70 STUDIES IN GEOLOGY, No. i 



genetic, but its reduction to the metallic state he ascribed 

 to sulphurous fumes emitted at the time of intrusion of the 

 dioritic rocks. Forbes says the problem would have been 

 easier if the deposits could have been shown to have had 

 their cupriferous contents injected into them at the time 

 of the dioritic intrusions as in the case of the copper veins 

 of Bolivia, Peru, and Chile; but he believed the facts in 

 hand point to the copper as originally present in the sedi- 

 mentary beds, probably not as metallic copper, but in a 

 state of combination, and subsequently reduced to the 

 metallic state. What facts he alludes to he does not specify, 

 and his own description of the occurrence is at variance 

 with such an interpretation. He recognizes clearly that 

 the copper content is confined to the bleached parts of the 

 strata, and he ascribes the bleaching to the magmatic 

 exhalations. Hence the more natural assumption would 

 seem to be that they also introduced the metal. The dis- 

 coloration of the mineralized rock he concluded was 

 "caused by the evolution of sulphurous fumes, disengaged, 

 and penetrating into the pores of the strata, at the time 

 of the eruption of the dioritic rocks of Comanche and the 

 Cerro de las Esmeraldas, situated respectively to the north 

 and south of the metalliferous district of Corocoro;" and 

 the prolusion of these rocks through the Corocoro strata 

 he thought caused the fault and the accompanying disloca- 

 tions of the strata. More specifically he considered the 

 ore bodies to have been calcareous sandstones impregnated 

 with copper oxide or carbonate. The sulphurous fumes 

 reduced the copper to the metallic form and were them- 

 selves thereby oxidized to sulphuric acid. The latter react- 

 ing with the calcium carbonate produced the gypsum which 

 so commonly accompanies these deposits. But it might be 

 added in comment on this suggestion that gypsum is not 

 confined to the mineralized parts of the Corocoro rocks but 

 is quite widespread and abundant in its occurrence beyond 

 the limits of copper mineralization. 



Mossbach thinks that the association of native copper 



