IX.] THE WORK OF RAIN AND RIVERS. 135 



But. perhaps the grandest results of river-denudation are 

 to be witnessed in the vast chasms through which some of 

 the rivers in Colorado flow. These narrow gorges, liounded 

 by steep wall-like cliffs, are known under their Spanish name 

 of cations (Fig. 32).! The Colorado River of the West, which 

 runs from the Rocky Mountains to the Gulf of California, 

 flows, during part of its course, at the bottom of a profound 

 chasm ; being hemmed in by vertical walls which, in some 

 places, are more than a mile in depth. Theie is no reason 

 to doubt that this gigantic furrow has been cut down by 

 the river which runs through it. The tributary streams 

 flowing into the river run, in like manner, through 

 smaller ravines, known as side-canons ; and, in fact, the 

 general arrangement of the canons at once suggests 

 that of the drainage-system of a country. Nothing can 

 show the amount of vertical erosion, effected by running 

 water, better than these gorges. Probably, they owe the 

 preservation of their peculiar form to the fact that the 

 country in which they occur is comparatively rainless 3 for, 

 if there were much rain, the sides could not retain their 

 position as perpendicular walls, and denudation w^ould 

 gradually convert the chasm into an ordinary river valley. 



To understand how running water usually effects denuda- 

 tion, it is instructive to watch, at the sea-shore, the behaviour 

 of the water which drains oft" a flat coast of mud, or fine sand, 

 as the tide retreats. Flat and smooth as the beach may seem 

 to the eye, the water soon finds out some slight inequalities of 

 surface, and runs down even the gentlest declivity. Particles 

 of sand carried down by the water begin to scour out 

 little grooves, and then to enlarge them into wider furrows. 

 Several streams may be seen uniting into one larger stream, 



^ From Powell's Exploration of the Colorado River of the Wat, 

 Washington, 1875, 



