778 DESCRIPTIVE GEOLOGY. 



granitic veins, bowever, are found fragments of the older crystalline schists, 

 which were caught in the mass at the time of the formation of the vein. 



As already mentioned, the later eruptive granites cover a considerable 

 area in the Pah-tson Mountains. They form the central body situated 

 nearly 2 miles northeast of Pah-keah Peak, and the western slopes extend- 

 ing down to the valley below. On the north and east, they are almost 

 entirely encircled by the later eruptive rocks of Tertiary age, which give 

 for the boundaries of the granite very irregular outlines. This granite 

 differs in a very marked manner from the rocks described as Archaean, but, 

 on the other hand, closely resembles nearly all the great bodies of granite 

 that form the more prominent ranges and isolated ridges lying in the 

 Nevada Basin between the Montezuma Eange and th^ Sierra Nevadas of 

 California, which have been regarded as most probably related to the 

 Jurassic upheaval. . 



In texture, this granite is a medium-grained, compact rock, having a 

 well-developed crystalline groundmass, which, however, is quite coherent, 

 breaking with some difficulty under the hammer, and forming little soil 

 where exposed to sub-aerial denudation. The prevailing color is a dark 

 purplish-gray, shading off in places to lighter tints. The individual min- 

 erals have a fresh unaltered appearance, and the feldspars are free from 

 coatings of ferruginous earthy material. It is composed of quartz, ortho- 

 clase, plagioclase, biotite, and hornblende. The quartz is in medium grains, 

 colorless or of a slight gray tint. Both the monoclinic and triclinic forms 

 are well disseminated tiirough the rock, in varying proportions, though the 

 latter appear to be always abundant, and to give a decided character to 

 the granite. Biotite is generally present in thin black laminae, and is the 

 only mica observed. Hornblende varies in amount in different localities, 

 although it forms everywhere a striking feature of the granite ; it is usually 

 present in short dark-green prisms. Titanite plays so important a part in 

 this granite, and is, at the same time, so uniformly disseminated throilgh 

 the mass, that it can hardly be regarded as an- accessory constituent, but 

 rather as an essential ingredient, although present in minute crystals and 

 in subordinate amounts. It occurs in bright amber-colored crystals, of a 

 sHghtly resinous lustre. It is abundant on the summit of Granite Peak, 



