160 ANNUAL BEPOET SMITHSONIAN INSTITUTION, 1917. 



Douglas regards the intrusive rocks of the eastern cordillera as 

 post-Devonian and pre- Jurassic in age; but this is apparently not 

 proved, some authors calling them early Tertiary. 



Conditions similar to those just described prevail in Chile. We 

 find here, however, a coast range of lower elevations, largely made up 

 of Mesozoic sediments with interbedded volcanics and a main western 

 range, plateaulike in the north, which is surmounted by a long line 

 of active volcanoes and often really constitutes the western margin of 

 the Altiplanicie. The basement on which the volcanic cones rest is 

 largely of Mesozoic sediments, more or less abundantly intruded by 

 granodioritic rocks. South of Concepcion tlie intrusive granitic 

 rocks increase in volimie and are bordered on the west by metamor- 

 phosed sediments of doubtful age in Chiloe Island and the Taytao 

 Peninsula. Quensel's^ researches have shown that a vast body of 

 quartz dioritic intrusive, similar to the batholith of British Colum- 

 bia, but of greater length, follows the coast from Puerto Montt down 

 to the extreme tip of the continent. 



On the east side this batholith is almost continuously adjoined by 

 Mesozoic sediments ^ in which great flows of " quartz porphyry," 

 " porphyrites," and their tuffs are embedded. These continue for 

 1,000 miles or more northward along the eastern slopes. The nomen- 

 clature is open to objection ; the rocks are rather rhyolites, andesites, 

 etc. 



In this Patagonian region the di,stinction between the coast, central, 

 and east Cordillera is less clearly marked. Pre-Cordilleras or front 

 ranges appear on the east side and are made up of granitic lacco- 

 lithic intrusions. East of these, again, are found vast table-lands of 

 basalt and other Tertiary eff'usives, which slope eastward and in 

 places reach almost across Patagonia. 



In southern Patagonia there is only one period of folding, involv- 

 ing Cretaceous and Tertiary beds, while farther north and indeed 

 through the whole chain of the Andes there are two periods of folds 

 ing, one Jurassic or older, the other late Mesozoic or early Tertiary. 



ARGENTINA. 



The recent work of Argentine geologists, such as R. Stappenbeck, 

 H. Keidel, and others, has given us a clear idea of conditions along 

 the eastern slope of the Andes. This is rarely a simple slope but 

 usually a succession of ridges, the more easterly of which are called 



1 Geologisch-pt'trographische Studien in der Patagonischen Cordillera (Upsala, 1911). 



2 Practically all of the sediments of the region of Magellan Straits and Ticrra del 

 Fuego are considered as belonging to the Mesozoic series. On the west coast the batho- 

 lithic rocks face the sea. 



