406 GEOLOGY OF THE OOMSTOCK LODE. 



Granite, close to Red Jacket mine. 



The orthoclastic feldspar gave reactions corresponding to perthite or loxoclase, 

 showing a mixture of amazonite with a triclinic feldspar. This mixture is also readily 

 recognizable under the microscope. The triclinic crystals are in part oligoclase and 

 in part answer to andesine.' Under the microscope I have noticed no crystals more 

 basic than oligoclase. 



Granular diorite, Bullion Eavine at Water Company's flume. 



All the feldspars tested gave reactions for andesine, with a tendency rather towards 

 labradorite than towards oligoclase. Under the microscope the maxinnim angles found 

 answered to labradorite, but the occurrence of zonal structure was noted.^ 



Granular diorite, TJtah^ 1950. 



In this specimen labradorite, andesine, and crystals of intermediate composition 

 were found. 



Porphyritic diorite, Ophir Eavine, south side. 



Labradorite and andesine only were detected, 



Metamorphic diorite, Amazon dump. 



All the feldspars tested were oligoclase, according with the microscopic results. 

 Quartz-porphyry, 1,000 feet south of Lawson's Tunnel. 



Amazonite and oligoclase were found, as well as a feldspar slightly more basic 

 than oligoclase, but not so much so as andesine. This mineral is therefore much more 

 closely allied to oligoclase than to labradorite. No angles of extinction exceeding 

 those of oligoclase were observed in the slide. Oligoclase was also found in the rock 

 described on page 109 (slide 304). 



Earlier diabase, Sutro Tunnel, north branch, 50 feet south of Ophir. 



All but one of the crystals tested proved to be labradorite. The exception was 

 an andesine. 



Earlier hornblende-andesite, North Twin Peak. 



A single feldspar had a composition intermediate between labradorite and ande- 

 sine. The remainder were characteristic labradorites. The zonal feldspar described 

 on page 61, and shown in Plate III., Fig. 13, is from the same cropping, though not 

 from the same specimen. 



Earlier hornblende-andesite, 1,200 feet northwest of Geiger Grade Toll House. 



The microscopic examination led to the supposition that anorthite, labradorite, 

 and oligoclase were all present, the hist, however, only as microlites. Tlie crystals 

 tested by Szabo's method proved to be andesine and a feldspar intermediate between 

 this and labradorite. No anorthite was met with. This fact, however, of course does 



' It is usual to regard audesine as peculiar to volcanic rocks, and the plagioclase of granite is 

 often supposed to be exclusively oligoclase. Professor Eosenbusch, however (Physiog. der Massigen 

 Gest., II., 121), mentions finding plagioclases iu granites which showed angles of extinction correspond- 

 ing to all of the feldspar species excepting anorthite. 



' For some remarks on the indications of zonal structure, see page 61. 



