GEOLOGICAL RErOET. 2G9 



Mountains, I found more indications of rocks of this period (see section IV), which, 

 therefore, must have a wide range. As they contain highly fossiliferous beds, it is 

 remarkable that they have never been noticed before. Underlying these Jurassic 

 strata there is a gypsum-bearing formation, mostly made up of red, shaly sandstones 

 which in all probability belongs to the Triassic period. 



All the strata of this district, even the most recent of the Tertiary formations, have 

 undergone some dislocations, but these latter, like tin* Tertiary formations of section 

 II, have been raised uniformly as a whole, and overlie, nearly horizontally, the older 

 rocks. 



The mountain ranges mainly consist of the upheaved older formations, which 

 have partly been considerably altered, in connection with igneous and metamorphic 

 masses, while in the valleys the more recent strata predominate. We hud, however, 

 Cretaceous and Tertiary strata crowning some of the main divides at South Pass, 

 Bryan's Pass, &c. 



THE IGNEOUS ROCKS. 



Effusions of igneous masses have taken place in this district at different times, 

 partly at an early period, probably toward the close of the Paleozoic era, partly at a 

 much later date. We have evidences of it in the unconformable superposition of the 

 strata of the various periods, combined with the difference in the miueralogical char- 

 acter of the eruptive rocks. These belong to at least two quite distinct groups, the 

 granitic group and the greenstones. 



Granites, composed of feldspar, quartz, and dark-colored mica or black hornblende 

 and granitic syenites, closely related to them, form the main body of the eastern chain 

 of the Rocky Mountains south of Fort Laramie, between the North and South Platte, 

 and much farther on. Similar rocks are extensively developed near our route. They 

 were observed in the mountains of the Laramie Peak system, on the divide between 

 Bitter and Horseshoe Creeks, and farther west, near Prele Creek; also in the mount- 

 ains south of the road, west of Deer Creek. They entirely form the Rattlesnake 

 Mountains from near the mouth of Sweetwater River to the Three Crossings, and part 

 of the Sweetwater Mountains. According to Colonel Fremont, the Wind River Mount 

 ains have also a granitic center. 



The granitic rock from Horseshoe Creek appears to be composed of reddish- 

 white orthoclase, milk-white oligoclase, quartz, and black mica, which are the normal 

 elements of true granites. That from the Sweetwater Mountains and Rattlesnake 

 Mountains is quite similar, and nearly all the specimens obtained from rocks in situ 

 present the same appearance. This, as well as the parallelism and close connection of 

 these ranges, indicates that they are only different spurs of one mountain system. A 

 specimen from Independence Rock contains whitish pellucid orthoclase, a little white 

 oligoclase, much quartz, and greenish-black mica ; also some particles of specular or 

 magnetic iron-ore, a frequent occurrence in connection with the eruptive rocks of this 

 district. Only at two points I observed somewhat different granites forming small 

 outcrops. 



Among the loose drifted pieces of granitic rocks we find a great diversity of 

 color and composition. They seem to originate from the neighboring mountains, but 



