130 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



was found that bars of ice frozen in a mould, or cut from a glacier, may be bent, 

 elongated, compressed, and twisted, without visible rupture, even when kept 

 continuously below the freezing temperature. These experiments were made by 

 Main, McConnel, Kock, and Mxigge and show that solid ice, made up of a collection 

 of irregular crystals, is decidedly plastic. McConnel calculated that the amount 

 of extension required of the ice in the Rhone Glacier, because of the more rapid 

 central movement when compared with its sides, amounted to 0.0029 millimeter 

 per hour, for each 10 centimeters of length. In his experiments with bars of 

 glacial ice, but one of the three bars tested showed as small amount as this and 

 that for only a portion of the experiment. In the case of single crystals they 

 were found capable of continuous yielding without rupture, providing the pressure 

 was applied at right angles to the optic axis, the movement appearing to consist 

 of a sliding between adjacent crystalline plates. When the force of compression, 

 or tension, was applied parallel with the axis, the result was exceedingly small, or 

 nil. 1 The verification of these results by other investigators leads to the conclu- 

 sion that ice is capable of showing a certain type of plasticity, although different 

 from that ascribed to it by Rendu and Forbes. An amorphous plastic substance 

 yields under a suitable stress in any direction without visible rupture. A crystal- 

 line substance which maintains its definite molecular arrangement will be 

 limited in the number of directions in which it may yield. If ice crystallized in 

 cubes it seems likely that it might have yielded without rupture in three direc- 

 tions. Had it crystallized in square prisms we may now conceive of a movement 

 in two directions. In the hexagonal system in which it actually crystallizes 

 the molecular cohesion measured in the direction of the main axis is of a suffi- 

 ciently different nature from that at right angles to it to permit of a gliding of the 

 basal plates without rupture and the destruction of their molecular arrangement. 

 This is plasticity in a crystalline substance. The experimental results were 

 obtained with moderate stresses and much below the freezing temperature. In 

 the case of a glacier under great stress and a temperature near the freezing 

 point it seems absolutely necessary chat the glacial granules should manifest 

 this property to a greater or less extent, regardless of the actual mechanics of 

 glacial movement. 



A number of phenomena, noted upon the Canadian glaciers, has convinced 

 the writer that a certain amount and kind of plasticity is a fundamental property 

 of glacial ice. The complicated patterns occasionally shown by the blue bands 

 are such as might arise from plasticity, but not from melting or rotating of the 

 granules. When similar effects are seen in ordinary rocks they are commonly 

 referred back to a plastic condition of the matrix. It was noted that when the 

 difference in the rate of movement between the centre and margins of the glacier 

 is sufficiently small, there are no marginal crevasses to be seen. This implies that 

 the ice permits a certain amount of stretching, without visible rupture. If ice 

 were absolutely incapable of yielding under tension, any appreciable difference be- 



>" On the Plasticity of Glacier and other Ice," James C. McConnel, Proc. Roy. Soc. of London, vol. 44, 

 1888, p. 331; "On the Plasticity of an Ice Crystal, " vol. 49, 1891, p. 323. 



