THE ICE AGE AND ITS WORK. 41 



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 special 

 conditions requisite for the production 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 certain peculiarities, 

 hitherto unnoticed, which clearly point to erosion, as opposed to 

 any form of subsidence and upheaval, 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 will be ad- 

 mitted 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. 



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 uni- 

 versally admitted that existing lakes can not be very ancient, 

 geologically speaking, since they would 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 rainfall, and many of them have no outlet. The 

 latter circumstance is a consequence of the former, since it indi- 

 cates that evaporation balances the inflow. This would have 

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

 vented the overflow of the water from the slight hollow first 

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

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

 the Grand Canon district, lays stress on this fact, " that the eleva- 

 tion of a platform across the track of a river rarely diverts it from 

 its course, for the stream saws its bed into the rocks as fast as the 

 obstacle rises." Scanty rainfall and great evaporation seem there- 

 fore to be almost essential to the formation of the larger plateau 

 lakes. Rarely, such lakes may have been formed in comparatively 

 well-watered districts, but the earth movements must in these 

 cases have been exceptionally rapid and extensive, and they are 

 accordingly found most often in countries subject to volcanic dis- 

 turbances. Such are the lakes of southern Italy, of Macedonia, 

 of Asia Minor, and perhaps those of Central Africa. 



Quite distinct from these are the subalpine lakes of those 

 mountain groups which have been subject to extreme glaciation. 

 These are characteristically valley lakes, occurring in the lower 

 portions of the valleys which have been the beds of enormous 

 glaciers, their frequency, their size, and their depth bearing some 



VOL. XLT. 4 



