THE ICE AGE AND ITS WORK. 245 



about to be subject to severe glaciation, and not only so, but 

 occurred on both sides of each range, as in the Alps, or all round 

 a mountain range, as in our lake district, or in every part of a 

 complex mountain region, as in Scotland from the Frith of Clyde 

 to the extreme north coast all in this very limited period of 

 geological time. We are further asked to believe that during the 

 whole period from the commencement of the Ice age to our day 

 such earth movements have never produced a single group of 

 valley lakes in any one of the countless mountain ranges and hilly 

 regions throughout the whole of the very much more extensive 

 non-glaciated regions of the globe ! This appears to me to be 

 simply incredible. The only way to get over the difficulty is to 

 suppose that earth movements of this nature occurred only at 

 that one period, just before the Ice age came on, and that the lakes 

 produced by them in all other regions have since been filled up. 

 But is there any evidence of this ? And is it probable that all 

 lakes so produced in non-glaciated regions, however large and 

 deep they might be, and however little sediment was carried down 

 by their inflowing streams, should yet all have disappeared ? The 

 theory of the pre-glacial origin of these lakes thus rests upon a 

 series of highly improbable suppositions entirely unsupported by 

 any appeal to facts. There is, however, another difficulty which 

 is perhaps even greater than those just considered. Whatever 

 maybe 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 ex- 

 treme slowness ; and all the evidence we have of surface move- 

 ments 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 com- 

 paratively 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 three thousand to five thousand feet deep and four 

 hundred 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 running water. 



* Principles of Geology, eleventh ed., vol. i, p. 353. 



