Page:Popular Science Monthly Volume 81.djvu/243

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237
WIND-GRAVED MESAS

By wind-action alone there appears now to be incontestable testimony that from the entire area of the vast arid region there has been lifted and exported by the winds in very recent geologic times a prodigious layer of rock not less than 5,000 feet in thickness. With a continuance of the present climatic conditions another volume of rock-materials of equal dimensions seems allotted to be similarly transported before the effects of the process shall become appreciably diminished.

Thus it is that the arid regions have introduced to us an erosive agent more potent than stream-corrasion, more constant than the washings of the rains, more extensive and persistent than the encroachments of the sea. The present conception of general eolic erosion, a sculpturing power in every way comparable to erosion by river and by ocean, appears destined to take its place among the first half-dozen great and new thoughts which shall especially distinguish geologic science of the twentieth century.