FRESH-WATER STREAMS. 639 



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 n o 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 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, ArsB. 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 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 wearing away of the 

 ridge that intervenes between two adjoining valleys. This takes place 

 about the higher portions, nearest the mountain-crest, where the de- 

 scending waters are most abundant. Gradually the ridge thins to 

 a crest, and finally becomes worn away for some distance, so that 

 two valleys (or more by the wear of more ridges) have a common 

 head. In Fig. 1077, ArsB represents 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 walls of rock. 



This is theoretically the history of valley-making, and the actual 

 history when the course is not modified by the structure of the rocks. 



A model of this system of erosion is often admirably worked out in 

 the earthy slopes along a road-side, — the little rill having its cascade- 

 head, then its torrent-channel, and below its flat alluvial plain, with 

 the winding rill-channel ; some of the ridgelets, in their upper parts, 

 worn away until two or more little valleys coalesce ; then, in some 

 cases, the head of the coalesced valleys widened into an amphitheatre, 

 and the walls fluted into a series of alcoves and buttresses. 



The system is illustrated on a grand scale among the old volcanic islands of the 

 Pacific, where the slope of the rocks at a small angle (five to ten degrees), from a centre, 

 has favored a regular development. Oc Mount Kea (Hawaii), nearly 14,000 feet high, 



