Every one is familiar with the fact that many rivers overflow their banks at certain seasons, and spread themselves out over a wide area, sometimes reaching to the foot of the hills that bound the valleys through which they flow. This is the origin of those Alluvial Plains so remarkable for their surpassing richness and fertility. In each successive year a thin film of sediment is deposited on the surface of the land; and thus in the course of ages a soil is formed capable of producing, season after season, the most luxuriant crops without manifesting any symptoms of exhaustion. The soil of the Alluvial Plain near St. Louis, on the Mississippi, is thus spoken of by a modern traveller: “As to the quality of the land, any given number of crops might be grown off it. Corn has been raised on it for a hundred years together--as far back as the settlement is known. To inquire about the system of farming in the West is not productive of information which would be of service on the continent of Europe. There is no system: the farmer scratches the ground and throws in the seed, and his bountiful harvests come up year after year without further thought or trouble. Thousands of centuries have made the soil for him, and it defies him to make too heavy demands upon it. It gives him all he asks, and is never known to disappoint or fail.”[42] The great basin of the Nile offers an admirable example of an Alluvial Plain on a scale of considerable magnitude. Even in the days of Herodotus, Egypt was regarded as the “gift of the Nile:” and the correctness of this opinion has been placed beyond all reasonable doubt by the investigations of modern science. The river bears along in its current, especially during the flood season, a large quantity of fine earthy sediment obtained by the process of Denudation from the mountains of central Africa. Once a year, between the months of July and November, it overflows its banks, and this sediment is deposited on the adjoining plains. Thus a new layer of rich soil is spread out every year over the existing surface; and the whole country is, in a manner, growing upward at the average rate, according to a rough estimate, of about six inches in the century. Near Cairo, where excavations have been made, the successive layers of annual deposit are distinctly visible to the eye. And it is worthy of remark that, although each one of these is no thicker than a sheet of paste-board, the stratum of alluvial soil which overlies the sands of the desert, and which to all appearance has come into existence by the very same process, is often forty, fifty, and even sixty feet in depth.