Page:Popular Science Monthly Volume 76.djvu/579

Rh us a group of results, not perhaps so nearly universal, but even more tangible and conspicuous. Many agents are at work producing mineral materials of such fineness that the winds can carry them. Oriental travelers and explorers know the sand storm as one of the most distressing and sometimes deadly visitations. In the desert mechanical disruption of rocks goes on rapidly and there is little moisture or plant life to hold the fragments down. The winds become factors of transportation in a manner little known by dwellers in moist and verdant lands. Dust storms are not confined to the Sahara, or Persia, or Turkestan. They occur in considerable numbers in our arid regions, where they sweep for hundreds of miles, last for many hours and carry incredible loads. Sand drifts a foot high have gathered in a half hour on railway tracks; thirteen cars of sand were taken from a single depot platform in Colorado; the same careful student who reports these facts estimates from 160 to 126,000 tons of sand carried in a cubic mile of air. This for a single storm may give us hundreds of millions of tons borne for hundreds of miles. Under such conditions the redistribution of surface materials by the atmosphere can no longer be held trivial.

We are now to remember that desert conditions furnish but a minor part of the dust that is available. Wherever in all geologic time there have been explosive volcanic eruptions, dust has been expelled, often in prodigious amounts, covering leagues of sea with floating pumice, littering the decks of vessels hundreds of miles away, destroying crops, darkening the atmosphere across wide seas, and enriching the sunset glows, it is believed, around the globe. Every rain storm purges the air of dust, much of local origin, no doubt, but some from remote and subterranean sources. Here is ceaseless accretion for all land surfaces and for all sea bottoms, and we have an impressive illustration of the interdependence and the cosmopolitan efficiency of every part of the earth's machinery.

Man never uncovers a soil surface with the plow or by the passage of hoof or vehicle, without exposing material to atmospheric migration, and it is some years since an expert road maker, in a highway convention, set forth the havoc wrought on macadam roads by winds.

From the point of view of natural scenery the winds' most conspicuous product is the dune. Many have seen a single example of a belt or field of sand hills, but the student of the earth finds in them no phenomenon of small range. He looks for them on the lee side of every river in a desert region and along all sand shores. He finds them invading the olive orchards of Palestine, the vineyards of France, the meadows of Holland, the forests of the Great Lakes and the fields of Cape Cod. The hand of man is put forth to stay the ravages of these flying cohorts and the organized skill of a government department joins in the task. Search is made for sand-binding grasses, in the same