212 NORTH-WEST AFRICA. Jurjura range. From it8 easily ascended summit a vast prospect is commanded of the northern plateaux and shotts between Batna and Ain-Beida, while on the southern horizon is visible a long blue streak marking the skirt of the Sahara. Eastward the Aures system is continued by the Jebel Sheshar and the Nememsha mountains, which, like the isolated ridges of the Tebessa plateau and Tunisian frontier, are noted for their natural fastnesses, often transformed into places of refuge by the natives. These south-eastern highlands of Algeria have a mean elevation of from 4,000 to 4,o00 feet, the Sheshar range culminating in the Ali- en-Nas, 6,2o0 feet high. South of the Algerian border chains the uplands terminate abruptly in extensive plains covered with (iuuternary alluvia, and forming a sort of strait between Mauritania and the Sahara highlands. Southwards the ground rises almost imperceptibly towards the Ras Shaab heights, which run south-west and north-east, parallel with the xVthis system, and which in their highest peak attain an elevation of '2,M0 feet above the Laghwat oasis. Beyond this point stretches the Sahara, which here consists mainly of Pliocene formations, originally deposited as alluvia by the running waters, and afterwards, doubtlessj distributed by the winds, like the yellow earth of North China. In some places the beds of this friable soil have a thickness, according to M. Holland, of 1,000 feet. Nevertheless, they are here and there broken by isolated masses of cretaceous rocks, some of which occupy a considerable space. The most extensive is the so-called Mzab plateau, which, although separated from the Algerian uplands by a tract of Quaternary alluvia, may be regarded as a sort of isthmus connecting the Mauritanian highlands with the Devonian plateaux of the interior of the Sahara, and with the crystalline rocks of the Jebel Ahaggar. Rivers of Algeria. Although it receives from the rain-bearing clouds a quantity of water at least equal to that carried off by such a river as the Nile, Algeria does not possess a single navigable stream. Its internal navigation is limited to a few skiffs and rowing-boats on the Seybouse. The development of large fluvial basins is prevented by the very relief of the land, the coastlands forming a narrow strip between the j)lateaux and the Mediterranean, while towards the south most of the streams flowing to the Sahara have their source on the inlfind slopes of the border chains. The total area of Mediterranean drainage may be approximately estimated at 80,000 square miles. All the rest of Algeria is distributed over closed basins, where the water either evaporates in saline lagoons, or else runs out even befot^ reaching the central depression. In fact, nearly all the Algerian streams are dry for a great part of the year, their beds presenting in the uplands nothing but bare rock or pebbly channels, in the lowlands strips of sand lashed by every breeze into whirlwinds of dust. The rivers, which retain a little moisture in summer, are closed at their mouths by compact sandbars, which present a solid path.to pedes-
