In recent geological times, the Great Basin was filled with water, forming a vast fresh-water lake known as Lake Bonneville, which drained into the Columbia River.  During the existence of this lake, soil materials were washed from the mountains into the lake and deposited on the lake bottom.  When at length, the lake disappeared, the lake bottom was exposed and is now the farming lands of the Great Basin district.  The soils of this district are characterized by great depth and uniformity, an abundance of lime, and all the essential plant-foods with the exception of phosphoric acid, which, while present in normal quantities, is not unusually abundant.  The Great Basin soils are among the most fertile on the American Continent.

Colorado River district.—­The fourth soil district lies in the drainage basin of the Colorado River It includes much of the southern part of Utah, the eastern part of Colorado, part of New Mexico, nearly all of Arizona, and part of southern California.  This district, in its northern part, is often spoken of as the High Plateaus.  The soils are formed from the easily disintegrated rocks of comparatively recent geological origin, which themselves are said to have been formed from deposits in a shallow interior sea which covered a large part of the West.  The rivers running through this district have cut immense canons with perpendicular walls which make much of this country difficult to traverse.  Some of the soils are of an extremely fine nature, settling firmly and requiring considerable tillage before they are brought to a proper condition of tilth.  In many places the soils are heavily charged with calcium sulfate, or crystals of the ordinary land plaster.  The fertility of the soils, however, is high, and when they are properly cultivated, they yield large and excellent crops.

California district.—­The fifth soil district lies in California in the basin of the Sacramento and San Joaquin rivers.  The soils are of the typical arid kind of high fertility and great lasting powers.  They represent some of the most valuable dry-farm districts of the West.  These soils have been studied in detail by Hilgard.

Dry-farming in the five districts.—­It is interesting to note that in all of these five great soil districts dry-farming has been tried with great success.  Even in the Great Basin and the Colorado River districts, where extreme desert conditions often prevail and where the rainfall is slight, it has been found possible to produce profitable crops without irrigation.  It is unfortunate that the study of the dry-farming territory of the United States has not progressed far enough to permit a comprehensive and correct mapping of its soils.  Our knowledge of this subject is, at the best, fragmentary.  We know, however, with certainty that the properties which characterize arid soils, as described in this chapter’ are possessed by the soils of the dry-farming territory, including the five great districts just enumerated.  The characteristics of arid id soils increase as the rainfall decreases and other conditions of aridity increase.  They are less marked as we go eastward or westward toward the regions of more abundant rainfall; that is to say, the most highly developed arid soils are found in the Great Basin and Colorado River districts.  The least developed are on the eastern edge of the Great Plains.