GEOLOGICAL IMPORTANCE OF DUNES. 
479 
is undermined, and the face of the hill is very steep and some¬ 
times nearly perpendicular. 
Geological Importance of Dunes. 
These observations, and other facts which a more attentive 
study on the spot would detect, might furnish the means of 
determining 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 hills of the African desert were originally 
coast dunes, and that they have been transported to tlieh’ pres¬ 
ent position far in the interior, by the rolling and shifting lee¬ 
ward movement to which all dunes not covered with vegeta¬ 
tion are subject. The present general drift of the sands of that 
desert appears to he to the southwest 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 Sahara. The latter, as has been 
before remarked, is probably the truth, though observations 
are wanting to decide the question.* There is nothing vio¬ 
lently improbable in the supposition that they may have been 
first thrown up by the Mediterranean on its Libyan coast, and 
thence blown south and west over the vast space they now 
cover. But whatever has been their source and movement, 
they can hardly fail to have left on their route some sandstone 
monuments to mark their progress, such, for example, as we 
have seen are formed from the dune sand at the mouth of the 
i 
Nile; and it is conceivable that the character of the drifting 
sands themselves, and of the conglomerates and sandstones to 
whose formation they have contributed, might furnish satisfac- 
* “ On the west coast of Africa the dunes are drifting seawards, and 
always receiving new accessions from the Sahara. They are constantly 
advancing out into the sea.” See ante , p. 16, note.— Naumann, Geognosie , 
ii, p. 1172. 
