belt of schists l)or(lers the San Lorenzo quartz diorite along the soiitli side of the Huniacao Playa. A photomicrograph of a specimen obtained from a small hill whicli projects above the playa level south of Huniacao, is shown in figure 42. It is a dark gray fuie-grained foliated rock consisting of bluish-greeii pleochroic hornblende, quartz, plagioclase, and a little pale green non-pleochroic augite. The latter sometimes has rims of hornblende around it, particularly in the case of the larger crys- tals. The schistose structure is due chiefly to tlie parallel alignment of the longer axes of the hornblende. Inclusions of metamorphosed tuff and andesite are of rather common occurrence in the marginal portions of the San Lorenzo quartz diorite. They vary in diameter from a few inches up to many feet in some instances. M ETAMO RPirOS J:D L 1MESTOXE The Collores limestone, whose outcrop occurs along a belt trending S. 53° E. for a distance of nearly six miles about 3/4 ^^^^^^^ northeast of the San Lorenzo batholith, has been recrystallized into a gray to white crystalline rock of medium texture, in which all fossil remains that may have occurred in it originally liave 1)een obliterated. In places, this lime- stone bas be(4i converted into an aggregate of lime iron silicates by con- tact metamor])hism. It is with these metamorphosed portions tbat the magnetite deposits of eastern Porto Rico are associated. Contact metamorphic phenomena in the Collores limestone can Ije studied to good advantage in the vicinity of the third magnetite de]iosit southeast of Juncos in the barrio of Collores de Piedras. Northwest of this magnetite deposit, Avhicli occurs very near the top of a northwest- southeast ridge, prominent ledges of the limestone crop out that range in mineral composition all the way from crystalline calcite associated with only small amounts of silicates to those in which silicates are the predominant constituents present. A thin sectio]i of the least altered rock consists mostly of calcite and a little ganiet intersected by fractures along wbich quartz, specular hematite, and possibly magnetite have been introduced, replacing the calcite. This was followed by the development of chlorite along fractures and between calcite grains, replacing the latter in part. Tlie final stage was the depositioii of later calcite in the form of minute veinlets cutting the other minerals. In the more altered rock, a photomicrograpli of which is shown in figure 43, garnet, showing pro- nounced optical anomalies, is tbe predominant mineral, having almost completely replaced the origiiuil calcite of the limestone. Magnetite and specular hematite liave 1)een introduced along fractures, accompanied bv the development of chlorite. Later, veinlets of calcite and quartz cut FKTTKE, i.;tl(HJK:y OF THE lll\UA('A() DhsTUIVT ;n'; the other Jnirlerrtl^^. 'V\w liiiirn-^toiie i.s cvcrlaiii by an aixh^^ite at xhU locality. Only tho upper purti<in dirpetly iiiulerneatli tlic aiulesit.o has Ik3C'ii olf<'ct»:Ml, as rthovo «1e>.'ribf.<-L Toward tho soriihoaHt. tlie lungiu'titp ih'ixisit »-ast ol: .fiiiieos is ^w,^ (ioedcMl by a belt ol* niftrtiuorphic roek whieli is hi'liovecl to bav<! timi derived froiri tiio (.'ollorcs l)m»>sto-|)(y fH-rhii};)s a toJTaeeous phase. Kii- rneroiis ytromiiieiit \(H\<te< crop ont; along the hillside. Large wigs oeeur ,,.-^^gwsf:;w^^^^^ m it in ].lacts. A niirr..sropi.- .'xainiiiatHHi of tie- m.-k r,-veals il,at it ermsi.st- largely of pymxtme ati.l gHrin-t, the gariH'f having develop.-d after the pn-rnxene. Cal.-ite, epidote. quartz, ami talc an- present in minor anionnfs. (Iiloril.' has developed along nniuite rra<-tnres. .\n^ other Hj)eeimpn. tak.ei from tbo wall^ <.r one of the vu-v ..onsisls birgplv of aimet an.l aniplubole'. tngetie^-r willi sonn- talo. Th.' garn.d: appar^^ enik formed first then the ainpbihole, and linally the tal,-. Hornblende and' pyr.)xene sehisfcs. nndouhledly derived from aielesitie tnlf-. overlie and underlie the motaniorpliosed iiitH^stone. tlH "IHMlFfV KVRVEV OF l-:U{T() RicO (ianit't roi-k. as.-<.cirtt»«(l with the Colloivs liiucstoiic, ivrtH iil^o ol)S<.rvwl iU om- hviilHy .near llie M.-iithoiiHt eiul of tlic Iniicstoiie belt. QiKiHz >tiotitr. ,itiriss.^^^^^^An tlie vicinity of Valiur<tii l>lay«. the Hiiii Lorenzo (jiifirtz diorite lias heoii eoiiverted iiitu a giiei-s by 1Ih> ii)fri!si(»i) of the Yahueoa u-ranite. The mieiss itself Is mi hv imiiierons totii.'-ue!> of the eoarw Yahiieoii granite and l)y aplite diken. The reek posses.^eri a mr'iliiiiu t-rystalliiie texture and n green ish-g-ray <:olor. It Hhn\v.< a din- tiiuily giH.i.<se.id .-tnietjire. in thin se<-tinu, it is seen (o r-ensisi prinei- Qtz: hloc Orth •Kft >! >, pally of ])higieelas(\, qnartz. some *)rth(iehise, and brown ish-greeij horn- hh-nde. ::\!a<iiietit<% titanite, biotite, now mostly altered to eldorite. and apatite ore tlse (diief aeeessory eonstitiieiits. Tlje hornblende has been affected must Ity the shearing produeetl by tJie preswnre exerie<l hv the invading gj'anite magma, but the qnai'tz also shows a pronomieed eata- clastie strncfore. The Mdspar^i lia\'e been affected the least. Herieite and kaolinite have developed from the feldspar, tbe plagioehise having been altered more than the ortboclase. Tlie hornblende lias been emi" FETTKE, GEOLOGY OP THE HUMACAO DISTRICT 179 verted in part into chlorite, epidote, and calcite. A photomicrograph of this rock is shown in figure 44. FOLDIISTG Tlie structure of the formations belonging to the older series is com- ])lex. The uplift which followed their deposition and which was accom- ])anied by the intrusion of the San Lorenzo batholith and numerous closely related stocks, produced a folded structure in them that is rather difhcult to unravel on account of the relative scarcity of outcrops and the massive character and weatliered condition of a large percentage of those that do exist. Eeliable strike and dip observations can be obtained at only rare intervals. For this reason it has been possible to show the trend of the folds only in a general way on the geologic map. In the western portion of the district the folds have a northw^esterly and southeasterly trend. Starting at the south, a syncline; followed by an anticline and another syncline, occur. Another anticline starts near the headwaters of the Turabo Eiver and trends slightly east of north, just east of the Turabo valley. As has already been pointed out in connection with the description of the Collores limestone, a syncline apparently exists northeast of the San Lorenzo batholith, between it and the Luquillo stock, which also trends in a northwesterly-southeasterly direction. FAULTING 1^0 direct evidence for the o(^currence of faulting on a major sc^ale was found within the limits of the Humacao district, althougli in sonn^ in- stances the abrupt change iu character of the formations within com- paratively short distances along their general strike and some of the topographic features of the district would seem to indicate that there is a possibility that displacements of considerable magnitude, due to fault- ing, may exist. In the few cases where faults were actually observed in outcrops under conditions that made a measurement of tlie displacement possible it was always found to be small. ALLUVIAL D7^:P0S]TS Unconsolidated alluvial deposits form tlie broad plains, knowii as playas, that occur along the lower courses of all the major streams of the Humacao district: For the most part, these have accumulated in estuaries formed by the drowning of the river valleys, but in some in- stances, particularly along the south coast^ they represent alluvial fans that have been built out into the sea by the streams is>^uing from the FliTTixi:. CFJHJKIY hF '111 F. fK^h'ACU) FHSTh'ICT ^•ii»H)<'lH uiiv Vn,-H> ilo|)n.<i!s hihI ilum^hx Inniimu^ Wrnm'^ alon: valh'VK. Siirh disswK'd alluvirti dt'iiosits an,' soim'tinics ciu-oi iiliiiost at iIh' head n! a strcani, a< is tlir- i-tm> with the (>x|Kisiiri' ill fi.mm. d:. ivliii-li ui-i-ur^ =i|.u!..- P,..,., |»nl>,v Cn-ok. i!' "^ ' ^ ;' . ^ ': -^"Tf^$^^^4 ^If^. ^, fH ii-^m^ -#::. _,, iKPl. -w : ' " -ji^- ''^ :^'^'' iiiil :V ;: »J ' T .:.,. ' • > '* "• 1 "'. ' . >:!^ . 3-2»'li •». .'HSi'iUiallv vuui;-^ from a uvnl, 'nni- <;l til lluiuarnt. .Ifstrirl; mIim'UII. While )M, U,y.<lU WCIV l.S-2 ^VIEXTIFIC SJ'UVKY OF PORTO U!('{} (liseoverod in tlie older series in soiitheasteni l^erto iiieo !)y the autlior, suffieieiit evidence has he(Mi uathered hy othtn- investigators in adjacent areas to lead to the conclusion that this series is hirgely confined to the u})])er Cretaceous, altlu)U<>ii tliere is a {)ossil)iIity that a |)ortion of the ]o\\"er ])art may be lower Cretact'oiis in age and sonu' Eocene sediments may he included in its up[)erinost niem!)ers. During Cretaceous times, Porto Rico was the scene of intense volcanic acti\ity, largely of the ex[)losive type. Enormous (juantities of frag- mental material were extruded, cliiefly of an andesiti(^ character, that ]»uilt u]) the great beds of volcanic tuffs ami breccias that now comprise the major portion of the ohUu; scries of tlie Humacao district. A por- tion of the eruptions, liowe\er, was of a quieter ty])e and resulted in the Java flows, also largely andesitic. that are interbeddtMl with the frag- mental rocks. I^ot all of tlie latter, liowever, readied tfie surface, as some are clearly of an intrusive cliaracter, having forced themselves as slieets and sills amongst tfie layers of tuif and breccia extruded by ])revi- ous eruptions. ]\luch of this volcanic del)ris, ]>er]ia])s the major portion of it. un(h)ubt- edly accumulated under sub-aerial conditions. Tlie oecurr<'nce of occa- sional beds of limesto]u\ such as the (N)llores, interstratihed with the volcam'c rocks, however, indicates that the district did ]H)t stand above sea-level throughout the (*retaceous |)eriod, but that oscillation with res|)ect to sea-le\el took place from time to tinu\ which resulted in at least j)artial sul)mergence of the area at iirtervals. Toward the close of the Cretaceous, a general uplift took place which was accompanied by the folding of tlie volcanic and a^soeiatsMl sedinuui- tary rocks tliat comprise the older series. Fresh accessions of magmatic material appeared which invaded the Cretaceous formations, and in the Humacao district resulted in the formation of the San Lorenzo batholith and the Luquillo and Caguas stocks. AMiile practi(^ally all of the volcanic^ rocks, both massivf^ and f rag- mental, are either amlesites or closely related types, the later intrusions sliow a nmch wider range in chemical and mineralogical composition. Ap])arently, the primary magnuis from which the various igneous rocks of the llumac^ao district ha\e betvn derived were of an amh^^itic or dioritic character. During the earlier stages of igneous activity. wIkmi cf)mpa ra- ti vely ive(' commum'cation with the sivrfacH^ pr(nailed, s'crv little differ- entiation occurred in this magma. Later on, however, wIkui new acces- sions of igneous material bceanu' largely of an intrusive chara(^ter. such differentiation did take ])lac(>. As a result, (]uartz (lio!'it(\s are followed by granites ami quartz nionzonites in the form of batholith^ and stocks. Firi'TK/j, aiu>L(>(;y of Tin: hi wiacao Dh^TRjcr iSo The (like rocks aceoiii])aiiyiiig' this [Period oi' io-neous activity show ev(vu greater variation, ranging all the way from aplites and granite pegma- tites at the one (extreme to diabase at the other. The diabase dikes that are occasionally foun<l cutting tlie (jiiartz monzonite stocks aj)])ear to be the last manifestations of igneous activity in the llumacao district. After the uplift which marked the dose of tlie Cretaceous, there is no direct evi(]ence that any (extensive ]^ortions of tiie ITmnacao district, with the (^xception of the i)Uiya areas, again ))ccome snl)merged below sea- level. Tlie later geological history of this area must, therefore, be inter- ])reted from its j)]]ysiogra])hic develojtment and from what took ])hu'e in other portions of the island. From tho fact tliat nowliere are tliere any igneous rocks found associated witli the Tertiary sediments tliat occur ah)ng th(^ north and south coasts outside tln^ Hunuu'ao district, it ai^pear's that volcaid(^ acti\ity came to nn end with the u])lift tliat marked the close of ih(^ Cretaceous. During tlu^ early Tertiary, (U'osion ^^'ore a large ])ortion of the lluma- cao distric^t down to a ])eneplain. It was at this time that the extcmsive erosion surface, rcdcrred to as the ui)per |)enej)lain in the (^irly ])a]*t of this re])ort, was developed. Tins was followed by anotlier uplift of ]Mn'- ha|)s 1000 f{\A. Erosion again set in and developed a lower ])ene|)lain over that ])ortion of the llumacao district lying b(4we(Mi the (iuaya]u\- ami llumacao drainage areas and that ])ortion occupi(Ml by the (\iguas lowland, the rest of the region reaching a matnre stagx^ of (U'osion. A slight sul)]nergt^iU'e followed the second ])artial pene})lanation, which affected the western portion of the island in ])articnhir, for along th(^ northern and southern coasts extensive marine deposits of middle and u])per Oligocene (rTul)l)ard, ID^ih p. 0(5; Loheck, lir^l\ p. :)1.S) age rests upon an erosion surface that is contem])orane(nrs with the lower ])ene- ])lain of the llumacao district. Then^ is no direct (Evidence, however, that any of the llumacao district itself ^s-as s\d)nHn'ge(l. as nowhere within its borders are there any Tertiary sedinu^nts. The submergence of a ])ortion of the northern a))d southern coastal margins during Oligocene tinu^ was followed by another niplift whidu in th(^ cas(^ of the Huma<?a() area, n^sulted in a rejuvenation of the streams and a marked increase in their eroding pow(MX Somewhat later, a drown- ing of the entire coast line occurred whidi r(\sult(Ml in tho formation of irumenms large estuaries that are now fillfMl with aUuvial (\i^\)V\^ and constitute the playa areas of the south(^ast(U'n coast. Lobeck (102-?, ])|). 346-:i48) attributes the drowning of the coast to a rise of s^a-Ievel, due to the nudting of i]w ice stored up in the great continental glacitvrs of Nortli America and Europe at the dose (vf the rieistocen<^ ej)0( h. 184 ."^CIEXTIFIC SmVEY OF PORTO NICO VnKvut cluiiiges ill tlio Hniuacao (listrict liave consisted in the deposi- tioii of alluviin]) along stream \alleys and in drowned hays and as al- luvial fans along the south eoast. Evidenee for a loeal eniergenc-c^ of ])er]ia|)s 25 to W feet ah>ng ])ortions of the eoast also exist in the form of wave cut terraees, raised ])eaehes, and eliffs ent into I'lHHMit ])laYa de|)0sits containing marine fossils. K(^()X()MJ(^ (;F.()LO(iY A]A(JX!7riTK I) K POSITS Magnetite deposits that liave attracted some attcMdion as possihie future sources of iron ore occur at a nund)er of localities in ihe ohhu" series of tlie llumacao district. The ])rincipal {)n(\^, which have ah-eady heen descrihed hy the writer in a })revious |)aper (Fettke, 1i>*^4) oeeur in association with the l)elt of ('(dlores limestone, trending approximately S. 5:]° E. wlneli l)egins It^ nndes east of J uncos and can he traced for a distanee of al)()ut (> nn'les toward the coast. Four outcrops of magnetit*^ were ot)ser\e(l along it. whicli are designated as Xos. 1, 2, i), and I, respectively, starting at tlie northwestern end ou tli(* ge()l()gi('al map accompanying tliis report. Otliers are j)rol)ah]Y present hut (^sca])e(l notice on account of t!ie thick mantle of resi<lual soil wide!) in most ])laces conceals the l)ed I'ock fornuitions. One deposit, D/lO ndle south of Torres, designated as Xo. o, lies outside the luain helt, heing al)out T/10 mile northeast of it. Other dcjxisits of a sindlar character oeeur in the older series, o or -1 nules west of d uncos, a short distance south of the duncos-Oaguas road, and in the Inlls north of Arrovo. The writer did not have an o{)|):)rtunity to examiiu' these personally, hut was in- formed \)y Professor dames F. Kemp that tliey nrv v(uy similar to those lying east of J uncos. Tlie outcrop at locality \o. 1 east of d uncos makes the most prondnent showing and this de])osit is prohahly the largest of those (wannned. hi- asnuu-h a>^ there are jio fuels availahle in I'orto KNco for the estahlish- nient of a local iron industry, the ores, if ex{)loited, will ha\'e to !)e shippcMl. The two nearest harhors at which shi{)])ing facilities are at pi'es(Mit avail- ahle are San duan and Fajardo Playa. Of these, San duan is hv far the l)ettei'. By way of the (Uiraho and Iiio Orande (h Eoiza vallev, the d uncos dej)osit is .'U miles distant from San duan. The electric and narrow gange steam railroad from San ehian to Oaguas covers tlie dis- tance to the mouth of the Ouraho lii\er. From there it is 10 ndles ui) tl)e ijuraho vallev to the magnetite deposit. Along the greater portion of the latter distance a light Jiarrow gauge track has alreadv heen eon- F/JTTKfJ, (UJOIJXjy OF THN III MACAO DISTRK'T IS5 sti'iu-ted for the transportation of siio-ar cane from the fields to the central '%) uncos." Bv way of the Jieadvvaters of the (xural)O Kiver, across the divide to the Rio Blanco valley, down it to Xagiiabo, and thence [)arallel to the coast to Fajardo Playa, the distance is al)out 27 miles. Ensenada Honda, p,,; 4s. h'clat'ioi) of wdffnctifc outcrop \o. 1 to ^urroundinn foniinf iou.^ Houth of I^Vjar(h) Ilarhor, is T ndles nearer, hut no shij)))ino- facilities are at })res(Mit availahle. Along all hut the iirst 1) miles of this distance light narrow gange tracks are already in existence h)r tlu^ liauling of cane. T\w onlv stec}) grades along this route occur after leaning the lu^ad- waters of the Guraho Biver and going down Pena ro])re (h-eek, a trif)u- tarv of the Bio Illanco. Along this tlie hanl would he down grade. TUv mniiuvtilv tli-|H,<ir at lu,-alily .Xo. 1 hv< 1% m\\v< east «.r .hitu-os uuii lUt vvv<l nf a ri.luv, s<.iilli ..f ilx' <;iiralM> vaJlev. that rise- appruxi^ rtivly :JM feel above the h.wlaiuis h> the iioiih ami >ova\\. 'Fipir.. 4H .nsistv uf ii >k.-.u.h inai), lja>«Hl ii|Hai lielil iiote^. whirh <!i.nvH tlio reliV niislii], <.r the miicrop u, the siiri-<niji<liii,a I'orjiiiil ions : while (iuiira 4«> a plinto^a-aph t.f the rid-e as it appeiiiv from tlio s.mili lonkino^ north. The oiitfrnp has a strike of \. (id ' \\\ and apparent I v <lips li'^- toward w iiurfheasi. jr eaii Ite irai-etl h.r a (lista.iiee «.l' 'HMM fee! ahaiiu- tlio vsi of the rid.uv. hut the hesi (h-veh)fimeiit of JiUl.mielih' ueeiirs at the .rthwesi taid. where hiruv huuhlers and, pnairincnt !ed-es nl' nearlv pure a-iiePile juake a v.-rv 'eoiKi.ieiious showiii.i!-. .\hieh ..f the niaguetiU' laMii !? ' i. appeariii.ii' in ihe uuter<.p ar the northwe-si end is line .yi-ained ami neMrly pure. lt]w>s('s<vi^ a port.us struetiire, thie iindonhtiMlly lo the h;-rtehiiig- out id: siJicales and perhaps snndl aintnmls id snlphides lyv wertiherin.i--. (h-hv^ hedral (awstals of uia,,a-rict:ite ai'e snnieiiine- seen liiun.!:^ (haisy ea,vilies. .Part of itif nia,2aK'tite is suiliiaently niaaaietie to piek up siriall tarks. Oeeasionallv a line' ji-rairied intcr^u'rowlh ot nu^.y-netite and speeulai' .heina.- tite oceui's. Qualitative aiuilyses slww only traces of titaninni. Jn several ].lares, a lirile nialaehite was untieed. This is presunujhly derived IroDi a sulpiudJe, sneli as eludeopyrde. and indieales thai in dei>th snurll ananijit.s of snlphides are proi.alily |n-eseui in associatitai with ihe nu-i.y- netda-. thirnet (teenrs with the uia<:iH'tite at the ivesl end oC ihe de|)osit. ruder the niierosenpe. it exldhd,s pruu.uuiiee«l opiieal araanalies and <:ranp1e-x twuriin.ii-. A little lihrous auiphihole has hecni huine.l frojn it by Ft:'l''l KL\ <!K<)IJ'H!y OF THH HI'Mli'Mt IH STRICT is; iiltcnvlioii. Ai lni. vn<i i-ml. the lua-nriit,. .H-.-urs with Juuch Inr^^t-r aoioiifiiM of -^ilu^Hi.-. A ihiii MH-liun of a s|.rriuN/n .<huv;> epidorc t.. I..- the piviUnmw.mi miin'ral willi which ihc nin^mici itc is associaietL i.m s^ihmII pati-lu's i,f y'linicf nrv rilsf. |ir.'.-wit. M.^iirc :.(i is si phnioiiiit-rugriVph that of iiui-iHi!t" an. I t'pidnk'. >;uv.>(>.h.il hv a (.f hf ro!IlMH'>: rnorphir r.uk ivhirh i> Ih-Hi'v.-I h. h.ave limc*ti>nc. pi-riiap^ ii 1ufrai-cou> jihan'. X out along thv hWhuU'. A I niii. phicMihoi iho strike; IK N. ;ir^ W. aiHi thr .lip :iN-^ X. K. I.arc.' vug.* ocrur it A iin(T..H-..pii- cvaiuiiialicu of ih.- au.l -arn..t. the s.niriH-t ha pill ftM't brvr.llil Ih mcteiuoi'pr.csc! '.<'<-^ m !'" rcvoalH that it >-un<l'^\^ jargclv of pvr . iIm:- pvri.K^'iie. Calcit.-. i-puluu., quart:-., am] tah- ai ,,.. ;,uinniiT<. (Iiloritc ha> P..v,.lupM-.I along unnuU- frai developed prc.«.irt i iss srft:\'nFH' ss(i,'ri-:y of I'oirro uir tuivs. AiH.tlier hiHH-iim.)i i;ik,.ii from tin- walls of uiii. ..f thv vui^h mnmnU hiruvlv ul* --aniet anJ ampliiholo with soim. U\\v. Thv -;:.ri!=.f Hiipiirenlly luniuMl first. Ihcn tiu' Hinpiiihul,., aiul liiiiilly the Inlr, llnnihleiule ainl pyrr.wuv .-.rhi.-^rs iiiHloul.tc.lly denv,..! irnii! luidcKiKi lu!l>, t.vi'riic and iiiiil(»rlic the nu'lamorpluKsed linicsloiu'. A tiiin scrtioji of a ,-4jH-riinen uhtaiiied from aln-v*' tii<> Jiiupstoin^ |h.^.m'>^-=i'- n iliHliucidy fnliatcrl >tructuiv. Ir i< iinde up cidctiy of pla-KHda.^i-. .|narf7., oiilai- cla-u, and a dci'j. .uavcu hoi uiilcudc. A little pymxv^iv ir~ al>u |. resold. I I I LJ^_J Fill' (.iitliiu's of a few ono-inal lVld..par .'ryslals. lanv ri,iH|d,.-i('l v altered <i kaolin, <eriMtv. and ..pideie. ran still he ]T.e.,miixe.L Mapieiiie and itaiiile are piv^enl as miner euiisritut-nls. .\ia)|,her >pinanHni taken frem I (MHid seme dislanee h..|(.u th.. liinestene alse in^<^v^^v. a sehi>tiw a|.- H-aranee. It is eompused ef an interhjeking^ nu.saie of pale lim^n »yruxeiu-, pda.--iuelar^e. enlioelase. and a lillle dark ,Haver= hiH-nhlende. A .eu larse ervstals uf pvri.xene full ef inrhisien^. inrludijiu^ smaller FfJTTK/'], aiJ()L()(JV OF THIJ HI MACAO DISTRICT IH!> pyroxene <i:rains, are ])rese]it. ^la^nietite oeeiirs in minor amounts. Hpidote lias (levelo])e(l alono- minute fraetures. At the west end, an angite andesite oecnrs as the Footwall of Ww mag-- netite deposit. It possesses a felsitie texture witli a tendency for the many fehlspar laths of the o'roumhiiass to he in parallel ali^iinnent, in- dicating- a tlowag-e structure. A few pheuocrysts of ])lagi()clase of an amlesine or acid lahradorite variety and a little augile are ])resent. The matrix consists chieHy of plagioclase. Titain'te is an accessory jnineral. Pale ^vreen horn])len(le aggregates occur ah)ng snudl fractures, as well as in places once occupied by augite |)hen()crysts. The ])lagioclase is clon(hMl \)y alteration products. In one ])lace north of the deposit, an outcrop ot nu'tam()rj)hose(l andesite tuff was ol)served. It ]){)ssesses a hue textui'e and (vxliihits a slight tendem-y towards a scliistose structure, (irains of augite, plagio- (dase, and orthoclase u]) to .4 millimeters in length, ai'c pi'esent. The matrix consists chiefly of an alloti'ioniorphic int(u*growth of (puirtz and ])ale grecui hornhlende. A little chlorite has develoj)ed from hoi'nhlende. Beyond the andesite tuff toward the northwest, therc^ is an occurrence of horid)lende andesite p{)r])hyry. Augite pheuocrysts with reaction rims of hornhhuide ai'e prominent. The nuitrix is made up i)i"(M]oniimint (d' a hrownish-green hornhlende with only minor amounls of ]>lagio(dase. It is coarse in tc>xture. :\ little magnetite and apatite^ are pi'esent. The J uncos deposit lies af)out 2J){)i) feet north of the San Loi'cnzo (juartz diorite hatholith. An olf-shoot of the latter, which is pai't of the Lu(piillo stock, crops out 500 t'eei north of ihe magni^ite occuri'cnce. The magnetite <le])r)sit at locality No. 2 is situated ahout 'A'-y\ mile< southeast of Xo. 1 on the south side of a ridge ahout 1<)0 feet helow its crest and 'M\^) U\A above the ^•alley at its base. The oulcro]) strikes X. 50° W . and can be traced U)V a distance of X\T) feet. Tlie dip appears to be 72° toward the northeast. ]\Iagnetite is the chief ore mineral pr(\^ent but sonu' fine grained specular liematite also occurs, (hirnet is thc^ j)redominating gangue mineral ai)pearing in the outcr(»j). Drusy cavities lined with tra])ezo- hedr(ms of garnet and sometimes small octahedrons of" m;ignetite as w(dl as crystals of (piariz are of connuon occurrence. \'einlets of magnetite and (piartz are seen cutting massive garnet rock. The garnet ])ossesses a reddish-brown color. A (puilitative analysis imT ica tes that it is the andradite vai'ietv silica, iron oxide, and linu* being the chitd' con- stituents. .Vlumina occurs in only snuill amounts and magnesia is al)sent. A green brecciat(Hl rock forms the north or hanging wall o!' the de- i)()sit. Tlie dark green |)ortion is intersected in all directions by streaks |()() .^CIEXTIFJC i^lRYEY OF PORTO RICO of light i-ulortHl niaterial. (Jiuler the microscope, the dark greeii por- tions are foiiiul to consist chiefly of pyroxene associated with small aiuouiits of garnet. The light portions represent an allotrioniorphic intergrowth of plagioclase, orthoclase, quartz, and some pyroxene, ft is thought tliat tliis rock represents a metamorphosed andesite tuff, l>i'<>h- al)ly a calcareous one. North of the tut! along the crest of the ridge, an augite andesite crops out. This iuis a felsitic fexture. Under tlie microscope, the plagioclase laths of the groundmass are seen to he in more or less parallel align- ment, indicating a flowage structure. A few hirge phenocrysts of augite can still he recognized. Dthers have been altered into aggregates of cpi<h)te or hrowuisli-green hiotite and secondary (puirtz. The Jiuiti'ix is (•oni])osed of plagioclase lafhs, hiotite. and augite. The above magnetite deposit is located al)out V-^ mile noi'theast of tlu^ San Lorenzo batliolitfi. Idle outcro]) of the maganetite deposit at hjcality iSo. v), which is t/o mile souttu'ast c>f Xo. 2, while it does not make a xery conspicuous show- ing at the siirface, is of considerable interest because the relationship of ihe magnetite and contact metamorphic minerals associated with it to the Coheres limestone is brought out more clearly tlian at any of the other localities. The magnetite associated with the metamorphosed lime- stone occurs very near the top of a nortli west-southeast ridge. Numercms outcroj>s of nearly jiure crystalline light gray and white limestone api)ear on the southwest iiank of this ridge. The strike of the formations is X. 54 "" \y. and tlie dip is :)2° N. E. It is estiuiated that the limestone has a thickness of at least 250 feet. A small qnarry has been opened in it at one place from which limestone has been taken for burning into lime for local use. Qualitative analyses of tlie limestone indicate that it sfiould yield a good grade of lime. A thin section of the crystalline gray limestone below tlie magnetite, rcNcals an interlocking mosaic of calcite crystals of medium texture with only occasional minute grains of iron oxide amongst them and a xei'Y little clilorite. A specimen of the metamorphosed limestone from the hanging wall of the deposit is composed of garnet^, pyi'oxene, and calcite—garnet being the most abundant. Garnet seems to have devel- op(?d somewhat later than pyroxene, and some of tfie calcite in the form of little veinlets cuts all three. Northwest of the deposit proper, promi- luMit ledges crop out that range in mineral composition all the way from crystalline calcite associated with only small amounts of silicates, to those in whicli silicates are the predominant constituents present. A thin section of the least altered rock consists mostly of calcite and a little FJJTTKE, (iEOLOay OF THl] lUMAC \0 Dh^TRK'T \\)\ garnet intersected by fractures along which quartz, specular hematite, and possibly magnetite have been introdnced, replacing tlie calcite. This was followed by the devek)pment of chlorite along fractures and between c-alcite grains^ replacing the latter in part. The final stage was the depositicm of later calcite in the forju of minute veinlets cuttijig the other minerals. The limestone is overlain by an andesite at locality Xo. ;] and under- lain by a metamorphosed and much altered andesite tnlf. The magnetite (h'])osit occurs in the upper portion of the limestone just below its con- tact with the overlying andesite. Tlie San J.orenzo batholith lies al)out V;^ mile to the southwest. The outcrop of the magnetite' deposit at locality Xo. 4 occupies neai'ly the entire crest of a hill about V/U ndles southeast of Xo, -"h It lias a length of nearly 200 feet and a maximum wi(hh of 80 feet at the western en(L At this end scmie coarse inicaceous hematite, associated with (piartz, as well as massive magnetite, are present. The outcrop^ wliile not very large in area, makes a V(^ry prominent showing. (Ireat boulder^ of massive magnetite derived from it are strewn along the hillside to tho south and have accumulated in tlie little \alley at the }>ase. A feldspathic andesite comprises the iiajiging wall of tlic de|K)sit. In fhiji section, it is seen to possess a felsitic texture and distinct fiowag<' structure, brought out by the parallel alignment of the iplagioclase latlis, A few phenocrysts of plagioclase and a light green ]iornl)lcnde (hn'ived possibly from augite are present. Occasional aggregates of epidote also occur. The matrix consists predominatingly of ])lagioclas(^ laths in more or less parallel alignment. Skeleton crystals of iron oxide, possibly magnetite, are prominent. A little hornblende is also pr(\<ent. Three hundred feet south of the deposit there is a small outcrop of a sericite phyllite, probably representing a metamor])]iose([ volcanic ash or shale, which has a strike of X. 59" AV. and a dip of 41° X. E. S(4'icit(\ quartz, and iron oxides are its chief constituents. One thcmsajid five hnndrcnl feet X. 73° W. from the deposit prominent ontcrops of gray crystalline limestone and associated garnet rock were o[)served. I'he main San Lorenzo batholith lies % mile to the soutliwest, wliile a small oiTshoot fr(mi it crops out only Va i^i^^ away. The magnetite deposit at locality Xo. 5, 9,10 mile south of Torres and about the same distance north of • Xo. 4, do(\< not belong to the same belt as the four deposits already descri))ed. The cmtcrop, whi(4i is not very conspicuous, being traceable only for about 50 ivvt, occurs on the ]K)rth side of a ridge just below its crest, wliicli rises 410 feet abo\e tlie PJ2 SCIKXT/FIC Sfin'E) OF l'(pRTi) RfCO (iiirabo vjillt'v, FiumH' :^2 is a |ihuto<;Trtph uf tlu' riil,u<' rt« H, Mp|H'rtrs fruin the north. Oil tin. iicrtlH-ast si<li' of tlu' rid.i.T. 220 W-A In'hnv tiu> liia.uiiotitc i)rus^ |iiM-t. u! (.'h'\atii«ii. <.iin-rn|)> of !iini'>toiH> aro cxposiMl. one Icdgv luiving a thickiies> ul" 1(1 ftH-r. Oil acroiiiir of the mussivp t-luiractor ol' the rock, no sf.rik*^ am! dip lueasuivnu'iit^ on tlic liiiH':^toiu« ilstdf wvrv pos^hh'. It: is ovt-rhiiiu liowev»«r. I»y an nuu'ilc aiidositx', ^howiiij^- tlowagf si nu-iuri' imdor the !rii<i'os<-op(\ ouf(Top|tiiiu UmIo-cs of whirh occur in such relalioii t<» the limestone as to load one to conclude that the r-trikc of the i-ontad of th(> two is X. is" W. and the dip :>'>'' S. W. This makes it appear thai a svnelinc rcrur^ hefwcen tlic depo^it^ of Xos. I and 5 and that the limestone at lr(^alilv Xo. o may In- the sanu' hed that the other four diiinsits areas^^ociat-'d with. W^^-o.*... ;*c^i A jiiirru-copic .'xainination of the limestmic reveal> a rather Hne^^ u-rained rock composed chiefly of an iiiterlockiii.u- jnnsiiie ol' calcite erys^ tals with occasional -rains of quartz, aviu'ite. aJid pla-iochise distrihulet! mnon-sf them. The latter undcnl.tedlv represent tulTacemis .materia! deposited contemporaneously with the ealean-ous constil ueiits. On.' hun- tired and seventv feet >outheast of ilie lunestone outcrops alon.u' the Inlh^ side a proniment. Icd.u^e of inunwi rock, dev.dopi'd from the limest.aie. was nol u-ed. The ^irnet, which is appareiilly an antlradttc variety. sho\v< hirefrineence and is np|icall\- nepitive. It was at (irst nustakeu for \a'su\danite in thin section, fmt posse»cs a liigher ind(>.\ of rtdd-action than the hitler inin<n'al. FETTKE, (UJOLOaV OF THE HI MA(\\(> DISTRICT YX\ (i()in<>- in the (lirectioii of dij), augite aiidesite and latite occur between the liniestoiie and magnetite (mt<TO})s over a belt about '^.')0 feet wi(U\ Whether tliese are extrusive or intruf^ivf^ sheets could not he deterniined from their field relations. The andesite wln'ch immediately oviudies tlie limestone, in thin scH'tioii, sbows a parallel alignmeiit of its plagioclase latbs, indieatin<^ a flowa<>e structure. The wall rocks of the maixnetite d(^|)osit itself are not exposed. A latite tuif ovcudies tlie nui.u'uetite, but it was not o})served iu direct contact with it. Tins in turn is succeeded by dacites and andesites toward the soutbwest. Tbe magnetite deposit lies IV:^ miles n(U'tlieast of the San Lorenzo batbolitb contact and l-/;; miles southeast of the lAKjuillo stock. The maguetite de|)osits described are cl(>arly of tlie type ordinarily termed contact metamor])hic deposits. In eacli case tbey [)oss(\>^s mineral associations and oc^-ur in such relation to the sun-ouinling rocks as to lead one to conclude that they have been formed by the re])lacement at high tem])eratures and ])ressures of either a nearly pui'e limestone, a tulfaceous lim(\stone, or a calcareous tuff. A study of ti\(^ geology oi' southeastern ]\u"to Hico leads one to tbe conclusion tbat tlu^ magma, wbich gave rise to the Saj] I^orenzo (puirtz diorit<^ batbolitb, as well as the Lufiuillo and other smaller satellitic stocks, and from wliicli tbe later intrusions that surr(uind, and in soim^ instances penetrat(> tlie (piartz diorite, were derived l)y magmatic differentiation, was also responsible for tbe (mianations tbat formed the magnetite deposits in tbe luiu'e cal- careous portions of the older series of eastern Porto liico. Tiic pi'oximity (d' the San Ijorenzo batbolitb and tbe liUipiillo stock, as wtdl as sniallci* olfsboots, to the magnetite deposits, indicates tbat tbe <piartz diorite undoubtedly underlies tbem at no very great <leptb. The metamorpbism ])roduced by the intrusion of tbe (piai'tz dioriti^ into the older series was of two kinds. In one case it involved simply a recrystallization of constituents already pres(Mit in tbe original rock with very little change in cbemical comj)osition. Tbe Ik^iI of tbe intrusi\«' and probablv tbe escaping water vap(n% as well as tbe lu'es^ure exerted by tfie invading mass, were largely responsible for tlu^se cbanges. Horn- blende s(^hists were in uuiny instances developed from andesite tulfs along the contact, as has already Ixn^n pointed out. In a tew ])laces, where tbe intrusions came in contact with sbales and sandstones, pbvllites and (piartzites were formed. The conversion of an ordinary limestone into a crystalline one, such as uow occurs in tbe vicinity of tbe magnetite deposits, with the obliteration of all traces of organic remains, can like- wise be attribute<l to the sanu^ cause. In tbe case of tbe formation of the magnetito deposit^ and tlx'ir ass(.- !1M ••IKSl'lFlv sfllVEY <»F I'OirFfP Rid) r-iatid ,^ilif.-it..>. nil rhe utlipr hnii.l, .'inaiirtt iojis hum the inii.i;-i!ui itself ii!frO(lu(-'0(! Inruv annmiits .if iniii.Tiil matter, eoiisistiu-' vhwih uf silica and iron o.\i<Ie<. viiirh n-«ft<'<l with hih! tu a .uT.-rti oxteiit ivi»lfu;i'.| the ongirinl crtlrii,- oi; th,. linn.stnnr- and vnlvnmms tulT with whiVh th.- .Icposifs are assoi-iated. The jiiiiir.falizetl eiiumariniis rr..tn rhi- (.lulino- .|iiiiriz dioritc ina.i(iiiu vn.^e aioji.u- ;frarfiire.< thruiiul, rhr Hirrnnndiiig- -rf.<'k> in rho ininKMliafe vieinity r.f tdie ininisivc imlil thiw eariie m idrti;cs wh./re llie rra.1iin>< inrcr.^eried the m-arhy hiiiestuKes Hud (^dc^aroous tulfs. The htun: r.- aeted wifli th." Ierru-siliee..ii,'< cmannt iuii^ miwh im.rp rea<lilv fiiaii the and(>>ire>aiid nrdiiuirv tiul'^ with whirh thev are iiitaa-hefhled and re|d,a«-.- th- IwlT *.ki-^- " "' 3%g iruii ore deposits ihiavrnre. the bells alonjr whieh the ealeareuiis iiiuld<aas Hf the older series erop ,,ut, ill the viididtv of the San Lorei.izn,hatJi(dit,h and its suellitie stocks. ar.> tlie ones whii-li >!)uuJd he pro>|)eeletl most thorough ly. Iloek syilal)Ie iV.r roai MJ'TAI. eojisirnctinii js alrundanl'ly distrihiits'd ilirtniuii- out the Ilhnnaeaii district. Ainonext l!a> various ty|H-.s that, have be.'ti n<v<l lor this |Hi]'|i"se iire ]n<'liid,etl fidsite. rhytilite, lalite-, daeite, ainlesit.-. qnartz diorite. andesitht InlTs and hna-eia.-. and raUearenus ivdl's. The andcsite.^ and latiles are prol)aldv Ihe la-st for nuiertdani road eonsirn<- tioii. Fi-uiv od shows a view id' one of the ]ar-vr rpiarrirNs located alon^^ the Hiiliraiw road .soiitliwesr of ('ayaia-. a short distanee nn tlie Qyehra^ dillas vailey. Idle roek obtained Itore is a hornblende bitiie. FF/rTKii aijouxn of the humacao district id:, LIMKSTOXi: The Collores limestone was tlie only one encounterei] within the ])or- (lers of tlie Hnniaeao district that is of siiffieient puritv to l)e snitahle for ])iirnin(2: purposes. The hest (levelojnnent was ol)serve(l in tho vicinity of the harrio of CoJlores de Piedras, where it has a strike of X. ^)A° W. and dips 32° toward tlie northeast. Prominent ledo-es of ^ray and wlute crystalline limestone, in some instances 40 feet thick, croj) out. The total thickness is estimated to he 250 feet. Qualitative analyses indicate a very pure limestone. There is very little residue after treatment with cold dilute hydrocliloric acid, and only slight amounts of ii'on oxides and ahimina and a trace of magnesium carhonate are present. A small (piarry has heen opened in it at one ]:)lace from which limestone ha^ heen taken for huivning into lime for local use. BRICK CLAY While the residual soils of the u])land ar(>a of tlie llumacan district are predominatingly of a plastic ferruginous clayey nature, they are not snitahle for making a good grade of huilding hrick. The writer did not come across a single locality in the district Avhere hricks were heing made from them. KAOLIN At several places along the military road southwest of Caguas and west of K. 45, rhyolite ])orphyry dikes are exposed that have heen almost C(un- pletely altered to kaolin. In some instances these are almost pure white in color, Avhile in other cases they are hadly stained hy limonite. The <puirtz phenocrysts of the original rlvvolite ])or|4iyry ai'c still ])laiiily visible in the white kaolin. These could prohal)ly he r.>mo\'ed by (^arefiil washing. There is a possibility that commercial (le])osits of kaolin may be present in this locality. THERMAL S PRIMES One thermal spring, the Yirella, located out on \\w ])laya northwest of Arroyo, lies within tlie borders of the Humacao district. ITodge (102t). pj). 225-22()) quotes the following analysis made l)y ^\v. Louis irernan- dez, of the Porto Pican Service of Sanitation, of the waters of this spring: Ich; SriEXTIFW survey of PORTO RICO Quantitative analysis Amon lit of water Tarts uer used. ("'ul)ic million. centimeters. Nitrates 02 100 Fi'ee aniinoiiia 04H2 .... Albuminoid ammonia 04S .... Total annnonia 01)12 KKK) Potassium (W,)! 5(X) Sodium 819 500 Litliium 00287 lO(KK) Sulphates 4598 1000 .Alau^nesium (K>12 1000 Calcium 1 . 0877 KKK) Olilorine l.:]5790 KM) Silicon dioxide :i078 KKK) (Vn-bonates as (^O, 1.00577 KK) Feiric oxide 00402 5.94702 1' »rts per million. Sodium nitrate i \7H9 Ammonium chloride l.*)74 Lithium chloride tM).*>4o Potassium chloride 1409 Sodium chloride 2.1991 Ma.irnesiinn suli)hate (K)4 < 'alcium sulphate 1 . K)88 Feri'ic cai'bonate 00759 Silicon dioxide *U)78 Calciiuji carbonate 1 .7721 5.82K)2 Total solids 5.8242 Q ua litativc ana his is I'romine neij^ative Iodine nej^ative HydrojLren sulphide ne,ij:ative Carbonic anhydride (free) nejj^atiA^e Free suli)hiU'ous anhydride ne,u:ative l*liosphoric aidiydride ne.irative P»oric acid negative Lithium (speetroscopically ) 1 positive Specific .gravity 1 . 0(K) Tem])erature .*>0° C. Alkaline to litmus and phen(»li)hthalein. FETTKIJ, GEOLOGY OF THE HUMACAO DISTRICT 197 Hodge attributes the high chloride content of the water from this spring to wind-carried salt spray from the near-by coast and to the fact that the w^aters issue through recent coastal plain deposits, which have not as yet been completely freed of their connate sodium chloride. Several small hath houses were at one time erected at the Yirella Spring, but these have now fallen into a state of disre])air. BIBLIOGRAPHY Daly, li. A. 1014. Igneous roeks and their origin, p. 90. New York. Berkey, C. p. 1915. (^Jeological reconnoissance of Porto Rico. Ann. N. Y. Acad. Sci., XXVr, pp. 1-70. Semmes, I). R. 1919. (Geology of the San Juan District, Porto Rico. Scientific Survey of I*orto Rico and the Virgin Islands, I, part 1. HouGE, E. T. 1920. Geology of the C'oamo-Guayama District, I^orto Rico. Scientific Survey of Porto Rico and the Virgin Islands, I, part 2. I.OBECK, A. K. 1922. The Physiography of I'orto Rico. Scientific Survey of I*orto Rico and tlie Virgin Islands, I, part 4. Mitchell, G. J. 1922. Geology of the I'once District, Porto Rico. Scientific Survey of I*orto Rico and the Virgin Islands, I, part o. HCCBARD, B. 1923. Geology of the Dares District, I*orto Rico. Scieutiftc Survey of l»orto Rico and tlie Virgin Islands, II, part 1. Fp:ttke, C. R. 1924. Magnetite deposits of eastern Porto Rico. Mining and Metallurgy. No. 1294-M. Feb., 1924. m I; ^mt M H5 B 528020 I UNIVERSITY OF MICHIGAN / 3 9015 03458 5615 I, ? 5" j». / ^\ f-, \ *^*fe.*a^*«*»- i i ,*l Ifc^^itJS^ . * 4.4^! >*^ ,