638 COAST-DUNES. 



vegetable network, wMcli 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 appHca- 

 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 prehminary 

 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 geographi- 

 cal 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 pro- 

 ceed, 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 mov- 

 ing and wandering propensities, and prevent them from trespass- 

 ing upon the fields he has planted and the habitations in which he 

 dwells. 



CoasirDmies. 



Coast-dunes are oblong ridges or round hillocks, formed by the 

 action of the wind upon sands thrown up by the waves on the low 

 beaches 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 conse- 

 quently 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 sohd matter it holds in 

 suspension. Hence, on all low, tide-washed coasts of seas with 

 sandy bottoms, there exist several conditions favorable to the 

 formation of sand deposits along high-water mark.* If the land- 



* There are various reasons why the formation of dunes is confined to low 



