MOVEMENT OF DUNES. 563^ 



erection of dikes and protection of dunes, there woidd now be 

 left of Holland little but the name. It is, as has been already 

 seen, still a debated question among geologists whether the coast 

 of Holland now is, and for centuries has been, subsiding. I be- 

 lieve most investigators maintain the affirmative ; and if the fact 

 is so, the advance of sea upon the land is, in part, due to this 

 cause. But the rate of subsidence is at all events very small, and 

 therefore the encroachments of the ocean upon the coast are 

 mainly to be ascribed to the erosion and transportation of the soil 

 by marine waves and currents. 



The sea is fast advancing at several points of the western coast 

 of France, and unknown causes have given a new impulse to its 

 ravages since the commencement of the present century. Between 

 1830 and 1842, the Point de Grave, on the north side of the Gi- 

 ronde, retreated one hundred and eighty metres, or fifty feet, per 

 year ; from the latter year to 1846, the rate was increased to more 

 than three times that quantity, and the loss in those four years 

 was about six hundred feet. All the buildings at the extremity 

 of the peninsula have been taken down and rebuilt farther land- 

 wards, and the hghthouse of the Grave now occupies its third po- 

 sition. ■ The sea attacked the base of the peninsula also, and the 

 Point de Grave and the adjacent coasts have been for thirty years 

 the scene of one of the most obstinately contested struggles be- 

 tween man and the ocean recorded in the annals of modern engi- 

 neering. 



Movement of Dv/nes. 



Besides their importance as a barrier against the inroads of the 

 ocean, dunes are useful by sheltering the cultivated ground behind 

 them from the violence of the sea-wind, from salt spray, and from 

 the drifts of beach sand which would otherwise overwhelm them. 

 But the dunes themselves, unless their surface sands are kept 

 moist, and confined by the growth of plants, or at least by a crust 

 of vegetable earth, are constantly rolling inwards, and thus, while, 

 on one side, they lay bare the traces of ancient human habitations 

 or other evidences of the social life of primitive man, they are, on 

 the other, burying fields, houses, churches, and converting popu- 

 lous districts into barren and deserted wastes. 



Especially destructive are they when, by any accident, a cavitji 



