376 I. Bowman — Physiography of the Central Andes. 



Between the western and the eastern Andes are ridges and 

 masses of rock of Devonian, Carboniferous, and Permian or 

 Triassic age, rising out of a great infilling mass of alluvial 

 material derived from the adjacent highlands. In northern 

 Bolivia the first foldings of the eastern Andes took place in 

 very early times, but the effects of erosion and of tectonic 

 changes were such as to bring the entire area below sea level 

 at the beginning of the Carboniferous period. Before the 

 close of this period renewed elevation again exposed the sedi- 

 ments to erosion, and from this time on the geologic history 

 of the region consists of erosion and renewed folding accom- 

 panied by the intrusion of those granitic masses which now 

 constitute the cores of the highest chains. So far as the 

 geologic record has been interpreted, the movements in the 

 eastern Andes seem to have been resumed in the Cretaceous 

 although the main central chain was outlined and its position 

 established at the end of the Paleozoic* 



The widely extended development of the Cretaceous hints 

 at the significant erosion that must have taken place on the 

 land area formed by the mountain -making movement at 

 the end of the Paleozoic. "Marine Kreiclefossilien warden in 

 fast alien Teilen der Anden gefunden . . ." f 



The western Andes, on the other hand, were a region of 

 sedimentation down to the end of the Mesozoic, when moun- 

 tain-making movements began. This movement, however, 

 must be distinguished from that now in progress and described 

 in the preceding chapter. The movements of to-day are 

 broad and regional in their effects, and distinctly non-moun- 

 tainous in character. 



In southern and eastern Bolivia the sediments are Cambrian 

 and Lower Silurian, with some traces of Devonian and 

 Carboniferous. The farther back geologic researches extend 

 the clearer it becomes established how general is the occur- 

 rence of Paleozoic strata. Silurian and Devonian rocks occur 

 widely distributed in every region of the continent, even in the 

 region which now has the most conspicuous mountain heights, 

 Silurian fossils occurring in the slates that form the highest 

 portions of the Nevadas of Quimsa Cruz, at 17,000 ft. Thick 

 strata of red Cretaceous sandstones also occur which, under 

 ordinary circumstances, are only preserved in troughs and folds. 

 This entire S3 7 stem of sediments "lies concordantly" except 



* Condensed and adapted froni the accounts of the geologic history of the 

 region in Expedition to Caupolican Bolivia, 1901-1902, by J. W. Evans 

 (The Geog. Jour., vol. xxii, pp. 631-634 et al., 1903); Siid- und Mittel- 

 Arnerika, by W. Sie vers (1903) ; The Continent of South America, by A. J. 

 Herbertson (1900) (Mills' International Geography, pp. 816-817) ; Arch- 

 helenis und Archinotis, by H. von Thering (1907). 



\ Archhelenis und Archinotis, 1907, p. 95. 



