The desert has given us the two great laws on which rests the entire scheme of ancient and modern landscape evolution. Arid regions have supplied the fundamental data for a general plan of land sculpturing that is in its nature strictly genetic. It is, indeed, as recently stated, a notable fact that the waterless waste should furnish us first glimpses of the mutation of land forms in the humid countries. One of the great principles thus formulated has been so suggestive of tangible results in the land of its birth that it has done more, perhaps, than any one factor to cast into shadow the other. The geologic processes whereby the second basic principle manifests itself have been until very lately almost wholly overlooked.

It is the arid region which has introduced to us an erosive agent more potent than corrasion, more constant that the working of the rains, . . .

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