Page:Africa by Élisée Reclus, Volume 2.djvu/510

 would appear to be from the north-east to the south-west, in accordance with the general course of the trade winds.

In the eastern Erg some of the crests are much higher than those of the French dunes on the Gascony coast. The Ghurd El-Khadem, measured by MM. Largeau, Say, and Lemay, has a height of 450 feet, and others are said to reach 490 feet, within 40 of that measured by Vogel near the "Lake of Worms," in

Fezzan. Duveyrier saw sandhills in the eastern Erg over 660 feet high, aud Largeau speaks of one attaining a vertical elevation of 1,650 feet. But their extreme altitude cannot be determined until the Sahara has been more completely surveyed. Seen from the neighbouring hills, those of the Erg present the appearance of enormous ocean waves suddenly solidified.

Besides the troughs between the sandhills, in several places deep depressions have been developed, resembling the craters of volcanic cones. Such is the