COROCORO COPPER DISTRICT OF BOLIVIA 35 



accretion and forced apart the shales along the bedding 

 planes and greatly disturbed their regular bedding. This 

 effect is doubtless confined largely to the proximity of the 

 surface and the underlying structure of the hills is in all 

 probability essentially like that of their continuation along 

 the strike to the south. 



Cupriferous beds extend through 1700 m. of the section 

 nearest the Corocoro fault but differ from those of the 

 Vetas formation in that none of them are plant-bearing nor 

 are they confined to the sandy layers. In the measured 

 section are 14 cupriferous horizons in shale, 4 in sandstone, 

 and 2 in gypsum. 



AGE OF THE VETAS AND RAMOS 



The age of the Vetas and Ramos beds of the Corocoro 

 district has been placed in periods ranging from Carboni- 

 ferous to Tertiary. Prior to 1915 these determinations 

 were little more than guesses based on insecure lithologic 

 correlations. 



D'Orbigny (Translation p. 10) states that since he did 

 not find any fossils in either series of rocks he is compelled 

 from their general relationships to include them provisionally 

 in the Carboniferous beds. 



Forbes (pp. 37, 38) comments on the absence of satis- 

 factory fossil evidence for determining the age of the strata, 

 but believes the balance of evidence in favor of the Permian 

 epoch. In this conclusion he is guided largely by their 

 lithologic similarity to the Permian rocks of Russia. He 

 was aware of the presence of plant remains but did not 

 utilize them in his correlation. He says: 



"Fossil plants are everywhere found in this formation ; but gen- 

 erally they are very indistinct. In some places, as at Pontezuelo, 

 large trunks of trees silicified are found in abundance." 



Sections of two specimens of carbonized wood were too 

 indistinct to permit of closer identification than that they 

 were coniferous. The occurrence of carbonized fossil wood 



