442 NORTH- WEST AFRICA. Other summits also, including perhaps the two culminating peaks, are probably of volcanic origin, lavas and ashes spread over the underlying granite rocks. For a long time the belief, based on the reports of the Tuaregs, prevailed that the Ahaggar also contained deposits of " black stones that burn," that is to say, coal. But these burning stones would appear to be certain porous lavas, which are filled with oil and lit up like lamps. The southern Ahaggar has not yet been visited by any European explorers, and still remains as little known as the plateau bordering it on the south, which is indicated on our maps by the name of the Southern Tassili, or Tassili of the Ahao'gars. From the reports of the Tuaregs it is known to be a rocky region, waterless, and destitute of vegetation, carefully avoided by the caravans and nomads. The camels which stray into these desolate uplands are said by the natives to perish of want, or else revert to the wild state, for no one will expose his life by going in search of them. The Ahaggar Waterparting. Lying in the very centre of the Sahara, the Jebel Ahaggar would constitute a waterparting for the surrounding fluvial basins, if the rainfall were copious enough to develop perennial streams beyond the limits of these highlands. Nevertheless there can be no doubt that the running waters descending from the Ahaggar Mountains lose themselves beyond the upland valleys in sandy beds, which, under different climatic conditions, formed the channels of large rivers draining in various directions. Northwards flowed the affluents of the Wed Igharghar ; to the south were collected all the streams which, through the common bed of the Tafassasset, went to swell the volume of the Niger ; the western valleys of the Tighehert, Tarhit, and other rivers belonged to the Messaura hydrographic system. It is still uncertain whether the basin of the Messaura drained to the Atlantic, as was supposed by Duveyrier, making its way to the Wed Draa through all the obstacles opposed by the dunes of Iguidi, or else is a tributary of the Tighehert and Niger, as might seem more probable from recent information regarding the general slope of the land. But according to the barometric measurements taken by Rohlfs in the Twat oasis, the Tighehert or Teghazert could never at any time have reached the Niger. To do so its waters would have to ascend o'er 330 feet in a space of about 480 miles. At the same time the data supplied by a traveller, whose rapid observations cannot be compared with those of other explorers, can, scarcely be accepted as offering a final solution of the question. The problem of the drainage of the Messaura basin, one of the most important in African geography, cannot therefore be regarded as yet solved. The Igharghar Basin, Even the Igharghar basin, although already explored by numerous travellers, is itself still insufficiently known ; nor can it yet be said with certainty to» belong altogether to the system of the Algerian shotts. There can be no doubt that a Wed Igharghar takes its rise on the northern declivity of the Jebel Ahaggar,
