Page:Africa by Élisée Reclus, Volume 2.djvu/519

 ranges. All are eruptive cones, which have risen above the crevasses of the plateau while covering the original sedimentary rocks with lavas and ashes, Over the slopes is strewn a fine layer of grit, as pleasant to walk on as the sandy paths of a garden.

Mount Tussideh, the culminating cone in this district, rising to an elevation of over 8,300 feet, shows on one of its slopes a secondary cone which formerly emitted

smoke. Near the southern talus, about 1,000 feet below the summit, lies the mouth of a crater, which according to Nachtigal has a circumference of "three or four hours," with a depth of over 160 feet. From the level surface of the ground at its mouth the fall is sudden at first, then diminishing gradually towards the bottom. Ridges of black lava converging from tho circumference to the centre are separated from each other by intervening lines of saline efflorescence, resembling the streaks of snow in.the crevasses of a mountain cirque. The centre of the crater is occupied by a small volcanic cone, which also terminates in a basin filled with a whitish substance called "natron" by the Tibbus.