THE STRENGTH OF THE EARTH'S CRUST 683 



of area than for more local adjustments. The absence of a notable 

 continental gradient is suggestive therefore of a deep zone of weak- 

 ness, least resisting in its central portions, and of very marked 

 plasticity in comparison with the rigidity of the lithosphere above. 

 This does not involve, however, the conception of a truly fluid 

 zone, but merely that of a comparatively plastic solid. 



The existence and nature of this zone of weakness is seen to 

 enter vitally into the theory of isostasy and must of course bear 

 with equal importance on other branches of terrestrial dynamics 

 as well. It is proposed therefore to elevate it to equal rank with 

 the other shells of the earth and to name it for that quality which, 

 from the standpoint of diastrophism, is its most significant feature 

 as compared to the zones above and below. This is its inability 

 to resist stress-diflerences above a certain small limit. Its name, 

 therefore, is the sphere of weakness — the asthenosphere. 



[To be continued] 



