FORMATION OF DUNES. 563 
feet, and sometimes even much higher. Strong winds, instead 
of adding to their elevation, sweep off loose particles from their 
surface, and these, with others blown over or between them, 
build up asecond row of dunes, and so on according to the 
character of the wind, the supply and consistence of the sand, 
and the face of the country. In this way is formed a belt of 
sand-dunes, irregularly dispersed and varying much in height 
and dimensions, and sometimes many miles in breadth. On 
the Island of Sylt, in the German Sea, where there are several 
rows, the width of the belt is from half a mile toa mile. There 
are similar ranges on the coast of Holland, exceeding two miles 
sand moist, and in the depressions, fresh water is met with near the surface.” 
-—_FORCHHAMMER, in LEONHARD und Brony, 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 a heap of drift-sand was not moistened to a greater height than eight 
and a half inches, after standing with its base 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 evap- 
oration 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 métres only, and the water rises to the height of a métre in them.— 
LAURENT, Mémoire surle Sahara, pp. 11, 12, 13. 
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 has been thought to be by some of those disin- 
terested philanthropists of other nations who were distressed with fears that 
French capitalists would lose the money they had 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. 
