bussell.] ^EOLIAN 8 AND. 139 
is the bare surface of the extensive lacustral deposits in southwestern 
Idaho. The prevailing winds, which are westerly, on crossing- these 
lake beds, cut as the}^ are into bad lands, become charged with dust, 
which is carried eastward and finally deposited. That much of the reo- 
lian sand was derived from this source is also indicated by a comparison 
of it with the material forming the lake beds. A characteristic sam- 
ple of the finer of the lacustral deposits, when subjected to a mechan- 
ical analysis in the manner already described gave the results presented 
in column No. 3 of the table given on page 136. As respects the size 
of the constituent particles, it will be seen that there isno conspicuous 
difference between the soils numbered 1 and 2 and the lacustral deposit. 
When the latter is examined undera microscope it is seen to be composed 
largely of fine angular and subangular grains of quartz sand, identical 
with those composing the principal part of the soils; the portions 
caught on 50- and 100-mesh sieves also are loosely cemented fragments 
of the same material, as in the case of the soil sample No. 1. This sim- 
ilarity is, in fact, so close that it suggests another hypothesis, and one 
which I considered and abandoned while in the field, namely, that 
the waters of the Pliocene lake which occupied the western part of the 
Snake River Plains extended far eastward, and that much of the fine 
yellow soil to the east of Malade River is a remnant of the sediment 
deposited in it. To what extent the soil in question is really lacustral 
sediment not removed and redeposited by the wind is difficult to deter- 
mine, as the passage is indefinite from the area where lacustral condi- 
tions were clearly present at the west to the area at the east where there 
is an absence of evidence of the former presence of a lake after the 
surface lava sheets were spread out. 
No doubt much of the dust derived from the lake beds referred to, 
and from the fine alluvial deposits is carried far beyond the borders of 
the Snake River Plains, but it is only in situations where other deposits 
are not being formed and where freedom from removal by rain wash is 
assured that the characteristic seolian soil can be expected to occur. 
The grasses, sagebrush, etc., growing on the plains serve to entrap the 
dust and preserve it from being again removed by the wind. 
In addition to the sources of supply of dust cited above, the fact 
needs to be borne in mind that volcanoes of the type of the Cinder 
Buttes, Market Lake crater, Kuna Butte, etc., contributed large quan- 
tities of volcanic dust and lapilli to the surface covering of the neigh- 
boring portions of the plains. Where this material is present in con- 
siderable quantity the soil is brown, but when seolian dust or lake 
beds predominate the soil is a characteristic light yellow, and is fre- 
quently nearly white. It is the fine, homogeneous, light-yellow soil 
that is generally present on the broad plains at a distance from the 
mountains. 
