﻿M. A. Heim on Glaciers, 491 



has penetrated them, and appear silvery white, and along these 

 the grains can be easily separated (primary capillary fissures), 

 irregular surfaces of division transverse to the grains come into 

 view on the application of greater force. The latter are surfaces 

 of division already traced in the substance, not new surfaces of 

 fracture ; they often help us to divide a grain into several pieces ; 

 by way of contrast to the above-mentioned we will name them 

 secondary capillary fissures. It is natural to suppose that these 

 secondary fissures are old primary ones not yet quite regelated, 

 and that in these two forms we have surfaces of division of the 

 glacier-grains in different states of development. The primary 

 capillary fissures are mostly smooth, curved or plane; the secon- 

 dary show for the most part (perhaps always) fine, acute eleva- 

 tions which accurately fit into depressions in the adjacent faces, 

 often all elongated in one direction ; and then the faces exhibit 

 parallel wrinkles. 



Between the explanation of glacier-motion by pressure, as given 

 by Tyndall, and the continual renewal of the network of fissures 

 there is, it seems to me, a deep and intimate connexion. Pro- 

 fessor Tyndall says, in relation to his experiments, " The ice, in 

 changing its form from that of one mould to that of another, was 

 in every instance broken and crushed by the pressure;" and then, 

 with respect to the glacier, continues, "but suppose that, in- 

 stead of three moulds, three thousand had been used, — or, better 

 still, suppose the curvature of a single mould to change by ex- 

 tremely slow degrees — the ice would then so gradually change 

 its form that no rude rupture would be apparent. Practically 

 the ice would behave as a plastic substance." 



I believe that just what is here called "rude rupture" takes 

 place also in the glacier, and occasions the breakiug up of the 

 ice into grains. Glacier-ice is so brittle that I cannot imagine 

 an alteration of its form without a continual shattering into frag- 

 ments (without " rude rupture"), even if the form of the mould 

 into which it is pressed be ever so slowly altered. In the gla- 

 cier also the ice mass is every moment broken into fragments 

 (the glacier-grains), and these are again cemented by regelation; 

 both processes proceed simultaneously and uninterruptedly 

 throughout the mass of the glacier — and, indeed, the more 

 actively the quicker the motion, and the more irregular the form 

 of the valley, to which the glacier must bend — in short, the 

 greater the relative motion of neighbouring portions of the ice. 



We cannot expect to be able to find, in support of this view, 

 forms of conchoidal fracture in the grain of the glacier. The 

 concentric streaks which characterize that are only grouped 

 around certain pronounced points of action of the force; while 

 the pressure in the glacier takes place almost uniformly through- 



