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GRAPHIC DESCRIPTION W 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 which 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-driver knows this 
formidable enemy, and so soon as he sees it looming in the horizon, 
he raises his hand to heaven, and 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 in 
these sand-storms; it was one of them that buried the army of 
Cambyses when it was traversing the desert. 
