2 H. C. COOKE 
amenable to solution are those of secondary enrichment; most 
important, because many of the most valuable deposits were con- 
centrated in this way; most amenable to treatment, because many 
of the factors of the problems are known, i.e., temperature, pressure, 
concentrations of solutions, and the chemical nature of the reacting 
solutions and of the minerals affected. 
The principles of secondary enrichment have been outlined by 
Penrose, S. F. Emmons,? Weed,3 Van Hise,* Lindgren,’ and others, 
and confirmed by the work of many investigators. In a-few cases 
the chemical reactions which take place have been studied in some 
detail. Brokaw® and McCaughey’ have investigated the solution 
of gold in surface waters; Van Hise,’ Leith, and their associates 
at the University of Wisconsin have studied the deposition and 
enrichment of the Lake Superior iron ores; Gottschalk and Buehler? 
have shown the reactions by which the sulphides of iron, lead, 
zinc, and other metals are taken into solution; and Allen’? has pre- 
cipitated pyrite and marcasite under natural conditions. 
The processes of the secondary enrichment of silver are of much 
interest both because they are of widespread occurrence and because 
a relatively small amount of secondary enrichment may render 
a low-grade deposit commercially profitable. There is a great 
variety of secondary silver minerals, and some of them may be 
deposited even at considerable depths. The mechanism of this 
downward migration has been discussed by many writers. Near 
the surface, some have supposed, the silver was dissolved by sul- 
phate solutions, others have regarded chloride waters as increasing 
t Penrose, ‘‘Superficial Alteration of Ore Deposits,” Jour. Geol., II (1904), 288. 
2. F. Emmons, “‘The Secondary Enrichment of Ore Deposits,” Trans. A..M.E., 
XXX (1900), 177. 
3 Weed, ‘“‘The Enrichment of Gold and Silver-Veins,”’ zbzd., p. 424. 
4Van Hise, ‘‘Some Principles Controlling the Deposition of Ores,”’ ibid., p. 27. 
5 Lindgren, ‘‘ Copper Deposits of the Clifton Morenci District,’ U.S.G.S., XLII 
(1905), 177. 
6 Brokaw, Journal of Geology, XVIII (1910), 321. 
7 McCaughey, Jour. Am. Chem. Soc., XXXI (1909), 1263. 
§ Van Hise and Leith, U.S.G.S. Monograph 52 (1011). 
9 Gottschalk and Buehler, Economic Geology, VII (1912), 15. 
7 Allen, Am. Jour. Sci. (4), XXXIII (1912), 160. 
