on the Motion of Glaciers. 41 S 



general appearance, and the unequal motion of the sides and 

 centre which their existence alone renders possible^ is the best 

 evidence of their origin, especially as their general direction 

 corresponds to what a " popular " view of the mechanism of 

 the case would indicate, and most accurately to the observed 

 results in the plastic models. This is the chief evidence which 

 I have for the origin of the ribboned structure being " a forced 

 separation of a half rigid mass*;" and it has this advantage 

 over Mr. Hopkins's speculations, that he does not profess to 

 offer any explanation f of this, the most elaborate and curious 

 peculiarity of a glacier. 



6. Yet something may be gleaned from Mr. Hopkins's 

 writings, showing a disposition to yield even this point. Ex- 

 actly in proportion as his statements have become more defi- 

 nite and tangible, and he has moderated the use of analysis, 

 with which he at first attempted to carry his point by storm, 

 his admissions to the viscous theory have become more fre- 

 quent and explicit. The combination of this passage in the 

 Second Letter (p. 166), "any internal constraint which might 

 be superinduced by the peculiar motion of the glacier, would 

 be relieved both by transverse fracture and by the sliding of 

 one part past another," with that in the Third Letter (p. 243), 

 " I am far from supposing that the origin of this structure 

 [the blue bands] is unconnected with the motion of the gla- 

 cier," seems as nearly an admission of the theory which I have 

 proposed of this structure as can well be imagined. Again, in 

 the Second Letter, Mr. Hopkins admits plasticity as an attri- 

 bute of glaciers with tliis qualification ; " I mean that plasti- 

 city which shall require for its development the continuous 

 application of force, possibly of great force, for a long period 

 of time ; " it is needless to say that it is of plasticity such as this 

 that I have always spoken in the case of glaciers, where the 

 forces are enormous and the motions slow. Yielding to the 

 slightest force in the shortest time is not plasticity, but fluidity ; 

 and hence the term secular plasticity, by which Mr. Hopkins 

 has attempted to raise a partition between his views and my 

 own, is, as Dr. Whewell has justly observed in the letter in 

 which he has generously defended my claims to this theory |, 

 not only superfluous, but improper, since the plasticity in 

 question does not require ages or even years for its develop- 

 ment, but is manifested (by the unequal motion of the centre 

 and side of a glacier) in a few days or even hours. 



7. Finally, Mr. Hopkins, admitting the plasticity, joins issue 

 with me on the metaphysic of a word. According to him the 



* Travels, p. 377- t Phil. Mag., March, p. 244. % Ibid, p. 218. 



