It will easily be understood that this accumulation along the shore cannot have taken place where the force and direction of the sands experience periodical or capricious changes; for then the sands cast upon the beach by the winds of the north and west would be driven back into the sea by the winds of the south and east. This is noticeable in many places where the nature of the coast is favourable for the production of such a phenomenon. But on other shores--as on the Atlantic littoral of France--the winds which blow most frequently and most violently are from the west and south-west. And it is there we encounter the Dunes. Those of Gascony are by far the most remarkable. Northward, they extend as far as the Point de Grave, which shuts in the mouth of the Gironde; southward, to the bank of the Adour, and even further, to the cliffs of Béarn. Here the basin of Arcachon constitutes one vast hollow; and some openings exist, moreover, in the department of Landes, between that basin and the Adour, for the overflow of the waters which descend from the interior. To the north and south of the 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 summit 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.