820 NORTH-EAST AFEICA. undeniable, apparently solid and tangible. We know it is mocking us, like an ignis fatuus ; but the most accurate knowledge of the physical laws which govern the phenomena will not brush the image from the retina. There is little wonder that the ignorant and inexperienced should have frequently yielded to the delusion. But life is always the price paid for such a mistake. Some years ago a company of soldiers perished from thirst in this very region. Disre- garding the warning of their guides, the unfortunate men, fresh from Egypt, and mad with thirst, broke from the ranks and rushed towards the seeming lakes of transparent water which were presented to their eyes on all sides. They pressed on eagerly towards the ever-receding phantom, and one by one fell prostrate, to leave their bones to bleach on the sands. On another occasion a detachment was sent across the desert to Berber on its way to Khartum. The soldiers, refusing to be checked by the guides, consumed all their supply of water when in sight of the El-Bok Mountains, confident of their ability to reach the imaginary lake. The heat was intense ; the men grew faint and in a few hours died one by one in horrible agony. It is not surprising that by the Arabs this strange phenomenon should have been named the bahr-esh-Sheitan, or " Devil's Sea." * Geological Features. The Surface of the Libyan desert is completely covered with sand, which accumulates in vast quantities in the depressions, leaving only the higher rock}' eminences partly exposed. In few places are the cliffs absolutely bare, being almost everywhere clothed with the yellow or reddish particles of disintegrated quartz. These quartzose sands are certainly of foreign origin, for the plateau itself presents nothing but limestone rocks and clays. These remains of primitive rocks have been brought from distant uplands by the action of the winds and, possibly, also of marine waters. By their ceaseless movement over the surface the shifting sands have imparted a remarkable smoothness to the surface rocks, which in many places exhibit the lustre of polished marble. All the scattered blocks are, as it were, varnished by the sand, which has rounded off their angles and softened their rugged outlines. Some of these boulders have thus acquired such brilliancy that observers have mistaken them for volcanic obsidians. The geologist Zittel supposes that the incessant friction may even have tended to produce a chemical modification in the very structure of the rocks ; for a large number of flints are met, in the centre of which is embedded a core of nummulitic limestone. Hence the stone has been apparently transformed from the outside inwardly, a phenomenon which can be attributed only to the constant friction of the sand on the surface. Amongst the myriads of nummulites covering the ground in dense layers, all those occurring on the surface have by this action of the arenaceous particles been entirely changed to flints, assuming a bluish or even a metallic appearance, whereas those lower down, being protected from the friction as well as from the action of light, remain white and retain their limestone formations. • "With Hicks Pasha in the Su«lan," p. 244.
