PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 247 



the region now occupied by the Andes. However, from the rela- 

 tion of the Carboniferous to the basal schists, and the most con- 

 servative extension of the known Carboniferous, it may be in- 

 ferred that the Carboniferous sea did not completely cover the 

 entire area but was broken here and there by island masses in the 

 form of an elongated archipelago. The presence of land plants 

 in the Carboniferous of Pisco warrants the conclusion that a sec- 

 ond island mass, possibly an island chain parallel to the first, ex- 

 tended along and west of the present shore. 



CRETACEOUS 



The Cretaceous formations are of very limited extent in the 

 belt of country under consideration, in spite of their generally 

 wide distribution in Peru. They 

 are exposed distinctly only on 

 the western border of the Cor- 

 dillera and in special relations. 

 In the gorge of Cotahuasi, over 

 seven thousand feet deep, about 

 two thousand feet of Cretaceous 

 limestones are exposed. The 

 series includes only a very re- 

 sistant blue limestone and ter- 

 minates abruptly along a well- 

 marked and highly irregular 

 erosion surface covered by al- 

 most a mile of volcanic ma- 

 terial, chiefly lava flows. The 

 character of the bottom of the 

 section is likewise unknown, 

 since it lies apparently far be- 

 low the present level of ero- 





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ALLUVIUM 



Fig. 166 — Geologic sketch map and 

 cross-section in the Cotahuasi Canyon at 

 Cotahuasi. With a slight gap this figure 

 continues Fig. 167 to the left. The sec- 

 tion represents a spur of the irain plateau 

 about 1,500 feet high in the center of the 

 map. 



sion. 



The Cretaceous limestones of the Cotahuasi Canyon are every- 

 where greatly and irregularly disturbed. Typical conditions are 

 represented in the maps and sections, Figs. 166 and 167. They are 



