550 GEOLOGICAL IMPORTAIS^CE OF DUNES. 



Geological Irnijortance of Dunes. 



These observations, and other facts which a more attentive 

 Btudj on the spot would detect, might furnish the means of deter- 

 mining interesting and important questions concerning geological 

 formations in localities very unlike those where dunes are now 

 thrown up. For example, Studer supposes that the drifting sand- 

 liills of the African desert were originally coast-dunes, and that 

 they have been transported to their present position, far in the 

 interior, by the rolling and shifting leeward movement to which 

 all dunes not covered with vegetation are subject. The present 

 general drift of the sands of that desert appears to be to the south- 

 west and west, the prevailing winds blowing from the northeast and 

 east ; but it has been doubted whether the shoals of the western 

 coast of ^Northern Africa, and the sands upon that shore, are 

 derived from the bottom of the Atlantic, in the usual manner, 

 or, by an inverse process, from those of the Sahai-a. The latter, 

 as has been before remarked, is probably the truth, though obser- 

 vations are wanting to decide the question.* There would be 

 nothing violently improbable in the a pi'iori 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-bot 

 toms, strevm with sand, and that its present supply of that ma 

 terial 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 conside* 

 able depths below the surface.f These observations have been co^ 

 firmed past all question by Desor, Martins and others, and the 

 facts and the obvious conclusion they suggest are at present not 

 disputed. 



* " The North African desert falls into two divisions : the Sahel, or western, 

 and the Sahar, or eastern. The sands of the Sahar were, at a remote period, 

 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 

 «wept clean." — Naumann, Oeognosie, ii., p. 1173. 



f Memovres sur le Sahara Oriental, p. 62. 



