418 NORTH-WEST AFRICA. which have carried away the soil, distributing it in thick alluvial deposits over the plains. Then flourished the forests whose petrified stems are still visible in many parts of the desert ; then lived the elephant and rhinoceros figured on the sculptured rocks in the Fezzan, Algerian, and Marocco highlands ; then the caravan routes were slowly traversed by pack oxen, since replaced by the camel. The rivers frequented by the crocodile are now dry, and all the large fauna have dis- appeared with the forests which afforded them a refuge. Nothing remains except a few flowing springs, and to obtain water, wells must be sunk in likely spots well known to the skilled eye of the nomad. But even this water is mostly brackish and disagreeable to the unaccustomed palate of the traveller. On arriving at the Dibbela wells, the first on the route from Lake Tsad to Fezzan, the people coming from the south, where good waters abound, always fall ill. But arriving from the north, after they have gradually become habituated to the taste of the desert waters, those of Dibbela seem well flavoured. The same phenomena of desiccation observed in the steppes and deserts of Central Asia and South Russia, have taken place in the regions south of the Atlas, only here the zone of regular rains appears, perhaps by a process of compensation, to have been enlarged at least in the southern districts of the Sahara. The Dunes. But however this be, the changes now going on are due almost exclusively to the action of sun and winds, and to the alternating temperatures. The great geological transformation of solid rock to shifting dunes is entirely the result of meteoric agencies. As soon as the softer rocks present an aperture through which the outer air can penetrate, the work of disintegration has begun. Dolomites, gypsums, and sandstones begin to crumble, and are slowly changed to sand or dust, the surface of the rock gradually corroded, leaving here and there the harder core, which thus develops into pyramids or pillars standing out in the midst of the sands. The argillaceous strata are attacked in the same way, every- where crumbling away except where preserved by the binding action of the roots of tamarisks and other shrubs. Once disintegrated, all this debris, whether of gypsum, limestone, silicious, or clay origin, immediately begins to move. "Wafted far and wide by the winds, it contributes to form in the depressions those argillaceous deposits which resemble the " yellow earth " of China, but which, for lack of the vivifying waters, are unable to yield the rich crops of that region. The particles of quartz, varying in size, are also borne from station to station, and deposited in the form of dimes, which are incessantly modified and displaced by the diverse action of the aerial currents. Thus the dunes are obviously of recent or contemporary formation, due to the peculiar influences of the Saharian climatic conditions. The hypothesis has been advanced that, after being formed by the disintegra- tion of the rocks, the dunes remain on the spot. They certainly do not travel as rapidly as might be supposed by those who have seen how they are at times blown
