100 GEOLOGY. 



er recede entirely to the lake, because, before it reaches 

 there, the fall will cease to be sufficient for the wearing 

 away of the shales underlying the limestone. 



184. Canons of Colorado. The case just cited of ero- 

 sion by water, though so striking, is by no means so 

 strong a one as some others ; for, while the rocks eroded 

 by the water of the fall are soft, there are instances of 

 vast and rapid erosion in solid tough rock. The stron- 

 gest example is in the canons of Colorado, which are riv- 

 ers running between perpendicular walls of rock, as rep- 

 resented in Fig. 38, in some cases standing even 6000 

 feet in height. These passages through the rocks were 

 actually worn by the water, incredible as it may at first 

 thought appear. We have some comparatively recent 

 erosions, which show that such monstrous erosions are 

 possible, if there be a sufficient length of time allowed 

 for them. For example, the River Simeto, having been 

 dammed up by an eruption from Mount Etna in 1603, 

 cut a passage through hard blue basaltic rock in a little 

 over two centuries, which was from fifty to several hun- 

 dred feet in width, and in some places fifty feet deep. 

 It is rather difficult to realize that water can accomplish 

 such an amount of erosion, but a little reflection on some 

 common results with which we are familiar will help us 

 to do this. We see the stone steps of public buildings 

 very soon worn by the friction of the many feet that pass 

 over them. But this friction is not constant ; it is so in- 

 termittent that it occupies but a small portion of the time 

 that passes from day to day. If it were constant, the 

 stone would soon be so much worn as to require being 

 replaced by another. Now the friction of water erodes 

 stone like the friction of footfalls; and if it be constant 

 through century after century perhaps age after age 

 the erosion, we can see, will be great in amount. 



185. Rocks Disintegrated by Frost. As there are crev- 

 ices and interstices in rocks, the water enters into these, 

 and in cold weather becomes frozen there. 'In freezing, 



