638 



DYNAMICAL GEOLOGY. 



the ocean. It is, therefore, a part of the markings in which the geo- 

 graphical history of the globe is registered. 



Second. The gathering drops make the rill, and the rill its little 

 furrow ; rills combine into rivulets, and rivulets make a gully down 

 the hill-side ; rivulets unite to form torrents, and these work with 

 accumulating force, and excavate deep gorges in the declivities. Other 

 torrents form in the same manner about the mountain-ridge, and 

 pursue the same work of erosion, until the slopes are a series of 

 valleys and ridges, and the summit a bold crest, overlooking the erod- 

 ing waters. 



2. Progress of erosion in the Formation of Valleys or River-courses. — 

 The mist and rains about the higher parts of mountains are usually 

 the main source of the water. As the first-made streamlets are 

 gathering into larger streams, through the course of the descent, and 

 are largest below, the torrent has its greatest force toward the bot- 

 tom of the steep declivity ; and there the valley first takes shape and 

 size. 



Let A B (Fig. 1076) represent a profile of a declivity. As the 

 erosion goes on, a valley is formed along I m, on the principle just 

 stated, so that the course of the waters on the profile corresponds to 

 A 7 m. At m. the most of the descent of the declivity is made : the 

 waters have, therefore, but little eroding power at bottom ; and they 

 flow off at a small angle to B, along the line m B. At m, moreover, 

 the stream, ceasing to erode much at bottom, commences to erode 



Fig. 1076. 



Fig. 1077. 



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B 



laterally during freshets, undermining the cliffs on either side, when 

 the rocks admit of it, thus widening the valley and making a " flood- 

 plain," or " bottom-lands," through which the stream when low has 

 its winding channel. 



The river, in this state, consists of its torrent-portion, A m, and its 

 river-portion, m B. Along the former, a transverse section of the 

 valley is approximately V-shaped, and along the latter nearly U-shaped, 

 or else like a V flattened at bottom. The river-portion usually ex- 

 hibits, even in its incipient stages, its two prominent elements, — a 

 river-channel, occupied by the waters in ordinary seasons, and the 

 alluvial-flat, or flood-ground, which is mostly covered by the higher 



