JSoseph Le Conte— Genesis of Metalliferous Veins. 11 
process revealed, but the true vein deposits have hitherto been 
nomena at Sulphur Bank and at Steamboat Springs.* At 
Steamboat Springs, alkaline carbonates predominate, and there- 
fore the surface deposit is in the form of a thick cake of sinter, 
only stained here and there with metallic sulphides and metallic 
oxides ; while at Sulphur Bank alkaline sulphides predominate, 
and therefore the deposit is rich in metallic sulphides; but there 
1s nocrust of silica formed, because the acidification of the sul- 
phides by contact with air neutralizes the alkali, and prevents the: 
silica from reaching the surface. In place of a deposited crust we 
have here a chalky silicdous residue of acid decomposition of 
surface rocks. Doubtless all gradations between these extremes 
may be found. The richness or poorness of veins in metals 
depends on similar differences in ascending waters of previous 
geological epochs. No doubt, however, the abundance and 
rapidity of circulation of the waters was also an important ele- . 
ment in determining the results, for, as we have already shown, 
very slow circulation is favorable for deposit in conduits ; abund- 
* This Journal, vol. xxiv, p. 23, 1882, and vol. xxv, p. 424, 1883. 
