170 DESORIPTiyp] GEOLOGY. 



Yampa River. From the Sugar Loaf, an isolated flat-topped hill near the 

 forks of the Yampa River, was obtained a gray sanidin-trachyte, which 

 contained none of the grains of quartz, which are so unfailing an accompani- 

 ment of most of the other trachytes of this region. It has a more massive 

 habit than the trachyte of Whitehead Peak, but contains like it large crys- 

 tals of sanidin, imbedded in a gray porous groundmass. Associated with 

 the sanidin are numerous hornblendes and black biotites, while the ground- 

 mass is made up of micro-crystalline feldspar, and hornblende. 



To the north of Whitehead Peak, an outlying western spur of Steves 

 Ridge, which forms a secondary parallel elevation between the main ridge 

 and Steves Fork of the Little Snake River, is formed of a still more char- 

 acteristic quartziferous trachyte. This rock bears a remarkable resemblance 

 to the famous trachyte of the Drachenfels on the Rhine, containing large, 

 well-defined crystals of sanidin-feldspar, often an inch or more in length, 

 in a rough, gray groundmass, associated with crystals of mica and a few 

 hornblendes. Like the trachyte of Whitehead Peak, the weathered sur- 

 face of this rock is full of rounded cavities, from which the grains of cracked, 

 glassy quartz, in which it abounds, have fallen out. Some varieties of the 

 rock present a white color, with a porous, almost earthy texture, from which 

 the harder sanidin crystals can easily be separated. These sanidin crys- 

 tals possess remarkably distinct, well-defined crystalline faces, having a 

 dull, smooth, compact surface, and resembling the orthoclases of some of 

 the porphyritic granites or felsitic porphyries. Under the microscope, no 

 augite or microscopical quartz could be detected. It discloses, however, 

 some titanite and apatite prisms, while in the dark quartz grains are seen 

 well-defined glass-inclusions, and the groundmass is made up of feldspathic 

 particles. A thin section of one of the larger sanidins shows that it is 

 made up of smaller crystals of sanidin, with a few striated plagioclases, 

 and also contains some hexagonal and' rhombic sections of quartz, but 

 neither glass- nor fluid-inclusions. 



• From the eastern spurs of Steves Ridge, toward the head of Little 

 Snake River, was obtained a trachyte, almost identical with that from the 

 summit of Whitehead Peak, in which olivine is present, with a considerable 



