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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



the conclusion that this change has been 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 Co- 

 manche and the Cerro de las Esmeraldas, situated respectively to the 

 north and south of the metalliferous district of Corocoro, and the 

 protrusion of which through these Permian beds I consider as 

 having caused the fault itself and the accompanying dislocations of 

 the strata. 



The sandstone I suppose to have been, previously to this disturb- 

 ance, calcareous, and more especially so in the cupriferous parts, in 

 which I regard the copper as having been present in the state of 

 oxide or carbonate associated with carbonate of lime. Sulphurous 

 acid, by combining with the oxygen of the oxide of copper to form 

 sulphuric acid, would reduce the copper to the metallic state, whilst 

 at the same time the sulphuric acid thus formed, acting upon the 

 carbonate of lime, would produce the sulphate of lime (or gypsum) 

 invariably accompanying these deposits. 



It would have much simplified our ideas as to the geological age 

 and origin of the occurrence of copper in South America if these 

 deposits could have been shown to have had their cupriferous con- 

 tents injected into them at the time of this dioritic eruption, which, 

 as previously has been stated, is the direct cause of all the copper- 

 veins which I had previously met with in Peru, Chile, and Bolivia. 

 The question deserves further investigation ; but the facts in hand 

 appear contrary to this view, and to point out the copper as originally 

 present in these sedimentary beds, probably, not as metallic copper, 

 but in a state of combination, and subsequently reduced to the 

 metallic state as before explained, — in corroboration of which it may 

 be mentioned that these dioritic rocks can be everywhere proved to 

 have been accompanied by a great evolution of sidphurons acid and 

 other gases, by which the rocks in immediate contact have very 

 generally been greatly metamorphosed. The supposition that the 

 sandstones were calcareous is only in accordance with the frequency 

 of calcareous beds met with in the unaltered parts of this formation. 



The eruption of these dioritic rocks may, however, have possibly 

 been the cause of our finding certain beds (or rather portions of 

 beds), to the west of the fault, containing metallic silver, and im- 

 pregnated with arsenic, sulphur, &c, by which arsenides, sulphides, 

 &c. of these metals have been formed as domeykite, condurrite, 

 copper-glance, <fec. 



One of these compounds, occurring in the Veta del Pmen Pastor 

 (previously mentioned), in the form of grey metallic grains dissemi- 

 nated in the sandstone in a similar manner to the usual occurrence 

 of native copper before described, was analysed by me and found to 

 be domeykite, the analysis affording — 



Copper 



Silver 



Arsenic 



71-13 

 0-46 

 28-41 



100-00 



