84 GEOI.OOY OF TfJE COMSTOCK LODE. 



founds the greater part of his diagnostic points of difference between pro- 

 pyhte and andesite upon the color and structure of the hornblende, and its 

 distribution in the rock. What has been taken for green fibrous hornblende, 

 however, in a groat majority of the propylite slides of the collection of the 

 Exploration of the Fortieth Parallel proves to be not hornblende, but chlo- 

 rite. This mineral, which is probably the rhipidolite of G. Rose, is, like horn- 

 blende, green, fibrous, and strongly dichroitic, but it occurs largely in 

 spherolitic and felt-like masses, extinguishes light when either of the 

 jn-incipal sections of the polarizing apparatus is parallel to the fibers; 

 and, when the Nicols are crossed, usually shows only dark-bluish tints, 

 very different from tiiose commonly tnxnsmitted by hornblende. In one of 

 the slides, indeed, there is abundant green fibrous hornblende, but the rock 

 is a granular diorite from Mount Davidson, while in the section from Storm 

 Canon, Fish Creek Mountains, there is both chlorite and hornblende, but 

 the latter is certainly uralite. 



It has been shown in the preceding section of this chapter that chlorite, 

 which is a decomposition-product of hornblende, augite, or mica, is fre- 

 quently diffused through the groundmass and any feldspars which may 

 have become porous through decomposition. This fact, combined with the 

 mistake of chlorite for hornblende, explains the distinctions based upon the 

 "■reenish hvie of the propylitic rocks, upon the color and structure of the 

 masses mistaken for hornblende, and upon the distribution of supposed par- 

 ticles of that mineral through groundmass and feldspars. Seemingly con- 

 clusive proof has also been offered elsewhere that epidote in the "Washoe Dis- 

 trict is not an immediate product of the decomposition of hornblende, but 

 of chlorite; which explains its absence in the comparatively fresh rocks recog- 

 nized as andesitic. 



In one limited area of hornblende-andesite, not represented among the 

 slides of the Fortieth Parallel, minute spiculfe of hornblende occur, distrib- 

 uted tln-ough the groundmass; but they are brown, each microlite is solid, 

 and tliey arc not grouped in crystal-like aggregates. In almost all cases 

 the andesitic hornblendes when fresh are black-bordered; but while the 

 magnetite usually resists decomposition longer than the hornblende sub- 

 stance, it sometimes yields first. The hornblendes of the diorito-porphyries. 



