THE ICE AGE AND ITS WORK. 293 



rests upon a series of liiglily improbable suppositions entirely unsup- 

 ported by any appeal to facts. Tiiere 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 wliicdi liave led to the formation of mountain masses, there 

 can be no doubt tluit 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 tlie other hand, the action of rivers in cutting down 

 rocky barriers is comparatively rapid, especially when, as in all moun- 

 tainous countries, they carry in their waters large quantities of sediment, 

 and during floods bring down also al)undanceof sand, gravel, and large 

 stones. - - 



It is in fact only on account of this powerful agency that we do not 

 tind valley lakes abounding in every mountainous country, since it is 

 quite certain that earth-movements of various kinds nuist 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 lor 

 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, that sufficiently rapid and extensive eartli-movements 

 actually did occur in the sub alpine valleys of these countries, we must 

 decline to accept such a hypothetical and unsatisfactory explana- 

 tion. - 



5. The contours (Old outlines of the lakes Indicate erosion rather than 

 submergence. — While collecting facts for the present articles it occurred 

 to me that the rival theories of lake formation — erosion and submerg- 

 ence — were so different in their mode of action that they ought to pro- 

 duce some marked difference in the result. There must be some' criteria 

 by which to distinguish the two modes of origin. Under any system of 

 earth-movements a valley bottom will simi)ly become submerged, and be 

 hardly more altered than if it had been converted into a lake by build- 

 ing an artiticial dam in a convenient situation. We should find there- 

 fore merely a submerged valley with all its usual peculiarities. If 

 however the lake basin has been formed by glacial erosion, then some 

 of the special valley features will have been destroyed, and we shall 

 have a distinct set of characters which will be tolerably constant in all 

 lakes so formed. Now I find that there are three such criteria by which 

 we ought to be able to distinguish the two classes of lakes, and the 

 application of these tests serves to show that most of the valley lakes 

 of glaciated countries were not formed by submergence. 



The first point is that valleys in mountainous countries often have the 

 river channel forming a ravine for a few miles, afterward opening out 

 into a flat valley, and then again closing, while at an elevation of a 

 hundred or a few hundred feet, at the level of the top of the ravine, 



