66 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. EM,W0,B f$£3S£ 



diorites, etc., with which it is closely united. This is overlain by Cretaceous Rudistes limestones, 

 at the base of which is an arkose of fragments of granite and serpentine derived from the base- 

 ment complex. No age can therefore be assigned to the basement rocks. They may be the 

 equivalent of the late Jurassic rocks of the western United States or much older (Hayes '01). 

 Steinmann ('05), citing observations by Gabb ('73), Kloos, and Martin, believes that radiolarian 

 chert occurs with diabase, variolite, and serpentine in San Domingo. In several other Antil- 

 lean islands schistose basement-rocks occur, but ultrabasic rocks are apparently not present, 

 though in the continuation of the tectonic axes through the Lesser Antilles, gabbros occur as 

 inclusions in the volcanic ejectmenta (Lacroix '04), and a series of plutonic rocks ranging from 

 granite to peridotite occur in the crystalline complexes exposed in the inner curve of islands. 

 Hogbom's ('05) statement concerning the rarity of potash in these rocks enables Suess (IV, 

 p. 464) to correlate them with the granodiorite masses of the Andean system. Hogbom con- 

 cludes that in spite of their plutonic habit they are Cretaceous, occurring among tuffs and 

 breccias of that age. 



Passing into South America we find a continent unique in the rarity of ultrabasic and 

 basic plutonic rocks. The Andean zone swings into northern Venezuela, and opposed to it 

 is the foreland of Brazil and Guiana. The massif, which is composed of ancient gneisses and 

 schists, in British Guiana, is covered by sandstones resembling those of the Triassic in New 

 Jersey, but, owing to the occurrence of certain fossils in its extension into Venezuela and Brazil, 

 a Cretaceous age has been suggested. It is invaded by huge flat-lying sills of quartz-dolerite 

 like that forming the Palisades on the Hudson, and therefore correlated with them by C. Brown, 

 though Harrison ('09) is inclined to correlate them with Tertiary eruptions for rather indefinite 

 reasons. Somewhat similar sills are associated with basaltic lavas in the Trias-Jura sand- 

 stones, etc., of Southern Brazil (White 'OS). 



Following round the massif of Brazil, and as its marginal folds, is the Andean chain. Some 

 serpentines have been found in the crystalline massif itself in Brazil (Hussak '17), and in French 

 Guiana (Lacroix '98), but Steinmann ('05) has commented upon the absence of such rocks 

 of post-Paleozoic age from the greater part of the Andean chain, though in Columbia some 

 serpentines and gabbros have been found (Iddings '13), and a small mass of norite in southern 

 Peru invades the Mesozoic andesites. Though there was considerable folding and intrusion of 

 granodiorites at the close of Jurassic times, the general absence of basic and especially of all 

 ultrabasic plutonic rocks seems to stand in definite relation to the structure of the range. 

 "Throughout the Peruvian Cordilleras, inverted folds are the exception rather than the rule, 

 and great zones of overthrusting appear to be entirely wanting. Any directional movement of 

 the folding, moreover, is hard to determine" (Douglas '20). Farther south, however, near 

 Aconcagua, Schiller ('07, '12) has indicated the presence of overfolding and overthrusting of 

 the Jurassic sediments and lavas directed toward the west, and on the eastern side of the range 

 in the north of the Argentine, Keidel ('16) has described overfolds directed toward the north- 

 east, involving Cretaceous rocks. Older than these in the Argentine, a complex of diorites, 

 norites, gabbros, and peridotite, occurs chiefly in the ancient schists but also in the Paleozoic 

 rocks of the pre- Cordillera chains east of the Andes (Romberg '94, Steiglitz '11). These rocks 

 occur as pebbles in the overlying Mesozoic sandstone, and are therefore pre-Mesozoic (perhaps 

 Altaid). In Patagonia, Quensel ('12) describes an interesting series of intrusions on the eastern 

 ranges and foothills of the Andes. The former consist of laccolitic masses, probably of very 

 early Tertiary age. These contain a varied series of plutonic rocks from subalkaline gabbros 

 and diorites, to orthoclase-gabbro, monzonite, and aegyrine-syenite. In one laccolitic mass 

 the rocks may be strongly differentiated, while in an adjacent one they may be almost uniform. 

 One may show alkaline features, another, though widely differentiated, may show no relation- 

 ship with the alkaline rocks. Apparently there was extensive differentiation before intrusion, 

 and some differentiation also occurred in situ. In the eastern foothills (pre- Cordillera) there 

 is a more uniformly alkaline character, the rocks being essexites, and these are believed to 

 antedate the rocks of the eastern flank of the cordillera. They invade the Cretaceous calcareous 



