GEOLOGICAL REPORT. 273 



2. On North Platte River, some 15 miles above Fort Laramie, and some miles 

 farther west, where the road strikes Horseshoe Creek. The rocks there are partly like 

 those of the quarry, partly siliceous altered sandstones, &c. 



3. Captain Stansburv obtained Carboniferous fossils at a point some distance south 

 of the road, not far from Prele Creek. 



Similar rocks are considerably developed near these localities, but have not fur- 

 nished any fossils to our collection. 



l\Tmkm formation,— Hitherto in this part of the Rocky Mountains no strata have 

 been conclusively identified with the Permo-Carboniferous formation of the Eastern 

 Kansas or the truly Permian period. 



Strata of generally much altered rocks, the exact age of which could not be deter- 

 mined on account of the scarcity or total absence of fossils, but which apparently be- 

 long to the Paleozoic periods, are largelv developed in the mountain-ranges of this dis- 

 trict, They cap the granites of the Black Hills, far north and south of Fort Laramie. 

 On our routes, we found them on the Platte River, from some miles above Fort Lara- 

 mie to the upper end of the canon near the first crossing of the river road. Part ol 

 these rocks have already been mentioned among the Carboniferous. I noticed highly 

 altered light-colored sandstone-, partly calcareous, or veined with agate, siliceous lime- 

 stones with secretions of Hint and jasper, marbles of various colors, some purple sand- 

 stones, &c. They contain traces of fossils, but more perfect ones were only obtained 

 at the localities mentioned above as Carboniferous. Such rocks also crop out at numer- 

 ous points south of the river, and toward Laramie Peak. On the upper road we find 

 them on Bitter Creek, and on the mountains east of La Bonte Creek. 



On La Bonte Creek, some distance south of the road, strongly tilted rocks form 

 several ranges of hills, parallel to each other and to the higher mountains in the south. 

 They present a uniform dip oif the latter, and, therefore, steep escarpments in one 

 direction, and more gentle slopes in the other. Sandstones prevail there of white, gray, 

 and brown colors; others are purple or dark brick-red, the latter mostly rather soft. 

 They are interstratitied with arenaceous and argillaceous shales and slates; gray, green, 

 bluish, reddish, &c. While the sandstones form the hills, the shaly strata have been 

 more easily eroded, and correspond to the intervening valleys, which are partly occu- 

 pied by more recent Tertiary formations. They trend generally to north-northwest, 

 and dip strongly to east-northeast over 60°, and at some places they are even vertical. 

 Near the road the trend and dip are much disturbed by local manifestations of the 

 subterranean agencies. A short distance from La Bonte Creek we find formations 

 which underlie Jurassic strata, and are provisionally referred to the Triassic period ; 

 and the question arises whether these sandstones and shales are not, perhaps, of the 

 same age, or hold an intermediate position between them and the more calcareous 

 portion* of the Carboniferous formation. Further west, near Prele Creek, the mount- 

 ains south of the road are mainlv composed of gray and white and some light red and 

 purple sandstones, with few interstratiiications of slates and pure or siliceous lime- 

 stones. Their dip is variable but strong. They are apparently also Paleozoic. Cap- 

 tain Stansbury obtained there some Carboniferous fossils. The valley of Platte River 

 is o-enerally occupied by more recent formations, and the higher mountains with the 



