464 
SAND DUNES AND PLAINS. 
up from the depths of the sea by the waves, and heaped in 
knolls and ridges by the winds. The sand with which many 
plains are covered, appears sometimes to have been deposited 
upon them while they were yet submerged, sometimes to have 
been drifted from the sea coast, 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 admixture of other mineral particles, and 
of animal and vegetable remains,* and they are, also, usually 
somewhat changed in consistence by the ever-varying condi¬ 
tions of temperature and moisture to which they have been 
exposed since their deposit. Unless the proportion of these 
latter ingredients is so large as to create a certain adhesiveness 
in the mass—in which case it can no longer prop-erly be called 
sand—it is infertile, and, if not charged with water, partially 
agglutinated by iron, lime, or other cement, or confined by 
alluvion resting upon it, it is much inclined to drift, whenever, 
* Organic constituents, such as comminuted shells, and silicious and 
calcareous exuvite 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 Witt wee, Physikalische Geographies p. 142. 
The desert on both sides of the Nile is inhabited by a land snail, 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. 
Forchhammer, in Leonhard und Beonn’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 [Hcemalopus ostralegus ], which carries his prey to the top of 
the dunes to devour it.” 
