240 THE ANDES OF SOUTHERN PERU 



latitudes. Among geologic agents a broad ocean current of 

 relatively uniform flow would produce the most uniform effects 

 throughout a geologic period, in which many thousand feet of 

 clastic sediments were being accumulated. A powerful ocean cur- 

 rent would also work on flats (in contrast to the gradient re- 

 quired by near-shore processes), and at the same time be of such 

 deep and steady flow as to result in neither ripple marks nor cross- 

 bedding. 



The increasing volume of shallow-water sediments of uniform 

 character near the end of the Silurian, indicates great crustal 

 stability at a level which brought about neither a marked gain 

 nor loss of material to the region. At any rate we have here no 

 Devonian sediments, a characteristic shared by almost all the 

 great sedimentary formations of Peru. At the beginning of the 

 Carboniferous the water deepened, and great heavy-bedded lime- 

 stones appear with only thin shale partings through a vertical dis- 

 tance of several hundreds of feet. The enormous volume of 

 Silurian sediments indicates the deep and prolonged erosion of 

 the land masses then existing, a conclusion further supported (1) 

 by the extensive development of the Silurian throughout Bolivia 

 as well as Peru, (2) by the entire absence of coarse material 

 whether at the top or bottom of the section, and (3) by the very 

 limited extent of older rock now exposed even after repeated and 

 irregular uplift and deep dissection. Indeed, from the latter very 

 striking fact, it may be reasonably argued that in a general way 

 the relief of the country was reduced to sea level at the close of 

 the Silurian. Over the perfected grades of that time there would 

 then be afforded an opportunity for the effective transportation 

 of waste to the extreme limits of the land. 



Further evidence of the great reduction of surface during the 

 Silurian and Devonian is supplied by the extensive development 

 of the Carboniferous strata. Their outcrops are now scattered 

 across the higher portions of the Andean Cordillera and are pre- 

 vailingly calcareous in their upper portions. Upon the eastern 

 border of the Silurian they indicate marine conditions from the 

 opening of the period, but at Pasaje in the Apurimac Valley they 



