40 THE POPULAR SCIENCE MONTHLY. 



THE ICE AGE AND ITS WORK. 



BY ALFRED R. WALLACE, F. R. S. 



EROSION OF LAKE BASINS. 



III. 



~T AKES are distributed very unequally over the various parts 

 - ^ of the world, and they also differ much in their position in 

 relation to other physical peculiarities of the surface. Most of 

 the great continents have a considerable number of lakes, many 

 of great size, situated on plateaus or in central basins ; while the 

 northern parts of Europe and North America are thickly strewn 

 with lakes of various dimensions, some on the plains, others in 

 subalpine valleys, others again high up among the mountains, 

 these latter being of small size and usually called tarns. The 

 three classes of lakes last mentioned occur in the greatest profu- 

 sion in glaciated districts, while they are almost absent elsewhere ; 

 and it was this peculiarity of general distribution, together with 

 the observation that all the valley lakes of Switzerland and of our 

 own country occurred in the track of the old glaciers, and in sit- 

 uations where the erosive power of the ice would tend to form 

 rock-closed basins, that appears to have led the late Sir Andrew 

 Ramsay to formulate his theory of ice-erosion to explain them. 

 He was further greatly influenced by the extreme difficulty or 

 inadequacy of any possible alternative theory a difficulty which 

 we shall see remains as great now as at the time he wrote. 



This question of the origin of the lake basins of the glaciated 

 regions is especially interesting on account of the extreme diver- 

 gence of opinion that still prevails on the subject. While the 

 general facts of glaciation, the extent and thickness of the old 

 glaciers and ice-sheets, and the work they did in distributing 

 huge erratics many hundred miles from their sources and in cov- 

 ering thousands of square miles of country with thick layers of 

 bowlder clay and drift, are all admitted as beyond dispute, geolo- 

 gists are still divided into two hostile camps when the origin of 

 lake basins is concerned ; and the opposing forces seem to be ap- 

 proximately equal. Having for many years given much attention 

 to this problem, which has had for me a kind of fascination, I am 

 convinced that the evidence in favor of glaciation has not been 

 set forth in all its cumulative force, while many of the arguments 

 against it seem to me to be either illogical or beside the point at 

 issue. I have also to adduce certain considerations which have 

 hitherto been overlooked, but which appear to me to afford very 

 strong if not conclusive evidence for erosion as against any alter- 

 native theory yet proposed. I shall, therefore, first set forth, as 



