GEOLOGICAL SURVEY OF THE TERRITORIES. 213 



date, July 17, the pools left by the fi'eshets had Hot quite disappeared. 

 From a short distance above the ford, the river flow^ over a basalt-bot- 

 tom for some miles, occasionally cutting rather deeply and leaping over 

 small falls, but generally having low banks. 



Thus far, we had been accompanied by a large wagon from Fort Hall, 

 which carried some of our heaviest supplies, so that our pack-animals 

 might become accustomed to light loads before receiving heavy ones. 

 Having now reached the limit of convenient wagoning, though it would 

 have been possible to take vehicles even to the very base of the Teton 

 range, we sent the wagon back, and spent one day in re-arranging packs 

 and in examining the proposecl route up T^ton Eiver, or Pierre's Eiver, 

 as it is more properly called, since this latter name was long since com- 

 monly applied to it among traders and trappers, as well as upon maps 

 of the region. The trappers of the present day, having little or no 

 knowledge of the names used by their predecessors, have, of course, 

 applied their own terms to the landmarks with which they are familiar ; 

 but the adoption of these by geographers would be contrary to all rules. 



Upon examining Pierre's River about six miles south of our camp, at 

 a point not far above its junction "with Henry's Fork, Mr. Stevenson re- 

 ported it as occupying a deep though narrow channel, walled on either 

 side by perpendicular cliffs of basalt, 60 feet in height, which frequently 

 come to the water's edge, so as to make travel along the bottoms im- 

 possible. Our guide, Beaver Dick, stated that these clilfs increased 

 rapidly to several hundred feet in height, rendering the stream unap- 

 proachable by stock for over thirty miles. As we were not in condition, 

 being so heavily loaded, to make so long a march without water, we 

 turned northward for a few miles, to pass up the valley of the next 

 tributary, which Beaver Dick called the middle fork of the Snake. 

 Since neither by position nor by size is this stream entitled to be con- 

 sidered a fork of the main Snake, we have applied to it another name. 

 Falls River, for reasons which will appear later in this report. Camping 

 just above where we struck the stream, we found it about 3 feet deep 

 and from 50 to CO feet wide. On the opposite bank, a bluff of basalt, 

 about 30 feet high, showed a distinct prismatic structure through the 

 upper 10 feet, while the central portion was amorphous and the lower 

 5 feet, again prismatic, the whole looking as if the upper and lower 

 portions had been affected by rapid cooling and consequent contraction 

 of the surfaces of the sheet of lava, while the central portion, cooling 

 much more slowly, did not reach the same state of tension. Just above 

 camp, on the south side of the stream, there is a considerable bluff of 

 light-pink porous porphyry, with an indistinct bedding, apparently up- 

 turned nearly to verticality, with a nearly north and south strike. The 

 upheaval evidently occurred before the tlow of the basalt. 



As we leave the valley of Henry's Fork, July 20, the country rises, 

 and becomes more broken and rolling. The abundant growth of grasses, 

 sedges, and other flowering herbs shows plainly that only slight irriga- 

 tion would be necessary to make this valuable farming-land ; and some 

 crops would succeed without that. Most of it would be fine laud for 

 stock-raising, the only exceptions being those portions along deeply- 

 caQoned streams, where water is difficult of access. We bore a little to 

 the southward to-day, toward the upper waters of Pierre's River. 

 About eight miles from camp, two rocky knolls, rising about oO feet 

 above their connecting saddle, and perhaps 350 or 400 feet above the 

 nearest creek, mark two points of the rim of an old, broken-down crater, 

 which faces nearly due east. The lava is a laminated mixture of quartz, 

 obsidian, and feldspar; but no mass seemed to be «?i sltn, so that direc- 

 tion of structure could not be determined. 



