Non-plasticity of Glacier Ice. 589 



and with the cavity in which it moves ; and thus the congre- 

 gation of these huge and rigid blocks moves forward and 

 adapts itself more or less completely to the twistings and 

 changings of size and shape of the glacier valley, with much 

 of the outward character of a fluid or a plastic tnass ; but there 

 is no more real plasticity than there is in a cartload of road 

 metal when shot from the cart, which, notwithstanding the 

 rigidity of every broken angular stone, assumes the general 

 character and outline which a mass of plastic mud would if 

 shot from the same cai't. 



This power, then, of adapting itself to its bed, viz. by being 

 broken up into fragments of such a size as to be small in pro- 

 portion to the whole mass and the cavity in which it moves, 

 it is due to myself to remark that I clearly perceived, and 

 have distinctly enunciated in my original paper on the mecha- 

 nism of glaciers, published in the Transactions of the Geolo- 

 gical Society of Dublin in 1838. The passage will be found 

 in pages 10 and 1 1 of that paper, which, having been published 

 fully nowhere else, and that journal having a very limited cir- 

 culation, has I am aware become comparatively little known. 



It appears strange how Prof. Forbes, after remarking upon 

 the annual re-appearance of the same crevasses in the same 

 spots of the glacier, should have conceived any such thing as 

 plasticity necessary to account for the motion of translation ; 

 and equally strange does it appear, how obstinately both he 

 and Mr. Hopkins reject the idea of any essential aid being 

 had from the lifting of the masses by hydrostatic pressure, 

 although both admitting (I believe) that there may be, and 

 most probably are, great cavities in the bottom of glacier val- 

 leys by which the ice becomes hooked on to its bed, and from 

 which neither of these gentlemen's views seem capable of de- 

 taching it, without the aid of this, which I believe to be not the 

 sole but one of the most important causes of the motion of 

 translation of glaciers. 



It is quite true, as has been said by Prof. Forbes and others, 

 that Saussure noticed this cause of motion ; so he did, incir 

 dentally, in one single sentence in his three quarto volumes; 

 but it is perfectly plain that Saussure looked upon the lifting 

 of the ice by hydrostatic pressure solely as a possible and con- 

 tingent event, and not as an efficient cause of glacier motion 

 in daily and hourly action. 



While, on reviewing my paper of 1838, I can now mark 

 some mistakes, and that I perhaps attached an undue import- 

 ance to some minor phaenomena, — an error into which those 

 who, like myself, have no opportunity of obtaining measures 

 of the phsenomenon which they wish to explain are prone tp 



