scattered by thousands in the desert. They are small sandstone hills merely rising some 60 feet above the plain, but at some points attaining a relative elevation of 660 feet, or from 1,630 to 2,000 feet above the sea. The granite heights in the interior reach an altitude of over 2,160 feet, some of the peaks even rising to nearly 3,300 feet.
Fig. 91. — Korosko Desert.
Scale 1: 2,800,000,
The sandstone rocks of Nubia present the most diverse forms. Some stand out like regular towers, others in the form of pyramids, whilst others again, whose central portion has disappeared, resemble volcanic cones. Consisting of horizontal layers of quartzose sandstone of varying density, they offer more or less resistance in different places. In one place the summit crumbles away, elsewhere the base leaving the crest crowned as if with a table; several rocks . are also pierced with openings through which light is visible. The very names that the nomads and caravan leaders give to these sandstone heights are a proof of the variety of their forms. They imagine they see in them palaces, animals, and processions of warriors. Thanks to these fantastic outlines, the guides of the caravans can always determine their whereabouts in these endless labyrinths of breaches winding between the rocks. The various colours of the stone also assist them in finding their way. Certain strata are shaded with green, yellow, pink, or blue; whilst others, in which ferruginous sands predominate, are of a brilliant red. Jasper, chalcedony, and siliceous crystals are embedded in the walls. But on each journey the guide finds some changes. The sands produced by the disintegration of the rocks shift their position according to the direction of the wind, which carries it in a cloud above the crests, and scatters it now on one side, now on another, forming rounded heaps which blend in graceful curves with the coarser sands at the base. Shifting dunes of sand, some of which are as much as 166 feet high, move here and there through-
