Non-plasticity of Glacier Ice. 591 



rather, is it not certain that they would be pressed down more 

 or less under their own enormous weight? that their sides, 

 where free, would bulge outward, filling up the crevasses be- 

 tween, and presenting to the eye all the outline of plastic 

 masses, in place of the keen, cutting, cliff-like, and often over- 

 hanging sides which these invariably show ? 



It does appear singular to me, how one of Prof. Forbes's 

 great perspicacity can lay any stress, or ground any argu- 

 ment upon such experiments, eii petit^ as he and others have 

 made on plastic pitch, &c. in artificial troughs of a few inches 

 or feet in length. These bodies are confessedly plastic, which 

 it remains to be shown that ice is ; and all these experiments 

 only prove (if they prove anything) that it is possible to si- 

 mulate and represent in some degree to the eye the general 

 form and motion of a glacier by that of a small plastic body; 

 but this is only to show that all bodies which flow, viz. which 

 move in a continuous train or stream, no matter how, do pre- 

 sent certain characters in common: this is true of a cartload 

 of broken stone or road metal, when shot out, as much as of 

 a mass of plastic mud. In a word, such experiments are illus- 

 trations in which there may be accidental and superficial resem- 

 blances, but in which the conditions are so entirely different, 

 that there is no analogy and therefore no base for argument. 



If an advocate of the plasticity theorj' will crystallize a mass 

 of soda or of alum, or some other cheap salt, in a form to be 

 interpenetrated with water or with water and foreign matter, 

 — as clay, sand, &c. — which can easily be done, and show that 

 this mass, in pieces of a few feet in length even, has any traces 

 of plasticity, the experiment would bear more upon the ques- 

 tion. It would remain however to show that glacier ice was 

 in an analogous condition. 



In conclusion, I cannot help remarking two instances of 

 approximation to my previously enunciated views in the most 

 recent papers of Prof. Forbes and of Mr. Hopkins which I 

 liave seen, viz. in the Phil. Mag. for this month, and in Jame* 

 son's Journal, last number. 



In treating of the motion of translation of secondary gla- 

 ciers, Prof. Forbes for the first time admits hydrostatic press- 

 ure to a prominent place as a cause of their motion, or rather 

 the want of it, by want of water, as the cause of their slow 

 motion ; while Mr. Hopkins urges in a forcible manner the 

 internal freedom of motion of the whole glacier's mass as due 

 to the dislocation of its parts and the reproduction of cre- 

 vasses. 



It seems to me that much light might possibly be thrown 

 upon the subject of the coloured bands of glacier ice (which I 



