396 Jones — Review of Chamberlin's Groundwork 



pressures and so crystalline aggregates must become 

 plastic. 



Temperature increase, of course, materially facilitates 

 yielding and so, as far as yielding by flowage is con- 

 cerned, and where temperature and pressure both 

 increase, a material will yield under a lower pressure than 

 it would if pressure were increased alone. The fact of 

 yielding implies differential pressures. True cubic com- 

 pression no longer exists when yielding takes place. 

 Certainly any terrestrial material we know of must be 

 in a potential yield-state under the conditions of temper- 

 ature and pressure at no very great depth beneath the 

 surface of the earth whatever its physical state as regards 

 homogeneity or heterogeneity, crystallinity or non- 

 crystallinity and yet such a yield-state would be one of 

 high rigidity in the ordinarily accepted sense of that 

 term, that is, for any short time differential stress. 



The term elasticity may have the same implication. A 

 material may rebound under one set of stress conditions 

 and not rebound under another set of conditions. Both 

 these terms imply simply dependent states. 



A material then, under any set of conditions as regards 

 temperature and pressure, may have properties of rigid- 

 ity and elasticity of a high order in so far as short-time 

 stress differences are concerned and yet be non-rigid and 

 non-elastic for stress differences of long duration and 

 large magnitude, and these latter are the types of stress 

 conditions we are concerned with in diastrophic move- 

 ments. The statement that the earth is more rigid and 

 elastic than steel should be qualified, therefore, with the 

 statement that it possesses these properties for certain 

 stress conditions — for example those imposed by tidal 

 pulsations. 



Now Chamberlin's interpretation of the evidences of 

 this rigidity — that is the rigidity shown under tidal stress 

 and seismic vibrations, implies that this state of rigidity 

 and elasticity holds good for stress differences of all 

 magnitudes and of all time durations. Under this inter- 

 pretation the terms rigidity and elasticity become prac- 

 tically absolute instead of dependent. But such an 

 interpretation is essential if he is to postulate the non- 

 existence of a yield-state anywhere within the earth for 

 almost any condition of stress. Under this conception 



