Page:Africa by Élisée Reclus, Volume 2.djvu/572

 from the normal disposition of the dunes, whose long incline slopes towards the marine wind, while the more abrupt declivity is turned in the direction of the continent. The prevailing atmospheric current in this region is a sea-breeze derived from the deviation of the regular trade-winds. The mean height of the 4 Iguidi dunes ranges from 300 to 350 feet, although numerous crests rise to a

still greater elevation. Throughout the sands are disseminated little black particles, or rather crystals, derived from the disintegrated rocks.

South of the chain of sandhills follow the El-Eglab mountains, consisting of granite and porphyry masses, which rise to heights of from 1,000 to 1,300 feet above the plains — heights which appear prodigious in contrast with the dead uniformity of the surrounding waste. Farther east stretches to an unknown distance the dangerous Tanezruft region, so much dreaded by the caravans owing to the general absence of water. But towards the south winds the bed of a torrent, which bears the name of the Wed Sus, like the river on the Marocco frontier, and which occasionally presents to the traveller's gaze a slender liquid streak.