THE STRENGTH OF THE EARTH'S CRUST 41 



reach maximum values at depths not exceeding 64 km. Is this 

 because the earth shell below the zone of compensation is strong, 

 but for some unrelated reason free from large stress-differences, or 

 is there an absence of such stress-differences because this shell is 

 too weak to bear them? If the latter is true, then the relations 

 of amplitude to wave-length which have been developed in this 

 chapter offer additional proofs of the reality of the existence of the 

 asthenosphere. 



The geologic evidence on the evolution of continental structures 

 and elevations leads to the conclusion that the distribution of 

 stress-differences must be in reality the result of the existence of a 

 zone which cannot carry large distortional strains, as may be seen 

 upon brief consideration. 



The internal activities of igneous intrusion and of tangential 

 compression do not in themselves work toward isostatic equilibrium, 

 but merely toward accentuation of relief. Erosion and sedimenta- 

 tion, while tending to destroy this relief, are not agents tending to 

 create, but to destroy, such isostatic relations as have developed. 

 All of these activities work on a continental or subcontinental as 

 well as on an orogenic scale, as seen in the Cenozoic history of the 

 broad Cordilleran province, yet while the orogenic departures from 

 isostasy are vertically very great, the continental departures are 

 very moderate. For the latter there must be then some more 

 narrowly limiting condition. This corresponds to the incapacity of 

 a deep zone, the asthenosphere, to carry large stress-differences and 

 the incapacity of the lithosphere in spite of its greater strength to 

 act effectively after the fashion of a beam for loads of great span. 

 The orogenic structures, on the other hand, give maximum stresses 

 much nearer the surface, in the stronger lithosphere; because of 

 their shorter wave-lengths they do not produce in it bending 

 stress as in a loaded beam and affect comparatively little the 

 deeper-seated asthenosphere. 



If, then, it is known from the preceding theoretical considera- 

 tions that the limits of strength of the Hthosphere and astheno- 

 sphere determine the limits of the departures from isostasy, the 

 analysis of the nature of these departures may lead in turn to a 

 knowledge of the distribution of strength. 



