116 GEOLOGY OF THE COMSTOCK LODE. 



showing close partings in a pinacoidal direction. It tiansmits light too 

 feebly to permit of exact determinations of angles of extinction, but angles 

 of about 30° were noted. 



The occurrence of the rock is purely local, and I regard it as a mere 

 modification of the diabasitic rocks, and as not sufficiently independent to 

 be classified as "•abbro. 



YOUNGER DIABASE. 

 Slide 466. Chollar, 1,900 foot level; 40 feet east of incline. 



The only variety. — Thls Is a bluisli-black fiue-graiued rock, without a trace 

 of porphyritic structure. Under the microscope it seems to be composed of 

 plagioclase, augite, and magnetite. The feldspars present lath-like forms 

 of nearly equal size ; they give angles of extinction corresponding to labra- 

 dorite, and show no distinguishable inclusions besides augite microlites. The 

 augite is mostly granular, and with the magnetite fills the interstices between 

 the feldspars. It is somewhat dichroitic. 



The slide is considerably obscured by clouds of a smoky brownish sub- 

 stance, which possesses no visible structure and no dichroism, but shows 

 aggregate polarization. It is the formation of this substance which turns 

 fresh fractures of the black dike from the bluish color known by draughts- 

 men as "neutral tint" to a smoky brown after a few hours' exposure. There 

 is but one variety of the black dike, and it is almost impossible to distinguish 

 slides of this rock from difterent parts of the Lodk. A characteristic field 

 of this slide is shown in Fig. 29, Plate V. A specimen of the diabase from 

 Orange, N. J., showed a tendency to the same alteration in color after a few 

 days' exposure, and a slide from it exhibits the same peculiarities. 



EARLIER HORNBLENDE-ANDESITES. 

 Slide 309. Edge of plateau, northwest of Ophir Hill. 



Typical rock. — This is a porphyHtlc rock, in which crystals of feldspar 

 and hornblende are separated out in a bluish-gray groundmass. Under 



