probably would be the sinuosities of the stream, and the wider would its waters spread. The greater also would be the probability of the stream, on the cessation of the flood, not returning to its original channel, which might have become obliterated or filled up, but of its flowing along some new course, it may be miles away from its former channel. Even when not flooded so as to overflow their banks, rivers along which a larger body of water flowed than there does at present, would, so long as they were not confined within deep valleys, have a tendency to wander over a much wider tract of country than that now occupied by their valleys. The tendency of all rivers to produce sinuosities in their course is well known; but Mr. Fergusson, in his excellent paper on recent changes in the Delta of the Ganges,[1] has called attention to the fact that all rivers oscillate in curves, the extent of which is. directly proportionate to the quantity of water flowing through them.
But rivers in a state of flood, or passing even at a moderate speed over soft or incoherent soil, are always turbid, owing to the presence in their waters of earthy matter which they are transporting towards the sea. The character of the solid matter thus transported by water in motion is entirely dependent on its velocity. A velocity of 300 yards per hour is sufficient to tear up fine clay; of 600 yards, fine sand; of 1,200 yards, fine gravel; and of a little over two miles per hour, to transport shivery angular stones of the size of an egg.[2] Considering the small velocity requisite to remove the finer particles of the soil, and to retain them in suspension, a river such as has been supposed, must have been excessively turbid, so long as any fine earthy particles were accessible to its waters, or to those of the streamlets delivering into it.
The amount of solid matter suspended in turbid water is. greater than might be imagined. Mr. A. Tylor has calculated that the detritus carried down by the Ganges is equivalent to what would result from the removal of soil a foot in depth over the whole of the area which it drains in 1,791 years,[3] and that brought down by the Mississippi to one foot in 9,000 years. Other estimates fix this at one foot in 6,000 years, while the sediment contained in its stream has been estimated at from 11245 to 11300 of the weight of the water.[4] Taking this latter proportion, an inch of rain
- ↑ Quar. Journ. Geol. Soc., vol. xix. (1863), p. 321.
- ↑ "Encyc. Brit."—Art. "Rivers." Lyell, "Princ. of Geol," 10th ed., yol. i. p. 348. Lubbock, "Prehistoric Times," 4th ed., p. 382.
- ↑ Quar. Journ. Geol. Soc., vol. ix. (1853), p. 48.
- ↑ Lyell's "Princ. of Geol.," vol. i. p. 458. Geikie, Geol. Mag., vol. v. p. 250.
