GEOLOGICAL SURVEY OF THE TEEEITORIES. 255 



up to the very summit, where this deposit is just pierced by an outcrop 

 of tlie gray trachytic lavas aud red basalt, partly vesicular, though 

 mostly compact, which form the luicleus of the ridge. We here stood 

 upon oue of the highest points in that neighborhood, about 8,054 feet 

 above the sea; so that we were entirely at a loss as to the source from 

 which had flowed the large river which had distributed su(;h immense 

 amounts of gravel and sand. We could obtain no local data which 

 should enable us to judge whether the stream had flowed northvrard or 

 southward, but the general levels of the country would imply a north- 

 erly drainage. The deposit is evidently very ancient, but no consider- 

 able consolidation had taken place. 



Looking westward, a heavy mass of mountains, broken by very few 

 water-courses, at least on their eastern face, lies between us and the 

 basin of Jackson's Lake, and I believe that this is here properly the 

 " main range" of the Eocky Mountains. It is certainly part of the highest 

 mass-connection between the Wind Eiver aud Big Horn Mountains, on 

 the east, which really form the northern termination of what is the main 

 range larther south, and the range west of the Three Forks of the 

 Missouri, which there bears the name of Eocky ]Mountains, and the con- 

 tinuation of which really appears to be the main range farther north, so 

 far as the best maps indicate. 



Above this mountain-mass the Tetons loom up grandly. A peak of 

 that range, some fifteen or twenty miles north of Mount Hayden, is 

 second only to that peak in elevation. 



Passing westward the quartzite-gravel continues for several miles, 

 though the mass of all tlie ridges is composed of a coarse volcanic 

 breccia. The slopes are here much flatter and more rounded, with heavy 

 growths of fine spruce, of which large areas have been burned and 

 have fallen, so that we found some large patches absolutely impassable 

 with animals, and were obliged to make considerable detours to avoid 

 them. The small streams heading here have flat valleys, with some 

 good meadows. 



Descending the valley of Coulter's Creek, we found that the high 

 divide between the head of this creek and the head of Snake Eiver con- 

 sisted of rapidly-disintegrating volcanic rocks, mostly conglomerates of 

 trachytic porphy:«f»^, obsidian, &c., like those which form the very i)re- 

 cipitous banks of the streams. Many bare spots on the slopes indicate 

 old land-slides, some of them of large size. The main valley is narrow 

 and in some parts deeply canonetl, so that, with both steep, rocky 

 slopes and badly-interlocked fallen timber, we had great difticulty in 

 making a passage out to the valley of the main Snake. This stream, we 

 found, after a course of about twelve miles from its ultimate sources, 

 running through a broad graveled channel, L'OO feet or more in width, 

 Avith three considerable terraces on either bank, along which we rode to 

 the junction, of Barlow's Eiver. The gravel is in some places washed 

 away by the stream, so as io expose a solid river-bed of volcanic rocks. 

 Near the junction, the terraces are at the levels of about 25, 80, and 140 

 i'eet above the present level of the bottoms. These are also developed 

 t(r some extent along Barlow's Eiver, which enters the Snake at a sharj), 

 reversed angle, through a narrow canoned valley, from the upper slopes 

 of which we could seeithe broad flats about the mouth of Heart Eiver. 

 Though this fork brings at this season the most water to the united 

 stream, and has the longest course, draining the largest area, yet the 

 relative forms of the valleys at once decided that this could not properly 

 be called the main Snake liiver. 



On the broad lower terrace, about a mile west of the mouth of Barlow's 



