830 NORTH-E-iST AFRICA. Bahr-Yusef has been gradually deflected eastwards by the ranges of dunes skirting the left bank. But the progress of the sands is extremely slow, and may perhaps be compensated by the erosions caused by the action of the current along the right bank of the Nile. Moreover the sands themselves may be cultivated like other lands, wherever they can be brought within the reach of the irrigating waters, bringing down the rich alluvia of the stream. West of the oases, the Libyan desert has not yet been traversed by any explorers in the direction of the Kufra oasis and of Fezzan. An inhospitable region at least 400,000 square miles in extent, inaccessible even to travellers provided with all the resources of modem industry, occupies this part of the continent, completely separa- ting Egypt and Cyrenaica from the lands comprised within the Tsad basin. The natives of the Egj'ptian oases are unable to give strangers any information regard- ing these mysterious and terrible regions bounding their horizon, and into which they are careful not to penetrate. A few confused legends destitute of all historic value are, however, kept alive amongst them regarding strange events supposed to have occurred in these frightful solitudes. In the year 1874, Rohlfs, Zittel, and some other German explorers, vainly attempted to make their way in a straight line across this region to Fezzan. In anticipation of a long journey they organised a whole caravan of camels, carrying water in iron chests lined inside with tin. But after a six days' march from Dakhel, they became convinced that it would be impossible for the camels to traverse the endless ranges of dunes barring the route in the direction of Fezzan. Hence they turned northwards to seek a refuge in the Siwah oasis. This point was reached twenty-two days after leaving the last watering-place, and throughout the whole of this expedition nothing was met except sand and rocks, and the " devil's water " shown by the mirage. The part of the Libyan desert lying nearest to the Egyptian oases resembles that skirting the Nile Valley. Here the surface is still varied by a few limestone hills, interspersed with ranges of dunes and stunted scrub. But when the traveller reaches the region of quartzose sandstones all vestiges of vegetation disappear. Nothing now meets the eye except sand and stratified rocks, alternating with deposits of a very rich iron ore. The land rises gradually in the direction of the west, and towards the parting-line between the limestones and sandstones the plateau attains an elevation of 1,460 feet. Here begins the ocean of sand, which stretches for unknown distances in the direction of Fezzan. In the north it extends for no less than 240 miles towards the Siwah oasis. The vast dunes of this region, produced by the weathering of the sandstone rocks, have a mean altitude of over 300 feet, hence exceed in elevation the largest sand-hills of Europe. Some are even said to attain a height of 500 feet. Disposed in the direction from south to north, or from south-south-east to north-north-west, perpendicularly to the polar winds, the ranges follow each other like the ocean waves under the regular action of the trade-winds. Secondary systems of dunes, which may be compared with the false cones occurring on the flanks of Etna, are developed by the irregular atmospheric currents, and these are usually disposed
