JEBEL AHAOOAB. 44 1 Ahugpar highlands, is also a fraj^ent of this broken plateau. West of the depressions, through which formerly flowed the wators of the Igharghar river system, other rugged plateaux belong to the same Devonian formation ; but they are no longer disjMJsed in the direction of the Eu«t Sahara highlands. The Mudir, which forms the waterpurting between the Wed Igharghar and the rivers of Twal, develops a long triangular mass in the direction of the west. Towards the eastern extremity of this plateau rises the Ifettessen jieak, which, like the great crest of the Adrar system, is supix)sed by Duveyrier to be a volcanic cone. North of the Devonian plateaux, which follow successively from the neighbour- hood of Rhat to and beyond Insalah, all the rocky formations rising above the alluvial plains, or which are not covered by the sands, belong to the chalk systems. Such is the Tinghert plateau, that is, the "Limestone," a long Lamada, which forms the western and south-western prolongation of the •* Red Hamada " of Trijwlitana. Such is also the Tademait, which develops a sort of circular rampart round the north side of the Twat oases, and unites with the plateau of El-Golea in the Algerian Sahara. Towards the south and west the Tademait tenninates in bold hemllandn and steep cliffs, presenting an effectual barrier to the encroaching sands. Rut on the opposite declivity the hanuula slopes towanls the north-east, and is here furrowed with ravines, which are (K-casionally flooded with torrents flowing to the Wed ^fiya. In this direction the zone of nnky uplands has in many places been invaded by the advancing dunes. Altogether the cretaceous formations in the regions to the south of Algeria are disjxjsed in the form of a vast horseshoe, sweeping round the basins of the Wed Miya and Lower Igharghar. Jebp:l Ah.vggar. The chalk formations of Tinghert and Tademait and the Devonian plateaux of Muidur and the Northern Tassili are followed in the south by the crystalline rocks of the Ahaggar system, enclose<l by i.«w)latod rocky groups of the same origin. Towards the east rise the Antief hills, whose highest peaks, from 0,000 to over 6,000 feet, were observed by Barth during his journey from Rh4t to Agades ; in the north the Eguereh plateau, where the nonnal granite rocks show faults of volcanic origin ; in the north-we.st the bnlcn, or " crest " of Ahenet, which is continued in the direction of the Twat oases. Viewed as a whole, the central group, of circular form, present* a circumference of over 360 miles, consisting of suiKTimix>sed plateaux, which rise in successive stages from altitudes of 1,600 or l.SOO feet to over 6.600 fwt above sea- level in the region of winter snows. According to our maps, which arti for the most part a reproduction of that traced on the sands by the Targui sheikh, Othraan, for his friend Duveyrier, the Ahaggar is dominated in the centre by the cubninating plateau of Atakor, crowned by the twin Watellan and Hikcna peaks. The whole system terminates northwards in the Tifedest headland, whoae hist spur, the volcanic cone of Udan, by the natives commonly called the " Noee of Ahaggar," rises abruptly above the surrounding (iuatemar>' alluvial fonnationa. 60— AF
