Читать книгу Essay on the Theory of the Earth - Georges baron Cuvier - Страница 42

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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.

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.

The late M. Bremontier, inspector of bridges and highways, who conducted extensive operations upon these downs, estimated their progress at sixty feet yearly, and in some places at seventy-two feet. According to this calculation, it will only require two thousand years to enable them to reach Bourdeaux; and, from their present extent, it must have been somewhat more than four thousand years since they began to be formed[107].

The overwhelming of the cultivated lands of Egypt, by the sterile lands of Libya, which are thrown upon them by the west wind, is a phenomenon of the same nature with the downs. These sands have destroyed a number of cities and villages, whose ruins are still to be seen; and this has happened since the conquest of the country by the Mahometans, for the summits of the minarets of some mosques are seen projecting beyond the sand[108]. With a progress so rapid, they would, without doubt, have filled up the narrow parts of the valley, if so many ages had elapsed since they began to be thrown into it[109]; and there would no longer remain any thing between the Libyan chain and the Nile. Here, then, we have another natural chronometer, of which it would be as easy as interesting to obtain the measure.