220 THE ANDES OF SOUTHERN PERU 



existed, and where small glaciers still linger. The plucking action 

 of ice has an excellent chance for expression, since the granite 

 may be quarried cleanly without the production of a large amount 

 of spoil which would load the ice and diminish the intensity of its 

 plucking action. 



As a whole the Central Andes passed through a cycle of ero- 

 sion in late Tertiary time which was interrupted by uplift after 

 the general surface had been reduced to a condition of topo- 

 graphic maturity. Upon the granites mature slopes are not de- 

 veloped except under special conditions (1) of elevation as in the 

 small batholith above Chuquibambilla, and (2) where the granite 

 is itself bordered by resistant schists which have upheld the sur- 

 face over a broad transitional belt. Elsewhere the granite is 

 marked by exceedingly rugged forms : deep steep-walled canyons, 

 precipitous cirques, matterhorns, and bold and extended escarp- 

 ments of erosion. In the shale belt the trails run from valley to 

 valley in every direction without special difficulties, but in the 

 granite they follow the rivers closely or cross the axis of the 

 range by carefully selected routes which generally reach the limit 

 of perpetual snow. Added interest attaches to these bold topo- 

 graphic forms because of the ruins now found along the canyon 

 walls, as at Torontoy, or high up on the summit of a precipitous 

 spur, as at Machu Picchu near the bridge of San Miguel. 



The Vilcapampa batholith is bordered on the southwest by a 

 series of ancient schists with which the granite sustains quite dif- 

 ferent relations. No sharp dividing line is visible, the granite 

 extending along the planes of foliation for such long distances as 

 in places to appear almost interbedded with the schists. The re- 

 lation is all the more striking in view of the trifling intrusions 

 effected in the case of the seemingly much weaker shales on the 

 opposite contact. Nor is the metamorphism of the invaded rock 

 limited to simple intrusion. For several miles beyond the zone 

 of intenser effects the schists have been enriched with quartz to 

 such an extent that their original darker color has been changed 

 to light gray or dull white. At a distance they may even appear 

 as homogeneous and light-colored as the granite. At distant 



