1867.] Ice Marls in North Wales. 49 



and more grinding power than elsewhere, and thus hollows must be 

 formed. And a hollow once formed the ice is there so much thicker 

 and the pressure so much greater, and thus the hollow may increase 

 more rapidly the deeper it goes. Then there comes the objection, 

 that when the hollow is deep the ice at its bottom will be motionless, 

 the upper layers sliding over the lower ones. 



But who really knows this ? It is a pure supposition ; and 

 there seem to be as good arguments on one side as on the other. 

 And who, of all our philosophers previous to direct observation, 

 would have supposed that glaciers could flow at all, and retain their 

 form and continuity ? The fact seems to be, that these huge 

 ancient glaciers, spreading over hundreds of miles of flat country 

 half-a-niile thick, are too vast for us to say what they could have 

 or could not have done. 



It is proverbially hard to prove a negative, and at present there 

 is really no positive theory before the world, except Professor 

 Ramsay's, that in any way explains either the overwhelming pro- 

 portion of lakes situated in glaciated regions — or the fact that so 

 many of the great lakes of Switzerland and Italy are situated 

 exactly where they should be if they were ground out by glaciers, — 

 or that the size and depth of the lakes correspond to the admitted 

 size and thickness of the ancient glaciers. Many who oppose this 

 theory will perhaps say that they admit it to be good as regards the 

 smaller lakes and tarns, but uphold the elevation and subsidence 

 theory for the larger ones. But this will in no way avoid the 

 difficulties of distribution I have already pointed out, since the large 

 lakes are very numerous and, as well as the small ones, abruptly 

 cease before reaching the limits of the ancient glaciers, — limits, it 

 must be remembered, traced before this theory was enunciated, and 

 by men who even now do not all adopt it. Again the lakes form 

 such a continuous series in position, form, and magnitude, that the 

 presumption is against their having been formed by two quite dis- 

 tinct processes. Lakes have, no doubt, been sometimes formed by 

 disturbance, tilting, or subsidence ; but these are evidently excep- 

 tional causes, and are not to be assumed in any particular case unless 

 they can be proved. 



In connection with this subject, I may allude to one main point 

 of difference which has existed among geologists almost since the 

 subject first attracted attention, and which still exists. It is the 

 question whether the glacial phenomena, so abundant over 

 the whole of the northern half of North America, have been pro- 

 duced by enormous aerial glacial masses, covering at once or at 

 different times the whole country, — or by icebergs floating down 

 over it and grating along a shallow sea-bottom. Agassiz first pro- 

 pounded the " glacier " theory, and still upholds it. Sir W. Logan 

 supports the same theory, and Professor Ramsay of course considers 



VOL. IV. E 



