COAST DUNES. 
465 
by any chance, the vegetable network which, in most cases, 
thinly clothes and at the same time confines it, is broken. 
Human industry has not only fixed the flying dunes, but, 
by mixing clay and other tenacious earths with the superficial 
stratum of extensive sand plains, and by the application of fer¬ 
tilizing substances, it has made them abundantly productive 
of vegetable life. These latter processes belong to agriculture 
and not to geography, and, therefore, are not embraced within 
the scope of the present subject. But the preliminary steps, 
whereby wastes of loose, drifting barren sands are transformed 
into wooded knolls and plains, and finally, through the accu¬ 
mulation of vegetable mould, into arable ground, constitute a 
conquest over nature which precedes agriculture—a geograph¬ 
ical revolution—and, therefore, an account of the means by 
which the change has been effected belongs properly to the 
history of man’s influence on the great features of physical 
geography. I proceed, then, to examine the structure of 
dunes, and to describe the warfare man wages with the sand 
hills, striving on the one hand to maintain and even extend 
them, as a natural barrier against encroachments of the sea, 
and, on the other, to check their moving and wandering pro¬ 
pensities, and prevent them from trespassing upon the fields he 
has planted and the habitations in which he dwells. 
Coast Dunes. 
Coast dunes are oblong ridges or round hillocks, formed by 
the action of the wind upon sands thrown up by the waves on 
the beach of seas, and sometimes of fresh-water lakes. On 
most coasts, the supply of sand for the formation of dunes is 
derived from tidal waves. The flow of the tide is more rapid, 
and consequently its transporting power greater, than that of 
the ebb ; the momentum, acquired by the heavy particles in 
rolling in with the water, tends to carry them even beyond the 
flow of the waves ; and at the turn of the tide, the water is in 
a state of repose long enough to allow it to let fall much of the 
solid matter it holds in suspension. Hence, on all low, tide- 
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