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The rains which fall, the vapours which are condensed, and the snows which are melted, upon the ridges and summits of mountains, descend, by an infinite number of rills, along their slopes, carrying with them some portions of the materials of which these slopes are composed, and tracing slight furrows by their passage. These rills soon unite in the deeper gutters with which the surface is marked, run off by the deep valleys which intersect their bottom, and thus form streams and rivers, which carry back to the sea the waters it had formerly supplied to the atmosphere. On the melting of the snows, or when a storm takes place, these mountain torrents become suddenly swollen, and rush down the declivities with a velocity proportioned to their steepness. They dash violently against the bases of those taluses of fallen fragments which cover the sides of all the high valleys, carrying off the already rounded fragments of which they are composed, and which thus become smoothed, and still farther polished, by attrition. But in proportion as they reach the more level valleys, where their violence is diminished, or when they arrive at more expanded basins, where their waters are permitted to spread, they throw out upon their banks the largest of those stones which they had rolled down. The smaller fragments are deposited still lower; and nothing reaches the great canal of the river excepting the minutest particles, or the most impalpable mud. It often happens, also, that before these streams unite to form great rivers, they have to pass through large and deep lakes, in which their mud is deposited, and from which their waters come forth limpid.