The sediment annually deposited by the Nile varies in thickness in different years. The mean thickness of the annual layers at Cairo has been calculated not to exceed that of a sheet of thin pasteboard, so that "a stratum of two or three feet must represent the accumulation of a thousand years."[68] Such thin laminæ must resemble greatly some of the more fissile shales in the Carboniferous system, which were, perhaps, formed by as slow a process, and in their aggregate depth probably took many thousand years to accumulate. But those fluviatile depositions of the Nile vary little in kind, for when cut through they are found regularly stratified down to their base, which rests upon the great underlying sand. They show us how the argillaceous seams of the coal-measures may have originated; but the diversity of character in these Carboniferous rocks indicates a more varied kind of sediment, and probably more rapid and active transporting currents. A closer analogy to such a condition of things meets us on the shores of the New World. [Footnote 68: Lyell's _Principles_, p. 262.] The Mississippi, so magnificent in all its proportions, has raised a delta which covers a tract of about 14,000 square miles, equal to almost half the area of Ireland. The lower parts of this delta are formed of low, shifting banks, traversed by innumerable streams that diverge from the main river, and alternately throw down and remove vast quantities of earthy sediment, intermingled with rafts of drift-wood. These swamps are covered with a rank growth of long grass and reeds, and for about six months of the year are more or less submerged below the waters of the river, while liable at the same time to continual inundation and encroachment from the sea. The higher parts of the delta, though also subject to a similar periodical submergence, nourish a more luxuriant vegetation. Vast tracts of level sandy soil are densely overgrown with pine, which is used extensively for making pitch. Large districts of the swampy ground are covered with willows, poplars, and thickets of the deciduous cypress, an elegant tree that rises more than 100 feet above the soil. When in hot seasons these swamps get dried up, "pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left. At the bottom of all these 'cypress swamps' a bed of clay is found, with roots of the tall cypress, just as the underclays of the coal are filled with stigmaria."[69] In this way a thick accumulation of vegetable matter goes on forming for years, until either the river changes its course, and inundating the swamp gradually covers it over with sand and mud, or until, owing to oscillations of the earth's crust, the district is either permanently submerged, so as to be silted over, or elevated to nourish a new and different kind of vegetation. [Footnote 69: Lyell's _Elements_, p. 386.]