RICH DEPOSITS NEAR SURFACE. 1191 



part of a fissure receives abundant lateral waters which have been trans- 

 ported a comparatively short distance under conditions of low pressure and 

 temperature; whei-eas the solutions lower down have taken a longer journey 

 under conditions of high pressure and temperature. If Le Conte's explana- 

 tion be satisfactory, one would expect the most insoluble constituent to be 

 precipitated at the greatest depth. In the case of the lead-zinc-iron deposits 

 this would make the galena abundant at depth, the sphalerite abundant 

 at a higher level, and the iron sulphide the dominating constituent at the 

 highest levels. In the case of the copper-iron deposits the rich sulphides 

 of copper would be in the lower levels and the cupriferous pyrites at the 

 higher levels. 



From the foregoing it is apparent that there has been a general under- 

 standing that a rich upper belt has been produced in many ore deposits. 

 My explanation of a rich upper belt is, mainly, that oxidized soluble com- 

 pounds are produced in the belt of weathering, and that these in situ or 

 lower down react upon the lean sulphides and tellurides. In this way a rich 

 belt is formed. Later, in consequence of denudation, these rich sulphides and 

 tellurides pass into the belt of weathering. Here they are again exposed 

 to the oxidizing forces, are largely transformed in situ to oxides, carbonates, 

 etc., and a belt of rich oxidized products is formed above the ground water. 

 In part, when oxidized, the materials are taken into solution, again trans- 

 ported downward, and again react upon the sulphides and tellurides. In 

 arid regions, where the amount of downward-moving water is small, the 

 oxidized products formed from the rich sulphides and tellurides are likely 

 to remain in large part in situ ; but in humid regions, where water is abundant, 

 the metals, after oxidization, are in large measure carried downward and 

 again react upon the sulphides and tellurides below and further broaden and 

 enrich this belt. Thus, under different climatic conditions, we may have 

 a rich oxidized zone, a rich sulphide and telluride zone, or both, in varying 

 proportion. 



It is notable that the vertical distribution of the sulphides in the rich 

 zone is precisely that which should be expected from the affinity of the 

 metals for sulphur In the lead-zinc-iron districts the galena is found at 

 the highest horizon, lower down is the sphalerite, and still lower the iron 

 sulphide. In the copper-iron lodes the high-grade copper sulphides, 

 chalcocite and bornite, are at higher levels; lower these are intermixed 



