562 HEIGHT OF DUNES. 
in a continuous, irregular bulwark, but deposited in isolated 
hillocks, they have a tendency to assume a crescent shape, the 
convexity being turned seawards, or towards the direction from 
which the prevailing winds proceed. This fact, the geological 
bearing of which is obvious, is not noticed by previous French 
writers or even by Andresen, though a semi-lunar outline has 
been long generally ascribed to inland dunes. It is, however, 
evident that such a form would naturally be produced by the 
action of a wind blowing long in a given direction upon a mass 
of loose sand with a fixed centre—such as is constituted by the 
shrub or stone around which the sand is first deposited—and 
free extremities. 
On a receding coast, dunes will not attain so great a height 
as on more secure shores, because they are undermined and 
carried off before they have time to reach their greatest dimen- 
sions. Hence, while at sheltered points in South-western 
France, there are dunes three hundred feet or more in height, 
‘those on the Frisic Islands and the exposed parts of the coast of 
‘Schleswig-Holstein range only from twenty to one hundred feet. 
‘On the western shores of Africa, it is said that they sometimes 
‘attain an elevation of six hundred feet. This is one of the 
very few points known to geographers where desert sands are 
‘advancing seawards,* and here they rise to the greatest altitude 
‘to which sand-grains can be carried by the wind. 
The hillocks, once deposited, are held together and kept in 
‘shape, partly by mere gravity, and partly by the slight cohesion 
of the lime, clay, and organic matter mixed with the sand ; and 
it is observed that, from capillary attraction, evaporation from 
lower strata, and retention of rain-water, they are always moist 
-a little below the surface.t By successive accumulations, they 
gradually rise to the height of thirty, fifty, sixty, or a hundred 
*“° 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,”’—NAUMANN, Geognosie, ii., p. 1172. 
+ ‘‘ Dunes are always full of water, from the action of capillary attraction. 
Upon the summits, one seldom needs to dig more than a foot to find the 
