102 DESCRIPTIVE GEOLOGY. 



schists and gneisses, with hornblende as the predominating constituent, 

 which have been so persistent along the entire western edge of the range, 

 give way to lighter-colored mica-gneisses; still, however, carrying some horn- 

 blende. At Cherokee Butte, the connecting mass between Elk Mountain 

 and the main range, occurs an altered Archaean granite, which is of special 

 interest, as it differs from anything observed elsewhere in the range. It is 

 a hard, medium-grained rock, composed chiefly of gray quartz and white 

 and flesh-colored feldspars, both monoclinic and triclinic species being 

 present. Micas, in minute plates, are scattered sparingly through the mass, 

 but hornblende appears to be absent, at least in the specimens collected. 

 Professor Zirkel has given a detailed description of the quartz under low 

 magnifying power, which indicates worn fragments and rounded and polished 

 grains, closely resembling the quartz-grains in the sedimentary gray wackes 

 of Germany, which is especially noteworthy here, as the habit in the field 

 is such as to suggest a metamorphic product. It has also many lithological 

 characteristics like the granite from Granite Mountain, in the Pah-Ute 

 Range, Nevada. 



On the western slopes of Cherokee Butte is found a typical gray gneiss, 

 the relations of which with the altered granite were not determined. It is 

 of medium texture, somewhat friable, and finely laminated. Hornblende is 

 present, but in a very subordinate amount; thin sections develop titanite. 



As already mentioned, Elk Mountain stands nearly isolated from the 

 main mountain mass, and has a higher elevation than any point in the 

 northern end of the range, except Medicine Peak. The flanks of the north- 

 em portion are covered with the sedimentary beds, which rise in many 

 localities to within 1,200 or 1,500 feet of the top, but the rest of the mount- 

 ain belongs to the Archaean series. The summit, with four points of nearly 

 equal height, is an irregular-shaped mass, which descends abruptly, but 

 regularly, toward the valley of the Medicine Bow, while on the west side 

 deep, rugged canons penetrate the slopes. In lithological habit, there is 

 considerable variety in the rocks, but they are all a hard compact, distinctly- 

 bedded series of gneissic strata. The recorded strikes of the beds differ 

 somewhat, but all have a prevailing northeast and southwest direction; 



