falling on a square mile of ground, and flowing off it in a turbid state, would carry with it at least forty-three tons of sediment; and were we to assume an annual rainfall of fifty-four inches—which, though exceptional, is by no means unknown even in the British Isles—about 2,300 tons of fine earthy matter would be removed from a square mile of country in a single year. Taking a cubic yard of solid ground as equal to a ton in weight, this would involve the removal of one foot in depth from the surface in about 450 years. If, however, a portion of the rainfall were delivered by springs, or fell on hard or rocky ground, so as not to be rendered turbid, of course the effect would be proportionally diminished. Sir Archibald Geikie[1] has estimated that practically, at the present day, the Thames (apart from about 450,000 tons of chalk and other matter carried away annually in solution), lowers its basin at the rate of one foot in 11,740 years; the Boyne, one foot in 6,700 years; the Forth, one foot in 3,111 years; and the Tay, one foot in 1,482 years. It is, however, with water moving with far greater velocity than that merely sufficient to keep fine sediment in suspension, that we have to deal in this hypothetical case; and we may readily suppose the streams, at more or less regular intervals, liable to violent floods, eroding the chalk and the superimposed clays and gravels, and carrying with them not only the finer particles and sand, but the pebbles, large and small, of the gravel, and the flints washed out of the chalk.
Let us now consider what would be the condition of the surface of a broad shallow valley, on the cessation of a flood such as that which has been supposed. In certain parts removed from the main current, and where the water had been nearly stationary, we should find deposits of fine mud or clay; in others, where the water had still moved with sufficient velocity to retain the clay and fine silt in suspension, the heavier particles of sand would have accumulated; in others, again, the smaller stones and pebbles; while near the main current, especially on the inner side of any curves which it had made, and where of course its velocity had been diminished, we should find the larger flints and pebbles, probably to some extent intermixed with part of the finer materials. In the beds of mud and sand, we should probably find the shells of some of the molluscs inhabiting the waters, and also those of terrestrial species, washed in from the inundated land surface, or brought down from the banks of the tributary rivulets; while
- ↑ Geol. Mag. (1868), vol. v. p. 250.
