Page:Africa by Élisée Reclus, Volume 2.djvu/513



Ain-Taiba cavity, about 60 feet deep, with a circuit of from 500 to 600 fect, which is flooded and fringed with sedge. In the vicinity is a similar formation partly choked with sand. In several places stony concretions aro found, attesting the former presence of mineral springs which have long disappeared. But among the stones scattered over certain regions of tho Subara there are many the origin of which has not yet been explained. Such aro the crystals in the form of pyramids, stars, or crosses, the chaplets, “fish scales,” and blackish nodules varying in size from a cherry to a hen’s egy, hollow inside or filled with sand. Such also are those vitrified silicious tubes usually about 12 inches long and terminating in a sort of hurd core. They occur in such large numbers in the southern parts of the Air district that they cannot be regarded as of meteoric origin. The Sahara is altogether a vast field in which geologists have still many things to discover.

Tn the Iguidi and certain parts of the Erg districts the phenomenon of the “singing sands” is not unfrequently heard, as on the slopes of the Serbal in the Sinai highlands, and elsewhere in the Asiatic deserts. The deepest silence is often suddenly broken by a vibrating sound like that of a distant trumpet, lusting a few seconds, then dying away, and again breaking out in another direction. This is no hallucination, for it is heard by animals as well as men, many of whom, ignorant of the cause, are stricken with fear by this mysterious “ music of the sands.” The effect is evidently due to the crumbling or frietion of myriads of molecules from time to time yielding to upward pressure. But it still remains to be explained why the phenomenon is not heard in all parts of the sandy regions, but restricted to certain districts. This, aguin, may perhaps be due to the different constitution or crystallographic nature of the vibrating particles.

CLIMATE OF THE Satara.

These vast sandy tracts are the best evidence of the extreme dryness of the climate. As observed by Carl Ritter, “ The Sahara is the south of the world,” although situated entirely north of the equator, and although Mascat, Aden, Tajura, and some other spots on the surface of the globe have a higher average temperature. A characteristic feature of its climate is the enormous oscillation between the extremes of heat und cold. While the sands are heated to a tempera- turo of 170° F., and even 190° F. in the sun, or from 136° F. to 146° F, in the shade, the radiation at night lowers the glass to 26° or 28°, that is, from four to six degrees below freezing-point. Such is the intense dryness of the atmosphere that fogs are ulmost unknown, although a slight dew gathers occasionally on the plants about sunrise, either after rainy days or during any abrupt lowering of the temperature. In this atmosphere destitute of humidity, arms never rust and flesh never becomes putrescent. Heavy showers are extremely rare, and in the Tuareg country ten or twelve years pass before the watercourses are flushed and vegetation renewed by a tropical downpour.

Limited north and south by two zones of regular rainfall, the Sahara presents no fixed laws for the recurrence of its meteorological phenomena. It forms a sort