324 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



From the latter point the expedition crossed the T6ton Pass, following 

 the western base of the T6ton Eange into the country north of this dis- 

 trict. Notwithstanding the haste with which the expedition prosecuted 

 its march, the salient geological features were seized upon, and wher- 

 ever later exploration has touched Captain Kaynolds's trail, the work has 

 been rather an amplification according to the better opportunity for more 

 extended examinations. The exj^editions of this survey have also ex- 

 jilored considerable portions of the district ; that of 1871 passing along 

 the western border, and the Snake Eiver expedition in the season of 

 1872 traversed stiU more extensive portions in the central part of the 

 district in the region of the Teton Eange. It is to the latter expedition 

 to which Prof. Frank H. Bradley was attached as geologist, that we owe 

 much concise information relating to the geological histoiy of a large 

 area of territory on the headwaters of the Snake or Lewis Eiver, an in- 

 teresting account of which is embodied m the Eeportof the United States 

 Geological Survey, 1872. In the following season Capt. W. A. Jones, of 

 the Engineer Corps U. S. A., in command of an expedition to which was 

 attached Prof. Theodore B. Comstock, traversed the extreme northeast 

 corner of the district. Professor Comstock's report is accompanied by a 

 maj) on which is indicated the occurrence and distribution of the geologi- 

 cal formations over the eastern section of the district lying between the 

 Upper Snake and Wind Elvers. Other expeditions have visited the re- 

 gion for geographical and other purposes at dates remote and late. 



With the exception of the southwestern section, we had already much 

 information in regard to the geological and geographical character of 

 the district 5 mainly, indeed almost wholly, derived from the explorations 

 conducted under the auspices of this survey. Yet the facilities for ob- 

 servation during the past season have enabled the accumulating of a con- 

 siderable mass of details in further exposition of the regions already 

 partially studied, while in the southwest a tract of virgin ground was 

 explored by this division of the survey during the past season. 



GENERAL SURFACE FEATURES. 



Drainage. — The district lies almost wholly within the system of the 

 Snake Eiver drainage, the sources of Wind Eiver rising in the continental 

 water-shed near the northeastern corner. The main Snake Eiver trav- 

 erses the district in an irregular Z-shaped course, rising in the volcanic 

 area in the northern portion, making its exit near the southwestern cor- 

 ner of the district. It thus makes two great bends within this territory, 

 the first in its passage across the Snake Eiver Eange south of the T6tons, 

 whence it flows northwesterly to the northern boundary, where it is 

 again suddenly deflected in a south and southwesterly course to the point 

 where it leaves the district. Nearly half of its course passes through the 

 great volcanic-floored x)lain of the Snake Basin, the upper half lying 

 within the mountainous area, traversing in graceful windings beautiful 

 mountain basins, its current interrupted by serious rapids only at two 

 points, the one in its wild x)assage of the gorge through the Snake Eiver 

 Eange, and the dalles at Taylor's Bridge or Eagle Eock, below the lower 

 great bend. Its principal tributary is the Henry's Fork, which rises in 

 the water-shed far to the north, beyond our present limits, and joins the 

 main stream in the northwestern portion of the district through a curious 

 delta system in the midst of extensive flats, which are occupied by bea- 

 ver-ponds and thickly grown with willow. Pierre's Eiver, which lies 

 wholly within the district, draining its northern section west of the Teton 

 Eange, also forms a delta in the beaver-ponded flats on joining Henry's 

 Fork a short distance above the confluence of the latter stream with the 



