If the size which Herodotus attributes to the Sea of Asoph, which he makes equal to the Euxine[99], had been less vaguely indicated, and if we knew precisely what he meant by the Gerrhus[100], we should there find strong additional proofs of the changes produced by rivers, and the rapidity with which they are made; for the alluvial depositions of rivers alone have, since the time of Herodotus, that is to say, in the course of two thousand and two or three hundred years, reduced the Sea of Asoph[101] to its present comparatively small size, shut up the course of the Gerrhus, or that branch of the Dnieper which had formerly joined the Hypacyris, and discharged its waters along with that river into the gulf called Carcinites, now the Olu-Degnitz, and reduced the Hypacyris itself to almost nothing[102]. We should possess proofs no less strong of the same kind, could we be certain that the Oxus or Sihoun, which at present discharges itself into the lake Aral, formerly reached the Caspian Sea. But we are in possession of facts sufficiently conclusive on the point in question, without adducing such as are doubtful, and without being exposed to the necessity of making the ignorance of the ancients in geography the basis of our physical propositions.[103] _Progress of the Downs._ 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. In 1802, the pent up pools overwhelmed five fine farming establishments at the village of St Julian[105]. They have long covered up an ancient Roman road leading from Bourdeaux to Bayonne, and which could still be seen forty years ago, when the waters were low[106]. The Adour, which is known to have formerly passed Old Boucaut, to join the sea at Cape Breton, is now turned to the distance of more than two thousand yards.