136 GEOLOGY OF THE COMSTOCK LODE. 



Exploration of the Fortieth Parallel. Slide Ko. 213, specimen No. 22,684, Crown Point 

 Eavine, Washoe. 



A somewhat more granular rock than the preceding, but of" the same 

 color. The slide shows that it is slightly less decomposed. In a few cases 

 feldspars can be detected with striations not entirely obliterated, and with 

 rectilinear outlines, such as are ordinarily met with in andesites. Several 

 brown apatites are visible. The patches of decomposition products show 

 outlines here and there which are suggestive of hornblende and augite. 

 Besides quartz and epidote, this slide contains some calcite. I regard this 

 and the preceding rock as entirely indetei-minable from the specimens and 

 slides, but from a study of their associations on the spot J believe them to 

 be hornblende-andesites. 



Exploration of the Fortieth Parallel. Slide No. 214, specimen No. 22,686. Crown 

 Point Eavine, Washoe. 



A gray coarse-grained rock, the feldspars of which are opaque, giving 

 it a superficial resemblance to pre-Tertiary rocks. Under the microscope a 

 glance shows it to be augitic. The slide contains several sections of the 

 undecomposed mineral with characteristic octagonal outlines and appropriate 

 angles and cleavages, as well as some longitudinal sections, giving angles 

 of extinction running up to above ,^0°. The color of this augite is the com- 

 mon brownish -yellow, not unlike the tint of bamboo. Much of the augite 

 has been decomposed to chlorite of fibrous structure, which shows dark 

 bluish tints between crossed Nicols, aggregate and sometimes spherolitic 

 polarization, and extinction when the microlites are parallel to the principal 

 sections of the Nicols. That the chlorite is a derivative of the augite is clear, 

 for in some cases augites are only in part converted into chlorite, and in 

 others the pseudomorphs are perfect, even retaining traces of the cross- 

 fractures of the augite prisms. I found but one mass of decomposition 

 products that might with any probability be referred to hornblende. The 

 feldspars are triclinic, and some of the large crystals show labradorite 

 angles of extinction. Apatite and magnetite are also present. The ground- 

 mass contains no glass, but seemed to me to show traces ofa felt-like struct- 

 ure, much obscured, however, by particles of clilorite and epidote. This 



