248 GRAPHIC DESCRIPTION C/ AFRICA. 



It is true that large stretches of the plain of Sahara are covered 

 by waves of sand, which were once sandy bars and dykes of the sea ; 

 but the whole desert is by no means the product of the ocean alone. 

 Very much of the sand is of local origin, formed from the soil of 

 the desert plain by the sudden changes of temperature and the 

 action of the wind. There are many such centres of sand radiation, 

 and the mechanically powdered fragments of rock are found in 

 every phase of transition from crumbled stone to fine drift-sand. 

 The ground above Khartoum, to the west of the Nile, consists partly 

 of rose-colored granite, and the whole surface of the rifted slope of 

 rock is bestrewn with fragments of different sizes. 



DUST WHIRLWINDS. 



Dust whirlwinds of considerable size are sometimes observed 

 in the Russian steppes; but the best known phenomena of this kind 

 are the high sand pillars of Sahara. Even in Australia these rotary 

 dust pillars are met with, generally being seen upon shadowless 

 plains. It is thought that these Australian whirlwinds are the 

 channels vv'hich carry the heated air from the ground to the higher 

 strata. 



Instead of the rolling waves and cool breezes of the sea, this 

 funeral region only gives out burning gusts, scorching blasts which 

 seem to issue from the gates of hell ; these are the simoon or poison- 

 wind, as the word signifies in Arab. The camel-Sriver knows this 

 formidable enemy, and so soon as he sees it looming in the horizon, 

 he raises his hand to heaven, r.nd implores Allah ; the camels them- 

 selves seem terrified at its approach. A veil of reddish-black invades 

 the gleaming sky, and very soon a terrible and burning wind rises, 

 bearing clouds of fine impalpable sand, which severely irritates the 

 eyes and throat. 



The camels squat down and refuse to move, and the travelers 

 have no chance of safety except by making a rampart of the bodies 

 of their beasts, and covering their heads so as to protect themselves 

 against this scourge. Entire caravans have sometimes perished m 

 these sand-storms; it was one of them that buried the army of 

 Cambyses when it was traversing the desert. 



