Mechanical forces at work in the disintegration of rocks--Rains Landslips--Effects of frosts--Glaciers and icebergs--Abrading power of rivers--Suggested volume on the geology of rivers--Some of its probable contents--Scene in a woody ravine--First idea of the origin of the ravine one of primeval cataclysms--Proved to be incorrect--Love of the marvellous long the bane of geology--More careful examination shows the operations of Nature to be singularly uniform and gradual--The doctrine of slow and gradual change not less poetic than that of sudden paroxysms--The origin of the ravine may be sought among some of the quieter processes of Nature--Features of the ravine--Lessons of the waterfall--Course of the stream through level ground--True history of the ravine--Waves and currents--What becomes of the waste of the land--The Rhone and the Leman Lake--Deltas on the sea-margin--Reproductive effects of currents and waves--Usual belief in the stability of the land and the mutability of the ocean--The reverse true--Continual interchange of land and sea part of the economy of Nature--The continuance of such a condition of things in future ages rendered probable by its continuance during the past. The forms of decomposition noticed in the last chapter were chiefly of a chemical kind. Their effects were observable alike on the surface of the earth and below ground; in the latter case we saw them excavating caverns and long irregular chasms, in the former we noted the production of debris which if undisturbed went to the formation of soils. It must be borne in mind however, that in these operations other forces than simply those of a chemical kind come into play. The percolation of water and the removal of insoluble particles on the exposed parts of rocks rank as mechanical processes. So also do those by which new surfaces of mineral masses are brought within the sphere of the chemical agencies, such as the action of frosts, rains, rivers, and waves. In short, as already noticed, any subdivision of the forces at work in effecting the decomposition of rocks must ever be more or less arbitrary; but it remains nevertheless useful, if we bear in mind that the exactly defined boundary lines are of our making, not Nature's. With this caution we may proceed to examine what are termed the mechanical agencies in the disintegration of mineral masses, and in so doing, we shall find that the chemical forces are not less helpful to the mechanical than the latter to the former.