56 STUDIES IN GEOLOGY, No. i 



marine fossiliferous rocks of this age in both the Eastern 

 and Western ranges that bound the high plateau of Bolivia 

 and on the latter near Lake Titicaca. Throughout the 

 Tertiary progressive folding went on accompanied by mod- 

 erate uplift. This is shown by the mature slopes of both the 

 Eastern and Western ranges in the erosion cycle preceding 

 the present. This elevation was not sufficient to bring any 

 part of the region to the snow line nor to interfere with the 

 trade wind circulation. Subsequent elevation brought 

 about the semiaridity of the present high plateau and the 

 desert along the Pacific coast. It is believed that at the 

 time the flora found in the Vetas series lived the altitude 

 of the region could not have been above 6500 feet, and it 

 was probably not as high as this.. Hence the country was 

 over a mile nearer sea level than is the present surface of 

 the Bolivian plateau. Toward the close of the Tertiary the 

 region of the plateau was progressively depressed. The 

 nature of this subsidence can only be conjectured. Whether 

 it was of the nature of a geosyncline, or an enormous rift 

 or graben, or was due to block faulting, or to differential 

 warping along a region of major faulting is unknown. 

 Bowman has advanced the theory that it was o>f the nature 

 of a graben, but there is no direct proof. 



During Pliocene time the Corocoro district was the scene 

 of rapid accumulation of Continental sediments derived 

 from the rising mountain mass to the west. Coincident with 

 the inception of sedimentation were igneous intrusions and 

 volcanic outbursts that contributed their quota to the elastics. 

 The igneous activity continued throughout the period of 

 sedimentation. The deposition of the Vetas series with its 

 evidence of an abundant flora was followed by the Ramos 

 series with its barren mud-cracked and ripple-marked 

 gypsiferous red beds, which might be interpreted as indicat- 

 ing that the Eastern Range had at that time been elevated 

 sufficiently to have modified the climate west of the divide 

 and brought about the partial extinction of the flora repre- 

 sented in the Vetas series. Following the deposition of the 



