The Sahara. 29 
growth analogous to that of a salt-marsh—salsolas, salicornes, tama- 
risks, brooms, &c. 
Desert of the Dunes.—The soil of this desert is incoherent moving 
sand, without vegetation, and where the camel alone can walk with 
ease. A day’s march to the west of Biskra there is a good specimen 
of this species of desert. There the dunes recall those of Holland ; 
but it is between Tugeurt and Wady Souf that the desert of the 
dunes shows itself in all its aridity. It is the desert par excellence, 
which in all times has produced on all people an impression of awe 
and terror. The yellowish white plain is deeply undulated, these 
undulations being the dunes raised by the wind. Their height is 
very variable, sometimes attaining to 50 feet ; and the two slopes 
are unequal, that which is opposed to the wind being steeper than 
the other, which is a gentle declivity. The sand is sufficiently firm 
to bear walking without sinking much. When the wind rises, the 
blown sand produces a kind of mist, which, of course, becomes 
thicker in proportion as the hurricane is severe. Consequently, as 
might be foreseen, these dunes are not permanent, for they change 
their places, though slowly; and in fact the sand does not move 
far from its starting point. It is not here as on the sea-shore, where 
the winds, being uninfluenced by local circumstances, are more constant 
and more intense, impelling the dunes always in the same direction. 
In the desert the wind frequently changes its direction, and the dunes 
change their forms, undergoing every kind of reconstruction. The 
general aspect preserves, nevertheless, its principal features so long 
that guides can still recognize their way. But guides being rare, 
and it being easy to lose one’s self in this labyrinth of dunes, the 
French Government has established landmarks, to guide the cara- 
vans, like those planted in winter among the snows of the Alps and 
the Jura. 
Origin of the Sand of the Dunes.—What is the origin of this sand ? 
Does it come from the sea, as has been long supposed, or is it pro- 
duced in place? M. Vatonne, mining engineer, in his journey to 
Rhadamis, has settled this question, having demonstrated that the 
dunes are the result of sandy formations decomposed in place. In 
Tunis, sands of the Cretaceous epoch generally provide the materials; 
and in Souf the Quaternary deposits. Between Guemar and Chott- 
Melrir, along the route followed by the caravans, relics of the original 
surface exist, unconcealed by the dunes; these surfaces being pre- 
served by a crust of gypsum which prevents their destruction. 
These unburied relics consist of stratified friable sand, which, de- 
prived of its protecting cover, easily disintegrates under the influence 
of atmospheric agencies and is driven along by the winds. Now, as 
this destructive action is carried on from year to year, and from age 
to age, it follows that the mass of the dunes is continually 
augmenting. 
Age of the Sahara.—If the Sahara is the bottom of a vanished sea, 
it is interesting to inquire if this disappearance has been caused by 
a sudden upheaval or by gradual and successive elevations, and at 
what epoch this extraordinary phenomenon has changed the aspect 
