248 



THE ANDES OF SOUTHERN PERU 



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|:;.-':;| Alluvium II [ ||| Ihick Conql and Sandstone p^^ Umestanc |§H Volcanic. 



penetrated and tilted by igneous masses, apparently the feeders 

 of the great lava sheets that form the western summit of the 

 Cordillera. From the restricted development of the limestones 

 along a western border zone it might be inferred that they rep- 

 resent a very limited marine in- 

 vasion. It is certainly clear that 

 great deformative movements 

 were in progress from at least 

 late Palaeozoic time since all the 

 Palaeozoic deposits are broken 

 abruptly down in this direction, 

 and, except for such isolated oc- 

 currences as the land Carbonif- 

 erous at Pacasmayo, are not 

 found anywhere in the coastal 

 region today. The Cretaceous 

 is not only limited within a 

 relatively narrow shore zone, 

 but also, like the Palaeozoic, it 

 is broken down toward the west, 

 not reappearing from beneath 

 the Tertiary cover of the desert 

 region or upon the granite-gneisses that form the foundation for 

 all the known sedimentary strata of the immediate coast. 



From these considerations I think we have a strong suggestion 

 of the geologic date assignable to the development of the great 

 fault that is the most strongly marked structural and physio- 

 graphic feature of the west coast of South America. Since the 

 development of this fault is so intimately related to the origin of 

 the Pacific Ocean basin its study is of special importance. The 

 points of chief interest may be summarized as follows : 



(1) The character of the land Carboniferous implies a much 

 greater extent of the land than is now visible. 



(2) The progressive coarsening of the Carboniferous deposits 

 westward and their land derivation, together with the great thick- 

 ness of the series, point to an elevated land mass in process of 



Fig. 167 — Geologic sketch map and 

 cross-section in the Cotahuasi Canyon at 

 Taurisma, above Cotahuasi. The relations 

 of limestone and lava flows in the center 

 of the map and on a spur top near the 

 canyon floor. Thousands of feet of lava 

 extend upward from the flows that cap 

 the limestone. 



