AIR WATER SOILS. 747 



objeoty of which " were to ascertain how far tlie geological couilitions, particuhirly 

 beneath the broad lava-flooreil phiiiis l)or(lering Snake River, favor the hope of 

 obtaining flowing water by drilling wells, and where test wells should be put down 

 in order to determine the correctness of inferences based on geological and other con- 

 ditions." Especial attention is given to the physiography and geology (including 

 soils) of the region, which is described as "a generally flat area bordered by rugged 

 mountains" extending "in a curved course concave to the north entirely across the 

 southern portion of Idaho. The length of this belt measured along its medial line is 

 about 350 miles, and the width is generally from 50 to 75 miles. Its area, as esti- 

 mated by Lindgren, is 34,000 square miles. Snake River traverses this rudely crescent- 

 shaped plain throughout its length in a great curve concave to the north, which has 

 a radius of some 175 miles. In a far-reaching view the Hat country through which 

 the Snake River finds its way presents the broader features of a vast level-fioored, 

 stream-eroded valley. A nearer acquaintance, however, shows that it is a built-up 

 plain, formed principally of lava sheets, and does not owe. its major surface features 

 to erosion. . . . 



"The geological discussion of the Snake River Plains may be i-onsistently divided 

 into 2 portions, the first dealing with the older rocks which were upraised into 

 mountains and deeply eroded before the partial filling of the Snake River Basin, and 

 the second with the lacustral and stream dejiosits, lava sheets, etc., now forming the 

 l)lains themselves. The former, in geological language, embraces mostly, if not 

 entirely, paleozoic rocks; and the second, tertiary and recent rocks. Between the 

 formation of these 2 great rock divisions there was a long interval, during which deep 

 erosion and the develoi)ment of a rugged topography took place. . . . 



"The climate of the Snake River Plains has for its leading characteristics aridity, 

 piexailing high temperatures in summer, and severe cold in winter. One of the 

 most marked features in the atmospheric conditions at nearly all seasons is the great 

 range in temperature between day and night. The mean annual precipitation is 

 about 13 in., but many local variations occur. Nearly all the water that reache.s the 

 thirsty lands comes in winter and spring. During the growing season the soil is 

 invariably parched, and successful agriculture without irrigation is seemingly impos- 

 sible. . . . However, over the higher i)ortion of the plains lying southwest of Big 

 Butte the rainfall may be sufficiently above the average, or the soil may retain 

 enough moisture, or possibly the slow melting of the winter snow may furnish the 

 proper conditions to i^ermit the growing of wheat and other cereals without irriga- 

 tion. . . . x\s agriculture is at present entirely dependent on irrigation, cultivated 

 land occurs only where water from streams and springs can be obtained. . . . 



"In general the soil of the i^lains is a fine, yellowish-w'hite, silt-like material, 

 largely a dust deposit, which mantles the surface not only on level tracts but covers 

 hills and broad depressions alike. This material is similar to the celebrated loess 

 of China, except that it usually occurs as a comparatively thin layer, and resembles 

 also the dejiosit bearing the same name in the Mississippi Valley. Like each of these 

 formations, it is of exceptional fertility if properly irrigated. . . . 



"The soils of southern Idaho are of 2 classes — sedentary and transportea. Of the 

 transported soils, there are again 2 ])rincipal subdivisions — wind-deposited and 

 water-deposited. Of the wind-carried or ajolian soils there are 2 principal varieties, 

 one consisting mainly of fine quartz sand and the other of volcanic dust and lapilli. 

 Of the water-deposited soils there are again 2 varieties — those laid down in lakes, 

 principally in southwestern Idaho, and those dejiosited by streams. The principal 

 part of the soil covering the plains at a distance from the niountains and outside the 

 canyons is of a-olian origin, and consists mainly of fine quartz particles." 



It is stated that what are commonly termed alkali incrustations are absent from 

 the soils of the region under normal conditions, although the occurrence of two 

 chemically-formed subaerial deposits are described, one a white or yellowish-white 



21781— Ko. S— 03 3 



