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 certain elevation, oppose to them an insuperable obstacle. This circumstance 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 Gascony 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 narrowed, 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 hills are separated from one another by valleys, locally named _laites_ or _lettes_. These valleys, where the pluvial waters flow and accumulate, exhibit a striking contrast, in their freshly-blooming verdure, to the naked, barren Dunes. The general aspect of the landscape may, therefore, be compared to that of the ocean. There is the same broken surface, the same extent of undulation, the billows of sand being upheaved by the wind like the billows of the sea, and sharing in their mobility. You must see, says a writer, in order to form an idea of those colossal masses of fine sand, which the wind incessantly skims, and which travel in this way towards the inland country: you must see their contours so softened that they look like mountains of plaster of Paris polished by the workman's hand, and their surface so mobile that a little insect leaves upon it a conspicuous track; their slopes, at every degree of inclination; their everlasting sterility--not a blade of grass, not an atom of vegetation; their solitude, less imposing than that of the mountains, but still of a truly savage character. You must see, from the summit of one of these ridges, the ocean on your right hand, and on your left the extensive lakes which border the littoral; and, in the midst of this tumultuous sea of tawny sand, green grassy valleys, rich and fertile pastures, smiling oases of verdure, where herds of horses graze, and cows half-wild, guarded by shepherds scarcely less wild than they.[12]