EASTERN ANDES: CORDILLERA VILCAPAMPA 217 



The highly mineralized character of the bordering sedimentary 

 strata, and the presence of numbers of complementary dikes, 

 nearly identical in character to those in the parent granite now 

 exposed by erosion over a broad belt roughly parallel to the con- 

 tact, supplies a basis for the inference that the granite may under- 

 lie the former at a slight depth, or may have had far greater meta- 

 morphic effects upon its sedimentary roof than the intruded 

 granite has had upon its sedimentary rim. 



The physiographic features of the contact belt are of special 

 interest. No available physiographic interpretation of the topog- 

 raphy of a batholith includes a discussion of those topographic 

 and drainage features that are related to the lithologic character 

 of the intruded rock, the manner of its intrusion, or the depth of 

 erosion since intrusion. Yet each one of these factors has a dis- 

 tinct topographic effect. We shall, therefore, turn aside for a 

 moment from the detailed discussion of the Vilcapampa region 

 to an examination of several physiographic principles and then 

 return to the main theme for applications. 



It is recognized that igneous intrusions are of many varieties 

 and that even batholithic invasions may take place in rather 

 widely different ways. Highly heated magmas deeply buried be- 

 neath the earth's surface produce maximum contact effects, those 

 nearer the surface may force the strata apart without extreme 

 lithologic alterations of the displaced beds, while through the 

 stoping process a sedimentary cover may be largely absorbed and 

 the magmas may even break forth at the surface as in ordinary 

 vulcanism. If the sedimentary beds have great vertical variation 

 in resistance, in attitude, and in composition, there may be af- 

 forded an opportunity for the display of quite different effects 

 at different levels along a given contact, so that a great variety 

 of physical conditions will be passed by the descending levels of 

 erosion. At one place erosion may have exposed only the summit 

 of the batholith, at another the associated dikes and sheets and 

 ramifying branches may be exposed as in the zone of fracture, at 

 a third point the original zone of flowage may be reached with 

 characteristic marginal schistosity, while at still greater depths 



