692 DESCRIPTIVE GEOLOGY. 



feldspars, with rounded quartz-grains, are porphyritically enclosed in the 

 groundmass. Mineralogically the composition would appear to be the same 

 as the coarse-grained granite in which it occurs. Zirkel calls attention to 

 an interesting point, "the microgram tic structure" — as brought out in the 

 microscopical study of the rock. The quartz-grains carry fluid but no 

 glass-inclusions. 



Diabase penetrates the Archaean crystalline beds of Granite Mountain 

 in several narrow dikes. They are especially noticeable, as the occurrence 

 of diabase rocks cutting granite bodies has seldom been observed in our 

 field of exploration. The dikes were seen on the northeast slopes, near the 

 summit, with sharp straight lines of contact, striking approximately east and 

 west. They are characterized by a fine-grained unifomi texture and a dark- 

 gray color. They are exceedingly compact, breaking under the hammer 

 with a rough, angular surface, free from all tendency to conchoidal fracture, 

 with a habit peculiar to fine-grained rocks wanting in quartz and an amor- 

 phous base. Mineralogically the rocks are made up of triclinic feldspar and 

 augite ; many of the former are brilliant acicular needles, with a vitreous 

 lustre, while the latter are rarely well developed. Small black grains are 

 visible here and there, which are probably magnetite. Yellow particles of 

 pyrite are occasionally seen as an accessory mineral. Under the micro- 

 scope may be detected apatite prisms, and in the augites are several glass- 

 inclusions. • 



On the east side of the range, about 15 miles to the northward of Granite 

 Mountain, occurs a very considerable body of granite. It extends in a 

 north and south line for 8 or 10 miles, forming the water-shed along the 

 main ridge for some distance, and falling off toward Grass Valley until 

 concealed by Quaternary deposits. Here, as at Granite Mountain, the 

 beds referred to the Triassic curve around the granite, resting upon it uncon- 

 formably. Rounded conical hills, rugged, uneven spurs, with a great diversity 

 of outline, characterize the topographical features of the region, the rock- 

 masses possessing a hard, compact texture, with a prevailing light-red color. 



Region south of Granite Mountain.^ — South of Granite Mountain, 

 for a distance of over 40 miles, no granite is known along the Pah-Ute 



^ From field-notes furnished by Mr. Clarence King. 



