V . GLACIAL EROSION OF LAKE BASINS 111 



disappeared ? The theory of the pre-glacial or inter-glacial 

 origin of these lakes by earth movements thus rests upon 

 a series of highly improbable suppositions entirely unsup- 

 ported by any appeal to observed facts. 



There is, however, another difficulty which is perhaps 

 even greater than those just considered. Whatever may 

 be the causes of the compression, elevation, folding, and 

 other earth-movements which have led to the formation 

 of mountain masses, there can be no doubt that they have 

 operated with extreme slowness ; and all the evidence we 

 have of surface movements now going on show that 

 they are so slow as to be detected only by careful and long- 

 continued observations. On the other hand, the action of 

 rivers in cutting down rocky barriers is comparatively 

 rapid, especially when, as in all mountainous countries, 

 they carry in their waters large quantities of sediment, and 

 during floods bring down also abundance of sand, gravel, 

 and large stones. A remarkable illustration of this 

 erosive power is afforded by the river Simeto in Sicily, which 

 has cut a channel through solid lava which was formed 

 by an eruption in the year 1603. In 1828, Sir Charles 

 Lyell states, it had cut a ravine through this compact 

 blue rock from fifty to several hundred feet wide, and in 

 some parts from forty to fifty feet deep.^ The enormous 

 canon of the Colorado, from 3,000 to 5,000 feet deep 

 and 400 miles long, which has been entirely cut through 

 a series of mesozoic and palaeozoic rocks during the 

 latter portion of the tertiary period, is another example of 

 the wonderful cutting power of sand- laden running water. 



It is, in fact, only on account of this powerful agency that 

 we do not find valley lakes abounding in every mountainous 

 country, since it is quite certain that earth-movements of 

 various kinds must have been continually taking place. 

 But if rivers have always been able to keep their channels 

 clear, during such movements, among the mountains of 

 the tropics and of all warm countries, some reason must 

 be found for their inability to do so in the Alps and in 

 Scotland, in Cumberland, Wales, and southern New 

 Zealand ; and as no reason is alleged, or any proof offered 

 ^ Principle's of Gtologij, 11th ed., vol, i., p. 353. 



