ANTILLEAN-CARIBBEAN REGION 



685 



Barbuda, only Quaternary limestone is exposed. Volcanic rocks crop out 

 on Anguilla, St. Martin, and St. Bartholomew. Oligocene limestone rests 

 unconformably on the basement rocks of Anguilla, and Eocene or Oligo- 

 cene limestone covers the volcanics on St. Martin. The basement of St. 

 Martin consists of well-stratified, strongly folded and metamorphosed tuff, 

 tuff-breccia, and somewhat crystalline limestone, intruded by a quartz 

 diorite-pyroxene diorite complex. According to G. A. F. Molengraaff 

 (1931) the sedimentary material of the basement may be of Cretaceous 

 age, but Christman (1953), as a result of work on St. Martin, St. 

 Bartholomew, and Antigua, states that "there is apparently no Cretaceous 

 basement in the Lesser Antilles." Miocene limestones are nearly horizontal 



i and have been deformed to a less degree than the Oligocene and upper 

 Eocene. 



I The oldest rocks of St. Bartholomew consist of volcanic debris and an 

 overlying upper Eocene limestone; both are intruded by an andesite 

 porphyry. Also found intruding the Eocene limestone beds are a volcanic 



< agglomerate and a dacite porphyry ( Christman, 1953 ) . See cross section 

 of Fig. 42.4. 



Antigua and Desirade likewise belong in the outer islands of the Lime- 

 stone Caribbees. Both have volcanic basement rocks. In Antigua, gently 

 1 dipping tuffs in the central plain are overlain conformably by the Antigua 

 j limestone of middle Oligocene age. The tuffs become coarser to the south- 

 iwest. At Crab Hill, Christian Valley, and St. Luke's Quarry, intrusive 

 andesite porphyries cut the series (Christman, 1953). See section in Fig. 

 ,42.4. Desirade possesses a basement of intrusive granodiorite, with con- 

 temporaneous andesite and rhyolite flows. Miocene limestone uncon- 

 formably overlies the basement. 



Grande Terre and Marie Galante are the southernmost, and also the 

 innermost, of the Limestone Caribbees. The latter is covered by a cap of 

 recent limestone (Woodring, 1928). Grande Terre, however, has a base- 

 ment of granodiorite which is overlain unconformably by lower Miocene 

 tuffs and limestone (Senn, 1940). 



To summarize, the outer islands of the Limestone Caribbees are char- 

 acterized by a basement of lava flows and coarse volcanic debris of late 

 'Eocene age, or younger, and andesitic to dioritic rocks of post-late Eocene 



age which intrude the volcanics. Oligocene and younger beds .ire mostly 

 limestone, and volcanic debris is fine-grained (as the Central Plain tuff ol 

 Antigua), where present. Apparently, these islands were active volcanic 

 centers in late Eocene and Oligocene time and have since received vol- 

 canic debris only sporadically and from a distance. They have not been 

 disturbed much by crustal deformation in post-Oligocene time. 



Inner Volcanic Arc 



The inner arc of the Lesser Antilles, stretching from Saba to Grenada 

 is characterized by Recent or subrecent volcanic activity. Tuffs of Oligo- 

 cene age on Martinique (Senn, 1940) and Carriacou (Trechman, 1935) 

 represents the oldest beds identified in the inner arc. Apparently volcanic 

 activity started here about in early Oligocene time and continued with 

 few interruptions to the present. As in the older (pre-Oligocene) vol- 

 canics of the Greater and Lesser Antilles, andesites predominate, with 

 basalts and dacites also present. The andesites and basalts of the more- 

 recent volcanoes contain hypersthene as a common constituent, whereas 

 the mineral seems to be extremely rare in the pre-Oligocene volcanics. 

 The significance of this mineralogic variation is not apparent. 



The volcanoes do not rest on the crest of the swell toward the south; 

 there they are found some 30 to 40 miles west of the crest. At the north 

 end of the island arc, however, they are approximately at the crest, and it 

 happens that here the islands are made up largely of sedimentary rocks. 



According to Hess (1938), sonic soundings show a series of peaks on 

 the western flank of Aves swell, parallel to tire Lesser Antilles arc and 

 250 kilometers west of it. Profiles across the peaks strongly suggest sub- 

 merged volcanoes. The lack of seismic activity along the greater part of 

 the Aves swell in the vicinity of the peaks suggests that if they are vol- 

 canoes, they are extinct. 



Margarita and the Dutch Leeward Islands 



The following is abstracted from Maxwell's report (1948). The north- 

 ern part of Margarita is composed of paraschists intruded by quartz 

 diorite and serpentinized peridotite. A zone of slightly metamorphosed 

 sediments lies south of the schist area, and unmctaniorphosed sediments of 



