COROCORO COPPER DISTRICT OF BOLIVIA 55 



tinuous series of outcrops which extend from Peru to Puerto 

 Montt in southern Chile. The eastern belt enters south 

 central Bolivia from the desert of Atacama, shows itself at 

 the Cerro de las Esmeraldas south of Corocoro, at the hill 

 of Comanche to the north of that place, and at the south 

 end of Lake Titicaca near Tiahuanaco. The rock at 

 Comanche he describes as intrusive in the red sandstone. 



Steinmann (pp. 362-367) considers the line of dioritic 

 intrusions which includes Comanche genetically related to 

 the porphyries of the eastern range of the Bolivian Andes, 

 and places the age of all as approximately the same as that 

 of the Potosi plant-bearing tuffs which are now definitely 

 determined as Pliocene * though regarded by Steinmann as 

 Miocene or early Pliocene. 



The inclusion of fragments of rock identical in character 

 with the Mirikiri rock and various varieties of fresh volcanic 

 rocks in the Vetas series proves that igneous activity had 

 commenced in the region prior to the deposition of the oldest 

 exposed Corocoro sedimentaries and that in part at least 

 the fragments were derived from rocks originating from 

 the same magma as the Mirikiri rock. On the other hand, 

 the Mirikiri rock is regarded as intrusive into the red rocks 

 of the Corocoro region. Hence the general period of 

 igneous activity is more or less coincident with that of the 

 deposition of these rocks, although it may have had its 

 inception prior to the beginning of the sedimentation. This 

 conclusion is in harmony with Steinmann's correlation of 

 the igneous activity of the Corocoro region with that of the 

 eastern Andes, that is, in late Tertiary time. 



RESUME OF GEOLOGIC HISTORY 



The latest general submergence of this region was in 

 Upper Cretaceous time as is shown by the presence of 



* Edward W. Berry : Fossil Plants from Bolivia and their bearing 

 upon the age of the uplift of the Eastern Andes. Proc. U. S. 

 Nat. Museum, Vol. 54, 1917, pp. 103-164. 



