The beds of rivers are the lowest parts of great chasms, formed by the same revolutions which produced the mountains. Running waters unceasingly wear away their beds and banks in places where their declivity is very rapid; they hollow out and deepen their channels in mountains composed of rocks of moderate hardness; they draw along stones, and form accumulations of them in the lower part of their course; and thus their beds are often gradually elevated in the plains, while they are deepened and depressed in the mountains. But these changes, though continually going on for thousands of years, could only give form to the banks of rivers; they in no wise created the banks themselves. Many great rivers flow with an almost imperceptible declivity. The river of the Amazons has only ten feet and a half declivity upon two hundred leagues of its course, making one twenty-seventh of an inch for every thousand feet. When a river is obstructed in its course by a bank of solid rocks, and finds beneath them a stratum of softer materials, its waters wear away the softer substance, and thus open for themselves a subterraneous passage, more or less long. Such are the causes which have formed the magnificent Rock Bridge in Virginia, an astonishing vault uniting two mountains, separated by a ravine two hundred and seventy feet in depth, in which the _Cedar Creek_ flows. In Louisiana, trees, or rather whole forests, have been observed to fall on a river, covering it nearly with vegetable earth; and thus giving rise to a natural bridge which for leagues has hid the course of the river.