SILVER ORES. 1169 



as in the cerussite."" Essentially the same phenomenon has been observed 

 by Spurr in the Monte Cristo district of Washington. He says that here 

 "the very richest ores in the district are the bunches of sulphides which are 

 residual in the oxidized ores." h Such cases as these may be partly explained 

 by the abstraction of the silver as sulphate from the lead carbonate, but I 

 suspect it to be mainly explained by the reaction of the oxidized silver salts 

 upon the galena, producing a galena richer in silver than originally existed. 

 Where the silver is largely changed to the sulphate and chloride, and 

 is not largely precipitated as cerargyrite, the upper part of the silver veins 

 in the belt of weathering may be greatly depleted in silver as a result of 

 this leaching process. That the silver is not thrown down as cerargyrite 

 may be due to a deficiency of chlorine in the descending solutions, or to the 

 fact that the solutions are of such a character or so abundant that they are 

 capable of dissolving the silver chloride. Illustrations of this process of 

 depletion may probably be found in the Cripple Creek district, where the 

 upper parts of the veins which carry free gold are deficient in silver, while 

 the original telluride contains a certain amount of the latter metal, indi- 

 cating- that it has been leached out. c 



It has already been fully explained (see pp. 1088-1089, 1139-1140) 

 that gold occurs extensively (1) in the native form free from sulphides and 

 tellurides; (2) in the native form closely associated with sulphides and 

 tellurides; and (3) as a telluride. In the belt of weathering gold is very 

 largely found in the metallic form, associated with the oxidized products 

 of the base metals, especially with oxide of iron. Much of such gold was 

 originally associated with the sulphides or the tellurides, or was united 

 with tellurium as a telluride. 



If the gold is in the metallic state, and merely inclosed in sulphides 

 of iron or other sulphides, it is freed by the oxidation of these sulphides. 

 Where it is united with tellurium it is necessary that this element be 



« Emmons, S. F., Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 

 12, 1886, p. 553. 



b Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Eept. TJ. S. 

 Geol. Survey, pt. 2, 1901, p. Sol. 



« Penrose, R. A. F., jr., Mining geology of the Cripple Creek district, Colo.: Sixteenth Ann. Kept. 

 U. S. Geol. Survey, pt. 2, 1895, pp. 131-132. 

 MON XLVII — 0i 74 



