315. Rivers are the causes of waste most visible to us, and most obviously capable of producing great effects. It is not, however, in the greatest rivers, that the power to change and wear the surface of the land is most clearly seen. It is at the heads of rivers, and in the feeders of the larger streams, where they descend over the most rapid slope, and are most subject to irregular or temporary increase and diminution, that the causes which tend to preserve, and those that tend to change the form of the earth's surface, are farthest from balancing one another, and where, after every season, almost after every flood, we perceive some change produced, for which no compensation can be made, and something removed which is never to be replaced. When we trace up rivers and their branches toward their source, we come at last to rivulets, that run only in time of rain, and that are dry, at other seasons. It is there, says Dr Hutton, that I would wish to carry my reader, that he may be convinced, by his own observation, of this great; fact, _that the rivers have, in general, hollowed out their valleys_. The changes of the valley of the main river are but slow; the plain indeed is wasted in one place, but is repaired in another, and we do not perceive the place from whence the repairing matter has proceeded. That which the spectator sees here, does not therefore immediately suggest to him what has been the state of things before the valley was hollowed out. But it is otherwise in the valley of the rivulet; no person can examine it without seeing, that the rivulet carries away matter which cannot be repaired, except by wearing away some part of the surface of the place upon which the rain that forms the stream is gathered. The remains of a former state are here visible; and we can, without any long chain of reasoning, compare what has been with what is at the present moment. It requires but little study to replace the parts removed, and to see nature at work, resolving the most hard and solid masses, by the continued influences of the sun and atmosphere.[161] We see the beginning of that long journey, by which heavy bodies travel from the summit of the land to the bottom of the ocean, and we remain convinced, that, _on our continents, there is no spot on which a river may not formerly have run_.[162] [Footnote 161: Theory of the Earth, vol. ii. 294.] [Footnote 162: _Ibid._ p. 296.]