300 D. Burns — On the Mechanics of Glaciers. 



in a general way, I am not prepared to discard it as haAung no share 

 in \he greater progress of glaciers in warm weather. 



When Mr. Oroll passes on to speak of the denuding power of 

 glaciers, I am quite staggered, and realize my fourth and greatest 

 difficulty in accepting his theory. He says : " As regards the denud- 

 ing power of glaciers, I may observe that, though a glacier descends 

 molecule by molecule, it will grind the rocky bed over which it 

 moves as effectually as it would do did it slide down in a rigid mass 

 in the waj?^ generally supposed ; for the grinding effect is produced 

 not by the ice of the glacier, but by the stones, sand, and other 

 materials forced along under it. But if all the resistances opposing 

 the descent of a glacier, internal and external, are overcome by the 

 mere weight of the ice alone, it can be proved that in the case of 

 one descending with a given velocity the amount of work performed 

 in forcing the grinding materials lying under the ice forward must 

 be as great, supposing the motion of the ice to be molecular, in the 

 way I have explained, as it would be suj)posing the ice descended 

 in the manner generally supposed." 



It requires some fortitude to accuse Mr. Croll of writing nonsense 

 on such a subject, yet, I must confess, I can make nothing else of 

 the above. It is a mystery to me how it has been so long uncor- 

 rected by its author or unchallenged by others, as it has, so- far as I 

 am aware. There could not be a more powerfully denuding engine 

 than a great glacier barbed with stones and propelled by gravitation 

 or other force over the ground ; nor could there be a weaker one 

 than a great glacier, which, as a solid mass, is stationary, but on 

 whose interior and on whose surface liquid molecules are here and 

 there moving through infinitesimal distances. It sounds like a joke 

 to say that " the grinding effect is produced not by the ice of the 

 glacier, but by the stones, sand, and other materials forced along 

 under it." Will a stone that is held in the crystalline grasp of 

 millions of ice molecules be forced along by a few dozen water 

 molecules frickling through a fraction of their diameter along its 

 surface? A stone thus '' forced along" may be supposed to scoop 

 out valleys if the exigencies of geologists demand it ; but the force 

 that moves the stone would not serve to tickle the sole of a mite. I 

 do not see how any geologist can accept this theory, and ascribe any 

 geological effects to glaciers whatever, except the negative one of 

 protecting the rocks where they prevail from atmospheric influences. 

 To my thinking it were much more reasonable to ascribe the 

 presence of the Quaker Stone in Darlington to the prevalence of 

 Scotch mists out of the north-v;"est, in the days when Noah was 

 a boy, than to "suppose it was lifted over the Pennine Chain by a 

 glacier acting as described by Mr. Croll. 



That gentleman's fallacy becomes, if possible, still more apparent 

 in the concluding sentence of the j^aragraph I have quoted. When 

 we regard the glacier as moving as a solid, we get the potential 

 energy stored in the ice on the mountain top used up in doing work 

 in grinding and polishing the rock beneath ; but if we regard the 

 glacier as moving molecule by molecule, this energy is gradually 



