234 THE ANDES OF SOUTHERN PERU 



strata, upon small granite intrusives and upon old and greatly- 

 altered volcanic rock. 



The Cordillera Vilcapampa has an axis of granitic rock which 

 was thrust upward through schists that now border it on the west 

 and slates that now border it on the east. The slate series forms 

 a broad belt which terminates near the eastern border of the 

 Andes, where the mountains break down abruptly to the river 

 plains of the Amazon Basin. The immediate border on the east 

 is formed of vertical Carboniferous limestones. The narrow foot- 

 hill belt is composed of Tertiary sandstones that grade into loose 

 sands and conglomerates. The inclined Tertiary strata were lev- 

 eled by erosion and in part overlain by coarse and now dissected 

 river gravels, probably of Pleistocene age. Well east of the main 

 border are low ranges that have never been described. They 

 could not be reached by the present expedition on account of lack 

 of time. On the extreme western border of that portion of the 

 Peruvian Andes herein described, there is a second distinct border 

 chain, the Coast Range. It is composed of granite and once had 

 considerable relief, but erosion has reduced its former bold forms 

 to gentle slopes and graded profiles. 



The continued and extreme growth of the Andes in later geo- 

 logic periods has greatly favored structural and physiographic 

 studies. Successive uplifts have raised earlier deposits once 

 buried on the mountain flanks and erosion has opened canyons on 

 whose walls and floors are the clearly exposed records of the past. 

 In addition there have been igneous intrusions of great extent 

 that have thrust aside and upturned the invaded strata exposing 

 still further the internal structures of the mountains. From sec- 

 tions thus revealed it is possible to outline the chief events in the 

 history of the Peruvian Andes, though the outline is still neces- 

 sarily broad and general because based on rapid reconnaissance. 

 However, it shows clearly that the landscape of the present repre- 

 sents but a temporary stage in the evolution of a great mountain 

 belt. At the dawn of geologic history there were chains of moun- 

 tains where the Andes now stand. They were swept away and 

 even their roots deeply submerged under invading seas. Re- 



