642 DYNAMICAL GEOLOGY. 



when the rocks admit of it, thus widening the valley and making a 

 " flood-plain," or " bottom-lauds," through which the stream when low 

 has its winding channel. With further progress in the erosion, Ano B 

 becomes the channel for the stream. 



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

 river-portion, oj»B. Along the former, a transverse section of the val- 

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

 or else like a V flattened at bottom. The river-portion, o m B, usu- 

 ally exhibits, 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 

 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 the widening thereby of the river-valley. 



These steps are illustrated about the volcanic mountains of the 

 Pacific, the more recently extinct of which, like Mount Kea, on 

 Hawaii, have their valleys confined mainly to the lower slopes, while 

 the older mountains are cut through the summit with profound gorges. 

 Mount Loa, which is still in action, has few valleys of erosion in any 

 part, although, like Kea, nearly 14,000 feet in height. 



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

 where the mists and rains are most abundant and often almost per- 

 petual through the year, the next step is the working down of a preci- 

 pice under the summit, or toward the top of the declivity, making 

 the course of the waters A p q B, and later, A r s 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 descend them in many 

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

 chiseled out, by these means, into a narrow, crest-like ridge or peak. 

 Each separate descending rill frequently makes its own valley-like 

 recess in the side of the precipice ; and together they may face it 

 with a series of deep alcoves and projecting buttresses. 



The next step in the progressing erosion is the thinning and wearing 

 away of the ridges that intervene between adjoining valleys, in the 

 higher regions where the descending waters are most abundant. By 

 this means two valleys (or more by the wear of more ridges) are 

 often made to have a common head. In Fig. 1077, A r s B repre- 

 sents the course of the stream, as in Fig. 1076; and A e f B the 

 eroded ridge, which has lost at e much of its height. The erosion, 

 continuing its action around the precipitous sides of the united head 

 of the valleys, may widen it into a vast mountain amphitheatre, out 

 of which the stream may pass below, between closely approaching 



