COAST DUNES. 559 
sistence by the ever-varying conditions 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 considerable adhesiveness in the mass—in which case 
it can no longer properly 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, by any chance, the vegetable net- 
work which, in most cases, thinly clothes and at the same time 
confines it, is broken. 
Human industry has not only fixed the flying dunes by plan- 
tations, but, by mixing clay and other tenacious earths with the 
superficial stratum of extensive sand plains, and by the applica- 
tion of fertilizing substances, it has made them abundantly pro- 
ductive of vegetable life. These latter processes belong to agri- 
culture 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 trans- 
formed into wooded knolls and plains, and finally, through the 
‘accumulation of vegetable mould, into arable ground, constitute 
a conquest over nature which precedes agriculture—a geogra- 
phical 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 terrestrial 
surface. 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 propensities, 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 
