640 DYNAMICAL GEOLOGY. 



the valleys extend about half-way to the summit, having made only this much of prog- 

 ress 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 

 2,000 to 3,000 feet, directly under the peak ; and the ridges between the valleys, though 

 1,000 to 2,000 feet high, are reduced in the interior to mere knife-edges, impassable 

 except as they are balustraded 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 cas- 

 cades before him, and a range of buttressed walls of remarkable grandeur. 1 Something 

 of this buttressed character of precipices is seen in Fig. 1079. 



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 water- 

 fall and precipice, whose height increases rapidly, from the force of the 

 falling waters, until some other similar impediment below limits the 

 farther erosion. Many waterfalls and rapids are thus made in the 

 cascade-portion of a stream ; and they are not absent from the river- 

 portion. Another effect of this cause is that the stream is set back 

 for some distance above a waterfall, and has in this part more or less 

 extensive flood-plains. 



If the rocks are in horizontal strata, and easily worn, the waters 

 work rapidly down to the level of the river-portion, so that the cas- 

 cade and torrent portion are each short, or are hardly distinguishable. 

 The streamlets descending the walls of such soft rocks will easily 

 widen the head of the valley into an extensive amphitheatre ; while, 

 in the farther course of the valley, beyond the limit of the rainy 

 region, the valley may be only a narrow gorge, with nearly vertical 

 walls, hundreds, or perhaps thousands, of feet deep. Here in these 

 depths, the stream meanders through a ribbon of alluvial land, rich in 

 verdure at one season, and in others mostly flooded. Examples of all 

 these peculiarities of river-valleys might be described, from among the 

 rivers of North America, especially the streams of the Mississippi 

 Valley and those of the slopes of the Rocky Mountains, where the rocks 

 are in general stratified, and often not far from horizontal in position. 



The features of the remarkable canon of the Colorado, between the meridians of 111° 

 and 115° W. longitude, have been described by J. S. Newberry, in the Reports of Ives' 

 Expedition. The principal facts are these: A length of 200 miles, and, through the 

 whole, nearly vertical walls of rock, 3,000 to 6,000 feet in height; these rocks lime- 

 stone and other strata of Carboniferous age, others of older Paleozoic, and below these 

 generally the solid granite, making from 500 to 1,000 feet of the gorge; and, in some 

 places, the granite rising in pinnacles out of the waters of the stream; finally, all 

 the tributaries or lateral streams with similar profound gorges or chasms. The view 



i See the Author's Expl. Exped. Geol. Rep., pp. 290, 384, and Am. J. Sci., II. ix. 48 

 and 289. 



