126 STUDIES, SCIENTIFIC AND SOCIAL 



in advance of the glacier is an impossible one. The only 

 other theorj^ is, that the lake was filled up b}^ alluvium 

 before the ice age, and that the glacier re-excavated it. 

 I have, 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 cumula 

 tive 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 favourable 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 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 geo- 

 graphical, which its supporters have not attempted 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 pecu- 

 liarity of their distribution which points directly and un- 

 mistakably to ice-erosion. 



A crucial test of the two theories is then suggested and 



