236 H. W. TURNER GEOLOGY OF THE SILVER PEAK QUADRANGLE 



of a dull or bluish color, precisely like some of the quartz-augen in the 

 white granite where it has undergone crushing. The dull color is proba- 

 bly due to the innumerable dots, visible only under the microscope in the 

 thin-sections. These dots when examined with a high power are in part 

 resolved into minute cavities containing one or more gas bubbles in a 

 fluid, but most of the dots are indeterminable. One of the thin-sections 

 examined showed plainly a crushing of the rock, the large quartzes being 

 broken and faulted. Two specimens assayed for the precious metals by 

 the Selby Smelting and Lead Company gave the following results : 



Gold Silver 



in ounces. in ounces. 



No. 493 0.03 0.13 



No. 500 none none 



It should be stated that no mica or feldspar was observed in quartz 

 vein number 493, which alone contains gold and silver. 



The following detailed description of thin-sections of the assayed 

 granitic quartz taken from two of these veins indicate their appearance 

 under the microscope: 



Specimen 493. — Locality : Mineral ridge, on the same spur as the Mary mine. 

 Specimen was taken from the vein lying just east of and overlying the Crown- 

 ing Glory ledge. 



Macroscopically, a dull bluish white quartz showing irregular fractures. 

 Microscopically, a quartz rock evidently composed originally of large quartzes 

 with uniform orientation throughout, but these grains have been crushed and 

 faulted. The lines of shearing cut the large quartz grains in various direc- 

 tions. Most of the quartzes show undulous extinction. 



Specimen 500. — Locality: 7.5 kilometers east of Red mountain, on Mineral 

 ridge. The vein is about one meter thick. 



Macroscopically, a bluish quartz containing some feldspar and white mica 

 and possessing a gneissic structure. Microscopically, a quartz-gneiss in which 

 the grains dovetail and are often elongated in one direction, producing a 

 gneissic banding. The quartzes are turbid, and this appears to be due to in- 

 numerable minute dots mostly arranged in parallel rows, but these rows cut 

 the gneissic banding at an angle. This quartz must have undergone deforma- 

 tion either when consolidating or subsequently. The lines of dots were un- 

 doubtedly formed at the time of consolidation, and as these are not parallel to 

 the gneissic banding, "the inference may be drawn that the gneissic structure is 

 due to stresses exerted after consolidation. At one point there are phenocrysts 

 of feldspar which show narrow lamellar twins with small extinction angle on 

 the trace of the twinning plane. The index of refraction of this feldspar is 

 greater than that of the balsam. It is probably oligoclase. 



Many instances have been recorded of veins or dikes of quartz similar 

 to those above described. Lehmann 10 has found them in Germany, 



10 Lehmann: Untersuchung ueber die Entstchung die altkrystal-linischen Schiefer- 

 gesteine. Bonn, 1884. 



