310 THE JOURNAL OF GEOLOGY. 



and various other minerals. The native silver is formed, proba- 

 bly, only after a preceding oxidation of the sulphide. Deposits 

 carrying other unaltered silver-bearing minerals, such as the 

 various silver sulphides, arsenides, antimonides, tellurides, etc., 

 are, when exposed to surface influences, affected in much the 

 same way as the silver in argentiferous galena. 



Alteration of zinc deposits — In the case of zinc, the most com- 

 mon ore is the sulphide known as blende. This mineral, like 

 galena, is generally oxidized on the surface, and forms by other 

 chemical changes the carbonate (smithsonite), the basic car- 

 bonate (hydrozincite), and the basic silicate (calamine), in a 

 manner similar to that described in copper and lead ores. 



In the cases of both lead and zinc, oxidized ores are very 

 desirable for metallurgical purposes, and are much sought after. 

 To be sure, the carbonates, sulphates, etc., of lead and zinc con- 

 tain less of these metals than the pure sulphides, but they occur 

 in a more concentrated form than the sulphides, and, therefore, 

 the ores containing them frequently carry as much or more of 

 the metals than the ores containing the sulphides. Moreover, 

 the oxidized ores are much more easy to treat and, therefore, 

 have an additional value over the sulphide ores. 



Alteration in gold deposits. — In the case of gold deposits, sur- 

 face alteration has a most marked effect, and probably in no class 

 of deposits is the change of more geologic and economic impor- 

 tance. The typical unaltered condition of gold in nature is in 

 association with iron pyrites in quartz, the gold being some- 

 times in such association with the pyrites that it cannot be sep- 

 arated by mechanical means, while in rarer cases, it can be so 

 separated. The effect of surface oxidation on such a deposit, 

 is first to convert the iron pyrites into a hydrated sesquioxide of 

 iron, which premeates the white quartz, with which the pyrites 

 is usually associated, and turns it into a rusty brown mass. The 

 next stage is the gradual leaching out of the hydrous sesquiox- 

 ide by the action of surface waters. The iron is, in this way, 

 finally removed altogether, and the remaining product is a pure 

 white quartz, containing the gold which was originally in the 



