ll 



EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



ii were observed in situ. With them I found pseudomorphous green- 

 z, shaped after feldspar. Near the South Pass I noticed fragments of a 

 ning white mica. As I have not seen any similar rock in these mount- 

 ;i bowlders at Bryan's Pass, southcasl from the first locality, and again 

 the same direction on Cache-la-poudre Creek, a tributary of South Platte 

 i.se that such granites form part of the western branch of the Rocky 

 lending from the Park Mountains to the Wind River Mountains. On 

 Of Sooth Pass I observed, however, granite, in situ, similar to that of the 

 fountains. 



nstones evidently date from a later period than the granites, in which 



ly form dikes. They art* composed of white feldspar and green horn- 



[tpear to be related to the diorites. Some of them are finely crystalline, 



tinerals can readily be distinguished. In others the hornblende prevails 



u only ]>e seen on the weathered surface of the rock. 



. form a homogeneous mass of dark greenish-gray 



mixture of the white with the green mineral, the 



be mistaken for basalt, and I should not be surprised 

 mtains which have been described as basalts should, 

 to be such dioritic greenstones. The lithological 

 character ot part ot these greenstones, especially of the latter description, is such that 

 loose pieces ot them cannot well be distinguished from metamorphic slates, hornblende 

 slates, and the like, and their eruptive origin is only proved by their position in dikes. 

 Similar rocks, which are evidently metamorphic, occur on our route and in the adja- 

 cent districts. On Horseshoe Creek I noticed pieces of a hornblende rock, with a 

 peculiar concretionary structure like the " schaalstein " of Germany, in which the 

 hornblende seems to envelop numerous small concretions of the size of lentils, and 

 which therefore presents an undulated surface. If not found together with other horn- 

 blende rocks winch appear to be eruptive, I would at once put it down as metamor- 

 phie; but, under the circumstances, not having seen it in situ, I hesitate to express a 

 decided opinion. 1 



< Greenstones have been observed at various points, between Horseshoe Creek and 

 the Rocky Ridge, a short distance east of the South Pass. They are best exposed on 

 Sweetwater River, in the Rattlesnake Mountains, where they cross the granite in 

 numerous dikes, and can easily be seen on account of the bareness of these mountains. 

 At Devil's Gate I noticed one on each side of the road, and several others near the 

 Gate. One vein has, in former times, filled a large portion of the gap, and may have 

 given origin to it by its disintegration. 



From the limited number of observations which could be made in regard to the 

 relative position of strata of different age and the igneous rocks, it has not been pos- 

 sible to determine the exact period of the eruption of the granites and greenstones. 

 The granites are undoubtedly very old. Similar rocks in the eastern hemisphere are not 

 positively known to have disturbed any other than Paleozoic formations, and this seems 

 also to be the case here. The Carboniferous strata have certainly been tilted by the 



