THE WORK OF SNOW AND ICE. 313 



crystals continue to lie in all directions. Hence glacier ice, unlike pond 

 ice, cannot usually be split along definite planes, except where cleavage 

 planes are subsequently developed by extraneous agencies. 



IVhile the crj^stals of a glacier usually have their principal axes in 

 various directions^ there appears to be a tendency for them to approach 

 parallelism in certain positions, especially in the basal parts of a glacier 

 near its terminus. Observations on this point are not so full and critical 

 as could be desired, but it is probable that the parallel orientation is 

 partly general, and due to the vertical pressure of the ice, and partly 

 special and local, and connected ^yith the shearing planes and foliation. 



The bearing of this partial parallelism of the crystals on shearing and 

 foUation is supposed to reside in the fact that a crystal of ice is made up 

 of a series of plates arranged at right angles to the principal axis of the 

 crystal. These plates may be likened to a pile of cards, the principal axis 

 being represented by a line vertical to them. If a cube be cut from a large 

 crystal of ice, it ^ill behave much like a cube cut from the pile of cards. If 

 the cube be so placed that its plates are hoHzontal (Fig. 291a), and if it be 

 rested on supports at two edges and heavily weighted in the middle, it 

 will sag, the plates sliding slightly over one another so as to give oblique 

 ends, but in this case the cube offers considerable resistance to deforma- 

 tion. If the cube be so placed that the plates stand on edge, each reach- 

 ing from support to support (Fig. 2916), it T\ill offer very great resistance 

 to deformation; but if the plates be vertical and transverse to the line 

 joining the supports, as in Fig. 291c, the middle portion will sag under 

 very moderate weighting by the shding of the plates on one another, and 

 in a comparatively short time the middle portion may be pushed entirely 

 out, dividing the cube. These properties have been demonstrated by 

 McConnel ^ and Miigge, and they appear to throw Hght on certain phases 

 of the action of glaciers that are most pronounced in their basal parts, 

 and are best illustrated in arctic glaciers. 



The Probable Fundamental Element in Glacial Motion. 



Melting and freezing.— It has already been shown (p. 279) that the 

 initial or fundamental cause of glacial motion must be operative at the 



^On the Plasticity of Glaciers and other Ice. Proc. Roy. Soc, Vol. XLIV, 1888 

 pp. .331-67 (with D. A. Ividd); Vol. XL VIII, 1890, pp. 259, 260; Vol. XLIX, 1891,' 

 pp. 823-43. 



