GEOLOGICAL SURVEY OF THE TEREITOEIES. 135 



breccias, and it is quite-possible that they even conceal the great mass 

 of compact trachyte rocks. At any rate, so far as the eye can reach, 

 the true trachyte rocks are exposed only in the form of cones, here and 

 there, while the great mass on the surface is the breccia. They are 

 continually disintegrating, so that all over the sides of the mountains 

 and among the foot-hills there are immense quantities of debris ; not 

 unfrequently huge masses are gradually broken off from the sides of the 

 mountains by the combined action of water and ice, leaving a vertical 

 wall 50 to 200 feet or more in height. 



From our camp on the east side of the lake, we ascended Mounts 

 Doane and Stevenson. Between the lake shore and the summits of 

 these peaks, there is a succession of ridges, which measured 8,500, 8,800, 

 9,000, 9,200, 9,400 feet, &c. These peaks, with an intermediate lower 

 portion, form a part of the rim of a huge crater, and on the inner side 

 the layers of trachyte appear like strata, inclining from the crater 10^. 

 The rocks are somewhat varied in texture, more or less compact, but 

 mostly very compact hornblende trachyte. J^ear the summit the rocks 

 are slightly porous, true basalt, as if they had not been subjected to 

 much pressure. Some of the rocks are red or ashen-gray, and have a 

 slaty cleavage; the volcanic breccia rises to the height of 2,000 to 2,500 

 feet above the lake. 



On the east side, the proofs of the former elevation of the lake may 

 be seen 300 to 500 feet high on the sides of the mountains. The little 

 streams that cut through the lower hills, along the borders of the lake, 

 expose 150 to 200 feet of stratified, recent deposits. Near Steam Point 

 the waters of the lake have washed the shores for two or three miles, 

 so as to expose 100 to 150 feet of strata, composed of volcanic sand and 

 gravel at the bottom, passing up into fine sand, and at the top consider- 

 able thickness of coarse sandstone and conglomerates. All these modern 

 deposits have been permeated and in part cemented with, the silica 

 of the old hot springs. We have said enough about the modern lake 

 deposits to establish the fact that they are worthy of attention, and 

 form a portion of the geological history of this basin. We shall only 

 allude to them hereafter as we meet them in our travels. 



One of the most remarkable localities for extinct springs is on the 

 east side of the southeast arm of the lake, at the head of Alum Greek, 

 and marked on the map ''Brimstone Basin." For half a mile before 

 reaching this spot the air is filled with a disagreeable sulphurous smell. 

 The deposit is mostly silica, though there is a good deal of sulphur 

 mingled with it. In the bed of the little stream that passes through 

 the basin are numerous small springs, from which bubbles of gas are 

 constantly escaping, probably sulphureted hydrogen. The little creek 

 which passes through the basin rises in the higher ridges ten miles dis- 

 tant, and, as it passes through the spring deposit, is rendered turbid 

 like milk. The channel is coated with a creamy-white material, silica 

 and sulphur; old pine logs, which must once have formed large trees, 

 now lie prostrate in every direction over the basin. It covers an area 

 of about three miles in extent, and, in some instances, a vertical thick- 

 ness of 50 feet was exposed. Not a trace of any spring could be found 

 ■«eith a temperature above ordinary spring-water. From all appear- 

 ances, this basin must have been active within a comparatively modern 

 period. It is true, however, that these springs are continually becoming 

 extinct, and have done so ever since the great period of volcanic activity 

 in this region. 



The hot-spring district, above and below Steam Point, is quite inter- 

 estiug, as showing the remains of what was once a very important group. 



