The geographical features of France, of Europe and of the entire world had become modified, from century to century. Seas had replaced continents, and new deposits at the bottom of the ocean covered the vanished ages, forming new geological strata. Elsewhere, continents had taken the place of seas. At the mouth of the Rhone, for example, where the dry land had already encroached upon the sea from Arles to the littoral, the continent gained to the south; in Italy, the deposits of the Po had continued to gain upon the Adriatic, as those of the Nile, the Tiber, and other rivers of later origin, had gained upon the Mediterranean; and in other places the dunes had increased, by various amounts, the domain of the dry land. The contours of seas and continents had so changed that it would have been absolutely impossible to make out the ancient geographical maps of history. The historian of nature does not deal with periods of five centuries, like the Arab of the thirteenth century mentioned in the legend related a moment ago. Ten times this period would scarcely suffice to modify, sensibly, the configuration of the land, for five thousand years are but a ripple on the ocean of time. It is by tens of thousands of years that one must reckon if one would see continents sink below the level of seas, and new territories emerging into the sunlight, as the result of the secular changes in the level of the earth’s crust, whose thickness and density varies from place to place, and whose weight, resting upon the still plastic and mobile interior, causes vast areas to oscillate. A slight disturbance of the equilibrium, an insignificant dip of the scales, a change of less than a hundred meters, often, in the length of the earth’s diameter of twelve thousand kilometers, is sufficient to transform the surface of the world. And if we examine the ensemble of the history of the earth, by periods of one hundred thousand years, for example, we see, that in ten of these great epochs, that is, in a million years, the surface of the globe has been many times transformed.