Review of the Controversy Regarding the Motion of Glaciers. 145 



cier must move at different rates, and tliat these rates will be deter- 

 mined by the amount of heat which each part receives. A region 

 exposed to the sun's rays must, therefore, move faster than a region 

 sheltered from them, inasmuch as it is constantly covered by a film of 

 water, from which Mr. Croll supposes the heat to be conducted. This, 

 however, is not the case. In the Mer de Glace the rate of movement 

 increases gradually from the higher parts to the lower. The glacier,. 

 in fact, moves as a whole, and not molecularly ; with a differential mo- 

 tion, it is true, but not with such a differential motion as Mr. Croll's 

 theorv requires, and the result of the motion of the whole is, that its 

 central and superficial portion is carried along at the rate of about 

 a yard per day. 



This, however, is only true of the glacier as a whole. A cubic yard 

 cut out of the more swiftly moving part, and kid upon the rock beside 

 it, would show no more motion than the rock itself, though exposed to 

 the same access of heat. Yet, on Mr. Croll's theory, both should com- 

 port themselves alike. Indeed, if the principle upon which his hypo- 

 thesis is founded were correct, any lump of ice exposed to<a heat above 

 32° should soften and flow outward, until it had spread itself into a 

 thin layer, without melting, except at the surface. Nor is there any 

 reason for limiting this result to the rate observed on the Mer de Glace. 

 Still greater mobility might be looked for, and ice must be more plastic 

 than butter to bring about the necessary consequence of Mr. CrolF's 

 principle. It is needless to add, that such results are never observed. 

 A lump of ice, even in summer, shows no tendency to spread thus;. 

 and if the absence of viscosity has proved a stumbling-block in the 

 way of Forbes' Viscous Theory, the absence of all evidence of liquidity 

 is an equally fatal objection to Mr. Croll's Liquid Theory. Again, it is 

 impossible to admit that a glacier, moving as on this Liquid Theory 

 it is supposed to move, can scratch and groove the rock-bed over which 

 it passes as glaciers are known to do. Mr. Croll, it is true, claims this 

 as a legitimate result of his theory, when he says : 



"As regards the denuding power of glacier, I may observe that 

 though a glacier descends molecule by molecule, it Avill grind the 

 rocky bed over w^hich it moves as effectually as it w^ould do did it 

 slide down in a rigid mass in the Avay generally supposed, for the 

 grinding effect is produced not by the ice of the glacier but by the 

 stones, sand, and other material 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 pei-formed in forcing the grinding materials lying under the ice 



