russell.] CHEMICALLY FORMED DEPOSITS. 141 
surface is covered with tine yellowish silt, composed almost entirely of 
fragments of quartz, and not distinguishable from true reolian soil. 
RESIDUAL SOILS. 
On the mountains of southern Idaho and on the steptoes which rise 
through the lava of the Snake River Plains there is usually an abun- 
dance of line material forming a soil mantle which agrees in the nature 
of its constituent fragments with the rocks on which it rests. In many 
instances there is a gradation from fine soil, with occasional angular 
rock fragments into the solid rock beneath. These are typical exam- 
ples of residual soils. 
SUMMARY. 
The soils of southern Idaho are of two classes — sedentary and trans- 
ported. Of the transported soils, there are again two principal sub- 
divisions — wind deposited and water deposited. Of the wind-carried 
or seolian soils there are two principal varieties, one consisting mainly 
of fine quartz sand and the other of volcanic dust and lapilli. Of the 
water-deposited soils there are ag"ain two varieties — those laid down in 
lakes, principally in southwestern Idaho, and those deposited by 
streams. The principal part of the soil covering the plains at a dis- 
tance from the mountains and outside the canyons is of asolian origin, 
and consists mainly of fine quartz particles. 
CHEMICALLY FORMED STTBAERIAI, DEPOSITS. 
A surface efflorescence of alkaline salts, etc., is unknown on the 
Snake River Plains. In a few localities, however, on the flood plains 
of streams — as, for example, along the border of Snake River to the 
west of Shoshone Falls — the nature of the plants indicates an alkaline 
soil, and in a few instances white surface incrustations are formed 
during the summer season. These incrustations, as is well known, are 
due to surface evaporation. The water from below rises, owing to 
capillary attraction, as the surface is dried, and being in turn evapor- 
ated, leaves its contained saline matter. Without attempting an 
extended discussion in this connection, it may be stated from observa- 
tions made in Idaho and elsewhere that soils do not become alkaline 
if well underdrained; but if there is an abundance of subsoil water it 
will be drawn to the surface during long-continued droughts and evap- 
orated, leaving the salts it previously held in solution at the surface. 
The obvious suggestion furnished in this connection by natural condi- 
tions is that irrigated land should be well underdrained. 
Although what are commonly termed alkaline incrustations are 
absent from the soils occurring on the Snake River Plains, there is an 
analogous substance frequently present. I refer to the whit* 1 deposit 
to he found almost everywhere on the under sides of loose stones or in 
