FORMATION OF DUNES. 
473 
shape, partly by mere gravity, and partly by the slight cohe¬ 
sion of the lime, clay, and organic matter mixed with the 
sand; and it is observed that, from capillary attraction, evap¬ 
oration from lower strata, and retention of rain water, they 
are always moist a little below the surface.* By successive 
accumulations, they gradually rise to the height of thirty, 
fifty, sixty, or a hundred feet, and sometimes even much 
higher. Strong winds, instead of adding to their elevation, 
sweep off loose particles from their surface, and these, with 
* “ Dunes are always full of water, from the action of capillary attrac¬ 
tion. Upon the summits, one seldom needs to dig more than a foot to find 
the sand moist, and in the depressions, fresh water is met with near the 
surface.”— Forohhammer, in Leonhard und Bronn, for 1841, p. 5, note. 
On the other hand, Andresen, who has very carefully investigated this 
as well as all other dune phenomena, maintains that the humidity of the 
sand ridges cannot be derived from capillary attraction. He found by 
experiment that drift sand was not moistened to a greater height than 
eight and a half inches, after standing a whole night in water. He 
states the minimum of water contained by the sand of the dunes, one foot 
below the surface, after a long drought, at two per cent., the maximum, 
after a rainy month, at four per cent. At greater depths the quantity is 
larger. The hygroscopicity of the sand of the coast of Jutland he found 
to be thirty-three per cent, by measure, or 21.5 by weight. The annual 
precipitation on that coast is twenty-seven inches, and, as the evaporation 
is about the same, he argues that rain water does not penetrate far beneath 
the surface of the dunes, and concludes that their humidity can be explained 
only by evaporation from below .—Om Klitformationen, pp. 106-110. 
In the dunes of Algeria, water is so abundant that wells are constantly 
dug in them at high points on their surface. They are sunk to the depth 
of three or four metres only, and the water rises to the height of a m&tro 
in them.— Laurent, Memoire sur le Sahara , pp. 11, 12, 18. 
The same writer observes (p. 14) that the hollows in the dunes are 
planted with palms which find moisture enough a little below the surface. 
It would hence seem that the proposal to fix the dunes which are supposed 
to threaten the Suez Canal, by planting the maritime pine and other trees 
upon them, is not altogether so absurd as it is thought to be by some of 
those disinterested philanthropists of other nations who are distressed with 
fears that French capitalists will lose the money they have invested in that 
great undertaking. 
Ponds of water are often found in the depressions between the sand 
hills of the dune chains in the North American desert. 
