﻿I860.] 



FORBES BOLIVIA AND PERU. 



45 



It would appear very probable that this had been formed in situ 

 by the action of arsenical vapours on the metallic copper dispersed 

 in the bed. That such a result can be thus produced may easily be 

 experimentally demonstrated by holding a small piece of Corocoro 

 copper-ore over a heated crucible containing arsenic. 



Only a certain portion of the beds appear to have been so affected ; 

 and the spotted portion in fig. 2, p. 41, is supposed to represent 

 this line of arsenical and other impregnation, from which it will be 

 seen that the Veta del Buen Pastor and Yeta de Rejo are altered 

 from the surface ; but the main bed, or Yeta Umacoia, is, I believe, 

 not affected at the surface ; but it is so deeper down, since we find 

 that the native copper from the Mina Cimbani contains arsenic, 

 as seen from an analysis which is given a little further on : and I 

 am informed that silver has been met with in depth in the Mina 

 de Quimse Cruz ; possibly this impregnation of arsenic, silver, &c. 

 might (or in greater depth be found to) present itself as a vein of 

 these metals. 



The metallic copper of Corocoro is not only found as small grains 

 in the sandstones, but also in nodules, irregular lumps, and plates 

 or sheets interposed between the beds of sandstone, occasionally 

 assuming crystalline and beautiful dendritic forms. In the Socabon 

 de la Paz, on the Yeta Umacoia (main seam), pseudomorphic cry- 

 stals of native copper are found as hexagonal prisms without ter- 

 minal planes ; an analysis of one of these by Mr. Kroeber is an- 



nexed : 



Copper 98-605 



Silica 0-015 



Silver (trace) 



Iron (as lost) 1-376 



Metallic matter (insoluble in 



N0 5 HC1) 0-004 



100-000 



Some are solid ; but others, when sawed through, exposed a nu- 

 cleus of carbonate of lime, which would lead to the inference that 

 these pseudomorphs had been formed by the action of a solution of 

 copper on crystals of carbonate of lime, and by some subsequent 

 chemical change the carbonate of copper so formed had been reduced 

 to the metallic state. 



As is well known, the Permian Kupfer-Schiefer, or cupriferous 

 bituminous shales of Thuringia, are characterized by the occurrence 

 of the rare metal vanadium entering into their chemical composi- 

 tion ; it appeared to me, therefore, of considerable interest to know 

 whether this also was the case here in Corocoro, where strata oc- 

 curred of very different mineral character, but supposed to represent 

 the same geological epoch. Not having a laboratory at my command, 

 Mr. Kroeber kindly undertook the examination ; and his analysis of 

 the washed copper-ore from the Mina de Cimbani, previously men- 

 tioned, afforded the following residts : — 



ft 



