THE ICE AGE AND ITS WORK. 299 



Summary of the evidence. — As the subject bere discussed is very 

 complex, and the argumeut essentially a cumulative one, it will be well 

 briefly to summarize its main i)oiuts. 



In the first place, it has been shown that the valley lakes of highly 

 glaciated districts form a distinct class, which are highly characteristic, 

 if not altogether peculiar, since in none of the mountain ranges of 

 the tropics or of non-glaciated regions over tlie whole world are any 

 similar lakes to be found. 



The special conditions favorable to the erosion of lake basins and 

 the mode of action of the ice tool are then discussed, and it is shown 

 that these conditions have been either overlooked or ignored by the 

 opponents of the theory of ice erosion. 



The objections of modern writers are then considered, and they are 

 shown to be founded either on mistaken ideas as to the mode of erosion 

 by glaciers, or on not taking into account results of glacieratiou which 

 they themselves either admit or have not attempted to disprove. 



The alternative theory — that earth-movements of various kinds led 

 to the production of lake basins in all mountain range, and that those 

 in glaciated regions were preserved by being filled with ice — is shown 

 to be beset with numerous difficulties, i^liysical, geological, and geo- 

 graphical, which its supporters have not attempted to overcome. It is 

 also pointed out that this theory in no way explains tlie occurrence of 

 the largest and deepest lakes in the largest river valleys, or in those 

 valleys where there was the greatest concentration of glaciers, a pecu- 

 liarity of their distribution which points directly and unmistakably to 

 ice erosion. 



A crucial test of the two theories is then suggested, and it is shown 

 that both the sub-aqueous contours of the lake basins, and the super- 

 ficial outlines of the lakes, are exactly such as would be i^roduced by 

 ice erosion, while they could not possibly have been caused by sub- 

 mergence due to any form of earth movements. It is submitted that 

 we have here a positive criterion, now adduced for the first time, which 

 is absolutely fatal to any theory of submersion. 



Lastly, the special case of the Lake of Geneva is discussed, and it is 

 shown that the explanation put forth by the auti-glacialists is wholly 

 unsupported by facts and is opposed to the known laws of glacier motion. 

 The geologists who support it themselves furnish evidence against their 

 own theory in the ancient alluvium at Geneva on which the glacial 

 deposits rest, and which is admitted to be mainly derived from the 

 distant Alj)s. l>ut as all alluvial matter is necessarily intercepted by 

 large and deep lakes, the presenceof this Alpine alluvium immediately 

 beneath the glacial debris at the foot of the lake indicates that the 

 lake did not exist in preglacial times, but that the river Rhone flowed 

 from the Alps to Geneva, carry with it the old alluvium, consisting of 

 mud, sand, and gravel, which it hadbrt)ught down from the mountains. 

 Still more conclusive however is the fact that the three special features 



