100 GEOLOGY OF THE COMSTOCK LODE. 



decomposed, but not in such a mannei' as to obscure its original constitu- 

 tion. When fresh it consisted essentially of well-developed crystals of tri- 

 clinic feldspar and hornblende, disposed porphyritically in a groundmass 

 mainly composed of feldspathic grains. A little mica, a small amount of 

 black ore (probably magnetite), and numerous colorless crystals of apatite, 

 were subordinate mineral ingredients. 



No undecomposed hornblende now remains. It has been replaced by 

 chloritic material, epidote, quartz, and calcspar, but in such a way as to 

 leave the larger portion of the hornblende crystal outlines undisturbed. 

 All, or nearly all, the hornblendes seem to have been crystals of consid- 

 erable size and sharp definition, and there is nothing to indicate that they 

 possessed a fibrous structure. Some of the hornblende crystal outlines 

 are completely filled with the chlorite. This substance sometimes shows 

 an excessively fine, fibrous, imperfectly spherolitic structure. In other 

 cases the fibers near the peripheries of former hornblendes ai'e arranged at 

 right angles to the crystal face. These fibers are of nearly equal length, 

 and they form a zone just within the crystal section. The chlorite is grass- 

 green, and very slightly dichroitic, varying between more and less yellow- 

 ish green shades. Between crossed Nicols it behaves almost like an isotropic 

 substance and shows, besides black, only dark purple tints. The chlorite 

 is not confined to the hornblende sections, but is diffused through the rock 

 in veins and patches. It also occurs in narrow borders about magnetite 

 and apatite, as if these minerals had mechanically obstructed its move- 

 ments. 



The epidote occurs in a similar way both without and within the horn- 

 blende sections, which it sometimes wholly and sometimes only partly fills. 

 It is noteworthy that this mineral when it occurs in small patches is usually 

 finely granular, and that within certain limits, the larger the area, the 

 coarser the grain. When, as is often the case, the occurrences are wedge- 

 shaped, the granulation grows coarser from the point to the base. This 

 seems to indicate a more or less continuous recrystallization of the 

 mineral. 



The relations of the chlorite and epidote in this slide are extremely 

 interesting, for it affords abundant proof that the epidote has formed at the 



