GEOLOGICAL SUEVEY OF THE TERRITORIES. 49 



ber of quite importaut branches, whicli also have cut deep canons for 

 ten or twelve miles back to the divide, flow into Slough Creek on either 

 side. The sides of the valley are everywhere from 2,500 to 3,000 feet in 

 ■ height, and are much more irregular than in the caiion of Middle Fork. 

 In the granites are seams of feldspar and quartz two feet in width. So 

 abundant in some localities are these seams that they become a notice- 

 able feature. 



The valley for the first twelve miles from its source is from one-fourth 

 to half a mile in width. The upper portion of the valley is volcanic, 

 but seems to have cut down to the granites, so that in the bottom we 

 travel for the most part over huge granite-bowlders. 



About twelve miles down the stream we came to an open bottom three-' 

 fonrths of a mile wide, covered thicklj'^ with sage. This continued about 

 three miles, then the valley closed up again within granite walls, and the 

 waters of the stream formed a beautiful cascade. Again the valley 

 expanded out and limestones were revealed on either side in regular 

 strata, from 100 to 300 feet in thickness. Again the valley closed up in 

 a granite caiion, and soon opened out into the valley of East Fork. It 

 is plain that the valley of Slough Creek is purely one of erosion. 

 Through the volcanic rocks and the limestones the waters have carved a 

 clean smooth channel, but immense masses of granite have been left in 

 the lower portion of the valley. The surface of the metamorphic or 

 granitoid rocks seems to have been very irregular, and when the lime- 

 stones have been stripped off by denudation, the valley would be 

 obstructed by masses 50 to 150 feet high, over which the waters seem 

 to have rolled for ages without making much impression. At any rate, 

 the evidence is clear that the volcanic rocks and the limestones yield 

 far more readily to meteoric agencies than the granites. 



We may say, in conclusion, in regard to the rocks of this district, 

 that we find a series of limestones probably of Silurian, and containing 

 some of Carboniferous age, resting upon an irregular surface of meta- 

 morphic gneiss, and upon the irregular surface of these limestones reposes 

 a greater or less thickness of volcanic trachyte and conglomerate. We 

 may judge of the different elevations at which we find the granite where 

 the peaks capped with limestone are 1,000 to 1,500 feet higher than the 

 valleys with the rocks holding the same relations. We thus see that 

 while depression of some portions of the surface may come in as an 

 element, yet elevation has really been the most prominent direction of 

 the force. 



We left this most interesting region with regret. I do not believe 

 that there is, at the present time, a more novel or interesting portion 

 of our continent for exploration than that about the sources of the 

 various branches of the Yellowstone, from the Lower Cafion to the 

 mouth of the Big Horn Eiver. The numerous large streams like the 

 Bowlder, PLOsebud, Rock Creek, Clark's Fork, Pryor's Fork, «&c., cut 

 deep and most picturesque gorges down the sides of the mountains until 

 they flow out into the plains. We have probably expressed the geology 

 of this region in general terms in the present report, but the amount of 

 interesting detail which must yet be wrought out before the formations 

 can be colored properly on a map must be very great. 



We returned to our camp below the mouth of East Fork, and the 

 following day pursued our way up the main valley of the Yellowstone. 

 For a detailed description of this valley, its falls, canons, hot springs, 

 &c., the reader is referred to the Report of the United States Geo- 

 logical Survey for 1871. The exploration of the present season de- 

 veloped but little that was new in this region. More careful instruinental 

 4 G s 



