1186 A TREATISE ON METAMORPHISM. 



although Hoover thinks that in this strange country the downward concen- 

 tration is more mechanical than chemical. Thus the secondary concentra- 

 tion by descending waters is no less an important part of the genesis of the 

 gold ores of Australia than the first concentration by ascending waters. 



It has already been fully explained (p. 1144 et seq.) that the lead and 

 zinc deposits of the Mississippi Valley furnish clear cases of the importance 

 of the action of descending waters in enriching ores first concentrated by 

 ascending waters. 



The Leadville deposits furnish an instance of the decrease of the rich- 

 ness in silver with depth. Emmons says: "There is a fair foundation for 

 the generalization that in the deposits, as developed at the time of this 

 investigation, the ores were growing poorer in silver as exploration extended 

 farther from the surface."" 



The ores early mined contained scarcely any zinc. Emmons states of 

 the ores of the deeper levels later mined that the "sulphide ores consisted 

 of mixtures of pyrite, galena, and zinc blende, the latter in fairly equal 

 amounts." The sulphide bodies immediately below the belt of oxidation 

 are richer in zinc blende than the ore at greater depth. The material rich 

 in zinc below correlates with the ores low in zinc near the surface, the zinc 

 sulphide of that horizon having been oxidized, and the zinc carried down 

 and redeposited. 6 



Another case of the diminution of the richness of sulphurets with 

 depth is furnished by the nickel mines of Lancaster Gap, Pennsylvania, 

 which were not worked beyond a depth of about 75 meters, presumably 

 because "the ore decreased in richness as depth was attained." 



The San Juan district of Colorado gives excellent illustrations of 

 decrease of values with depth. The upper parts of the lodes were usually 

 very rich in silver, not infrequently ores running 300 ounces or more per 

 ton. In the deeper workings of the veins the values in silver usually ran 

 down, in many cases very rapidly, so that, at a depth of a few hundred 

 meters the amount of silver became so low that work upon many of the 



o Emmons, S. F., The geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, 

 vol. 12, 1886, pp. 554-555. 



6 Emmons, S. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst. 

 Min. Eng., 2d ed., 1902, pp. 439-440. 



<"Kemp, J. F., The nickel mine at Lancaster Gap, Pennsylvania: Trans. Am. Inst. Min. Eng., 

 vol. 24, 1895, p. 626. Discussion by E. E. Olcott, p. 884. 



