96 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



preceding report upon the topographical features of our district this 

 subject has been more fully discussed. Geologically, this low area is of 

 the greatest importance, and furnishes very acceptable hints as to the 

 early history of the country. 



PROZOIC. 



It will be remembered that the subsidiary range west of the "Wind 

 Biver Mountains is composed of Prozoic rocks. They disappear under 

 the metalliferous schists of the Stambaugh region and remain hidden 

 for some time. We next find them at the western termination of the 

 . Sweetwater Hills. Here they appear as a grouj), trending approximately 

 east and west, composed of disconnected hills. Bare of vegetation and 

 rugged, they present a very typical aspect. The granites composing 

 these hiUs are entirely structureless, forming huge, irregailar masses. 

 Eed and flesh-colored orthoclase, white and gray quartz, and musco^dte 

 are the constituent minerals. Toward the southern edge of this group 

 we find that either loose drift or Tertiary strata reach directly to the 

 granites, covering whatever older formations may be beneath them. On 

 the north side Carboniferous strata rest upon them, preventing the 

 appearance of Silurian beds which undoubtedly occur there. 



An interesting feature was observed in these granites, which I failed 

 to notice in the subsidiary range, the occurrence of basaltic dikes. 

 Varying in width from 4 inches to 10 feet these eruptive rocks closely 

 fill fissures in the granite, striking as a rule northeast and southwest. 

 Although the granites decompose and disintegrate but slowly, we find 

 some localities where the dikes project for several feet above them. Gen- 

 erally they appear on the surface simply as black bands, conforming 

 entirely to the details of weathering and cleavage shown by the rock 

 enclosing them. In lithological character, mode of appearance, and 

 general relations to younger formations, these granites appear to be 

 ahnost absolutely identical with those farther west. 



Following the line of granitic outcrops of this region in an easterly 

 direction we find them covered by Palaeozoic and subsequently these by 

 Cenozoic formations. Curving somewhat to the north we cross the 

 Sweetwater Valley and find ourselves in the continuation of the granites 

 just described. It is formed by the Granite Hills directly north of the 

 Sweetwater. By all explorers who have ever visited tliis region has 

 been noted the remarkable barrenness of these hills. Fremont says : * 

 "Except in the crevices of the rock, and here and there on a ledge or 

 bench of the mountain, where a few hardy pines have clustered together, 

 these are perfectly bare and destitute of vegetation." Dr. Hayden, in 

 his report upon this region,t describes the Granite Hills as follows: 

 "All around the flanks of these granite ranges the same Tertiary beds 

 jut up without any interruption, and are smooth and even, so that the 

 granite masses seem to rise abruptly out of the plains. A few stunted 

 pines struggled for existence among the crevices, and some rare shrubs 

 and ferns were all the vegetable life observed." The absolute nakedness 

 of these hiUs is truly remarkable. Without any definite rock structiu-e 

 they rise directly from the plain, presenting their bald, gray and red 

 forms, which rise to a relative elevation of 300 to 1,100 feet. Smooth, 

 rounded surfaces of considerable extent render the ascent of some of the 

 higher peaks almost an impossibility. Toward the centre of the group 

 or range we find the hiUs connected with each other by ridges, mostly 

 bare, but sometimes covered with short grass. 



*Eep. Espl. Exp. to the Eocky Mts., 1842. Wasliington, 1845. 

 tEep. U. S. Geol. Surv., 1870, p. 30. 



