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Page:Popular Science Monthly Volume 38.djvu/761

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rents, leaping from ledge to ledge, or dashing round and over masses of rock in their wild mountain homes. Lower down the current slackens, some of the impetuosity is lost, but it still glides swiftly over its rocky bed. Still lower down the current becomes slower, the stream broadens out, and the bed loses its rocky and

PSM V38 D761 Differences of unglaciated and glaciated lands.jpg
Ideal view of an Old Unglaciated Country, showing the Form assumed by the Eminences when Erosion has proceeded to a great extent. (United States Geological Survey.) (Chamberlin.) A Country, in contrast with the adjacent Figure, in which the Drainage has been disturbed by Glacial Deposits, and the Streams are beginning to wear new Channels. (Chamberlin.)

rugged character; while as the mouth is approached the current becomes sluggish, broad bottoms appear, a greater width to the stream is apparent, and all signs point to the end of its career. As with the course of a river, so with its life. In early days, before the channel is well defined, it is a foaming torrent. Later on it smooths its bed and becomes more stable in position. As years and centuries pass away, the rougher places are leveled, and the stream then flows placidly in its course over its well-worn, often deeply excavated channel. The Ohio has reached this last stage in its history, for at only a single place in all its course from Pittsburg to its mouth does its channel show signs of a rocky character. The reason for this single exception will soon become clear.

An examination of the geological structure of the country through which the Ohio flows shows none but the extreme end of the valley to be of later age than the Carboniferous. Portions are, indeed, far older; but the area covered by these, though perhaps extensive enough to allow the development of some system of drainage, was never large enough to develop a stream of any