1190 A TREATISE ON METAMORPHISM. 



by the reactions above stated, as at Ducktown, Tenn., or partly dissolved 

 and transported below, as at Butte, Mont. Thus for a certain depth the 

 ores may increase in richness. This exception, however, does not affect 

 the common rule as to diminution of richness with increasing depth. 



Penrose," in 1894, in discussing- the superficial alteration of ore 

 deposits, says : 



As a result of these various changes, certain materials are sometimes leached 

 from the upper parts of ore deposits, which have become porous by alteration, and 

 carried down to the less pervious unaltered parts. Here they are precipitated by 

 meeting other solutions, or in other ways, and hence the richest bodies of ore in a 

 deposit often occur between the overlying altered part and the underlying unaltered 

 part. This is not always the case, but it is true of some copper, silver, iron, and 

 other deposits.* 



De Launay, c in 1897, emphasizes the frequent occurrence near the 

 surface of rich deposits, which in some cases are oxidized products and in 

 others are sulphides. He explains the richness of the deposits by the 

 abstraction of more soluble material. This frequently results, he thinks, in 

 transforming a low-grade product into a rich ore. By this process a poor 

 sulphide may be changed to a rich sulphide, as, for instance, cupriferous 

 pyrites or chalcopyrite may be transformed to covellite or chalcocite by 

 abstraction of iron sulphide. It is a natural deduction from de Launay's 

 explanation d that the volume of the material is decreased, although he 

 does not make this point. 



De Launay further emphasized the point that the metallic material of 

 veins may have been repeatedly transferred from one place to another, and 

 suggests that a part of the material now found in veins may have been 

 transferred from vein material which was once above the present surface of 

 denudation. 



Le Conte e suggested that the rich belt may be explained by supposing 

 that precipitation by ascending waters does not occur at great depth, 

 because the solutions do not get saturated until comparatively near the 

 level of underground water. But it is to be remembered that the upper 



Penrose, R. A. P., jr., The superficial alteration of ore deposits: Jour. Geol., vol. 2, 1894, pp. 

 2S8-317. 



6 Penrose, cit. p. 294. 



"Launay, M. L. de, Contributions a l'£tude des gltes m^talliferes: Annales des Mines, 9th ser., 

 vol. 12, 1897, pp. 151-152. 



d Launay, de cit. p. 194. 



e Le Conte, Jos., On the genesis of metalliferous veins: Am. Jour. Sci., 3d series, vol. 26, 1883, p. 12. 



