1188 A TREATISE ON METAMORPHISM. 



zone is shallowest and is overlapped by the blende zone, and this by the 

 chalcopyrite zone."" Thus in this upper belt there is a definite order of 

 prominence of these three minerals — galena, blende, and chalcopyrite. 



In addition to these specific instances of the production of a rich 

 upper belt, some general statements have been made which need to be 

 referred to. One of these is made by Douglas in reference to sulphuret 

 mines as a whole. Says he, in the conclusion of his discussion as to the 

 copper resources of the United States, with reference to the various Appala- 

 chian deposits, "Like all sulphuret mines, they became poorer as depth 

 was attained." b 



With the exception of the San Juan and Monte Cristo districts, the 

 above illustrations of secondary enrichment and diminution of richness with 

 depth are the same as originally published by me.' Recent articles by other 

 authors have given many other instances of secondary enrichment which 

 fall in line with the above illustrations, but which in this treatment of 

 principles need not be repeated. Some of ,the more notable contributions 

 upon this subject are those by Emmons/ Weed," Ransome/ and Bain. 3 



Upon the hypothesis that the rich ores of the upper parts of deposits 

 usually result from secondary action of descending solutions upon material 

 no richer than the deeper parts of the deposits, the extent and richness of 

 such deposits may give an approximate idea of the minimum denudation 

 which a district has suffered. For example, at Ducktown, Tenn., to 

 produce the rich oxidized products of copper above the level of ground 

 water, and the very rich sulphides at and near it, from the lean, cuprifer- 

 ous pyrrhotite below, which bears about 2 per cent of metallic copper, 



"Spurr, cit., p. 841. 



''Douglas, James, The copper resources of the United States: Trans. Am. Inst. Min. Eng., vol. 19, 

 1891, p. 694. 



('Van Hise, C. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng., 

 vol. 30, 1901, pp. 128-134. 



''Emmons, S. F., Secondary enrichment of ore deposits: Trans. Am. Inst. Min. Eng., vol. 30, 1901, 

 pp. 177-217. 



«Weed, W.H., Enrichment of mineral veins by later metallic sulphides: Bull.Geol. Soc. Am., vol. 

 11, 1900, pp. 179-206. 



/Ransome, F. L., Ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept. U. S. 

 Geol. Survey, pt. 2, 1901, pp. 229-397. Report on the economic geology of the Silverton quadrangle, 

 Colorado; Bull. U. S. Geol. Survey No. 182, 1901, pp. 265. 



tfBain, H. F., with Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and sine 

 deposits of the Ozark region: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, pp. 23-227. 



