The Veiled Man by William le Queux

Through the very heart of the barren, naked Saharan country, that boundless sea of red-brown arid sands, which, like the ocean itself, is subject to fitful moods of calm and storm, there runs a deep rocky ravine which has ever been a mystery to geographers. It commences near the shore of Lake Tsad, and extending for nearly eight hundred miles due north to Lake Melghir, is known as the Igharghar, and is the dried-up bed of a river, which, with its tributaries, once rendered this bare wilderness one of the most fertile spots on earth, but which, for upwards of two thousand years, has ceased to flow. Strangely enough, the country traversed by this great stony ravine is to-day the most arid and inhospitable in the world. The river, which, according to the legendary stories told in the market-places of the desert towns, must have been as mighty as the Nile, dried-up suddenly from some cause which has always puzzled geographers.

A portion of its course, about two hundred miles, half filled with sand, has for ages been used as the caravan route between the city of Agades, the capital of the Air country, and Temasinin, at the foot of the Tinghert Plateau; but the remainder is of such a rocky character as to be impassable, and has on many occasions served us as ambush when fighting the Ouled Sliman marauders, our hereditary foes.