320 NOETH-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 rocky 

 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 numraulitic 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. 

 ■"■ " Wiih Hicks Pasha in the Sudan," p. 244. 



