172 DESCKIPTIVE GEOLOGY. 



ently by a coating of ferruginous material, resulting from the decomposition 

 of the surrounding rock-mass. To the naked eye, the groundmass has 

 almost the appearance of a fine-grained sandstone, but is seen to be filled 

 with minute dark crystals and flakes of mica. The microscope detects 

 crystals of glassy sanidin, but neither hornblende nor olivine. Beneath 

 the trachytic rocks, at the head of Steves Fork, was found a small out- 

 crop of sedimentary rock, a compact, black, indurated clay, containing 

 fossil impressions. The fossils could not be identified specifically, but were 

 thought, when combined with the lithological character of the rock, to indi- 

 cate the horizon of the Colorado Cretaceous, and the beds have been, there- 

 fore, colored as such on the map. The microscope detects in this rock some 

 grains of quartz and fragments of long, slender, transparent crystals, together 

 with a few opaque grains of magnetite. 



Camel Peak, near the bend of the Little Snake River, is a remarkably 

 sharp, wedge-shaped ridge, rising abruptly about 2,500 feet above the val- 

 ley. It is composed of a light-gi'ay, compact rock, somewhat resembling 

 a basalt, but containing the same cracked grains of quartz which abound 

 in the trachytes. These quartz grains often occur as little spheres, from 

 'ihe size of a pinhead upward, which stand out upon the fractured surfaces, 

 covered with a greenish-white coating of decomposed material, and looking 

 like amygdaloidal inclusions in a basalt. In the bluish-gray, homogeneous- 

 looking groundmass, besides the quartz grains, only a few flakes of black 

 mica and occasional hornblendes or augites are visible. The microscope 

 detects the presence of small sanidins and much magnetite, with a preva- 

 lence of augite over hornblende. This rock, therefore, forms, as it were, an 

 intermediate step between the basalts and the trachytes, but from its asso- 

 ciation it has been classed with the latter group. From the broad bench- 

 like spur of Steves Ridge, to the east of Camel Peak, were collected a 

 number of diff'erent specimens, whose general habit resembles this rock, 

 which have, therefore, been also included in the trachytes. One of the 

 specimens collected from this ridge has been classed by Professor Zirkel as 

 a basalt. It is a dark-blue, compact rock, containing the usual large grains 

 of quartz, together with crystals of augite, and a few of, what are apparently 

 olivines, in an almost homogeneous groundmass. A second specimen, 



