Page:Popular Science Monthly Volume 82.djvu/480

476 is now measurable to something of its true proportions. A "sand-storm" or "dust-storm" is really a strong desert air-current two or three hundred miles in width instead of a mile wide as in the case of the largest rivers, running forty miles an hour instead of three or four miles, and sweeping along a thousand times as much sedimentative materials. Only by such comparison is the enormous erosive potency of deflative action fully comprehended.

Wind-formed deposits are mainly laid down far outside the confines of deserts—in the moist verdure-clad lands, or in the sea. Their magnitude is very great, as the enormous loess formations, the vast expanses of black soils of the steppes and prairies, and the extensive adobe clays

of many parts of the world amply attest. Continental deposits of this origin are just beginning to receive from scientists the attention which they merit.

The law of regional eolation will rank high among modern geological discoveries. What William Smith's discovery of characteristic fossils for identifying geologic terranes was to stratigraphy and historical geology, what Bunsen's theory of magmatic differentiation was to modern petrology, what Agassiz's hypothesis of continental glaciation was to recent geologic history, and what Powell's law of the base-level of erosion was to physiography of to-day, so the general theory of deflation, or regional eolation, is to the sciences of desert landscape sculpturing, and the formation of continental deposits as vast as any laid down on ocean borders. The theory adequately explains a grander host of