PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 235 



Fig. 157 — Outline sketch showing the 

 principal rock belts of Peru along the 

 seventy -third meridian. They are: 1, 

 Pleistocene and Recent gravels and sands, 

 the former partly indurated and slightly 

 deformed, with the degree of deformation 

 increasing toward the mountain border 

 (south). 2, Tertiary sandstones, inclined 

 from 15° to 30° toward the north and 

 un conformably overlain by Pleistocene 

 gravels. 3, fossil-bearing Carboniferous 

 limestones with vertical dip. 4, non-fos- 

 siliferous slates, shales, and slaty schists 

 (Silurian) with great variation in degree 

 of induration and in type of structure. 

 South of the parallel of 13° is a belt of 

 Carboniferous limestones and sandstones 

 bordering (5) the granite axis of the 

 Cordillera Vilcapampa. For its structural 

 relations to the Cordillera see Figs. 141 and 

 142. 6, old and greatly disturbed volcanic 

 agglomerates, tuffs and porphyries, and 

 quartzitic schists and granite-gneiss. 7, 

 principally Carboniferous limestones north 

 of the axis of the Central Ranges and 

 Cretaceous limestones south of it. Local 

 granite batholiths in the axis of the Central 

 Ranges. 8, quartzites and slates predomi- 

 nating with thin limestones locally. South 

 of 8 is a belt of shale, sandstone, and lime- 

 stone with a basement quartzite appearing 

 on the valley floors. 9, a portion of the 

 great volcanic field of the Central Andes 

 and characteristically developed in the 

 Western or Maritime Cordillera, through- 

 out northern Chile, western Bolivia, and 

 Peru. At Cotahuasi (see also Fig. 20) 

 Cretaceous limestones appear beneath the 

 lavas. 10, Tertiary sandstones of the 

 coastal desert with a basement of old vol- 

 canics and quartzites appearing on the 

 valley walls. The valley floor is aggraded 

 with Pleistocene and Recent alluvium. 11, 

 granite-gneiss of the Coast Range. 12, late 

 Tertiary or Pleistocene sands and gravels 

 deposited on broad coastal terraces. For 

 rock structure and character see the other 

 figures in this chapter. For a brief desig- 

 nation of index fossils and related forms 

 see Appendix B. For the names of the 

 drainage lines and the locations of the 

 principal towns see Figs. 20 and 204. 



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