Geology. - On rocks {rom the Venezuelan islands bet ween Bonaire and Trinidad and on some rocks {rom Northwestern Venezuela. By L. RUTTEN.

(Communicated at the meeting of O ctober 31. 1931.)

During a journey to the Antilles in 1930 I expected to get an opportunity to visit the small islands between Bonaire and Trinidad. Lack of time. how­ever. rendered this visit impossible. By the kindness of Professor R EIN HAR D and Professor BERGT in Leipzig and of the Curators of the British Museum in London I obtained afterwards a series of rocksamples from these islands. partly collected by the German prospector R. LUDWIG. partly by the English ornithologist P. R. LOWE. The examination of these rocksamples had a twofold interest : firstly very little was known about the geology of the islands . and secondly it beca me clear that the geology of several of them shows clear relations with that of the Dutch Leeward islands. chiefly with Aruba. The results of th is examination will be stated here; moreover the geological data. known from the islands. which are rather scattered in the geological literature. will be shortly recorded.

Aves Islands.

They consist of two groups of low coral islands. so called " keys" or "cayos" 1. 2). Phosphate and Guano have been reported from them 1). To the geologist a study of the coral reels of the islands may be of much interest; for the rest. however. it is almost certain that they do not show anything interesting. Probably the isles may be compared with the southern part of Bonaire: they have emerged only recently from the ocean . and young limestones cover the invisible older formations .

Los Roques.

As with Aves islands. most islands of this group are "keys". Only the Northernmost. El Roque. is hilly and shows a core of older rocks . El Roque has a N-E strike; in the N. and the E. it is covered by coral limestones. locally with phosphate and guano ; in the W. and SW. its surface ris es to 150-250 m. (W. SI EVERS . l.c.). According to BERGT (in: SI EVERS l.c.) . who examined long ago the rock-samples collected by R . LunwlG. the chief

1) W . SI EV ERS. Die Insein vor der Nordküste von Venezuela. Nach den bisherigen Quellen und unter Berücksichtigung des T agebuchs und der Gesteinssammlung Richard Ludwigs. Globus 74. 1898. p. 163-165. 291 - 294. 302-307.

2) CH. B. CORY. The birds of the Leeward Islands. Field Museum of Nat. Hist. Chicago. Ornith. Series I. S. 1909. p. 193- 255.

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rock is quartzmicadiorite. Besides occur: biotitegranite. aplitic granite. Hein eigenartiges Augitgestein" and amphibolite. I reexamined slides of all the

. .... 0 r-,.,

0

~ .. > 0

c.Q Cl P-o ;J .. ..

samples. collected by L UDWIG. The r t:sult ;,v u;,. lH ... , ...I Ü odong to rocks. very

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nearly related to those of Aruba and Curaçao, as a short description will show.

V . 208, D. 12486 (E I R 0 q u e 3) 1) is a granula r, hypidiomorphic rock with green hornblende (frequently accumulated in dark spots of the rock. often idiomorphic in the prism zone ) , plagioclase (oligoclase-andesine, of ten with zonar structure, beautifuIly twinned with rather good idiomorphism) and quartz (occurring as filling-mass, with strong wa ving extinction or cataclastic) . Brown, partly chloritized biotite, magnetite, titanite and apatite a re accessories. This bi 0 ti t e - b e a rin 9 q u art z hor n bie n­d e d i 0 rit e resembles by its mineral-combination, by the zonary plagioclases and by the pressure-phenomena the diorites of Aruba. 2)

V. 209, D . 12487. (EI Roque 107.) Biotite-bearing quartzhorn­bie n de di o 'r i t e. strongly resembling the foregoing, but with more biotite.

V . 211, D . 12489. (E I IR 0 q u e 107a) is a da rk, granular rock, macroscopicaIly resem­bling <l n <lmphibolite. Consists of green hornblende (more than 50 % of the rock , partly idiomorphic in the prism-zone ) <lnd pl<lgioclase (somewhat idiomorphic, of ten zonar with the most basic feldspar in the core, on the average about labrador). Accessories are some ore (grains of magnetite and some cubi of pyrite) and tra ces of quartz. The rock is certainly no amphibolite. It shows strong resemblance to malchites from Aruba , from which it differs by the a bsence or scarceness of quartz. Q u art z -free malchite:l) .

V. 214, D . 12492 (EI .Roque, 108) . Is a pebble of a very dark, finely fibrous phos­phorite.

V . 216, D . 12493 (E I R 0 q u e, 6). Consists of partly perthitic orthoclase, almost not idiomorphic aga inst totally xenomorphic qua rtz; moreover some acid plagioclase with traces of idiomorphism. On some pl <l ces in the slide myrmekite occurs. A p I i tic 9 r a n i t e, v ery weil agreeing with some rocks of Aruba.

V. 217, D. 12494 (E I R 0 q u e. 7) . Consists of perthitic orthoclase (partly idiomorphic against quartz) and quartz (xenomorphic, partly intergrown granophyrically with ortho­d ase), with some microcline, acid plagioclase and green biotite. Agrees fairly with rocks from Aruba. A p I i tic 9 r a n i te.

V. 218, D. 12495 (EI Roque, 15) . Is a splinter, showing the contact of a green with a very dark, coarsely granula r rock. The green part appears to consist entirely of opa I and spherolithic quartz; in the dark part occur la rge augites, large epidotes, smal.!, basic plagiocl<l ses and many fibres and needies of pale-green hornblende. M ay be the rock is a s a u s s u rit i z e d gab b r o. The rock does not agree absolutely with rocks from Aruba, but it resembles very nearly a saussurite-gabbro of cretaceous age Erom NW. V enezuela, which will be trea ted <l fterwa rds.

As a whoie , it is clear that the rocks from El Roque show very near resemblance to those from Aruba. The determinations of BERGT have in-

1) The "V " numbers refer to a coIlection in the Utrecht geological institute, the "D" numbers to the collection of slides in that institute, the "EI Roque" numbers are the numbers in the Leipzig collection.

2) J. H . WESTERMANN, Handel. XXIII , Nat. Geneesk. Congr. 1931 ; id. Leidsche Geologische Mededeelingen. Vol. IV, 1931.

3) Prof. BERGT, who originaIly took this rock for an amphibolite, wrote to me that he

suspected it now for a dioritic veio-rock.

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duced SIEVERS to suppose that the crystalline rocks of El Roque have an archaeic age:

"Das lässt auf ein archäisches Schieferg ebirge schliessen und erinnert einerseits an Aruba .... .. anderseits an Orchila." (W. SIEVERS, I. c. p. 165.)

Without doubt SI EV EI~S was right , when he compared El Roque with Aruba. We know now, however , that the rocks of Aruba, which are related to those of El Roque, are of Y oung-Cretaceous or Old-Tertiary age: quartzdiorite and its sequence of vein-rocks, absolutely comparable with those of Aruba haven intruded in Curaçao the "Midden Curaçao beds", which are Uppermost Cretaceous or Old Eocene 1). In consequence we may assume that the crystalline rocks of El Roque are of the same age.

The island of Orchila, situated between Los Roques and Blanquilla will be treated afterwards.

Blanquilla.

SI EVERS' publication (l.c.) does not give any reliable details on Blanquilla. In the beginning of this century the island has been visited by P. R. LOWE, who has mentioned it in different publications and who, in his book of 1911 has even given a geological sketchmap of the island 2). The island is very asymmetrie: in the E. and S. there are coral limestones, rising to 200' along a line, directed about from N . to S .; to the W. of this line there is a "granite landscape", whilst near the westcoast true dunes are found . LOWE repro duces some photo 's from the granite landscape which resembie very much the diorite landscape of Aruba . I was in the possibility to study a sample, collected by Mr. LOW E:

V . 162, D. 12179. is a granula r rock with biotite (often chloritized; forming dark spots with ore and apatite ) as only dark mineral. The commonest feldspar is a plagioclase (oligoclase-andesine, polysynthetically twinned, of ten clearly zonar, rather idiomorphlc). Orthoclase and quartz (strongly cataclastic or with wavy extinction) form a xenomorphic filling-m ass.: This bi 0 t i t e 9 r a n 0 d i 0 rit e resembles very much some rocks from Aruba.

LOWE'S publication has not been mentioned by the two compilators of the geology of Venezuela A . JAHN 3 ) and R. A. LIDDLE 4). Whilst JAHN

indicates "granitico" on his map, LIDDLE indicates "cretaceous" on Blanquilla.

1) G . J. H. M OLENGRAAFF, Geologie en Geohydrologie van Curaçao, Diss. Delft, '29. L. W. J. V ERMUNT and M. G. RUTTE N, These Proceedings, 34, 1931. p. 271-276 ; id. id. These Proceedings, 34, 1931. p. 558-563. P. PIJPERS, These Proceedings, 34, 1931. p. 1023-1027.

2) P. R. LOWE. On the birds of Blanquilla, Ibis (9), I, 1907, p. 111-122. Id. A naturalist on desert islands, London, 1911.

3) A JAHN, Esbozo de las form aciones geologicas de Venezuela, Caracas, 1921. 1) R. A . LIDDLE, The geology of V enezuela and Trinidad. Fort Worth, Texas, 1928.

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Los Hermanos.

SI EVERS (l.e.) mentions. that R. LUDWIG has found phosphate and diabase-like rocks on these islands. Afterwards LOWE 1) has visited thc islands; he mentions diorite from them and has reproduced some photo·s. I could examine one sample. coUected by Lo wE.

V. 203. D . 12481 is a dark rock with dioritic-gabbroid habit. It consists of green horn­blende (non idiomorphic. forming dark spots in the slide. with inclusions of magnetite). plagioclase (Iabrador-bytownite with some zonar structure ) and a fill ing-mass of quartz which sometimes ente rs the hornblende in minute channels. The rock is a hor n bie n d e­g a b b r 0 cl i 0 r i t e and may be compared very weU with some rocks from Aruba.

Los Testigos.

the last islands between Bonaire and Trinidad. consist . as Los Hermanos. of seven islands. from which R. LUDWIG has mentioned - without coUecting rocksamples - the occurrence of chlorite-schists (W. SI EVERS l.c. ). According to P. R. LowE (l.c . 1909) the islands. which have a height fr om 400-500' consist of granite. in one island covered by shale. I could ex amine three samples. coUected by LowE. of which ane is a piece of quartz. which . for the judgment of the geological composition is of no importance. The ot hers are:

V . 198. D. 12437. Is a hornblendegranodiorite with green hornblende. plagioclase (idiomorphic. oligoclase andesine with zonar structure) and a fiUing-mass of orthoclase and quartz.

V. 212. D . 12490. Is a da rk rock with lamprophyric structure. consisting of small crystals of plagioclase (oligoclase-andesine) and of grains and sm all crystals of augite and hom­blende. Some inclusions of rounded quartz-grains. so characteristic for many lamprophyres. are equally found . The rock is a sp e s s a r t i t e.

Both rocks from Los Testigos are again nearly related to rocks of Aruba. LIDDLE (l.c.) indicates on his map cretaceous sediments as weU on Los

Hermanos as on Los Testigos.

If we ex cept Orchila . we may say that all the islands between Aruba and Los Testigos. over a distance of 800 km .. possess several common characteristics. All seem to have risen rat her recently from the sea. young raised coral reefs. sometimes concealing the older rocks . occurring in Aruba. Curaçao. Bonaire. Aves. Roques. Blanquilla and Los Hermanos. In most of the islands we find comagmatic abyssal rocks . poor in potash and rather rich in silica. With these quartzdioritic rocks are associated in several islands characteristic veinrocks such as malchites. vintlites. spessartites.

1) P . R. LOWE. Notes on the birds collected during a cruise in the Carihoean sea. Ibis (9). lIl. 1909. p. 304- 347; id. I. c. 1911.

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kersantites 1) etc. Everywhere the plagioclases of the dioritic rocks show beautiful zonal structure, whilst the quartzes present signs of very strong pressure. For Curaçao it is certain and for Bonaire it is probable that the rocks are Youngest Cretaceous or Oldest Tertiary; we may assume that the age of the dioritic rocks on the other islands is about the same. The rocks differ strongly from the igneous rocks that have been described from the Venezuelan coast range 2) which are probably much older.

It would be of interest to have data on the geological composition of Los Monges, the small islands to the W . of Aruba. I did not succeed , how~ ever, to Eind anything a bout them in literature. To the E. of Los Testigos we do not have any definite knowledge of the quartzdioritic cretaceous rocks, the northern range of Trinidad having the same schistous com~ position as the coast range of Venezuela. It is. however, possible, that a granophyr, which has been mentioned from the NE. of Trinidad points to a manifestation of the dioritic magma 3) . The few geological data, tha. have been published on Tobago do not point to a relation between that island and Los Testigos 4 ).

Orchila.

In the island~row Aruba - Los Testigos the island of Orchila occupies a particular placc. According to SJ E VEI~S (I.c.) thc name of Orchila indicates scvera l isla nds, the largest of which has W ~E strike. In LUDWJG 's collection of rocksamples BERGT has identified different gneisses and other schists. It is easily understood that SJ EVERS took the rocks from Orchila for an "archaeisches Schiefergebirge" (I.c. p . 292). The results of a reexamination of LUD WJ G's samples are following here.

Two samples indicate a byssal igneous rocks.

V . 224, D . 10199. Orchila Ie is a peridotite~serpentine. Theserpentine

has arisen for the gr·catcst part from olivine, for the res t from monoclinie pyoxenc, of which still some remains a re present and from amphibole. the clear cleavages of which are still visible in the pseudomorphic serpentine.

V . 235, D . 12501. 0 re h i 1 a 5a is only a large fragment of a fairly twinned plagio­clase (oligoclase-andesine) . which is epidotized and sericitized. Possibly this fragment has belonged to a dioritic rock.

Several splinters indica te the presence of true gneisses.

V . 221, D . 10196. 0 r c h 1l a 8 consists of more or less clear layers of strongly cataclastic qua rtz , of orthoclase and of a lbite whils t several very thin layers are rich in

J) P. PIJPERS, 1. c. 2) L. RUTTEN, These Proceedings, 34, 1931, p. 1013- 1022. 3) G . A . WARI NG, T he geol. of Trinidad. John Hopkins Univ. Studies in Geol.. 7,

1926, p. 11-86, 112- 170. 4) E. H. CUNNINGHAM CRAIG, Prelim. Rep. on Tobago. Trinidad Council Papers, 9.

1907.

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muscovite and contain some biotite, and one layer is built up by epidote-prisms. The rock

is a b i mie a 9 n eis s.

V. 225, D. 10200. 0 r eh i I a 3a is a gneissic rock. containing quartz, much microclin.: with quartz-drops, less orthoclase and albite, much biotite and muscovite, and some epidote in grains and prisms. Thc colourless elements arc xenomorphic ; th ry <lre not strongly crushed . Epi dot e - b i mie a 9 n eis s.

V . 226, D. 10201. 0 r C h i I a 5 is a gneissic rock with quartz, albite and orthoclas~,

much muscovite, less biotite and some epidote. It differs from the foregoing samples by thc enormous crushing of all the colourless minerais. Cru s h e d b i m i c a 9 n eis s.

V. 227 , 0.10202. Orchila 2b is an epidotebimica albitegneiss, differ­ing from the foregoing samples by the absence of potash feldspar.

V. 229, D . 10194. 0 r c h i I a 3 shows layers of very strongly crushed quartz anc others of non-twinned albites. Moreover there are strings of epidote and zoisite. Ep I­a 0 t e - a I bit e 9 n eis s.

V. 220, D . 10195. 0 r c h i I a 4 is a very strongly crushed rock, consisting of quartz and layers of a lbite in minute grains with strings of epidote and with some sericitic muscovite. S t ron 9 I y cru s h e de pi dot e - a I bi t e 9 n eis s.

One rock from Orchila has thc composition o f a micaschist: V . 223, D . 10198. 0 r c h i I a 2 is a white schistous rock with thin layers of muscovitc and others of crush~d quartz, Rutile and epidote occur as accesories. Strongly crushed muscovit,.eschist.

The remaining rocks from Orchila, all more or less schistous, are of green colour.

V . 222, D. 10197. 0 r C h i I a Id is a dark green, schistous rock, consisting almost wtirely of long prisms of green hornblende, enclosing grains and crystals of rutile, while some acid plagioclase occurs as accessory . Am ph i bol i te.

V. '235, D . 12502. 0 r c h i 1 a Ib is an epi dot e hor nb I end e s c h i s t with more or less clear layers of epidote and green hornblende whilst chlorite, quartz and albite are accessories.

V. 238, D. 12504. 0 r c h i 1 a Ic shows in the slide an interchange of layers with .:hlorite anc! twinned albite-Iaths, whilst epidote, calcite and qua rtz a re accessories. C h lor i tea I bit e s c h i s t.

V . 237, D. 12503. 0 r C hila 4b is an epi dot e z 0 i s i tee h lor I t e s c h i s t with titanitc, quartz and untwinned albite as acccssories.

V. 2.39, D. 12505. 0 r c h i 1 a 9 has a groundmass, consisting almostentirely of minute grains of a lbite and small scales of chlorite; epidote and leukoxene being accessories. In this gruundmass occur la rge plagioclases (albitized, with epidote, chlorite and sericite), which undoubtedly bear the characteristics of phenoc rysts. C h lor i tea I bit e s c h is t (met am orphosed porphyrite) .

V . 210, D . 12506. 0 r c h i 1 a l a. In a, not clearly schistous groundmass of chlorite, epidote, long needies of green hornblcnde, some albite and quartz occur many large phenocrysts of plagioclase (albitized, with epidotc and needies of hornblende) . C h lor i t e­l' p i d .) t e - hor n bie nd e s c h i s t (m eta mor p hos e d por p h y rit e).

71 Proc(>cdings Royal Acad. Arnstprdalll. Vol. XXXIV. 1911.

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The rocksamples of Orchila belong clearly to different groups. th.: geological relation of which would be highly interesting to know. All in­formation thereabout. however. is lacking. The peridotite-serpentine and the possible dioritic rock might be comagmatic with the abyssal rocks of the other islands. It must be stated . however, that the occurrence of peridotites is not known from the ot her islands. The schistous serpentine. occurring in the coast range near Caracas (L. RUTTEN l.c.). seems to have other characteristics. All the gneissic rocks and the micaschist. although not identical with rocks from the Venezolan coast range (L. RUTTEN l.c.) . are probably nearly related to them. The green schists. on the other hand. which partly are undoubtedly metamorphosed basic surface rocks. although not identical with rocks of the basement of Aruba (J. H. WESTERMANN l.c.) seem to be nearly related to them . It seems probable to me that Orchila is the geologically highest elevated island of the row. where we find side by side the basement rocks of the cretaceOlIS cycle (the green schists and. eventually. the peridotite and the ? diorite) and the much older. gneissic rocks which may belong to the same cycle as the rocks of the coast range.

lt is weil known that the coast range. westward from Puerto Cabello. bends to the SW., running in the direction of Merida (Venezuelan Andes) . Hetween the row of islands under discllssion and the Andes is a broad zone. comprising chiefly the states of Falcon and Lara . wh ere folded cretaceous and tertiary sedimel)ts of great thickness are the surface-rocks. The facies of the Cretaceous and Tertiary m th is part of Venezuela is entirely different from that on the islands. and so are the orogenic phases 1). It is, therefore. comprehensible. that. generally. we do not meet the intrusive rocks. so characteristic for the islands. in North-Western Venezuela. a lthough magmatic rocks are not entirely lacking. They have been known a long time already from the Cerro de Santa Ana on the peninsuIa of Paraguana :!). The details are. however. very meagre. and it 'is not at all clear. wh ether the rocks . reported from here. belong to a cretaceous phase or are prae-cretaceous. as BENDRAT and SI EVERS (l.c.) suppose. Another locality. where granite has been fOllnd. is the small island of Toas at the entrance of the Bay of Maracaibo (R. A . LIDDLE. l.c. p. 91). From the description it seems to be certain that th is is a pre­cretaceolls granite.

On a wonderflIl excursion which my students and I made as guests of the Caribbean Petroleum Company under the guidance of Dr. L. KEHRER through the states of Falcon and Lara we visited two localities. where magma has intrllded cretaceOlIS sedimerits. the first at tos Algodones

1) See for ex . G . J. H. MOLENGRAAFF. I. c. and R. A . LIDDLE. I. c. 2) W. SIEVERS, Richard Ludwig 's Reisen <luf Par<lguan<l. Globus 73. 1898, p. 303-

309; T. A. BENDRAT. The rocks of the Cerro of Sta Ana on Paraguana. Venez. Amer. J. SC. (4). 37. 1914. p. 268- 272.

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between Baragua and Siquisique, the second to the NNE. of Rio Tocuyo. A short description of the rocks , collected at these localities will be given here.

v . 154 D . 12128, from Los Algodones is a gabbroid rock of quite normal structure, with diallage and basic plagioclase. G a b b r o.

v. 156, D . 12132, seemingly a dike in the foregoing gabbro. Los Algodoces. Is a sa II S S u rit e - g a b b r 0 with albitized plagioclases, much actinolitc, prehnite and zoisite.

V . 153, D . 12126, an igneous dike in cretaccous limestone, NNE. of Rio Tocuyo, is a granular rock with somewhat idiomorphic a lbite, with orthoclase and totdly xenomorphic quart z; with wcathered biotite and muscovite and with traces of tcurmaline and idio­morphic garnet. C a t a c I a st i ebi m i c a 9 r a n i t ~.

Although these rocks, the age of which is about the same as of those hom the islands do not agree very weIl with the quartzdiorites of the islands, they may belong to their parenty.

By the kindness of the geologists of the Caribbean Petroleum Company 1 received moreover two samples of granite from the territory W. of Mara­ca ibo. Both are regarded to be of precretaceous age.

V . 160, D . 12177, Rio Palrnar is a gray , granular rock with large, idiomorphic, zonally structured crystals of oligoclase-andesine and with some orthoclase and much quartz as filling-Ill ass. Biotite, pa rtly chloritized is the dark mineral; apatite and zircon are acces­sories. The qua rtzes show partly waving extinction and are partly cataclastic. Bi 0 t i t e· ÇJ r a n 0 - d i 0 rit e.

V . 161, D . 12178. Weil YOCIL, district of Ma raca ibo is a granitic rock, related to the former, but with more orthoclase. Bi 0 t i t e 9 r a n i t e.

It is remarkable that these two rocks, regarded as precretaceous show strong resemblance with the cretaceo-tertiary rocks of Aruba. It is possible that these granites belong already to the igneous sequence of the porphy­ritic rocks of the Sierra de Perija, to which possibly also the porphyry of the Cerro Tete on the Goajira peninsuIa belongs. Pebbles, derived from the Perija-porphyries are found already in the basal conglomerate of the Cretaceous of the Sierra de Perija .1 ).

Summary .

The row of islands from Aruba to Los Testigos, which topographically seem to form a unit , form certainly a unit from a geological point of view. On most of them are found abyssal and, partly also, hypoabyssal rocks with clear consanguinity, derived from a quartzdioritic magma. Their age i.,:

Youngest Cretaceous or Oldest Tertiary.

I) Sec, f. e. O. STUTZER, Streifzüge cines Geologen im Gebiet der Goajira lndianer, Berlin, 1927; id. Zur Geologie der Goa jira Halbinse!. Neues Jahrb. etc. Bei!. Bd. 59b, 1928, p. 304- 326.

71 *

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These rocks differ clearly from the igneous rocks . known from the Ca rib­bean coast range.

Rocks of about the same age as those from the islands occur on some places in NW. Venezuela; these rocks. however. are not clearly con­sanguineous with the rocks of the islands. It is not impossible. that the - imperfectly known - igneous rocks of the Paraguana peninsuIa belong to the same sequence.

Granitic rocks from the Maracaibo district are mineralogically and struc­turally nearly related with the rocks of the islands; it seems however. that they are older.