34 GEOLOGY OF TBE COMSTOCK LODE. 



chaFBcter. — Graiute does not play a large part in the geolog)' of Washoe. 

 Besides the small area laid down on the map, it has been struck by a tunnel 

 near McClellan Peak, and in the Bock Island and Baltimore mines; so far as 

 I know, nowhere else in the neighborhood. The rock is a fine typical granite, 

 consisting of orthoclase, quartz, biotite, a little oligoclase, magnetite, and 

 some accessory minerals. The apatites are colorless, the zircons are numerous 

 and beautiful, and the titanite occurs in typical rhombs, with well developed 

 cleavages. Finally, it contains a colorless regular mineral, seemingly in 

 ill developed rhombohedrons, which answers to sodalite. The microscopi- 

 cal characters of sodalite, however, are rather negative than positive, and 

 it may be some other physically similar mineral. 



Near the Bed Jacket the granite shows very distinct parallel partings, 

 suggesting, but by no means conclusive of, a metamorphic origin. Some 

 of the gi-anite has been mistaken for diorite, and a part of the metamorphic 

 diorite has been called granite; but these are errors which can readily be 

 avoided by careful inspection. 



ERUPTIVE DIORITE. 



General relations. — Tlic devclopmeut of dioritc iu tlic Wa.shoe District is 

 very extensive, ajid the variations of lithological character which it presents 



movpliic diorite occurs close to the granite. It is found as a very volcanic looking breccia, just east of 

 the Volcano at point 5,444, C. D. 6. The western portion of the small patch of this rock in C. 7 is 

 extremely similar to the eruplirt diorite of Mount Davidson. Earlier itornbleiide-aiideiite in a fresh state 

 is found on the north Twin Peak C. D. 4. An abandoned quarry .'iOO feet north of this point shows the 

 stages of its decomposition to admiration. The south Twin Peak isauoccurreuc*^ of loose texture and 

 gray color, somewhat rcsemblingtUe younger hornbleude-andesite of the Ctali neighborhood. The variety 

 with larye hornblende)) is well developed at point 5,G7S, about 1,000 feet east of the .Succor, D. 5. Other 

 varieties, including decomposition-products charged with epidote, may be found on the north llauk of 

 Cedar Hill Canon, say 500 feet west of the Brewery. Fresh augiteaudesite can be conveniently 

 examined at point G,1,V, close to the Forman shaft. The cuts of the Occidental Grade, say from the For- 

 man shaft road to the Prospect, show many beautiful examples of the decomposition aud disintegration 

 of blocks. The eroppings of breccia marked 6,.">69 on the Opliir Grade, B. 4, show many transitions and 

 the develiipmeut of epidote. Tonnger Iwrnblende-andesite is found a.s a purple porphyry at the (juarries 

 2,t)00 feet northeast of .Shaft III. of the Sutro Tunnel ; as a red porphyry (very angitic) at a quarry 2,000 

 feet east of the Occidental mill ; as a gray, somewhat granular looking mass with fine columnar structure 

 in the quarry close to the Utah; as a dense, black, glassy rock at point 6,728 E 2. The tufii modifica- 

 tion is best seen on the Sutro road, where it crosses the divide between Slonut Kmma and Mount Rose. 



