108 GEOLOGY OF THE COMSTOOK LODE. 



chromatic tints between crossed Nicols, and in one or two instances the 

 cleavage, make the diagnosis fairly certain. Besides the ilmenite there 

 ajjpears to be a certain quantity of magnetite, which is not improbably 

 titaniferous, for while the crystal forms are referable to the cube there is no 

 accompanying limonite. Finally, there are numerous well-crystallized 

 zircons and a few ordinary apatites. 



Slide 293. 700 feet southwest of Devil's Gate. 



Intermediate variety. — This Tock is intermediate in character between slides 

 295 and 429. It is crowded with green hornblende microlites, but not to 

 such an extent as to conceal the feldspar, which shows the angles of extinc- 

 tion proper to oligoclase. It also contains much quartz and ilmenite, as well 

 as many apatites and zircons. 



This slide is chiefly remarkable for the presence of tourmaline. It 

 occurs in grains and in imperfect prisms. These extinguish light parallel 

 to their principal axis. They are very highly dichroitic, showing a clear 

 brown color when parallel to the main axis of the polarizer, and an almost 

 absolute black at right angles to this direction. 



QUARTZ-PORPHYRY. 



Slide 354. 1,000 feet south of Lawson^n Tunnel. 



Typical variety. — Macroscopically this rock shows a purplish-gray ground- 

 mass in which are separated out porphyritically feldspar, mica, and quartz. 

 Under the microscope a few hornblendes, apatite, and iron ores also make 

 their appearance. 



The feldspars, which in this slide are fairly fresh, occur for the most 

 part in irregular grains, rendering it difficult or impossible in many cases to 

 determine the crystallographic orientation. The larger part of the feldspars 

 are unstriated, and of these many are certainly orthoclase, as determined 

 by the angles of extinction referred to the cleavages. I was unable to find 

 any unstriated feldspars which, tested in the same manner, gave angles 

 appropriate to either of the triclinic feldspars. Thei-e is also a considerable 



