192 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



we met with on our way to Virginia City in June. They probably ex- 

 tend across the country. ] rode up the valley for some distance, and 

 found the mountains to be limestones, alternating with white quartzites, 

 for six or seven miles. I also discovered a trap -dike. Near the mouth 

 of the valley there is an old hot-spring formation, of which nothing now 

 remains save the hard calcareous basins, overgrown with low bushes 

 and grass. The basins are on the side of a hill, and when the springs 

 were active must have resembled very closely the springs at Gardiner's 

 Eiver. There is a small stream of cold water flowing over it. Eeaching 

 the Beaver Head Eiver again, I proceeded up the stream, through a 

 rather picturesque canon, at whose mouth were towering masses of a 

 trachyte porphyry, which was vesicular, having a brown, vitreous matrix, 

 containing small, irregular cavities coated with blue chalcedony. This 

 rock rests upon white sandstones of loose texture, which are probably 

 of Tertiary origin. Crossing the river, our road led us close by expo- 

 sures of siliceous clay-slates, which were again succeeded by an igneous 

 rock of a greenish-black color, and specific gravity of 2.32, the cavities 

 being filled with masses of chalcedony varying from the size of a pin- 

 head to two inches in diameter. 



We also met with an old hot-spring formation, probably connected 

 with the one mentioned above as occurring in Black-Tail Deer Creek 

 Valley. The deposit is calcareous, very hard, and the springs must be 

 long extinct. The water, which is cold, flows over it, forming a small 

 cascade. I obtained some good specimens of calcareous tufa. We also 

 passed some beds of bright-red sandstone conglomerates, or pudding- 

 stone, as the pebbles were small. We obtained specimens of a breccia- 

 ted rock, which seems to be a friction breccia. The matrix is of a junk 

 color, and seems to be volcanic in its nature, while the fragments it 

 incloses are siliceous, and of a greenish-white color. It probably occurs 

 at the margin of the trachytic rocks found in the caiion. Our camp on 

 the 11th of August was on Horse Plain Creek, in a valley covered in 

 spots with quite an abundant deposit of alkali. Leaving here, the rocks 

 first encountered were granitoid gneisses, succeeding which were alter- 

 nate beds of limestones and quartzites, which continued, with the ex- 

 ception of a few igneous outbursts, until we reached the main divide of 

 the Eocky Mountains, a distance of about thirty miles. On Sage Creek, 

 in the foot-hills, there were beds of light-brown clay-slates, which were 

 fossiliferous. We crossed the divide on the 14th of September, over 

 reddish quartzites highly metamorphosed, probably, in part at least, by 

 contact with an outburst of igneous rock at the same place. We pro- 

 ceeded down Medicine Lodge Creek, camping on that stream in the 

 evening. We passed by a bed of old hot-spring deposit, resembling a 

 stratified limestone. It was about 60 feet in thickness. Near camp, 

 there was an exposure of purplish-colored volcanic rock, that I con- 

 sider a trachyte, upon which rested a dark basaltic rock. Beneath 

 these were white sandstones, very fine-grained and splitting into layers 

 of an inch in thickness. They are probably Pliocene in their origin. 

 Just before reaching the Snake Eiver Valley, we ascended a broad 

 plateau of basaltic rock, like that bordering on Snake Eiver. In crev- 

 ices in the rock, we found obsidian. We crossed Snake Eiver the second 

 time, finding it about 20 feet lower than when we crossed it in June. 

 We arrived at Fort Hall on the 19th, and left on the 21st, proceeding up 

 .'Lincoln Valley, between hills of Jurassic limestone. We camped in the 

 -evening at Twin Springs, where there are the remains of old hot springs. 

 Kear us there were two extinct craters, and the whole valley was overflowed 

 with lava. The following day we reached Bear Eiver, and turning up it 



