212 GEOLOGY OF THE COMSTOCK LODE. 



very usually result from the decomposition of hornblende, augite, and mica 

 is a well-known fact, and pseudomorphs of epidote, after these minerals, are 

 also common. The difference is great, for wliile chlorite contains little or 

 no ferric oxide and no calcium, epidote contains both, but is free from mag- 

 nesium. It would seem, therefore, as if epidote must be formed under 

 such conditions that ferrous compounds might be oxidized, or such that 

 ferric compounds, at all events, would not be reduced, and, further, under 

 conditions favoring the solubility of magnesian salts rather than those of 

 calcium. It appears to me somewhat difficult to suppose the bisilicates 

 exposed to a sulphidizing action so strong as to result in the formation of 

 pjrite, and yet not sufficiently reducing to prevent the formation of ferric 

 compounds. On the other hand, pyrite, though of very variable stability, 

 often oxidizes with great difficulty, and the oxidation of ferrous compounds 

 may sometimes be effected in its presence. Epidote might, therefore, form 

 in the presence of pyrite, but hardly contemporaneously with it. The 

 behavior of the salts of magnesium and calcium salts towards one another 

 is known to vary greatly with the physical conditions, especially with tem- 

 perature, and presumably also with pressure, and it is fuilher affected by 

 the concentration of solutions. Thus, Dr. T. S. Hunt^ found that when solu- 

 tions of the chlorides and carbonates of these elements are evaporated at 

 ordinary temperatures, calcium carbonate alone is first precipitated; while, 

 when the solution is boiled, magnesium carbonate first separates. This and 

 similar facts tend to the supposition that high temperatures would favor the 

 formation of magnesian chlorite rather than of calciferous epidote. 



Conditions under which epidote occurs. — Thc uudergrouud rocks at Washoe all con- 

 tain chlorite in abundance, but epidote is uncommon. Thus, a special search 

 was necessary to discover epidote in the underground diabases, while per- 

 haps half the augite-andesites from the surface contain it in considerable 

 quantities When it occurs at a considerable depth it seems to be either 

 close to the Look or near strong seams extending towards the surface, as in 

 one or two localities in the Sutro Tunnel On the surface epidote is extremely 

 common, tinging whole areas of the various rocks with its peculiar green, 

 and occurring in many and widely separated localities. Where epidote is 



' Chem. and Gcolog. Essays, p. 138. 



