COMPOSITION OF ORES. 543 



that owing to this difference in composition the former is more sokible, 

 but this coiikl only be satisfactorily determined by a practical experiment 

 which should be carried on for a sufficiently long time to imitate in some 

 degree the processes of nature. Before undertaking such an investigation, 

 which would require several years for its proper conduct, it would be advis- 

 able to gather data from various districts to determine whether in point of 

 fact the silicious limestones or dolomites are in general less frequently ore- 

 bearing than those of normal composition. 



From the above considerations it seems that in this district the main 

 cause of concentration of ore in the Blue Limestone has been its physical 

 or structural conditions, and that the influence of its peculiar composition 

 has been at best of minor importance 



COMPOSITION OF ORES. 



As at the time the materials for these investigations were gathered 

 underground explorations had not yet penetrated to depths which wei'e 

 beyond the oxidizing influence of surface waters, a great part of the ores and 

 vein materials collected were necessarily of secondary origin; their compo- 

 sition therefore affords only indirect evidence in regard to the composition 

 and genesis of the original deposits by indicating the agencies and processes 

 by which these alterations were effected. 



Independently of this evidence, however, there exist good a priori 

 grounds for the assumption that the original deposits were in the form of 

 sulphides : first, in the fact that, in ore deposits in general, oxidized ores 

 almost universally give place to sulphides in depth and beyond the reach 

 of surface waters; and, second, in that in the analogous deposits of the 

 adjoining Ten-Mile district oxidized ores similar to those of Leadville are 

 seen to result directly from the alteration of a mixture of galena, pyrite, 

 and zinc blende. 



Carbonate ores. — In the followiug table are given the analyses of three 

 speciuiens of carbonate ore, selected as being especially free from impuri- 

 ties, together with that of an average taken from a thousand specimens of 

 carbonate ore as it is delivered by the mines to the smelters, and containing 

 therefore a considerable admixture of what might be considered gangue. 



