THE STRENGTH OF THE EARTH'S CRUST 433 



winter, a phenomenon to be accounted for by the rate of recrystal- 

 lization. The parts of an ice crystal which are subjected to shear 

 and compression have the melting-point lowered. They melt, dis- 

 charge the strain, and refreeze. In the winter the general tempera- 

 ture is reduced, and a greater strain is necessary to bring the 

 melting-point down to the lower temperature. Until local melting 

 is produced the ice behaves like any other crystalline solid, as 

 a substance possessing elasticity of form. Beyond that point it 

 exhibits plasticity and behaves in some respects like a very viscous 

 fluid. In other respects, however, it exhibits properties quite 

 distinct from that of the usual conception of mere plastic flow, since 

 in the testing machine, or on the walls of a crevasse, ice will resist 

 strong shearing strains, and yet the glacier as a whole yields and 

 flows slowly under a moderate pressure-difference as shown by the 

 low gradient of its upper surface. Glacial motion appears to take 

 place, therefore, by the solution and growth of crystals, not by a 

 true viscous flow. The soKd and crystalline nature throughout as 

 opposed to viscous fluidity is furthermore shown, as Chamberlin 

 has noted, in the power of the glacial ice to shove over and abrade 

 its floor and to ride up slopes. ChamberHn adds that a dry glacier 

 is a rigid glacier. A dry glacier is necessarily cold, and a cold 

 glacier is necessarily dry.^ 



With ice subjected to slowly applied forces the elastic limit 

 is consequently dependent upon the point of yielding by recrystalli- 

 zation. We thus see an intimate relationship between tempera- 

 ture and variation in the elastic limit, the elastic limit for ice being 

 greater for low temperatures than for high temperatures. But the 

 modulus of rigidity, on the contrary, measures the elastic change 

 of form for unit-shearing force, change of form not accompanied 

 by crystallization, but marked by a capacity to spring back to the 

 original form upon the removal of the stress. 



T. W. Richards, in his studies on the compressibility of solids, 

 notes that they are almost as compressible and voluminous at 

 absolute zero as at ordinary temperatures. Under this conception 



^"A Contribution to the Theory of Glacial Motion," Decennial Publications 

 of the University of Chicago, IX, 203, 204 (1904); Chamberlin and Salisbury, Geology, 

 I (1904), 305- 



