78 STUDIES IN GEOLOGY, No. i 



capable of blanching red rock by one or the other process, 

 so that the mere fact of blanching tells little concerning 

 the origin or the character of the mineralizers. The 

 occurrence of the ore as impregnations of the country rock 

 is an element of form rather than of genesis and may mean 

 only that the mineralizing solution encountered porous 

 strata rather than open fissures as channels of circulation. 



Wholly lacking in the Corocoro deposits are Lane's 

 features 3 and 6. Yet the theory that he advanced to 

 explain the native copper deposits is based more essentially 

 on feature 3 than on all the other of the six characteristics 

 he enumerates, and feature 6 is consequent on 3. His cor- 

 roborative geologic evidence is all based on the facts of 

 Lake Superior geology and not on those of Corocoro. 

 Hence irrespective of the validity of his conclusions, they 

 apply specifically to the Lake Superior deposits; and 

 because of the departure of the Corocoro deposits from 

 them, in the essential features 3 and 6, those conclusions 

 are not applicable to the Bolivian district. 



The explanation of the Lake Superior copper deposits 

 offered by Van Hise, Leith, and Steidtmann * is also based 

 on a direct relationship between the mineralization and the 

 basic igenous rocks that constitute a large part of the 

 copper-bearing series of rocks. Hence, their arguments 

 apply only remotely, if at all, to the Corocoro region. The 

 same is true of other theories that have been advanced to 

 explain the Lake Superior copper deposits, their funda- 

 mental bases are geological conditions that do not obtain 

 at Corocoro. It would not aid our present problem to 

 present these various theories for critical examination ; 

 because, whether acceptable for the American deposits or 

 not, a theory acceptable for the Corocoro deposits must 

 conform to and be based on different geologic relations. 



* Van Hise and Leith : The Geology of the Lake Superior Region, 

 Mon. LII, 1911, U. S. Geol. Survey. 



