GEOLOGICAL SURVEY OF THE TERRITORIES. 23 



directly through a point of rocks that extends across the channel. The 

 old bed is now overgrown with trees and bushes, but is fifty feet higher 

 than the present one. The little creek must have changed its course 

 slightly, for some reason not apparent now, so that its waters were 

 brought against this point or wall of rock, and finding a fissure or open- 

 ing "through, it gradually wore its present channel. It is certainly as 

 perfect a natural bridge as could be desired. The opening under the 

 bridge is about one hundred and fifty feet wide and fifty feet high. The 

 old bed is about three hundred feet to the northwest. It is also plain 

 that the water at one time flowed over the top of the bridge, which is 

 fifty to one hundred feet lower than the top of the gorge, so that we 

 have here some of the intermediate steps which a stream takes in the 

 process of wearing out a gorge or channel. The rocks are mostly lime- 

 stone, quite pure, arenaceous limestone, and at the base very cherty 

 limestone. I found a few fossils in the canon in a blue limestone, as 

 Hemipronites crassas, Productus nodosus Myalina perattenuata, &c. I 

 think all the rocks are of carboniferous age, although some of them may 

 be Silurian. Whatever the age of the rocks may be, there seems to be a 

 great thickening of them as we go westward. This ridge, which is 

 twelve hundred feet above the base, is composed entirely of what I have 

 Usually classified as carboniferous rocks, and nowhere in the canon, 

 which is at least half a mile in length in a straight line, have the waters 

 worn through to the granites. On the west side of this ridge the beds 

 incline west, northwest, and southwest, and between it and the main 

 mountain range there is an interval of ten to fifteen miles in width, 

 with one or two ridges of limestone and sandstone dipping from the 

 mountains. This broad interval forms a beautiful, grassy valley, which 

 is a great resort for game, and will some time afford fine pasturage for 

 stock. The metamorphic rocks soon make their appearance. On the 

 northeast side of the ridges the White Eiver beds lap on the flanks in 

 many places, but here and there they are stripped off so as to reveal the 

 red-beds, Jurassic marls, and the cretaceous. Six miles of the valley 

 between the canon is covered with the modern deposits, and the remain- 

 der of the way to the fort, with the lignite beds, which are shown in high, 

 cut bluffs of ferruginous sands and sandstones. We were indebted to 

 the hospitable officers of Fort Fetterman for a very pleasant and instruct- 

 ive day. 



We will now notice briefly the geological character of some of the well- 

 known localities contiguous to but not immediately on the route. The 

 White Eiver group, which we have had occasion to mention so often, 

 extends from La Prele Creek eastward nearly to the Missouri Eiver. 

 It is therefore the prevailing formation in this region. Scott's Bluffs, 

 Chimney Eock, and Court-house Eocks are well-known landmarks on 

 the North Platte, belonging to the White Eiver group. A few miles 

 north, or northwest, of Fort Laramie is a group of high hills rising 

 above the tertiary beds, exposing a considerable thickness of carbonif- 

 erous rocks, with an extensive series of gneissic strata ; and here and 

 there a nucleus of feldspathic granites. Eaw Hide Butte, which gives 

 origin to several streams, as Eaw Hide Creek and a branch of the Mo 

 brara Eiver, rises above the surrounding country six hundred to eight 

 hundred feet, and exposes a nucleus of reddish feldspathic granite, with 

 gneissic strata inclining from its sides, with carboniferous limestones 

 reposing unconformably upon the upturned edges. In some cases the 

 limestones are elevated to the summits of the hills in a nearly horizontal 

 position. All these isolated mountains seem to have been islands in this 

 great tertiary lake. There is no doubt that all the formations that suc- 

 ceed the carboniferous period, as triassic, Jurassic, cretaceous and lower 



