556 



GEOLOGY AND MESTING INDUSTRY OF LEADVILLE. 



excess of carbonic acid. The chloride of silver, on the other hand, might 

 be assumed to have become insoluble in the long time that had elapsed 

 since it was formed ; or, if any were taken up, it would probably be thrown 

 down again in a short distance by a slight change in the character of the 

 descending solutions. Thus a gradual increase in the proportions of silver 

 over lead would be taking place in the zone which was being brought by 

 the erosion of overlying rocks nearer and nearer to the surface.^ 



COMPOSITION OF VEIN MATERIALS. 



In the remaining vein materials, which constitute the relatively value- 

 less portion of the deposits, iron and manganese, generally in the form 

 of hydrated oxides, are the most prevalent metals. Carbonate of iron is 

 very rarely found, and pyrite never among the oxidized ores. The com- 

 parative absence of zinc has already been remarked, yet this metal is quite 

 uniformly detected in the products of smelting. Arsenic and antimony are 



I In a paper read at the Chattanooga meeting of the American Institute of Mining Engineers in 

 May, 1885, by Mr. F. T. Freeland, superintendent of the Iron Silver Mining Company, the following 

 analyses are given of sulphide ore from the Minnie mine, situated on the southeast slope of Iron Hill, 

 adjoining the Colonel Sellers, made by William R. Boggs, jr. They are interesting as showing the rel- 

 ative distribution of silver in the different components of the unoxidized ore: 



His mixture, calculated from the relative amounts of Ph, Zn, and Fe, in column IV, would con- 

 sist of a little less than three parts of galena to one each of pyrite and blende. The tenor of silver 

 given for this mixture is, however, only a little over a quarter of what would result from a mixture in 

 these proportions of these three minerals, each having the tenor in silver given in its separate column. 

 It seems doubtful, therefore, if these separate values can be taken as a fair representation of the .aver- 

 age tenor of thedlfferent sulphides In the ore. That given for zinc blende is evidently abnormally high, 

 as may be seen on comparison with the pyrite column. This pyrite contaius an admixture of about 

 one-fifth of Its weight In zinc blende, and if this zinc blende ran as high in silver as thatglven in column 

 II, this fifth alone would yield nearly 20 ouuces of silver, whereas the whole mass is said to contain only 

 4i ounces. The silver tenor of the galena, on the other hand, represents a fair average of the galenas 

 found in the older mines, though these, as is shown by the assays in Appendix B, vary very much and 

 may contain over ten times as much silver as this. 



