636 



DYNAMICAL GEOLOGY. 



sion 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 I 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, more- 

 over, the stream, ceasing to erode much atbottom, commences to 

 erode laterally during freshets, undermining the cliff's 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 



Fig. 938. 



Fig. 939. 



b c 



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

 shaped, or else like a V flattened at bottom. The river-portion 

 usually exhibits, even in its incipient stages, its two prominent ele- 

 ments, — a river-channel, occupied by the waters in ordinary seasons, 

 and the alluvial fiat or fiood-ground, which is mostly covered by the 

 higher freshets. The two go together whenever the course of the 

 stream is not over and between rocks that do not admit of much 

 lateral erosion and a widening thereby of the river-valley. 



In the farther progress of the stream, A. no becomes the torrent- 

 portion, and o B the river-portion. Later, the valley commences from 

 the summit A. 



As the waters continue their work of erosion about the summits, 

 where the mists and rains are most abundant and often almost 

 perpetual through the year, the next step is the working down of 

 a precipice under the summit or towards the top of the declivity, 

 making the course of the waters Apg B, and, later, Ar«B. The 

 stream in this state has (1) a cascade-portion, and (2) a torrent-portion ; be- 

 sides (3) its river-portion. The precipices thus formed are sometimes 

 thousands of feet in height ; and the waters often descend them in 

 thready lines to unite below in the torrent. The mountain-top is 

 chiselled out by these means into a narrow, crest-like ridge. Each 

 separate descending rill frequently makes its own recess in the 



