GERLOGICAL SUEVEY OF THE TEREITORIES. 51 



and volcanic breccia, there are sedimentary rocks, and even the granit- 

 oid group, for the latter was well shown in the second canon. I have al- 

 ready described the existence of great thicknesses of Carboniferous and 

 Jurassic strata on the west side of this range around Mystic Lake. Upon 

 the east side, in some of the gorges or ravines of the Yellowstone drain 

 age, it is quite possible that some of the older rocks are exposed. The- 

 highest peaks, many of which are covered with snow all summer, are 

 composed of volcanic breccia ; on the north side of Trail Creek there is a 

 range of hills, as they may perhaps be called more properly. These 

 hills are really a group of broken ridges ; the anticlinal belt seems to 

 diverge, one portion passing up along the divide or water-shed, between 

 the sources of the Gallatin and Yellowstone, appearing in full force at 

 Cinnabar Mountain ; the other following along the north side of Trail 

 Creek, crossing the Yellowstone Eiver at the lower canons, and extend- 

 ing oft" on the northeast slope of the Snow Mountains, about the sources 

 of Big Bowlder, Eosebud, and Clark's Fork of the Yellowstone. The 

 amount of erosion in the interval, between these two portions of the 

 anticlinal, has been very great. E'ot that the valleys have been en- 

 tirely carved out of the mountains, for they were doubtless, in part at 

 least, and perhaps in all cases, marked out in the process of upheaval. 

 The valley of Trail Creek, which is a narrow gorge at the head, gradu- 

 ally expands out, near its entrance, to the immediate valley of the 

 Yellowstone, a distance of about twelve miles, so that it is about two or 

 three miles wide. We can now see, by fragments of ridges that are re- 

 maining, that portions of all the formations known in this portion of 

 the West, however much they may have been fractured by upheaval, 

 once extended across the broad interval. 



Should we ascend the high pine-covered ridge on the north side of 

 Trail Creek, we can look over into the next valley beyond, and along its 

 northern side, extending west or northwest nearly to Fort Ellis, we can 

 see the outcropping edges of the coal-beds, inclining north and north- 

 east in wave-like ridges, until they die out about ten miles distant, from 

 the reverse effect of the force which elevated the Crazy Woman Moun- 

 tains. The Yellowstone Eiver cuts directly through this ridge, and 

 thus forms its first canon, and the point of exit from the canon is called 

 the exit of the Yellowstone from the mountains. The walls on either 

 side are entirely of Carboniferous rocks. The view from this ridge near 

 the canon, down the Yellowstone Valley to the Crow agency, is very in- 

 structive. Above the canon the river flows nearly northward, but after 

 emerging from the canon it bends quickly around to the northeast and 

 east, and enters a lower gorge, cutting through Tertiary and Cretaceous 

 beds, about three miles below the mouth of Shield's Eiver. This valley 

 belongs to the old lake system ; is oval in shape, expanding from 

 one-fourth of a mile in width at the upper end to four or five iiiiles. 

 It is about ten miles in length and has an average width of three miles. 

 On the left side of the Yellowstone, the somber-hued rocks of the Creta- 

 ceous and Eocene Tertiary groups i)resent their basset edges like walls, 

 and recede to the northwest and north, in step-like ridges, for ten or 

 twenty miles. The thickness of these beds I could only estimate, and 

 I believe them to be in the aggregate 1,500 to 2,000 feet in thickness. 

 The inclination or dip varies much, sometimes 25^ to 30°, then 10^ 

 to 20°. Just below the mouth of Shield's Eiver, on the left side of 

 the Yellowstone, there is a nearly vertical bluff* of these beds, composed 

 of alternate layers of sandstone and arenaceous clay, all with the steel- 

 gray hue. The rocks are all of various textures and composition ; some 

 layers contain a considerable per cent, of clay, and the harder beds vary 



