GEOLOGICAL SURVEY OF THE TEERITOEIES. 189 



out, to the height of 3 and 4 feet, were situated in a bed of clay. 

 Their contents consisted of a rather thick mud of an extreme degree of 

 fineness and of a beautiful pink color. It contained — 



Iron, (abundant,) 

 Alumina, (abundant,) 

 Lime, 

 Silica. 



Our party again divided, one portion returning to the permanent 

 camp to bring up further supplies, another to make the survey of the 

 lake in the boat, while the remainder of us started on the 9th of August, 

 on our way around the lake by land. In the evening, after a ride 

 through low, marshy ground, we camped at the head of one of its south- 

 ern arms, at the base of a large reddish-colored mountain, which forms 

 one of the prominent landmarks, being visible from all parts of the 

 lake. The nest day we crossed the mountain and pitched our tents on 

 one of the small streams that contributes to form the Snake River. The 

 following evening we reached Bridge Creek, or the Upper Yellowstone 

 Eiver, at the head of the southeast arm of Yellowstone Lake. Leaving 

 here we proceeded down the eastern shore of the lake, which we found 

 to be not so thickly covered with timber as the western side, nor so marshy 

 as the southern shores. After leaving the head of the lake, we made 

 three camps before leaving it altogether. Back of our first camp, which 

 was on a rocky bluff, there was a high ridge of igneous origin, com- 

 posed mainly of volcanic breccia, in which I obtained good specimens of 

 wood-opal. Some of the pieces were inclosed in the center of a mass of 

 the breccia, which seemed to have flown over it in a melted condition. 

 Some of the sx)ecimens obtained were evidently the heart of the wood, 

 the center of which contained chalcedony and crystals of quartz in fis- 

 sures caused originally by the splitting of the wood. Our second camp 

 was in one of the small prairies so numerous on this side of the lake. 

 Here we were joined by the supply-train, and by the party in the boat. 

 In the lake opposite to us was Promontory Point, a point of land run- 

 ning out into the water between the southeast arm and one of the south- 

 ern arms of the lake. A piece of rock brought from it contained rhomb- 

 spar and crystals of calcite, the matrix being red from the presence of 

 iron, j^ear camp were two high volcanic peaks. Mounts Stevenson and 

 Doane. The summit of the former is composed of a light-gray trachyte, 

 containing acicular crystals of hornblende. The rock is identical with 

 that on Mount Washburne. Between our two camps was the site of an 

 old hot-spring basin, now extinct, to which was given the name of Brim- 

 stone Basin, from the sulphur which exists in it. The deposit, which is 

 mostly of a white color, fills a valley that is about a mile in length, and 

 a quarter of a mile in width. It extends up the side of the mountain in 

 deep ravines, in some of which there is a strong sulphurous odor, al- 

 though the hot springs are all extinct. The water flowing from these 

 beds is cold and impregnated with alum, which probably results from 

 the water passing through the sulphur and clay beds. It is acid in its 

 reaction. 



On the 19th of August we moved our camp farther down the lake to 

 Steamy Point. Just before reaching it we passed a small group of hot 

 springs and steam-jets, which were a few yards from the shore. There 

 was about them a deposit of sulphur, iron, and alum. One or two of 

 the springs contain chloride of sodium. The average temperature of 

 these springs was 183^ F., the highest being 198^ F., and the lowest 

 178<^ F. Our camp was situated on a high bluff on the edge of the lake. 



