RIVERS. 637 



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 descending waters are most abundant. Gradually the 

 ridge thins to a crest, and finally becomes worn away for some dis- 

 tance, so that two valleys (or more by the wear of more ridges) 

 have a common head. In fig. 939, ArsB represents the course of 

 the stream, as in fig. 938 ; and A e fB 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. 



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 (5 to 10 degrees) from a 

 centre has favored a regular development. On Mount Kea (Hawaii), nearly 

 14,000 feet high, the valleys extend about half-way to the summit, having 

 made only this much progress upward since the volcano became extinct. On 

 Tahiti, the old mountain is reduced to a mere skeleton. The valleys lead up to 

 amphitheatres bounded by precipices of 2000 to 3000 feet, directly under the 

 peak; and the ridges between the valleys, though 1000 to 2000 feet high, are 

 reduced in the interior to mere knife-edges, impassable except as they are balus- 

 traded by shrubbery ; and in some cases, adjoining the central heights, they are 

 worn down to a low wall or pinnacled crest, partially separating two of the 

 valleys. The traveller ascending one of the valleys along the bed of the stream 

 finds himself at last at the base of inaccessible heights, with numberless cascades 

 before him and a range of buttressed walls of remarkable grandeur.* Some- 

 thing of this buttressed character of precipices is seen in fig. 941. 



The nature of the rocks causes modifications in these results. If 

 there are harder beds at intervals in the course of the stream, or any 

 impediment to even wear, the impediment becomes the head of a 

 waterfall and precipice, whose height increases rapidly from the 



* See the Author's Expl. Exped. Geol. Rep., p. 290, and Amer. Jour. Sci. [2] 

 ix. 48, and 289. 



