222 



THE ANDES OF SOUTHERN PERU 





• 









~"'***-'^~~*^—^>^^^^^*. 





7^- 



f& 





*^^^^0*O\>-_ — SHALE 



^TC '-.\^ Z LIMESTONE 



^^O ...-- LIMESTONE 



'/ ^ SHALE 



■— — CONGLOMERATE 



Fig. 147 — Overthrust folds in detail on the 

 southwestern border of the Vilcapampa batho- 

 lith near Chuquibambilla. The section is fifteen 

 feet high. Elevation, 13,100 feet (4,000 m.). 

 For comparison with the structural effects of 

 the Vilcapampa intrusion on the northeast see 

 Fig. 142. 



such as the mica schists of the Cordillera Vilcapampa display. 

 The diversified sedimentary series is folded and faulted on a large 

 scale with broad structural undulations visible for miles along the 

 abrupt valley walls. Here and there, however, the strata become 

 weaker generally through the thinning of the beds and the more 

 rapid alternation of hard and soft layers, and for short distances 



they have absorbed notable 

 amounts of the stresses in- 

 duced by the igneous intru- 

 sions. In such places not 

 only the structure but the 

 composition of the rock 

 shows the effects of the in- 

 trusion. Certain shales in 

 the section are carbonaceous 

 and in all observed cases the 

 organic matter has been 

 transformed to anthracite, a 

 condition generally associated with a certain amount of minute 

 mashing and a cementation of both limestone and sandstone. 



The granite becomes notably darker on approach to the north- 

 eastern contact near Colpani; the proportion of ferro-magnesian 

 minerals in some cases is so large as to give a distinctly black 

 color in sharp contrast to the nearly white granite typical of the 

 central portion of the mass. Large masses of shale foundered in 

 the invading magma, and upon fusion gave rise to huge black 

 masses impregnated with quartz and in places smeared or in- 

 jected with granite magma. Everywhere the granite is marked 

 by numbers of black masses which appear at first sight to be ag- 

 gregations of dark minerals normal to the granite and due to dif- 

 ferentiation processes at the time of crystallization. It is, how- 

 ever, noteworthy that these increase rapidly in number on ap- 

 proach to the contact, until in the last half-mile they appear to 

 grade into the shale inclusions. It may, therefore, be doubted that 

 they are aggregations. From their universal distribution, their 

 uniform character, and their marked increase in numbers on ap- 



