138 SNAKE EIVER PLAINS OF IDAHO. [rttll.199. 
Judging- from both the mechanical and chemical analyses just given, 
it seems that the soil consists essentially of exceedingly fine quartz 
sand, to which a small percentage of calcium carbonate, together with 
some vegetable matter, has been added. One interesting result of the 
chemical analysis is the small amount of sodium found. 
The soil represented by sample No. 2 is brown in color. This is due 
in part to organic matter, but does not wholly disappear when a sam- 
ple is heated to redness. The particles which will not pass through a 
200-mesh sieve are aggregations of smaller grains, but are not readily 
disintegrated by rubbing and do not fall to pieces when treated 
with hydrochloric acid, and besides there is but slight effervescence 
when acid is used. The material passing through a 200-mesh sieve 
and that obtained by the partial breaking up of the coarser particles 
when rubbed appear under the microscope to be about the same as the 
grains composing sample No. 1, but are much more frequently col- 
ored. The larger part of the grains consists of quartz, but a consid- 
erable number are dark colored or opaque and indeterminate. Judg- 
ing from these facts, the soil represented by sample No. 2 consists 
mainly of fine quartz sand, colored and in part cemented into grains 
by some substance not readily soluble in acid, and, as indicated by 
qualitative chemical tests, consisting probably of ferric oxide. The 
opaque indeterminate grains are possibly volcanic lapilli. 
Near each of the localities where the samples referred to above 
were taken, as is the case throughout the soil-covered lava sheets 
beneath the Snake River Plains, there is an abrupt and conspicuous 
change from the soil to the lava and not a gradual transition from one 
to the other. Tt is evident from this, as well as from the mechanical 
and chemical analyses given above, and the composition of the lava, so 
far as known, that the soil is not derived, to an appreciable extent, 
from the rocks on which it rests. In many sections seen the soil is 
essentially homogeneous from top to bottom, excepting that much 
more organic matter, usually in the form of readily recognizable veg- 
etable fibers, is present at the surface than at a depth of a few feet. 
Also, as previously stated, the soil mantles both the smooth and the 
rolling portions of the plains, and occurs also on the sides and sum- 
mits of ancient lava craters. 
Judging from all the facts in hand, it is evident that the fine, usually 
light-yellow soil covering the Snake River Plains at a distance from 
the mountains is mainly and essentially a dust deposit laid down by 
the wind. The dust is derived to some extent from the mountains, 
where naked cliffs, bare talus slopes, etc., furnish material for wind 
transportation, but of more importance is the fine sediment deposited 
by ephemeral streams which spread out on the marginal portions of 
the plains and leave all of the debris brought by them from the moun- 
tains. Another and possibly the source of the most abundant supply 
