[49] The terms rapids, falls, and cataracts are rather loosely used. Many moderate rapids are incorrectly called falls. The “Falls of the Ohio” is an example. The term cataract is often applied to very steep rapids or falls.
[50] Gilbert, article on Niagara Falls, in Physiography of the United States.
[51] Gilbert. Am. Jour. Sci., Vol. XII. p. 99, 1876.
[52] For a brief account of this fall see Gilbert in Physiography of the United States.
[53] Gilbert. Science, Vol. VIII, p. 205, 1886.
[54] See Campbell, Jour. Geol., Vol. IV, pp. 567, 657.
[55] Russell. Rivers of North America, p. 280. The influence of joints on drainage is further discussed by Hobbs, Jour. Geol., Vol. IX, p. 469.
[56] See Willis. The Northern Appalachians, in Physiography of the United States.
[57] This process of adjustment has been well described by Davis in The Rivers and Valleys of Pennsylvania, Natl. Geog. Mag., Vol. I, p. 211 et seq.
[58] This sort of adjustment may be called topographic adjustment. A tributary is in topographic adjustment when its gradient is harmonious with that of its main.