THE ICE AGE AND ITS WORK. 257 



however, already given reasons why the glacier would not have 

 done so, and the very existence of this ancient alluvium in the 

 course of the ancient glacier is a proof that it did not do so. This 

 theory seems now to have no supporters. 



SUMMARY OF THE EVIDENCE. As the subject here discussed 

 is very complex, and the argument essentially a cumulative one, 

 it will be well briefly to summarize its main points. 



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 

 the 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 ig- 

 nored 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 glacier action 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 ranges, 

 and that those in glaciated regions were preserved by being filled 

 with ice is shown to be beset with numerous difficulties, physical, 

 geological, and geographical, which its supporters have not at- 

 tempted to overcome. It is also pointed out that this theory in 

 no way explains the 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 peculiarity of their distribu- 

 tion 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 subaqueous contours of the lake basins and 

 the superficial outlines of the lakes are exactly such as would 

 be produced by ice erosion, while they could not possibly have 

 been caused by submergence due to any form of earth move- 

 ments. 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 anti-glacialists 

 is wholly unsupported by facts and is opposed to the known laws 

 of glacier motion. The geologists who support it themselves fur- 

 nish 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 Alps. But as all alluvial 



VOL. XLV. 21 



