532 DESCRIPTIVE GEOLOGY. 



mineral constituents are well developed, and consist of both monoclinic 

 and triclinic feldspars, hornblende, biotite, and quartz. Many of the plagio- 

 clase crystals are quite large, and characterize the rock by their brilliant 

 wliite faces. Thin flakes of black biotite are everywhere scattered through 

 the groundmass, frequently adhering to the edges of the feldspars. The 

 hornblende appears to be a deep dark green. Quartz-grains are rare, but 

 are mainly in broken translucent masses ; the rock forming no exception to 

 most of the quartz-bearing trachytes in having the grains quite large, and 

 in showing under a high power no microscopical ones. Minute apatite 

 cr3^stals are seen under the microscope in great numbers. A marked pecu- 

 liarity of this rock is its coarse crystalline groundmass, to which it doubtless 

 owes the somewhat characteristic fracture. 



Region north of Fremont's Pass.^ — The granite body which ends at 

 Fremont's Pass agrees in trend with the general course of the range, and 

 extends northward for nearly 15 miles, finally falling away toward Ruby 

 Valley. In the pass, the granite possesses usually a coarse crystalline text- 

 ure, weathering readily into picturesque forms of domes and pillars, with 

 great diversity of outline, the whole more or less covered by granitic detri- 

 tus. The feldspar and quartz are intimately blended, giving the rock a 

 white or yellowish-white color, while the mica, although alfvays present, 

 varies greatly in amount. White Cloud Peak is the culminating point of 

 the granitic mass, and here possesses in general the characteristics of an 

 eruptive granite; that is to say, there are no distinct lines of bedding that 

 would ally it with the gneisses. • There are a series of divisional planes 

 which strike apparently with the range, and dip west, dividing the granite 

 into broad tabular masses 40 or 50 and sometimes 80 feet in thickness. 

 These planes, although traceable by the eye, when looked at in detail are 

 obscure or wanting, yet there is a further indication of parallel structure in 

 the granite, which appears in a hand- specimen by the arrangement of mica- 

 flakes. The laminae of the mica are by no means all parallel, but there is, 

 however, in certain strata a prevailing direction. Among feldspars, ortho- 

 clase largely predominates, although plagioclase is present. In the quartz, 

 liquid-inclusions are almost entirely wanting. Apatite prisms occur very 



^ From notes lurnished by Mr. Clarence King. 



