We shall have gained a great step if we can approximate to the number of millions of years in which the average aqueous denudation going on upon the land would convey seaward a quantity of matter equal to the average volume of our continents, and this might give us a gauge of the minimum of volcanic force necessary to counteract such levelling power of running water; but to discover a relation between these great agencies and the rate at which species of organic beings vary, is at present wholly beyond the reach of our computation, though perhaps it may not prove eventually to transcend the powers of man.
[1] Hull, Quart. Geol. Journ., vol. xxiv, p. 322, 1868.
[2] Tylor, Phil. Mag., 4th series, p. 268, 1850.
[3] Croll, Phil. Mag., 1868, p. 381.
[4] Vol. i, p. 442, 1867.
[5] Trans. Geol. Soc. Glasgow, vol. iii, p. 169.
[6] 1st ed., chap. x, p. 167, 1830; see also 10th ed., vol. i, chap. xv, p. 327, 1867.
[7] Principles, vol. ii, p. 237; also 1st ed., p. 447, 1830.
[8] Principles, vol. ii, p. 229, 1868.
[9] Principles, vol. i, p. 265, 1867.