96 GEOLOGY 01^ THE COMSTOOK LODE. 



position of hornblende. Embedded in it are numerous small grains and 

 microlites, which extinguish light at a large angle to the plane of the Nicols. 

 They are arranged at angles of about 60°, and it appears to me that the 

 object must be supposed to be a decomposed hornblende, filled with micro- 

 lites of the mineral which results from the decomposition of the black 

 microlites. This opinion is strengthened by the occurrence of a number of 

 intermediate stages, as they seem to be, between fresh hornblende and the 

 last mentioned chloritic mass. As the black microlites alter, the hornblendes 

 become in some cases grayish and less and less pellucid, not apparently 

 from want of transparency on the part of the minerals, but through irregular 

 refraction of light. 



The feldspar is undoubtedly for the mo.st part labradorite, many of the 

 finely twinned crystals showing angles of extinction of neai'ly 30° on each 

 side of the twinning plane. I see no evidence of the presence of any other 

 feldspar. There are a few grains of quartz, which contain some liquid in- 

 clusions. The apatites are few in number, colorless, and in no way remark- 

 able. I detected no other minerals in the slide. 



Slide 81. Utah, 1,950. 



Gray diorite with brown hornblende. This is thc freshcSt dioritC iu tllC ColleCtioU, 



the feldspar being as transparent as it ordinarily is in andesite. Unfortu- 

 nately the slide is not thin. The principal difference between this and 

 .slide 413 is that the majority of the hornblendes are brown, many of them 

 without a tinge of green. 



Slide 3G1. Savage 1,300. North drift, about 310 feet iu. 



Micaceous granular diorite. — lu this Tock, whicli is uot a porphyritc, but gran- 

 ular, the hornblende has been almost wholly replaced by biotite, which is 

 of the usual structure, and gives an interference figure nearly like that of 

 a uniaxial mineral. The slide contains much quartz and many beautifully 

 sharp zircons. In other res^^ects it is similar to slide 213. 



Slide 291. Chollar 1,700 ; 1,425 feet west of Combination shaft. 



Diorite containing tourmaline, etc. — Tliis is a dioHtc of tlic granular crystalHue 

 type, but of a very quartzose variety. The quartzes contain innumerable 



