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The trough of the colorado Desert extends from San Gorgonio Pass southeastward through the Salton Sink to the Gulf of California. It is separated from the Mohave basin by the San Bernardino, Cottonwood, Chuckwalla, and Chocolate mountains, and so forms an overlapping parallel axis of depression. This great axis is known to be closely related to widely extended geologic structures, and it is probable that the Death Valley axis has been similarly determined. In addition to these major controlling depressions, the entire desert area consists of a series of more or less nearly parallel ranges and intervening minor desert valleys. In the northern part of the area the trend of these features is nearly north and south, and in the southern part the trend swings to the southeast; but in the intermediate region there is an area of confused, broken ranges in which definite trends seem to be lacking. FAULT LINES. It is well known that the eastern front of the Sierra Nevada is a great fault scarp, but it is perhaps less generally known that the north face of the San Gabriel Mountains is a similar structural line, which extends southeastward through Cajon and San Gorgonio passes and into the colorado Desert. It is regarded as probable, too, that the great trough which has been described as extending northwest and southeast through the desert, along the line of the greater links, owes its origin to crustal movement. The basin range type of Structure, the tilted block first recognized by Gilbert, involves faulting along the uplifted edge, and recent geologic work in the desert has proven the existence of much faulting of the block type in the Bullfrog district.(a) It is not unlikely that when structural details shall have been worked out many of the strongest of the desert Springs will be found along these fault lines, and that their waters will prove to be of deep origin and independent of local rainfall and local drainage. It is difficult otherwise to account for many of the springs. CLIMATE. The physical feature that exercises the greatest control over the climate of the Southwest is the great Sierra, which gives rain to the lands that lie west of it and withholds it from the desert to the east. The winds, which are moist and cool along the coast, shed their moisture upon the high mountains, and are dry when they reach the inferior, where they absorb moisture from both the soil and the vegetation. _____ (a) Bull. U. S. Geol. Survey No. 303, 1907, pp. 50-52.

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