132 GEOLOGY OF THE COMSTOCK LODE. 



surrounded by thin shells of feldspar. The hornblende is in part perfectly 

 fresh, and so solid that the cleavages are almost imperceptible with low powers. 

 The color of the hornblendes is various, and seems to depend largely on their 

 position. Some crystals are nearly pure brown ; others a slightly brownish- 

 green, or of intermediate tints. A few are decomposed to calcite, quartz, and 

 epidote. A part of the crystals show no black borders, and others only a 

 very narrow line of magnetite. The mica is fresh biotite, giving the char- 

 acteristic interference figure, and like the hornblende shows a few glass 

 inclusions. It has a narrow black border. The groundmass is composed of 

 feldspar microlites and brown glass. In parts of the slide the arrangement 

 of the microlites seems wholly without ordei', while in others fluidal structure 

 is well developed 



Slide 462. 2,000 feet uorthwest of Geiger Grade Toll House. 



Black, glassy variety. — Tliis is a pltchy-bhick rock, with a glassy luster, showing 

 some large hornblendes, but resembling certain augite-andesites in appear- 

 ance more than any hornblende-andesite of the District. Under the micro- 

 scope the reason of this unusual appearance is plain, for it contains a large 

 amount of glass base, which is not the case with any other Washoe rock 

 of this species examined. Nearly all the feldspars seem to be labradorite, 

 only the minutest untwinned microlites giving angles of extinction proper 

 to oligoclase. A few sections give angles of extinction which might be re- 

 ferred to anorthite, but I failed to find any such in which extinction took 

 place at equal angles to the trace of the twinning plane, and suppose the 

 crystals to be labradorites cut in one of the uninvestigated zones. Many 

 of the feldspars at first sight appear to be simple crystals, but show on 

 closer examination a few exceedingly minute striae. The feldspars contain a 

 very unusilal abundance of glass inclusions, a large proportion of which 

 have polygonal outlines parallel to the sides of the feldspar section. They 

 also contain inclusions of the base, many of which assume fantastic forms, 

 some looking like ripple-marks, and others arranged as if the base had pene- 

 trated perpendicularly into the feldspar and spread between its zones. The 

 process must have been just the reverse, and the appearance is no doubt 

 due to an ineifectual effort of the feldspathic material to free itself from the 



