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Page:Popular Science Monthly Volume 70.djvu/194

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190
POPULAR SCIENCE MONTHLY

Andes, the Pacific, and the rivers Vitor and Tambo. Its length and breadth are about equal, perhaps fifty miles in extent. The mean elevation of the pampa is about four thousand feet, increasing toward the north. It is a great plain with occasional low hills, almost devoid of animal and vegetable life, except among the low hills facing the sea. It appears to have been formerly the bed of the ocean. The surface is composed of sand, sprinkled over with stones and small boulders, and ail occasional outcrop of rock. Scattered over the pampa, especially in its northern portion, are hundreds of crescent-shaped sand-dunes. Their form is always the same, approximately that of the new moon, unless some unusual object is encountered by the dunes in their journey across the desert.

PSM V70 D194 Sand dune on the desert of islay.png

A Sand-dune on the Desert of Islay.

Their motion seems to be always toward the north or northwest, in the same direction as that of the prevailing south and southeast wind. The convex surface is directed toward the wind, and the cusps lie in the direction of motion. Their size varies between rather wide limits. They are in general from one hundred to two hundred feet broad, and from ten to twenty feet high. They are composed entirely of a fine gray sand, and are moved along by the wind so perfectly that not only is the crescent form preserved, but none of the sand is left behind to mark the passage. A casual glance at the surface of the pampa detects little if any of the sand which enters into the composition of the 1 dunes. The same variety of sand is found, however, by digging beneath the surface. It appears that all the available surface sand has already been collected by the wind into these symmetrical heaps, and that, unless the surface is disturbed by some convulsion of nature, the dunes may all finally disappear among the hills on the north of the desert. This theory seems to be confirmed by the abundance of dunes in the northern part of