Mr. Hopkins on the Mechanism of Glacial Motion. 24- 1 



tension along M n is a maximum. Consequently there must 

 be a tangential force called into action between these elements, 

 acting on the former toxvards M and on the latter Jiom M. 

 Again, take another pair of contiguous elements terminating 

 originally in M and Q, and, after the motion, in M and y, 

 where Qg = Pjt;. Of these two elements along M g, the one 

 nearest to Mw will be extended the most, and therefore the 

 tangential force called into action will act from M on the ele- 

 ment most remote from M n, and towards M' on the other, 

 fience the tangential action between two elements for which 

 the line of separation is situated on one side of M n being 

 called positive, that for two other elements whose line of sepa- 

 ration is on the opposite side of M n, will be negative, and 

 therefore, when the line of separation coincides with M «, the 

 tangential action must = 0. 



The conclusions from this mode of reasoning are identical 

 with those deduced from the two independent investigations 

 previously given (articles 7 — 12, and 17, Second Letter). I 

 might have contented myself with an appeal to those investi- 

 gations, but it appears to me that the reasoning now employed 

 is better calculated to point out the fallacy of that by which, 

 as I conceive, the conclusion of Prof. Forbes has been arrived 

 at. 



1 maintain, then, that in the directions of maximum tension, 

 which may be considered as approximately perpendicular to 

 the transverse fissures, there is no tendency whatever in con- 

 tiguous particles of a glacier to slide past each other, or, there- 

 fore, to produce in those directions planes of discontinuity ; 

 and, consequently, that the alternate bands of blue and white 

 ice cannot have their origin in a system of fractures so pro- 

 duced, assuming always the approximate truth of the law di- 

 stinctly announced by Prof. Forbes, that the bands do approxi- 

 mate to perpendicularity to the general directions of the trans- 

 verse fissures, where the two sets of phaenomena are best de- 

 fined. 



4. But further than this, I contend that though we should 

 allow the bands of blue and white ice to exist in those direc- 

 tions in which there is the greatest tendency to form these 

 planes of discontinuity, those bands could not be thus ac- 

 counted for, because 1 conceive it to be mechanically impos- 

 sible, under the conditions to which I believe glaciers to be 

 subjected, that a system of planes of discontinuity, in which 

 the planes are parallel and at distances varying from the frac- 

 tion of an inch to a few inches from each other, could be pro- 

 duced by the internal forces of constraint independently of 

 some such molecular action as that to which crystallization 



