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There still exist, however, four causes in full activity, which contribute to alter the surface of our continents. These are, rains and thaws, which waste down the steep mountains, and precipitate the fragments to their bottoms; running waters, which carry off these fragments, and deposit them in places where their current is abated; the sea, which undermines the foundations of elevated coasts, forming steep cliffs, and which throws up great banks of sand upon the low coasts; and, lastly, volcanoes, which pierce through the solid strata from below, elevate these strata, or spread over the surface vast quantities of ejected matter[7].

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In every place where the broken strata present their edges on abrupt surfaces, there fall down to their base, every spring, and even after every storm, fragments of their materials, which are rounded by rolling upon each other. These collected heaps gradually assume an inclination determined by the laws of cohesion, and thus form, at the bottom of the cliff, taluses, of greater or less elevation, according as the fragments which have fallen are more or less abundant. These taluses constitute the sides of the valleys in all elevated, mountainous regions, and are covered with a rich vegetation, whenever the fragments from the upper parts begin to fall less abundantly; but their want of solidity subjects themselves also to slips, when they are undermined by rivulets. On these occasions, towns, and rich and populous districts, are sometimes buried under the ruins of a mountain; the courses of rivers are interrupted, and lakes are formed in places which were before the abodes of fertility and cheerfulness. Fortunately these great slips happen but seldom, and the principal use of those hills of debris, is to furnish materials for the ravages of torrents.