52 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
In the region examined below the mouth of Bruneau River the lava 
sheets forming the rim rock of Snake River Canyon on the north side 
do not extend south of the river. The south wall of the canyon is com- 
paratively low and composed of soft, white, horizontally stratified sedi- 
ments, which extend southward to the base of the mountains which 
define the border of the Snake River Basin in Owyhee County. A 
portion of the border of Lake Idaho is marked by the junction of its 
sediments with the rocks which formed its shore. The broad tract of 
exposed lake beds in Owyhee County is deepty sculptured and presents 
typical examples of u bad-land topography.' 1 
The precipitous northern wall of Snake River Canyon near the 
boundary between Ada and Elmore counties is about 150 feet high, 
and is composed of three principal varieties of rock, namely, in 
descending order, basalt, lapilli, and lacustral clay and sand. In 
general, the upper portion of the canyon wall is the edge of a lava 
sheet which underlies in part the thin covering of soil on the adja- 
cent plain, and in part another similar lava sheet. In at least one ;j 
locality the lapilli, which usually occurs below the basalt, rises higher 
than its surface, and forms the actual summit of the escarpment. The 
usual thickness of the basalt is from 20 to 40 feet. The thickness of 
the yellowish, earthy lapilli beneath the basalt is variable, in places 
being 180 feet, and at a distance of perhaps a mile from where the 
maximum exposure occurs, thinning out to but a few feet and even ; 
vanishing, the lake beds below thickening and occupying its place. 
From the locality referred to all the way to Shoshone Falls, the pres- 
ence of three stages in the filling of the basin is frequently clearly 
exposed. All of the members of the series vary in thickness, and j 
above the mouth of Bruneau River several sheets of basalt are some- I 
times present and the lapilli are wanting. The records of a few of j 
the numerous sections examined will serve to illustrate these features I 
in detail. 
At a locality in Ada County, about 7 miles below the mouth of Can- j 
3^on Creek, shown on PI. VII, the wall of Snake River Canyon reveals j 
the following section: 
Section of troll of Snake River Cannon 7 miles below month of Canyon Greek. 
Feet. I 
1. Black, compact, but somewhat scoriaceous basalt 20 
2. Yellowish, slightly compacted lapilli, with angular fragments of scoriaceouw 
basalt sometimes 2 inches in diameter, and reddish pebbles of granitic rock. . 100 
3. Thinly laminated lacustral clay 1 80 
4. Not exposed to the river, about 150 
Total 450 i 
A mile farther west the rim rock of basalt is absent, and the bed of [ 
lapilli, there about 180 feet thick, rises above the level of the basalt 
in the adjacent portions of the canyon wall. The lapilli here contain 
