WATER AS A MECHANICAL AGENT. 



181 



parts of the mountains, and especially those of the windward side, are the 

 source of the water. The slopes collect it as it descends into streamlets j. 

 these increase toward the foot, where the valley, as Mount Kea shows, first 

 takes shape. 



The diagram Fig. 162, although greatly exaggerated in angle of slope, — 

 that of the line AB, — will serve to illustrate the steps of progress. In the 

 early stage a valley forms toward the base of the mountain, having its bed 



162. 



163. 



along Im ; and later along no. On reaching o, the most of the descent of 

 the declivity is made : the waters from o to B have, therefore, little eroding 

 power at bottom, and commence to erode laterally during freshets, under- 

 mining the cliffs on either side, when the rocks admit of it, thus widening 

 the valley and making a " flood-plain," or " bottom-lands," by deposition of 

 the transported material in consequence of the slackened flow. The river, 

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

 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, omB, usually exhibits, even in its incipient 

 stages, its two prominent elements, — a river-channel, occupied at low water, 

 and the alluvial flat, ov flood-ground, which is mostly or wholly covered dur- 

 ing freshets. 



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

 the mists and rains are generally most abundant and often almost perpetual 

 through the year, the next step is the eroding about the summit and the con- 

 tinued deepening of the torrent-channel, making thus a precipice imder the 

 summit, or toward the top of the declivity ; in this stage, the course of the 

 waters is ApqB, and later, ArsB. The stream has now (1) a cascade- 

 piortion, and (2) a torrent-portion, besides (3) its river-portion. The preci- 

 pices of the cascade-portion may be thousands of feet in height ; and the 

 waters may descend in many thready lines, to unite below in the torrent. 

 The mountain cone, in such a case, may have its top chiseled into a narrow, 

 crest-like ridge or peak, with many vertical alcoves in the face of the preci- 

 pice that were made by the falling and leaping streamlets. 



The next step in the progressing erosion, as Tahiti illustrates, is the thin- 

 ning and wearing away of the ridges that intervene between adjoining valleys, 

 in the higher regions where the descending waters are most abundant. It is 

 in this way that two valleys (or perhaps more than two, by the wear of more 



