PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 237 



be referred to the early Palaeozoic, while some of them may date 

 from the Proteriozoic. 



The Silurian beds are composed of shale, sandstone, shaly 

 sandstone, limestone, and slate with some slaty schist, among 

 which the shales are predominent and the limestones least impor- 

 tant. Near their contact with the granite the slate series is com- 

 posed of alternating beds of sandstone and shale arranged in 

 beds from one to three feet thick. At Santa Ana they become 



E3 Granite 



Schist 



SlSandstone 



Basalt 



Porphyry 



Fig. 158 — Geologic sketch map of the lower Urubamba Valley. A single traverse 

 was made along the valley, hence the boundaries are not accurate in detail. They were 

 sketched in along a few lateral traverses and also inferred from the topography. 

 The country rock is schist and the granite intruded in it is an arm of the main 

 granite mass that constitutes the axis of the Cordillera Vilcapampa. The structure 

 and to some degree the extent of the sandstone on the left are represented in Figs. 

 141 and 142. 



more fissile and slaty in character and in several places are quar- 

 ried and used for roofing. At Eosalina they consist of almost 

 uniform beds of shale so soft and so minutely and thoroughly 

 jointed as to weather easily. Under prolonged erosion they have, 

 therefore, given rise to a well-rounded and soft-featured land- 

 scape. Farther down the Urubamba Valley they again take on 

 the character of alternating beds of sandstone and shale from a 

 few feet to fifteen and more feet thick. In places the metamor- 

 phism of the series has been carried further — the shales have be- 

 come slates and the sandstones have been altered to extremely re- 

 sistant quartzites. The result is again clearly shown in the topog- 

 raphy of the valley wall which becomes bold, inclosing the river 



