638 DYNAMICAL GEOLOGY. 



force of the falling waters, until some other similar impediment 

 below limits the farther erosion. Thus many waterfalls and rapids 

 are made in the cascade-portion of a stream, and they are not ab- 

 sent 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 

 cascade and torrent portion are each short or are hardly distin- 

 guishable. The streamlets descending the walls of such soft rocks 

 will easily widen the head of the valley into an extensive amphi- 

 theatre ; while in the farther course of the valley, beyond the limit 

 of the rainy region, the valley may be only a narrow gorge, hun- 

 dreds, or perhaps thousands, of feet deep. Here in these depths 

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

 dure 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 Missis- 

 sippi valley and those of the slopes of the Eocky Mountains, where 

 the rocks are in general stratified, and usually not far from hori- 

 zontal in position. 



The remarkable canon of the Colorado, between the meridians of 111 and 

 115° W. long., has already been partly described on p. 569 from the account fur- 

 nished by Dr. Newberry. The principal facts are these : — A length of 300 miles, 

 and through the whole nearly vertical Avails of rock, 3000 to 6000 feet in height; 

 these rocks limestone and other strata of Carboniferous age, others of older 

 Palaoozoic, and below these generally the solid granite, making from 500 to 1000 

 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 Avith similar 

 profound gorges or chasms. The A'iew represented in fig. 940 Avas taken at 

 the junction of the Colorado and the Green Rivers, near the meridian of 113J°. 

 It shoAvs Avell the narroAV and profound chasm in Avhich the waters of the Colo- 

 rado floAV, although not doing justice to the depth, Avhich at this place is about 

 3000 feet. Some distance up the stream the two rivers come together, the Colo- 

 rado from far to the right, and Green River from the left; and eA r ery where OA T er 

 the great plain there are the profound lateral chasms or side-canons of the tri- 

 butaries. 



Fig. 941 is another view from the same remarkable region, illustrating espe- 

 cially the side-canons. It is from the excellent Report of Lieutenant J. C. Ives, 

 the commander of the expedition with which Dr. Newberry was connected, 

 and is one among many A r iews equally grand and instructive giA'en in this 

 Report. 



NeAvberry attributes these profound gorges, and beyond doubt correctly, to 

 erosion, -each stream haA'ing made its oavu channel. The cliffs are so high that in 

 general no undermining can set back the Avails far enough to alloAV of alluvial 



