EASTERN ANDES: CORDILLERA VILCAPAMPA 219 



the line of contact which was once a prominent topographic fea- 

 ture. With reference to the effects of climate it may be said sim- 

 ply that a granite core of batholithic origin may extend above the 

 snowline or above timber line or into the timbered belt, whereas 

 the invaded rock may occur largely below these levels with obvi- 

 ous differences in both the rate and the kind of erosion affecting 

 the intruded mass. 



If we apply the foregoing considerations to the Cordillera 

 Vilcapampa, we shall find some striking illustrations of the prin- 

 ciples involved. The invasion of the granite was accompanied by 

 moderate absorption of the displaced rock, and more especially 

 by the marginal pushing aside of the sedimentary rim. The im- 

 mediate effect must have been to give both intruded rock and coun- 

 try rock greater height and marked ruggedness. There followed 

 a period of regional compression and torsion, and the develop- 

 ment of widespread joint systems with strikingly regular features. 

 In the Silurian shales and slates these joints are closely spaced; 

 in the granites they are in many places twenty to thirty feet 

 apart. The shales, therefore, offer many more points of attack 

 and have weathered down into a smooth-contoured topography 

 boldly overlooked along the contact by walls and peaks of granite. 

 In some cases a canyon wall a mile high is developed entirely on 

 two or three joint planes inclined at an angle no greater than 15°. 

 The effect in the granite is to give a marked boldness of relief, 

 nowhere more strikingly exhibited than at Huadquina, below 

 Colpani, where the foot-hill slopes developed en shales and slates 

 suddenly become moderate. The river flows from a steep and all 

 but uninhabited canyon into a broad valley whose slopes are dot- 

 ted with the terraced chacras, or farms, of the mountain Indians. 



The Torontoy granite is also homogeneous while the shales 

 and slates together with their more arenaceous associates occur 

 in alternating belts, a diversity which increases the points of at- 

 tack and the complexity of the forms. Tending toward the same 

 result is the greater hardness of the granite. The tendency of the 

 granite to develop bold forms is accelerated in lofty valleys dis- 

 posed about snow-clad peaks, where glaciers of great size once 



