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GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



somewhat like a dry clay, and easily recognized by its external appearance, 

 though, as will be seen, of very variable composition: 



Basic sulphates. 



These substances are somewhat complex basic sulphates, and might 

 be considered to be a mixture of jarosite with varying proportions of basic 

 ferric sulphate. They are evidently an alteration product of pyrite and 

 galena, although, while nodules of galena rich in silver are occasionally 

 found in them, pyrite has not yet been detected. The absence of zinc in 

 the specimens analyzed is noteworthy, and is in accordance with the obser- 

 vation already made, that it has been further removed from the original 

 ore bodies than the other metals, presumably on account of the ready 

 solubility of its sulphate. The persistent percentage of the alkalies, which 

 were found in sensibly the same proportions in three other specimens tested, 

 would suggest that the waters which produced this alteration reached the 

 ore bodies after passing through decomposed porphyry. Their chief in- 

 terest lies in the definite evidence they afford that they result from the 

 oxidation of sulphides. Similar products have frequently been observed in 

 old mine openings where large bodies of pyrite have been long leached by 

 surface waters. Copperas first formed gradually loses a portion of its 

 water on exposure to the air, and the protoxide of iron becomes sesquioxide. 

 Further exposure leads to the formation of limonite. 



Processes of alteration. Assuming that the metals in the original deposits 

 existed in the form of sulphides, it comes next in order to consider the 

 possible processes by which the combinations noted above may have been 

 derived from them. 



By oxidation all the metallic sulphides may be transformed into sul- 

 phates, 1 and the relative solubility of the latter is in inverse proportion to the 



1 Percy's Metallurgy of Silver aud Gold, Part I. London, 1880. 



