160 Ji. M. Deeley and G. Fletcher — Structure of Olacier-Icc. 



the constituent particles are able to slide over each other. But it is 

 only under certain circumstances that this can take place. While 

 a mass of Glacier-Tee is viscous in all directions, it has been found 

 that a single crystal of ice is only viscous in a direction at right 

 angles to the optic axis.^ A single crystal, or a portion of a crystal, 

 ■will yield to continuous transverse stress applied in a direction 

 parallel to the optic axis, but will not yield in a direction at right 

 angles to the axis — in brief, viscous shear may take place in one 

 plane only. Now, if all the crystalline grains constituting a glacier 

 had their optic axes arranged parallel with the direction of motion, 

 or if there were a large majority of grains so arranged, it would not 

 be difficult to account for the mode of motion of a glacier ; but 

 there is not any such relation between the optical structure of the 

 glacier grains and the direction of motion. If we imagine shear 

 to take place in any single grain, the motion will be stopped by 

 adjacent crystals exhibiting rigidity in that plane. Indeed, it does 

 not appear that a glacier in moving can make any use of the fact 

 that ice-grains are viscous in one plane, for the direction of that 

 plane varies in almost every grain. How, then, does the glacier 

 move ? Why does it, as a mass, exhibit viscosity ? 



When a plastic or viscous substance undergoes change of form 

 without change of bulk, the distortion in its simplest form may be 

 regarded as due to the formation of great numbers of parallel shear 

 planes. In such a case every molecule of each plane must change 

 its position with respect to every other layer of molecules. On the 

 other hand, if the substance be built up of a number of rigid grains 

 of all shapes and sizes, closely fitting and adhering to each other, 

 the nature of the change necessary to give rise to distortion is much 

 less simple. In such a case not only do we require shearing 

 between the interfaces of the particles but also a change of shape 

 of the particles themselves. And this must go on in ice without 

 producing more than local ruptures, for its tenacity and shearability 

 are sufficiently high to resist general fracture. To account for 

 Glacier-Motion, therefore, we have to show that the glacier grains 

 can not only increase in size but also change their shapes under the 

 smallest stresses, and also that they can, under similar conditions, 

 slide over each other without actual fractures resulting. 



We will first consider the question of change of shape and size. 

 Fig. 9 shows an ideal case of a number of particles lying between 

 two parallel planes, the upper of which is moving more rapidly than 

 the lower one. The small arrows near to or crossing the interfaces 

 indicate the direction in which shear must take place, and also show 

 those surfaces which, being pi'essed together, must be wasting, and 

 those surfaces which, being in tension, must be growing. Although 

 we shall deal with the case as though each crystal had rectilinear 

 motion only, it must be remembered that they will also have a 

 tendency to roll over each other as well. This, however, rather 



' " On the Plasticity of Glacier and other Ice," hy James C. McConnell, M.A., 

 and Dudley A. Kidd, Proc. Eoy. Soc, vol. xliv. No. 270. Also "On the Plasticity 

 of an Ice Crystal," by James C. McConnell, M.A., ibid. vol. xlix. No. 299. 



