MOVEMENT OF DUNES. 587 
advance of the 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 Gironde, 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 landwards, and the lighthouse of the 
Grave now occupies its third position. The sea attacked the 
base of the peninsula also, and the Point de Grave and the ad- 
jacent coasts have been for thirty years the scene of one of the 
most obstinately contested struggles between man and the ocean 
recorded in the annals of modern engineering. 
Movement of Dunes. 
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 over- 
whelm 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 populous districts into barren 
and deserted wastes. 
Especially destructive are they when, by any accident, a 
