THE BAYUDA STEPPE. 289 commanded by these heights is a mountainous country covered with hollows, or a few groves of green mimosas during the rainy season, and bounded to the west between Khartum and Ambukol by the depression of the Wady Mokattam, or the " Valley of Inscriptions," into which probably flowed an ancient arm of the Nile. The whole region, though much less barren than the atmur of Eastern Nubia, is termed the steppe or desert of Bayuda. Gekdul and Magaga, whose highest point, Ussub-Oramaneh, is a cupola of red porphyry, or erupted masses, around which the sandstone rocks, probably liquefied by the outpourings of lava, have spread over the sands in sheets of siliceous scoriao. According to Russegger, it is owing to the eruption of these ancient volcanoes that the Nile, formerly flowing to the west, was compelled to turn eastwards in order to effect its great bend of over 480 miles. In the western part of the steppe the ferruginous sands of the moun- tains, washed down by the rains, have covered the soil in thick layers. Here and there the sand has collected in the hollows where the wadies have deposited their alluvia ; the surface of the plain is thus streaked with long bands of diverse colours of the strangest appearance. The Jebel-Simrieh, formed of pink sandstone, and other hills to the west of the depression of the " Valley of Inscriptions," are less elevated than Magaga, and like it are not very long ; the valley of the Wady-Melek, whose bed, during the floods, gives passage to the waters of Dar-F6r, bounds these mountains on the west. On the banks of the Nile itself, in the space comprised between Marawi and New Dongola, there are nothing but sandstone cliffs, the crystalline rocks reappearing only at the Third Cataract. Here the heights on the left bank form part of the chains which rise in Eastern Nubia ; to the west they soon become lost under the sands, being succeeded by oases at a short distance from the river and parallel with it. In this respect the western zone of the Nubian region forms a complete contrast to the tracts beyond the Nile. To the north of Wady-Halfa, and nearly opposite the colossi of Ibsambul, lies a deep valley overlooked by the black or reddish walls of ancient volcanoes. This is the Wady-Jehenna, or " Valley of Gehenna," a terrible region which the Arabs shun as if it were still burning. In Western as well as in Eastern Nubia, the sandstones rapidly crumble away under the influence of the wind, rain, and heat, and change into loose sand which the aerial currents reform into dunes or taluses. In many respects the sands of Africa recall the snows of the great Alps ; like the snowtields they collect in the depressions and crevasses of the rocks, glide over the ravines in avalanches, crown the needle-like points of the peaks, and here and there project over the precipices, forming narrow strips which give way at the slightest shock. Between the dunes and the populations of the oases on the edge of the desert the struggle is incessant ; the sands, borne forward by the winds, surround the trees, cover up cultivations, block up the fountains, and encroach upon the inhabitable domain. But on his side the peasant utilises the sand by mixing it with his soil. The extent of ground he can render productive depends entirely upon the quantity of water at his disposal. 19-AF.
