1844.] PLASTICITY OF GLACIERS OBJECTIONS REMOVED. 51 



required that several difficulties, very obvious, and seemingly 

 insuperable, should be removed ; and the chief of these was the 

 texture of ice compared to the texture of lava the former 

 passing from a brittle solid into limpid fluid by heat, the latter 

 passing like sealing-wax through every intermediate degree of 

 viscidity. This difficulty could only be met by an exact deter- 

 mination of the question how far a glacier is to be regarded 

 as a plastic; mass ? Were a glacier composed of a solid crystal- 

 line cake of ice, fitted or moulded to the mountain bed which 

 it occupies, like a lake tranquilly frozen, it would seem impossible 

 to admit such a flexibility or yielding of parts as should permit 

 any comparison to a fluid or semifluid body, transmitting pres- 

 sure horizontally, and whose parts might change their mutual 

 position, so that one part should be pushed out whilst another 

 remained behind. But we know, in point of fact, that a glacier 

 is a body very differently constituted. Tt is clearly proved by 

 the experiments of Agassiz and others, that the glacier is not a 

 mass of ice, but of ice and water ; the latter percolating freely 

 through the crevices of the former, to all depths of the glacier ; 

 and as it is matter of ocular demonstration that these crevices, 

 though very minute, communicate freely with one another to 

 great distances, the water with which they are filled communi- 

 cates force also to great distances, and exercises a tremendous 

 hydrostatic pressure to move onwards in the direction in which 

 gravity urges it, the vast, porous, crackling mass of seemingly 

 rigid ice, in which it is, as it were, bound up. 



But farther than this, the experiments first announced in 

 the earliest of these letters, shewed, that whatever be the con- 

 stitution of a glacier, and whatever be the cause of its motion, 

 THE FACT is, that it does not move like a solid body sliding 

 down a bed or channel, but that the velocity of each part of 

 its breadth is different. It was demonstrated by the most 

 clear and plain geometrical measurements, that whilst the 

 centre of a glacier moves 500 feet, the side of the glacier moves 

 only 300 ; consequently, the portions of ice which started 

 together soon part company, and the central molecule has com- 



