136 
SNAKE R3VEE PLAINS OF IDAHO. 
[bull. 199: 
the surface of the smoother plains lying farther west. Although 
there is a suspicion that the soil on the smooth plains, especially when 
their surfaces are inclined in the direction in which streams from the 
mountains would flow if precipitation were sufficiently abundant, was 
deposited by the distributaries of ephemeral streams, it is evident that 
the rolling plains could not have been covered with a sheet of fine material 
in this manner. As is explained below, the soil on the rolling plains, 
which have in fact the typical aspect of rolling prairies, is decidedly 
different from the rocks beneath, and, as is plainly apparent, has not 
been formed by the disintegration and decay of the underlying basalt. 
It has manifestly been deposited on the lava from above, and must 
therefore have been spread out beneath a lake which occupied the 
Snake River Basin at a recent date, or else it was laid down under con- 
ditions essentially the same as those now existing. That a lake has 
not been present over the greater part of the lava-covered region 
lying east of the Malade since the surface lava sheets were spread out 
is shown by the absence of beach lines and other similar phenomena. 
The soil must therefore have been deposited from the atmosphere. 
^EOLIAN SOILS. 
One method of studying the nature of soils is to sift them and thus 
separate the grains of uniform size. The results obtained in this 
manner from two samples of soil from the Snake River Plains are 
presented below. No. 1 is from a locality about 8 miles southeast of 
Shoshone Falls and is a representative of the soil covering the smooth 
plains. No. 2 is from a locality about midway between Arco and Big 
Butte, where the surface is rolling and has never been crossed by 
streams. Each sample was taken about 1 foot below the surface. No. 3 
is a sample of the lacustral deposit which covers a great area in the 
northern part of Owyhee County, and is introduced here for compari- 
son. It will be referred to later. 
Mechanical analyses of soils and lacustral deposit from Snake River Plains. 
No. 1. 
No. 2. 
No. 3. 
Retained on a 20-mesh sieve a 
Per cent. 
0.0 
5.0 
6.5 
4.5 
82.8 
]'i r cent. 
5.7 
4.0 
5.5 
8.5 
76.0 
Per ct ni. 
0.0 
Retained on a 50-mesh sieve . . . 
4.5 
Retained on a 100-mesh sieve 
10.5 
Retained on a 200-mesh sieve 
6.0 
Passing through 200-mesh sieve 
79. 
« Number of meshes per linear inch. 
The material retained on a 20-mesh sieve, in the case of sample No. 2, 
contains about 5 per cent of easily recognizable vegetable fibers, and 
in the fractional parts of each sample coarser than that which passed 
