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SAND DUNES AND PLAINS. 
Sand Dunes and Sand Plains. 
Two forms of sand deposit are specially important in Eu- 
ropean and American geography. The one is that of dune or 
shifting hillock upon the coast, the other that of barren plain 
in the interior. The coast-dunes are composed of sand washed 
up from the depths of the sea by the waves, and heaped in 
more or less rounded knolls and undulating ridges by the winds. 
The sand with which many plains are covered appears some- 
times to have been deposited upon them while they were yet 
submerged beneath the sea, sometimes to have been drifted 
from the seacoast, and scattered over them by wind-currents, 
sometimes to have been washed upon them by running water. 
In these latter cases, the deposit, though in itself considerable, 
is comparatively narrow in extent and irregular in distribution, 
while, in the former, it is often evenly spread over a very wide 
surface. In all great bodies of either sort, the silicious grains 
are the principal constituent, though, when not resulting from 
the disintegration of silicious rock and still remaining in place, 
they are generally accompanied with a greater or less admix- 
ture of other mineral particles, and of animal and vegetable re- 
mains,* and they are, also, usually somewhat changed in con- 
* Organic constituents, such as comminuted shells, and silicious and cal- 
careous exuvie of infusorial animals and plants, are sometimes found mingled 
in considerable quantities with mineral sands. These are usually the remains 
of aquatic vegetables or animals, but not uniformly so, for the microscopic 
organisms, whose flinty cases enter so largely into the sand-beds of the Mark 
of Brandenburg, are still living and prolific in the dry earth. See WITTWER, 
Physikalische Geographie, p. 142. 
The desert on both sides of the Nile is inhabited by a land-snail—of which 
I have counted eighty, in estimation, on a single shrub barely a foot high—and 
thousands of its shells are swept along and finally buried in the drifts by every 
wind. Every handful of the sand contains fragments of them. ForcHmAM- 
MER, in LEONHARD und BRONN’s Jahrbuch, 1841, p. 8, says of the sand-hills 
of the Danish coast: ‘‘It is not rare to find, high in the knolls, marine shells, 
and especially those of the oyster. They are due to the oyster-eater | Jwmalopus 
ostralegus|, which carries his prey to the top of the dunes to devour it.”’ See 
also STARING, De Bodem van Nederland, i., p. 321. 
