1858.] on the Mer-de- Glace. 651 



sents wrinkles which are the representatives, in little, of the 

 protuberances upon the glacier. Or the coat sleeve is an equally 

 instructive illustration : when the arm is bent at the elbow the 

 sleeve wrinkles, and as the places where these wrinkles occur in 

 the cloth are determined, to some extent, by the previous creas- 

 ing, so also the places where the wrinkles are formed upon the 

 glacier, are determined by the previous scarring of the ice during 

 its descent down the cascade. The 



manner in which these crumples tend __««P„r5=*«»>^ fic.4. 



to scale off speaks strongly in favour of 

 the explanation given. Fig. 4 is a type 

 of numerous instances of scaling off 

 observed by the speaker, and recorded 

 in his note-book. By means of a 

 hydraulic press he was able to pro- 

 duce a perfectly similar scaling off in 

 small masses of ice. One consequence 



of this crumpling of the glacier would be the backward and for- 

 ward inclination of the veins as actually observed. The falling 

 backward and forward of the veins was also observed on the 

 wrinkles of the Glacier du Geant. It was also proved, by 

 measurements, that these wrinkles shorten as they descend. 



In virtue of what quality then can ice be bent and squeezed, 

 and change its form in the manner indicated in the foregoing obser- 

 vations ? The only theory worthy of serious consideration at the 

 present day is that of Prof. Forbes, which attributes these effects to 

 the viscosity of the ice. The speaker did not agree with this theory ; 

 as the term viscosity appeared to him to be wholly inapplicable as 

 expressive of the physical constitution of glacier ice. He had 

 already moulded ice into cups, bent it into rings, changed its form 

 in a variety of ways by artificial pressure, and he had no doubt 

 of his ability to mould a compact mass of Norway ice which stood 

 upon the table into a statuette ; but would viscosity be the proper 

 term to apply to the process of bruising and regelation by which 

 this result could be attained ? He thought not. A mass of ice at 

 32" is very easily crushed, but it has as sharp and definite a fracture 

 as a mass of glass. There is no sensible evidence of viscosity. 



The very essence of viscosity is the ability of yielding to a force 

 of tension, the texture of the substance, after yielding, being in a 

 state of equilibrium, so that it has no strain to recover from ; and the 

 substances chosen by Prof. Forbes, as illustrative of the physical 

 condition of a glacier, possess this power of being drawn out in a 

 very eminent degree. But it has been urged, and justly urged, 

 that we ought not to conclude that viscosity is absent because hand 

 specimens do not show it, any more than we ought to conclude that 

 ice is not blue because small fragments of the substance do not 

 exhibit this colour. To test the question of viscosity then, we 

 must appeal to the glacier itself. Let us do so. First, an 

 analogy between the motion of a glacier through a sinuous valley, 



