248 GEOLOGICAL SURVEY OF THE TEERITORIES. 



the montli of the creek, which is constantly extending itself into the 

 lake. Some large lagoons are here inclosed by narrow sand-bars, some 

 of which are scantily cos^^ered with pines. On this marsh, a little back 

 from its present border and a foot or so above its level, an old shore- 

 line is faintly marked. The only plainly-marked terrace is about 112 

 feet above the lake. It consists of thick beds of sand and gravel, which 

 form the hills just described. The gravel has formed a loose conglom- 

 erate, but the sand has been compacted into a pretty solid quartzite by 

 the siliceous deposits of the springs which filled it when it was beneath 

 the lake-waters. This consolidation, so diametrically opposed to the 

 disintegration just now described, was probably consequent upon the 

 pressure of the overlying lake, which caused a very slow percolation of 

 the spring- waters through the sand, except at the points of more active 

 ebullition. 



The present level of the lake is about 7,792 feet, at which the theo- 

 retical boiling-point of water is about 198.1°. Springs were observed in 

 active ebullition at various temperatures, from 198° to 182°, and one 

 even at 1G0°; in this latter case the ebullition was probably due to an 

 escape of gas, though that was not apparent. Quiet springs were of all 

 temperatures, up to 186°. Certain sulphur- vents gave 190°. 



For some six or eight miles north, along the divide, the ridges consist 

 mainly of obsidian-sandstone, inclosing heavy bands of reddish-brown 

 and variegated obsidian, having a nearly east and west strike. Most of 

 the subordinate ridges had about the same course. All of this district 

 is heavily timbered, with few openings, so that it became necessary to 

 climb trees when we wished to get any idea of the surrounding country, 

 even upon the highest crests. 



On September 9th we moved to the outlet. The lake is of a very irreg- 

 ular form, something like that of a well-filled purse, contracted in the 

 middle to quite a narrow passage. Besides the Geyser Creek, it receives 

 only one streaui of any size, which enters at the southern end of its more 

 eastern pocket, coming from the high plateau on the south, before 

 mentioned. The northern and eastern shores and the central part of 

 the southern ones are mostly precipitous, with deep water near them, 

 but along the western and parts of the southern shores one can ride 

 most of the way in the edge of the water, thus avoiding the swampy 

 ground and fallen timber of the steep hill-sides. The center of the lake 

 was apparently deep, though, as we did not put the boat together here, 

 we were not able to take any soundings. The water was clear, and the 

 shore sand and gravel quite clean, even from diatomaceous growth, ex- 

 cept at the mouths of the streams. At the entrance of the southeastern 

 tributary, a considerable marsh has formed, on the eastern side of which 

 small spring-deposits were noticed ; but no springs were seen here except 

 cold oues. 



Near the outlet, the rocks are mainly mottled black and red spher- 

 ulitlc obsidian-porphyry. 



At 6.o3 p. m,, (9th,) just as we were camping, we felt three slight 

 earthquake shocks. 



On the 10th we moved about five miles, to a camp at the north end of 

 Lewis's Lake. In leaving Shoshone Lake, the river has at first wide bot- 

 toms, but the valley soon becomes quite narrow, with steep banks. 

 About one and a half miles below the lake, the stream widens to between 

 600 and 700 feet, and becomes very shallow, with muddy bottom. Then 

 it narrows suddenly, and rushes through a rocky gateway, perhaps 75 

 feet across, formed by the projection into the stream of a mass consist- 

 ing of two layers of obsidian-sandstone, with a rapidly disintegrating 



