216 



THE ANDES OF SOUTHERN PERU 



EASTERN BORDER RANGES 



Fig. 142 — The deformative effects of 

 the Vilcapampa intrusion on the north- 

 eastern border of the Cordillera. The 

 deformed strata are heavy-bedded sand- 

 stones and shales and the igneous rocks 

 are chiefly granites with bordering porphy- 

 ries. Looking northwest near Puquiura. 

 For conditions near Pampaconas, looking 

 in the opposite direction, see Fig. 141. For 

 conditions on the other side of the Cordil- 

 lera see Fig. 140. 



granite on the one hand and almost unaltered shales and slates on 

 the other. Inclusions or xenoliths of shale are common, however, 

 ten and fifteen miles distant, though they are prominent features 

 in a belt only a few miles wide. The lack of more intense contact 

 effects is a little remarkable in view of the altered character of 



the inclusions, all of which are 

 crystalline in contrast to the fis- 

 sile shales from which they are 

 chiefly derived. Inclusions with- 

 in a few inches of the border 

 fall into a separate class, since 

 they show in general but trifling 

 alteration and preserve their 

 original cleavage planes. It ap- 

 pears that the depth of the in- 

 trusion must have been rela- 

 tively slight or the intrusion sudden, or both shallow and sudden, 

 conditions which produce a narrow zone of metamorphosed ma- 

 terial and a sharp contact. 



The relation between shale and granite at Colpani is shown 

 in Fig. 143. Projections of granite extend several feet into the 

 shale and slate and generally 

 end in blunt barbs or knobs. 

 In a few places there is an in- 

 timate mixture of irregular 

 slivers and blocks of crystal- 

 lized sediments in a granitic 

 groundmass, with sharp lines 

 of demarcation between igneous 

 and included material. The 

 contact is vertical for at least 

 several miles. It is probable 

 that other localities on the con- 

 tact exhibit much greater modification and invasion of the weak 

 shales and slates, but at Colpani the phenomena are both simple 

 and restricted in development. 



Fig. 143 — Relation of granite intru- 

 sion to schist on the northeastern border 

 of the Vilcapampa batholith near the 

 bridge of Colpani, lower end of the granite 

 Canyon of Torontoy. The sections are 

 from 15 to 25 feet high and represent con- 

 ditions at different levels along the well- 

 defined contact. 



