The Earth as Modified by Human Action eBook

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 Geographic, 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.  Forchhammer, 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 [Haemalopus ostralegus], which carries his prey to the top of the dunes to devour it.”  See also Staring, De Bodem van Nederland, i., p. 821.] and they are also, usually somewhat changed in consistence 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 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 by plantations, but, by mixing clay and other tenacious earths with the superficial stratum of extensive sand plains, and by the application of fertilizing 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 accummulation of vegetable mould, into arable ground, constitute a conquest over nature which precedes agriculture—­a geographical 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.