reviews 77 s 



which not only the original ore is leached, but the earlier enrichment 

 deposits migrate downward. At Butte, Mont., the region has passed 

 through several very pronounced changes in elevation since the forma- 

 tion of veins in Tertiary time. In early Tertiary time the present 

 topography of the region was blocked out, and mountain ranges and 

 deep valleys carved. This was succeeded by earth movements by which 

 the streams became clogged or the valleys dammed, forming lakes ; 

 while volcanoes broke out at numerous places and showered ashes and 

 scoria over the region. The valleys were silted up or in part filled 

 with volcanic debris before crustal movements drained the valleys and 

 altered the divides. More recent movement, possibly still continuing, 

 is marked by faults and a reversing of the stream courses. The old 

 valley at Butte is filled by hundreds of feet of debris, and a mountain 

 wall 2500 feet high marks a north and south fault-line. These changes 

 all caused a migration of water-level facilitating the processes of weath- 

 ering and enrichment, and the great bodies of rich copper ores of the 

 region are believed to be in part due to this cause." 



Charles R. Keyes. 



Enrichment of Mineral Veins by Later Metallic Sulphides. By 

 Walter Harvey Weed. Bull. Geol. Soc. Am. Vol. XI, 

 pp. 179-206, 1900. 



The author calls attention to the occurrence of localized masses of 

 exceptionally rich ore in mines of copper, silver and zinc, which he 

 undertakes to explain as the result of enrichment by processes subse- 

 quent to the deposition of the lower grade ore. The paper attempts to 

 show that these richer bodies of sulphide ore are formed by the redep- 

 osition of material leached from the vein, generally by superficial 

 waters, and to show the chemical and mineralogical changes involved, 

 as well as the physical conditions under which redeposition took place. 

 The ores in question are chiefly the high grade sulphides. 



He describes three zones ; that of oxidation, that of sulphide 

 enrichment, and that of primary sulphides, and refers to the writings 

 of DeLaunay, Prosepny, Penrose, Emmons and Kemp in this connec- 

 tion. In discussing the chemical and mineralogical changes supposed 

 to take place, he compares unaltered and altered ore and drainage 

 waters observed by himself and cites freely from the literature of the 

 subject, concluding that together they show that the original ore is 



