BOLIVIAN COPPER DEPOSITS 
377 
one is almost forced to the conclusion that the association with 
red beds is a fortuitous coincidence rather than a significant gene¬ 
tic factor. The blanching of the sandstones indicates that the 
mineralizing solutions were capable either of reducing the ferric 
oxide to which they owe their red color to the ferrous state or of 
dissolving and removing it. No studies of the chemical composi¬ 
tion of the bleached and unbleached rock have been carried on to 
determine which has happened. A priori^ one might expect most 
primary ascending mineralizing waters to be 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. 
The Corocoro deposits are unusual or anomolous primarily in 
the occurrence of native copper. But mining developments of 
recent years have called attention to the large quantities of sulphi- 
dic ores and the gradation of the one type of ore into the other, 
that is, have made less marked the line of separation between the 
native metal ores and the more common type of copper ores. 
Experimental chemical work has also demonstrated the ease with 
which copper may be precipitated from its solutions in the metallic 
state. Stokes showed that ferrous sulphate will precipitate cop¬ 
per from a solution of copper sulphate and Fernekes that fer¬ 
rous chloride acts in the same way on copper chloride solutions 
provided the hydrochloric acid is constantly neutralized. These 
particular reaction^ hardly apply to the genesis of the Corocoro 
native copper, because ore deposition seems to have taken place 
under conditions that produced a concomitant reduction of ferric 
oxide in the rocks, but they do show the readiness with which 
native copper can be precipitated. 
Despite many uncertain features and the lack of detailed and 
exact data, the geologic relations and the mode of occurrence of 
the Corocoro deposits are now sufficiently well established to rule 
out all syngenetic theories of their origin. They are due to the 
impregnation of porous strata along and adjacent to the Corocoro 
fault by ascending cupriferous solutions. If one could postulate 
reducing conditions within those strata, the chemistry of the native 
