the line of the system beginning in Tibesti. The escarpment of the plateau, limited to the south-west by the quaternary alluvia which constitutes the Saharian plains, seems to be continued in the same normal orographic direction.
Towards the centre of this escarpment rises a group of heights, designated, like so many others in this region, by the name of Adrar, or "the Mountains," in a preeminent sense. According to Duveyrier, Mount In-Esokal, culminating point of the system, and rising to a height of over 5,000 feet, is certainly a volcanic crest,
whose lavas have spread over the underlying Devonian formations of the plateau. Towards the west the Tassili is cut up into isles and islets, while on the north side the depressions of the wadies penetrate like gulfs and inlets into the mountain mass. The plateau is thus divided into a number of fragments, each of which is known by a separate name to the local Tuareg tribes.
Egueleh, the block lying nearest to the Wed Righ, is encircled on the east, north, and west by the Edeyen, or "Sands." The Khanfusa eminence (1,940 feet), the first Devonian rock occurring on the route from Tugurt to Ideles, in the
