russell.] ALLUVIUM. 135 
SOILS. 
As was stated in the introductory portion of this paper, the Snake 
River Plains are, as a rule, soil covered. The principal exception is 
where the recent lava is present. Where lava sheets of considerable 
antiquity are still at the surface, however, their roughness, especially 
the pressure domes and ridges which occur upon them, is not always 
concealed by the nearly universal sheet of fine debris in which the 
desert shrubs are rooted. 
ALLUVIAL SOILS. 
One source of the soil covering the lava sheets has already been 
described, namely, the alluvial deposits made by streams flowing down 
from the mountains. About the border of the plains, and extending 
out for many miles toward their central part in certain instances, the 
snrticial covering is composed of coarse material of the same litho- 
logical character as the rocks forming the neighboring mountains. A 
gradation from coarse to fine is easily traced as one leaves the moun- 
tains and journe\ r s out upon the plain. In making such a journey it 
will be noticed that at first the alluvial fans, stream terraces, etc., are 
pronounced, but as one proceeds they flatten out and become more and 
more indistinct, and at a distance, in general, of 8 or 10 miles from the 
bordering upland the plain is flat and has an even surface of soft 
yellowish-white silt-like material, broken only by occasional crags of 
basalt, or the summits of pressure ridges. Far from the channels of 
streams, and where even the courses of the ephemeral distributaries 
of wet-weather creeks and brooks are no longer distinguishable, the 
light-colored soil continues, and not infrequently and over continuous 
areas embracing several hundred square miles forms an even, unbroken 
surface. The origin of the soil at localities distant from all stream 
channels is the most interesting problem that this surface covering 
presents. 
The surface of the plains at a distance from the mountains, and from 
the channels of streams, inclusive of those dependent on unusually 
heavy rains, presents two quite well-marked characteristics. To the 
west of Malade River the broad, seemingly level interstream spaces 
are smooth and have a gentle downward inclination to the south. It is 
evident that if the ephemeral streams from the mountains to the north 
had sufficient volume they might easily continue across this space, 
which varies in width from 10 to 20 or more miles, and deposit fine 
sediment upon it. To the east of the Malade, and over nearly all of 
the eastern half of the Snake River Plains, the surface is undulating, 
and there is no direction in which surface water could flow for many 
miles without being ponded. This rolling surface, how r ever, is covered 
with a sheet of soil similar in its physical characters to that forming 
