142 GEOLOGY OP THE COMSTOCK LODE. 



presence of carbonic acid in the inclusions, which is, at all events, very 

 rare in quartz-porphyry, I should class it as a member of that group. 



Exploratiou of the Fortieth Parallel. Slide No. 227, .specimen No. 21,500. West Gate, 

 Augusta Mountains. 



Macroscopically a gray, granular rock. Under the microscope it bears 

 a strong resemblance to the preceding. The feldspars are almost opaque, 

 the hornblende and mica are wholly decomposed. The groundmass con- 

 tains some epidote and much chlorite, magnetite, and zircon. More than 

 one of the quartzes carry besides fluid inclusions, typical, fresh, colorless 

 glass inclusions which contain bubbles and are of sufficient size to remain 

 black between crossed Nicols. I noticed an apatite with good prismatic 

 cleavages. This rock seems to me an old quartz-porphyry. 



Exploration of the Fortieth Parallel. Slides Nos. 228 and 229, specimen No. 21,308. 

 Cortez Peak, Cortez Range. 



I entirely assent to Professor Zirkel's description of these slides. The 

 rock appears to me both macroscopically and microscopically to resemble 

 a porphyritic diorite in all respects. Slide 230 is a highly micaceous variety 

 of the same rock. 



Exploratiou of the Fortieth Parallel. Slide No. 231, specimen No. 22,717. Cross-spur 

 below graveyard, Virginia City. 



Macroscopically this is a greenish-gray, andesitic-looking rock, with 

 impellucid feldspars and brilliant hornblendes. Under the microscope the 

 slide shows a considerable number of hornblendes and some augites. The 

 hornblende is of the greenish-brown tint common among the andesites, but 

 brown enters very largely into the color. It is not fibrous, but decom- 

 position into chlorite has set in along the cleavages, and, in the longitudinal 

 sections, the cleavage prisms separated by chlorite might possibly be taken 

 for coarse fibers. It is a peculiarity of this rock that the iron ore has been 

 attacked more enei'getically than the bisilicates. Many of the smallest 

 grains of the ore, which is probably magnetite, may be seen throughout the 

 slide, converted into a slightly diaphanous, whitish substance, which in so 

 far resembles leucoxene ; but between crossed Nicols it looks more like cal- 



