BOLIVIAN COPPER DEPOSITS 
373 
more permeable sandstones, 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 carbonate 
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 miner¬ 
alized 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 “com¬ 
plete lack of fossils” in the Corocoro strata; he can hardly con¬ 
sistently 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 Corocoro 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 facies of the sediments. They frequently show a vein¬ 
like occurrence, and though this is not pronounced, their distribu¬ 
tion 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 in¬ 
herent in the nature of the mineralizing solutions and in part 
inherent in the rocks which they invaded. In support of the first 
conclusion he cites certain characteristics of the copper veins that 
are widely distributed along the west slope of the western Andes 
from central Chile to Peru. Most important is the small amount 
of the common gangue minerals which they contain, from which 
fact he concludes that the metalliferous solutions to which they 
8 Rivista Minera, 32 pp., Oruro, 1916. 
