COROCORO COPPER DISTRICT OF BOLIVIA 73 



some of those beds, preferably the more permeable sand- 

 stones, and where they found conditions favorable deposited 

 the copper. He next raises the question : "What were these 

 favorable conditions?" His answer is quite unsatisfactory. 

 He again calls attention to the replacement of calcium 

 carbonate by native copper in the aragonite twins. The 

 action of cupriferous sulphate solutions on calcium car- 

 bonate would explain the formation of calcium sulphate 

 but not that of copper instead of copper carbonate. The 

 latter is explained by assuming the calcium carbonate 

 derived from marine shells. Then the putrefying organic 

 matter contained by the shells would be available as a 

 reducing agent to reduce the copper to the metallic state 

 and the ferric iron of the strata to the ferrous state. The 

 carbonate of iron that would be formed in this way being 

 soluble in carbonated waters would be carried away leaving 

 the mineralized sandstones bleached and colorless. We 

 may suppose further, he says, that the reductive power of 

 the organic materials had not been sufficient to reduce the 

 ferric oxide until it had been aided by the carbonic acid 

 liberated in the reduction of the copper. In this same 

 paper Sundt comments on the "complete lack of fossils" 

 in the Corocoro strata ; he can hardly consistently advance 

 a theory of ore deposition based on the presence of marine 

 shells and putrefying organic matter within those beds. 



A still fuller consideration of the genesis of the Coro- 

 coro ores has been presented by Steinmann. To him the 

 epigenetic nature of the deposits is beyond question. The 

 ores are not restricted to one or more definite horizons, nor 

 are they restricted to any particular fades of the sediments. 

 They frequently show a veinlike occurrence, and though 

 this is not pronounced, their distribution in the beds is quite 

 irregular. Accepting then an epigenetic origin, he ascribes 

 the peculiarities of the occurrence to peculiar conditions 

 accompanying ore deposition. These were in part inherent 

 in the nature of the mineralizing solutions and in part 

 inherent in the rocks which they invaded. In support of 



