OF CENTRAL CANADA PART III. 155 



and there is scarcely a river, or small stream indeed, in any part of 

 Canada, that does not exhibit indications of having occupied at one 

 period a wider bed and high level than at present. Thi^ erosive power 

 of rivers has probably been assisted in many instances by a gradual 

 elevation of the surrounding land. Some of the grandest examples 

 of river erosion are exhibited by the canons of the Colorado and other 

 streams west of the Rocky Mountains. In some of these remarkable 

 ravines, the stream has excavated its channel, within almost perpen- 

 dicular walls of limestone and other rock, to a depth of a thousand 

 feet or more. 



The amount of detrital matters borne down by some rivers to the 

 sea, is exceedingly abundant. This is well shown by the formation 

 of deltas. The delta of the Mississippi on this continent, for example* 

 like all other deltas, is derived essentially from the sandy and other 

 matters brought down by the stream. On entering the sea, the 

 velocity of the river is necessarily checked, and the sediments are 

 thrown down. Much of the coarser matter is indeed deposited on 

 the bed of the river itself, raising this, and compelling the formation 

 of artificial banks, or levies, to prevent inundations. Finally, as a 

 well-known illustration of the immense amount of sedimentaVy 

 matters borne seawards by certain rivers, the case of the Ganges, 

 as described so fully by Sir Charles Lyell, in his " Principles of 

 Geology," may be here cited. That river, it has been demonstrated 

 by actual observation and experiment, conveys annually to the sea 

 an amount of matter that would outweigh sixty solid pyramids of 

 granite, supposing each, like the largest of the Egyptian pyramids, 

 to cover eleven acres at its base, and to stand 500 feet in height. 

 The delta of the Ganges, composed of mud, &c., thus brought down 

 by the river, extends for 200 miles along the coast, and commences 

 far inland. 



A considerable quantity of sediment is also produced by the slow 

 movements of glaciers in Alpine and other districts in which these 

 remarkable ice-rivers prevail. The glacier of the Aar, which covers 

 with its tributaries an area of only six or seven square miles, thus 

 furnishes daily, according to some recent researches of M. Collomb, 

 at least 100 cubic yards of sand. This is carried off by its terminal 

 stream or torrent. 



Action of the Sea (and of large bodies of Water generally). Vast 

 .in amount as are the sediments collected by rivers, they are far sur- 



