GEOLOGICAL RErORT. 



granites, and have been altered in eons 

 the limits between them and the stmt 

 cannot decide whether both hold the s; 

 I am strongly inclined to the opinion 

 by them. 



Other disturbances of the strata sn 

 changes of the formations. At some po 

 recent strata appear to be con formal >1 

 upheavals and erosions have taken plac 

 general continental upheaval is the clot 

 which the position of the more model 

 doubtful whether more than one of tin 

 of eruptive masses within this distri 



appearance 



the granites, except tin 



the lattermost probably coincides with a second great uplifting of the Rocky Mountain 

 chains, which seems to have occurred toward the close of the ( "retaeeous or early in the 

 Tertiary period, while the last great changes of the level do not seem to have been 

 accompanied by any violent disruptions of the strata and outbursts of eruptive masses 

 in this section of the country. 



THE METAMORPHIC ROCKS. 



Many of the older formations of this section have undergone considerable changes 

 in their lithological character, by the immediate influence or secondary consequences 

 of the eruption of the igneous rocks; but completely metamorphosed strata or origi- 

 nally crystalline schists are extensively developed only in the western part of this sec- 

 tion, between the Three Crossings of Sweetwater River and the South Pass. On Sweet- 

 water, above the crossings, I observed mica schist, mainly composed of dark-colored 

 mica and quartz, with a laminated texture, also gneiss, made up of white oligoclase, 

 quartz, dark-colored mica, and hornblende (!) with a coarse crystalline granitic tex- 

 ture, and other rocks of a similar character; also some hornblende rocks which may, 

 however, be of eruptive origin. On the Rocky Ridge, east of the last crossing of Sweet- 

 water River, I noticed more outcrops of gneiss (and perhaps granite?), and some of 

 the hornblende rock ; but the western portion of this ridge appears to consist chiefly of 

 argillaceous and silico-argillaceous schists, part of which assume a micaceous character, 

 without, however, changing into mica schists. They continue westward, and form 

 numerous outcrops on the eastern slope of the South Pass, until they disappear beneath 

 the capping Tertiary strata. According to Colonel Fremont they thence extend north- 

 westward in the Wind River Mountains. 



Similar metamorphic strata, but especially a hornblende slate, are extensively de- 

 veloped some distance south of our route, in the Medicine Bow Mountains. Tit the 

 eastern portion of this section, at least near the traveled road, there are only lew indi- 

 cations of metamorphic strata. I have mentioned that some of the hornblende rocks 

 near Horseshoe Creek may belong to that series, and perhaps also some south of the r< »ad, 

 near Prele Creek, where I observed a curious alternation of granite, a crystalline or 

 compact greenstone or hornblende slate and quartz rock. 



