GEOLOGICAL IMPORTANCE OF DUNES. 573 
probable in the a priort supposition that they may have been in 
part first thrown up by the Mediterranean on its Libyan coast, 
and thence blown south and west over the vast space they now 
cover. But inasmuch as it is now geologically certain that the 
Sahara is an uplifted bed of an ancient sea, we may suppose 
that, while submerged, it was, like other sea-bottoms, strewn with 
sand, and that its present supply of that material was, in great 
proportion, brought up with it. Laurent observed, some years 
ago, that marine shells of still extant species were found in the 
Sahara, far from the sea, and even at considerable depths below 
the surface.* These observations have been confirmed past all 
question by Desor, Martins, and others, and the facts and the 
obvious conclusion they suggest are at present not disputed. 
But whatever has been the source and movement of these 
sands, they can hardly fail to have left on their route some sand- 
stone monuments to mark their progress, such, for example, as 
we have seen are formed from the dune sand at the mouth of 
the Nile; and it is conceivable that the character of the drift- 
ing sands themselves, and of the conglomerates and sandstones 
to whose formation they have contributed, might furnish satis- 
factory evidence as to their origin, their starting-point, and the 
course by which they have wandered so far from the sea.t 
drifted to the west. In the Sahel, the prevailing east winds drive the sand- 
ocean with a progressive westward motion. The eastern half of the desert is 
swept clean.”—NAUMANN, Geognosie, ii., p. 1173. 
* Mémoires sur le Sahara Oriental p. 62. 
+ Forchhammer, after pointing out the coincidence between the inclined 
stratification of dunes and the structure of ancient tilted rocks, says: ‘‘ But 
I am not able to point out a sandstone formation corresponding to the dunes, 
Probably most ancient dunes have been destroyed by submersion before the 
loose sand became cemented to solid stone, but we may suppose that circum- 
stances have existed somewhere which have preserved the characteristics of 
this formation.”—LEONHARD und Bron, 1841, p. 8, 9. 
Such formations, however, certainly exist. Laurent (Mémoire sur le Sahara, 
etc., p. 12) tells us that in the Algerian desert there are ‘‘ sandstone formation” 
not only ‘‘ corresponding to the dunes,” but actually consolidated within them. 
‘* A place called El-Mouia-Tadjer presents a repetition of what we saw at El- 
Baya; one of the funnels formed in the middle of the dunes contains wells 
from two metres to two and a half in depth, dug ina sand which pressure, 
