564 ' GEOLOGY AND MINING 1NDUSTEY OF LEADVILLE. 



the sulphate have been immediately reduced to sulphide again by organic 

 matter or other reducing agency. 1 



That the direct replacement of the dolomite by sulphides is not impos- 

 sible, however, seems proved by the fact that galena, zinc blende, and 

 pyrite are found in nature as pseudomorphs after calc spar, 2 and the last two, 

 also, as pseudomorphs after dolomite. 3 The sulphates of the metals are more 

 or less soluble in water, especially when it contains some free sulphuric 

 acid. Their reduction to sulphide through the agency of organic matter 

 is a matter of common observation. 



The reactions by which the Leadville deposits might have been made 

 from solutions carrying the metals as sulphates are more readily conceiv- 

 able. In contact with dolomite containing organic matter the sulphates 

 would be reduced to sulphides with the formation of carbonic acid. The 

 waters thus charged with an excess of carbonic acid would dissolve and 

 remove the carbonates of lime and magnesia, which would be replaced by 

 the metallic sulphides. Any excess of sulphuric acid would form soluble 

 sulphates of lime and magnesia, which would also be carried away. If 

 these sulphates were reduced to sulphides they would render the waters 

 more capable of dissolving out the dolomite. 



The metals might have been taken up in the form of sulphates by waters 

 percolating through rocks, where they might have been brought into this 

 combination by the oxidation of sulphides or by the decomposition of sili- 

 cates. It might also be conceived that during their passage these sulphates 

 would be reduced to sulphides by contact with organic matter before they 

 reached the locus of the deposit. 



The most important objection to the hypothesis that the metals were 

 brought in as sulphates is that the lead sulphate is so insoluble compared 

 with that of iron or zinc; and yet the amount of galena in the ores was 

 probably greater than that of zinc blende. 



Sulphide of barium would be precipitated as sulphate of baryta in con- 

 tact with the limestones, owing to its relatively greater insolubility than the 

 sulphates of lime and magnesia. 



1 J. Roth, Allg. Geologic, p. 235. S J. Eoth, op. cit., p. 171. 3 J. Hoth, op. cit., p. 184. 



