572 GEOLOGICAL IMPORTANCE OF DUNES. 
fication of the windward slope is somewhat confused, while the 
sand on the lee side is found to be disposed in more regular 
beds, inclining landwards, and with the largest particles lowest, 
where their greater weight would naturally carry them. The 
lee side of the dunes, being thus formed of sand deposited 
according to the laws of gravity, is very uniform in its slope, 
which, according to Forchhammer, varies little from an angle 
of 30° with the horizon, while the more exposed and irregular 
weather side lies at an inclination of from 5° to 10°. When, 
however, the outer tier of dunes is formed so near the water- 
line as to be exposed to the immediate action of the waves, it 
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 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 north-east 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 re- 
marked, is probably the truth, though observations are wanting 
to decide the question.* There would be nothing violently im- 
* “©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, 
