30 GEOLOGICAL SURVEY OP THE TEERITORIES. 



great basin. We can also see how readily such, rivers as Camas, Medicine 

 Lodge, Grodins, and many others disappear in the plains, and find their way, 

 from ten to thirty miles, to Snake Eiver, underneath this basaltic floor. 

 Before leaving this interesting region, I wish to add a few general re- 

 marks in regard to what may be very jiroperly called the Snake Eiver 

 Basin. There is here a broad, nearly level plain, from seventy-five to 

 one hundred miles in width, and one hundred and fifty to two hundred 

 miles in length, surrounded on all sides by mountain ranges. This basin 

 follows the course of Snake Eiver, and is really an expansion of the val- 

 ley; and it at first extends from the northeast to the southwest, 

 bends around west, and then continues northwesterly toward Boise 

 City. The mountains on either side form a series of more or less lofty 

 ranges, some of the more prominent summits rising to a height of 10,000 

 feet. These ranges appear to the eye, from any one point of view, to 

 trend about north and south, but the trend of the aggregate ranges is 

 plainly a little west of north and east of south. Between these ranges 

 are valleys of greater or less breadth, varying from one to five miles in 

 width, oftentimes of great beauty and fertility, through which wind 

 some of the numerous branches which flow into Snake Eiver. The great 

 basin is entirely covered with a bed of basalt of quite modern date, (Fig. 

 4,) and this basalt has set to a greater or less distance up the valleys of all 

 these streams. It extends up the Port Neuf Valley twenty or thirty 

 miles. The American Falls are formed by the descent of Snake Eiver 

 over the basalt. I believe that this vast basin has been worn out of 

 the mountain ranges by erosion ; that the three buttes and other frag- 

 ments of ranges scattered over the i)lains serve as monuments in proof 

 of this statement. This basin was also the bed of a lake which proba- 

 bly originated during the Pliocene period. At any rate, I have been 

 unable to discover in the immediate vicinity of this basin any Tertiary 

 beds of older date than the Pliocene; and underneath the basaltic 

 crust there is a considerable thickness of the deposit. The effusion of 

 the basalt was one of the latest events, and must have merged well on to 

 our present period. The average elevation above the sea is from 4,000 

 to 5,500 feet. Our camp on the Blackfoot Fork was 4,324 feet, which 

 was at least twenty miles above Snake Eiver east; and, inasmuch as the 

 basin extends down Snake Eiver, the valley below the American Falls, 

 and near Boise City, cannot be over 4,000 feet, and may be less, while 

 near the northern rim the elevation is 5,730 feet. From the great 

 basin of Snake Eiver we ascended the hills that form the northern rim 

 over a divide 6,200 feet high, with hills on either side rising 1,200 to 

 1,500 feet higher. All these hills are capped with beds of basalt, which 

 incline southward toward the. basin at various angles, from 5^* to 10°. 

 Where the rocks can be seen they are plainly igneous, but as we ap- 

 proach Pleasant Valley the hills are so covered with a drift deposit that 

 it is seldom the underlying rocks can be seen. The surface here, for 

 miles in extent, is made up of short, abrupt hills, generally one main 

 sharp ridge, with a great number of side ridges extending from it. 

 These hills are covered over with grass. The rocks that are scattered 

 thickly over the surface, and enter largely into the composition of these 

 superficial deposits, are rounded bowlders of quartzites mostly. The 

 distance from our camp on Dry Creek, in the Snake Basin, was sixteen 

 and a half miles. The little stream that flows through Pleasant Valley 

 emerges from a canon, which has nearly vertical walls of basalt, with 

 an irregular bedding, but with jointage quite perfect, fracturing into 

 columnar masses. A vast amount of debris has fallen down the sides of 

 the walls and into the bed of the stream. Some of the rock is very 



