1844.] EXPLANATION OF THE VEINED STRUCTURE. 57 



in the last chapter of my book of Travels ; but not having it 

 by me, I cannot refer you to the particular passages. The 

 point in question is undoubtedly the least obvious and most 

 difficult part of the theory, but as I have no doubt of its exact- 

 ness, it will have a proportionate weight in deciding in its 

 favour the opinion of persons accustomed to mechanical theories. 

 It would be difficult to bring it home to the apprehension of 

 ordinary readers ; and, for this reason, I have dwelt upon it, 

 perhaps, too shortly in the chapter alluded to. 



You will readily admit, that if I shall demonstrate separate 

 reasons for the existence of each of the structures figured 

 above (the first a plan, the second a section), the result will 

 be the spoon-shaped structure which I have shewn to exist in 

 glaciers. 



(1.) The tearing asunder of the particles of the glacier 

 owing to the friction of the sides is nearly but not quite, parallel 

 to the sides ; for this reason, that the lines of greatest strain are 

 determined, not merely by the force of gravitation which urges 

 the particles forwards, but there is a drag towards the centre of 

 the stream, in consequence of the greater velocity there. 



^ Let A B be the side of the 



*~~"~---.^ glacier, whilst the particle a 



A^MMMI^M^MB II . ..^ L _ ^ MM^MMM^B^^BMMM*.}} 



a ^:V_7:::_-:=^ C a moves to a , the central par- 



~" B ' tide Z> moves to I', which, 



ft H 



"~P owing to the cohesive bond 

 between a and , must pro- 



c - ' D duce a strain oblique to the 



Rg ' 13> axis of the glacier. 



Or view the matter thus the movement of the ice stream 

 (considered just now solely as respects its surface), is effected 

 against a varying resistance. The line of particles in the direc- 

 tion a a present a greater force of opposition to the movement 

 of the particle a, than the line of particles b 13 present to the 

 movement of I. This is owing to the lateral friction acting 

 more powerfully in retarding the first than the second ; conse- 

 quently the virtual wall of the glacier, or plane of complete 



