370 GEOLOGY OF THE COMSTOCK LODE. 



It is strongly dichroitic, but except in dense masses appears nearly black 

 between crossed Nicols. It is fibrous, often spherolitic, and invariably ex- 

 tinguishes light parallel to the direction of the fibei's. It thus bears a con- 

 siderable resemblance to fibrous green hornblende, but the cases are very 

 rare, if they actually occur, in which a careful examination will not serve 

 to discriminate between the minerals. This chlorite is decidedly soluble. 

 It occurs in veinlets and diffused through the groundmass and through other 

 minerals when these have become pervious through decomposition. It is 

 especially striking as an infiltration in altered feldspai's, where, of course, it 

 is readily visible. All the stages can be traced, from the first inconsidei'able 

 attack upon the bisilicates or the mica through instances in which chlorite 

 occurs wholly or almost wholly as admirable pseudomorphs after the ferro- 

 magnesian silicates, and up to cases in which the secondary mineral is wholly 

 diffused through the mass of other products of decomposition. 



Epidote is usually in Washoe a product of the decomposition of chlo- 

 rite. Comparatively very few occurrences of epidote are explicable on the 

 supposition that the mineral is the direct result of the decomposition of the 

 primary silicates; none are inexplicable on the supposition that chlorite 

 represents an intermediate stage in the alteration, and hundreds of cases 

 show beyond question that epidote develops in chloritic masses, sending 

 characteristic denticles and fagot-like o£fshoots into the comparatively homo- 

 geneoiis chlorite. Several drawings illustrating these processes are shown 

 in Plate II. They are photographic in their fidelity. Epidote, too, is pos- 

 sibly soluble to a very slight extent, but certainly far less so than chlorite. 

 The veinlets of epidote are often, though perhaps not always, a result of 

 the alteration of chlorite. No evidence has been obtained that feldspars 

 are ever converted into epidote, and the dissemination of fresh hornbleude 

 particles in feldspars in any considerable number has not been observed. 

 In many cases, on the other hand, it can be shown that feldspars have been 

 impregnated with chlorite, from which epidote has afterwards developed. 

 Chlorite does not always change to epidote, and apjiears often to be replaced 

 by quartz and calcite. This is frequently visible in slides which also show 

 its alteration to epidote. No certain evidence of the altei'ation of epidote 

 has been met with. 



