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The downs or hillocks of sand which the sea throws up on low coasts, when its bottom is sandy, have already been mentioned. Wherever human industry has not succeeded in fixing these downs, they advance as irresistibly upon the land as the alluvial depositions of the rivers advance into the sea. In their progress inland, they push before them the large pools formed by the rain which falls upon the neighbouring grounds, and whose communication with the sea is intercepted by them. In many places they proceed with a frightful rapidity, overwhelming forests, buildings, and cultivated fields. Those upon the coast of the Bay of Biscay[104] have already overwhelmed a great number of villages mentioned in the records of the middle age; and at this moment, in the single Department of the Landes, they threaten ten with inevitable destruction. One of these villages, named Mimisan, has been struggling against them these twenty years, with the melancholy prospect of a sand-hill of more than sixty feet perpendicular height visibly approaching it.