216 GEOLOGY OF THE COMSTOCK LODE. 



which often result from the decomposition of feldspars; and mica and epidote 

 are counted among the products of alteration. 



Kaolin. — Kaolin is microscopically an obscure mineral. According to 

 Mr. H. Fischer it is amorphous, while Mr. A. Knop found it to consist of 

 delicate hexagonal plates of the rhombic system. Breithaupt named this 

 crystalline modification nacrite, and M. Des Cloizeaux pholerite. If saussurite 

 and pinitoid are really independent minerals, it is certain that these names 

 have also been given to mere mixtures resulting from the extraction of por- 

 tions of the silicic acid and of the stronger bases. 



Evidence of the microscope. — lu tlic Washoe rocks, as is usual elscwliere, the 

 first indication of decomposition is the appearance of calcite and quartz in 

 the more or less carious crystals. This is doubtless attended by the forma- 

 tion of soluble alkaline silicates, which, however, are not recognizable under 

 the microscope. As the process continues the striations are obliterated, and 

 the final result is a heterogeneous mass showing aggregate polarization, 

 sometimes only faintly translucent, and containing in a recognizable form 

 only grains of calcite and quartz. No amorphous substance has been ob- 

 served, nor any hexagonal lamellae answering to the description of nacrite. 

 Mica, too, appears to be absent, although occurring among the decomposi- 

 tion products of similar rocks at no great distance from Virginia. Chlorite 

 and epidote are common in decomposed feldspars, but in many cases it seems 

 certain that chlorite due to the decomposition of the bisilicates has merely 

 permeated the spong)^ mass; and epidote has repeatedly been observed 

 developing in patches of chlorite, which were surrounded by feldspar sub- 

 stances, just as it has been described and illustrated as occurring in altered 

 bisilicates. No case has been met with in which either minei'al was dis- 

 tinctly parasitic on feldspar. 



All lithologists agree that chlorite forms from the bisilicates, and that 

 feldspars become carious; it is also acknowledged that chlorite is diffused 

 through the portions of the rock mass in the immediate neighborhood of 

 the point at which it forms. It must therefore penetrate the feldspars where 

 these are partially decomposed, in all rocks in which the bisilicates are to. 

 any extent converted into chlorite. It is, of course, by no means necessary 

 that the point at which chlorite gained access to the feldspar should be 



