112 GEOLOGY OF THE COMSTOCK LODE. 



structure and mineralogical composition of this slide appear to me identical 

 with that of the rocks of the District described by Professor Zirkel as dacite 

 and by me as quartz-porphyries. The properties of the triclinic and ortho- 

 tomic feldspars are the same, the hornblende and mica are of the same char- 

 acter and of the same degree of decomposition, and the groundmass is 

 indistinguishable. Professor Zirkel draws special attention to the fluid 

 inclusions in the feldspars of this slide. 



EARLIER DIABASE. 

 Slide 349. Sutro Tunnel, north branch, 50 feet south of Ophir. 



Typical example. — This is a gray rock, which might readily be mistaken at 

 first glance for a diorite. On close inspection, however, a certain waxy 

 luster, characteristic of augitic rocks, is perceptible, as well as numerous 

 lath-like feldspars from 1""°- to 2""- long. Under the microscope it is plain 

 that the rock consists of triclinic feldspar, augite, and an iron ore. 



The larger feldspars are well developed; the smaller ones are granitoid 

 in structure, and appear to have occupied the interstices between the larger 

 crystals. The larger feldspars show polysynthetic twinning, according to 

 the albite law, the lamellse being of moderate thickness. In addition, many 

 of the individuals show pericline twinning, and in some cases polysynthetic 

 individuals are united as Carlsbad twins. The angles of extinction are all 

 within the limits appropriate to labradorite, and some of the macropinacoidal 

 sections recognizable by the shape, and by the angles of the two species of 

 twin lamellfe, give almost exactly the theoretical maxinmm angle of extinc- 

 tion on each side of the twinning plane. Vev}' few of the small feldspars 

 forming a sort of groundmass show crvstalHne outlines ; but almost all are 

 twinned, and many of them give angles of extinction indicating labradorite. 

 In fact, I was unable to find any evidence of the existence of any other 

 feldspar. There are a few fluid inclusions in the feldspars. 



A considerable portion of the augite is fresh. It is of the ordinary pale 

 brownish-yellow, only just perceptibly dichroitic, and in general exhibits 

 excellent cleavaores. Some well-defined octagonal cross-sections show not 



