94 GEOLOGY OF THE COMSTOCK LODE. 



do not disappear upon heating- the slide to 40^ C. on Vogelsang's table, and 

 are therefore probably aqueous. 



There is a considerable quantity of magnetite in this slide, characterized 

 by its square outlines and opacity. I observed no titanic iron. A few 

 crystals of apatite appear under the microscope, rather fewer than is usual 

 in the rocks of the District. They are colorless, and contain no determinable 

 inclusions. There are many minute zircons recognizable by their high refrac- 

 tion, brilliant polarization, and by their crystal form (the eight-sided prism, 

 terminated by the fundamental pjn-amid). One or two fragments of mica 

 appear in the slide — e. g., at 21-21. There are also a number of irregular 

 fi-agments of a mineral which can scarcely be anything but titanite. It 

 shows an uneven surface, brown color, perceptible dichroism, and high 

 refractive index. In polarized light it is only feebly chromatic. Plate IV., 

 Fig. 25, shows a characteristic portion of this slide. 



Slide 413. Union Shaft, 2,625 feet from surface. 



Dark diorite with some brown hornblende. — Maci'oscopically this is a vcry dark rock, 

 highly charged with scales of hornblende. It reminds one of freshly fract- 

 ured "No. 1" pig iron. Under the microscope it is seen to be composed 

 essentially of triclinic feldspar and hornblende, both minerals having con- 

 solidated nearly at the same time. A few grains of quartz, and an insig- 

 nificant amount of colorless apatite, complete the list of components. 



The hornblende is in part of a brown tint, very slightly tinged with 

 green; in part it is of a light and vivid blue-green color. Many of the 

 hornblende crystals show both colors ; the green variety occurring along 

 the edges and cleavages, and sometimes leaving only small irregular patches 

 of the brown mineral surrounded by the green. The structure of the two 

 varieties is distinctly diiferent. The brown mineral shows excellent cleav- 

 ao-es, but no tendency to fibration. In the green portions of the same indi- 

 viduals the hornblende seems to be composed of minute fibers, but the 

 tesselated appearance of the cross-sections is nearly obliterated. In fact all 

 the appearances are such as accompany a distinct alteration in mineral char- 

 acter. The brown hornblende is as usual very strongly dichroitic; the 



