206 I. Bowman — Physiography of the Central Andes. 



of variable degrees of induration, lying upon light-colored fel- 

 site. The unconformity is singularly regular and shows on 

 the one hand a smooth surface gradually descending beneath 

 the piedmont, while on the other it ascends with equally regu- 

 lar slope to the summit of the plateau fifteen to twenty 

 miles to the east. The line of unconformity as it appears at 



Fig. 



Fig. 6. Erosion surface between piedmont and underlying igneous rocks 

 east of Pica, Chile. The surface may be distinguished by differences of 

 shade in right background. 



10,100 ft. is represented in fig. 6, where the upper lighter pied- 

 mont deposit is shown resting upon the felsite. The ascent of 

 the slope leading to the summit of the plateau discloses a sur- 

 face thinly veneered with slabs and Hat stones of great dimen- 

 sional heterogeneity. This fragmental material is clearly the 

 result of the extremes of night and clay temperatures, for, in 

 the practical absence of vegetation and at these altitudes 8,500 

 to 15,000 ft, above the sea, solar radiation, especially during 

 summer, heats the rock to an incredibly high temperature, 

 while at night the rare atmosphere favors equally pro- 

 nounced terrestrial radiation under an unclouded sky. The 

 consequence is a continual rock peeling, a process only obscured 

 when the action has continued to the point when a protective 



