Beginning at the bottom, we may compute the period of the forty feet of sandstone variously, according to the river selected as the type of a transporting agent. Tried by the standard of the Nile, all other conditions being similar, such a deposit would require perhaps not less than 14,000 years; by that of the Mississippi, 5000; and by that of the Ganges, nearly 2000.[72] We come, then, to the superincumbent fire-clay and coal, representing an ancient soil and the forest that grew on it. The occurrence of these seams shows us that the river-bed had become a swampy tract clothed with vegetation; but who shall say how long it may have continued so? Like the sunk country of the Mississippi, it may have been submerged, and to some extent cut off from the sediment-transporting channels of the river, and thus, as a vast lake, have nourished a prolific growth of marshy and aquatic plants. If the temperature resembled that of our own country, the growth of peaty matter, other circumstances being favourable, might be comparatively rapid. If, however, as seems probable, the climate were more warm and humid, giving rise to a more luxuriant vegetation, and at the same time to a more rapid decay, a long interval might have elapsed without adding materially to the thickness of the vegetable accumulations, and the eventual entombment of peaty matter sufficient to consolidate into four feet of coal, might be owing in some measure to the submergence of the swamp beneath the waters of the river, whereby a quantity of detrital matter was deposited that arrested the process of putrefaction, and entombed the thickly matted plants which were growing on the spot at the time. Hence, until we know more of the conditions under which vegetation may accumulate at river-mouths in such a climate as the coal plants are conjectured to have enjoyed, calculations of the amount of time required for the formation of a great series of coal-bearing strata must be regarded as premature. In the present instance, we can but affirm that the growth of the four-foot coal-seam probably occupied many long years, even at the most rapid rate of accumulation known to us. The forest-covered swamp on which the plants grew was eventually invaded by muddy detritus brought down by the river; and during another period of indefinite extent--five hundred years or five thousand years--fine mud continued to settle down over the foundered forest, hardening eventually into twenty feet of shale.