THE ICE AGE AND ITS WORK. 287 



is especially interesting on account of the extreme divergence of opin- 

 ion that still prevails on the subject. While the general facts of gla- 

 ciation, 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 covering thousands of square miles of coun- 

 try with thick layers of l)owlder clay and drift, are all admitted as 

 beyond dispute, geologists are still divided into two hostile camps when 

 the origin of lake basins is concerned; and the opposing forces seem 

 to be approximately e(]ual. Having for many years given much atten- 

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

 convinced that the evidence in favor of giaciation 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 over- 

 looked, but which appear to me to aftbrd very strong if not conclusive 

 evidence for erosion as against any alternative theory yet proposed. I 

 shall therefore first set forth, as fully as the space at my command 

 will allow, the general evidence in favor of the ice origin of certain 

 classes of lakes, and the sjiecial conditions requisite for the produc- 

 tion of lakes by this agency. The objections of the best authorities 

 will then be considered and replied to, and the extreme difficulties of 

 the alternative theories will be pointed out. I shall then describe cer- 

 tain peculiarities, hitherto unnoticed, which clearly point to erosion, as 

 opposed to any form of subsidence and uj)heaval, in the formation of 

 the lakes in question. Lastly, the special case of the Lake of Geneva 

 will be discussed as affording a battle-ground that Mill be admitted to 

 be highly favorable to the anti-glacialists, since most of them have 

 adduced it as being entirely beyond the powers of the ancient glaciers 

 to have produced. 



1. The different kinds of lakes and their distribution. — To clear the 

 ground at the outset, it may be well to state that the great plateau 

 lakes of various parts of the world have no doubt been formed by 

 some kind of earth movements occurring subsequent to the upheaval 

 and partial denudation of the country. It is universally admitted 

 that existing lakes can not be very ancient, geologically speaking, since 

 they w^ould inevitably be filled up by the sediment carried into them 

 by the streams and by the wind. Our lakes must therefore be quite 

 modern features of the earth's surface. A considerable proportion of 

 these plateau lakes are in regions of little rain-fall, and manj' of them 

 have no outlet. The latter circumstance is a consequence of the for- 

 mer, since it indicates that evaporation balances the inflow. This 

 would have favored the formation of such lakes, since it would have 

 prevented the overflow of the water from the slight hollow first 

 formed, and the cutting of an outlet gorge which would empty the 

 incipient lake. Capt. Dutton, in his account of the geology of the 

 Grand Canyon district, lays stress on this fact, " that the elevation of 



