8o STUDIES IN GEOLOGY, No. i 



is true that many of the beds in the vetas contain carbonized 

 plant remains, but they are not coextensive with the cupri- 

 ferous beds of the Vetas and this material is lacking in the 

 Ramos. Hence it can not be called upon as the precipitat- 

 ing agent. On the other hand, the solutions reduced ferric 

 oxide or dissolved it wherever they deposited copper. Con- 

 sequently ore deposition took place in the presence of 

 ferric oxide and probably the mineralizing solutions were 

 being oxidized by it. The balance between the deposition 

 of native copper and copper sulphide seems to have been 

 delicate as both were deposited in large quantity. Just 

 what was the chemical character of the mineralizing 

 solutions and just what were the reactions that caused 

 the precipitation of the native copper throughout most of 

 the cupriferous beds are questions that, in the light of 

 present knowledge, can only be speculated on but not con- 

 vincingly or unequivocally answered. Steinmann's theory 

 of the oxidation of sulphides in those solutions by the 

 ferric oxide and the reaction of the resulting sulphuric 

 acid with the alkaline earths of the impregnated beds, leaves 

 the copper in a state and under conditions favorable to the 

 deposition of native copper. It is the most plausible of the 

 theories that have been reviewed. 



The source of the mineralizing solutions may be ascribed 

 with reasonable certainty to an underlying dioritic magma 

 of which the Comanche rock is an offshoot. Evidence of 

 igneous activity during the period of the geologic history 

 of the Corocoro district with which we have had to deal 

 was presented in the account of the geology of the district. 

 The period of mineralization coincided with the period of 

 consolidation of that magma and the mineralizing solutions 

 doubtless bore the usual relations to it which are so 

 generally recognized in the case of epigenetic deposits 

 associated with igneous rocks. 



