36 FORMATION OF THE DUNES. 



Teste de Buch the chain of sand-hills measures from 4400 to 6600 

 feet in width. At other points it is still wider ; but it narrows 

 towards its extremities, and both at the Point de Grave and near 

 Bayonne does not exceed 450 yards. 



Owing to their extreme shiftiness of soil, the Dunes can attain no 

 considerable elevation. The sand deposited by the wind on the sum- 

 mit of the hill is always in a state of precarious equilibrium. It has 

 a constant tendency to be precipitated down the other side ; and the 

 higher the summit the greater is this tendency, so that there comes 

 at last a moment when no further accumulation in height is possible. 

 The Dune may then extend its basis, may even increase twofold in 

 dimensions, but it no longer rises. 



Let us note, moreover, that owing to its density the sand cannot 

 be carried even by the most violent winds into the higher regions of 

 the atmosphere ; and that the Dunes, when they have reached a cer- 

 tain elevation, oppose to them an insuperable obstacle. This circum- 

 stance would consequently have a salutary effect, and the accumulation 

 of sand would be determined by a law of its own, if the Dunes, once 

 formed, had time to cohere. But this is not the case. Incessantly 

 does the wind undo or modify its work ; and the loftiest hills being 

 the most exposed to its violence, are quickly reduced to the common 

 level. In general, the greatest elevation of the Dunes corresponds to 

 their greatest breadth. Thus the culminating point of those of Gas- 

 cony is found in the belt situated between the lakes of Cazau and 

 Biscarosse, where the chain is from 7500 to 9000 yards across. 

 Their average height is 180 feet to 200 feet above the sea-level ; but 

 some of the hills in the forest of Biscarosse attain an altitude of 320 

 feet. In the neighbourhood of the mouths of the Gironde and the 

 Adour, where the chain is considerably nan-owed, the height of the 

 Dunes is only thirty to forty-five feet. 



The reader must not suppose that the Dunes consist of a single 

 series of sand-hills ranged along the shore. He will, however, have 

 conjectured, from our statements respecting their width, that they 

 really compose a chain of several more or less regular ridges. The 



