374 B. WILLIS ROLE OF ISOSTATIC STRESS 



there has ensued a decided uplift. The evidence of geologic history thus 

 testifies definitely to the fact that the earth's crust is characteristically 

 stable when lands are being reduced by erosion to a low grade plane, and 

 when, therefore, the isostatic stress is approaching a maximum. And 

 yet, that characteristic stability is interrupted by episodes of mountain 

 growth which result in reducing the isostatic strain to a minimum. 

 These relations of stability and instability are clearly not those which 

 would result from the slow yielding of the lithosphere in consequence of 

 fatigue induced by a small stress within the elastic limit. 



Conclusion : Isostasy can not Produce, but may Direct, Earth 



Movements, whose Cause is to be Sought in the Effects 



of the periodic activity of internal heat 



I conclude, then, that the isostatic stress is incompetent to produce 

 deformation in the lithosphere, in so far as that deformation requires a 

 force in excess of the elastic limit; and, further, that the appeal to time, 

 as a factor which may enable a stress within the elastic limit to do the 

 work, fails because the work done is not of the character which would 

 result from fatigue of materials. Nevertheless, I recognize that there is 

 a definite isostatic stress which affects those zones of the lithosphere in 

 which erosion and deposition modify the equilibrium toward which large 

 masses of unlike density tend. This stress is unquestionably related to 

 the movements involved in continental uplift and mountain growth. It 

 is oriented in the direction in which the movements take place, and its 

 role is, in my judgment, to direct the movements which it is incompetent 

 to produce. 



The views which I have thus expressed have troubled for many years 

 my speculations on the nature of the forces which produce earth move- 

 ment. I would like to have believed with Dutton, and later with Hay- 

 ford, that isostasy Avas competent to cause that undertow which seems to 

 be implied by erosion and uplift; but the facts are too strongly opposed 

 to that view. They call for the action of a far greater force, characterized 

 by periodic activity. That force I believe to be the internal heat of the 

 earth, alternately accumulated in and carried away from the astheno- 

 sphere, as postulated by Chamberlin and Barrell. The transformation of 

 the heat energy into the mechanical energy which displaces mountains is 

 brought about by molecular changes in the rocks, resulting in recrys- 

 tallization and elongation in the direction in which the isostatic stress is 

 oriented. 



Thus isostasy is the rudder, not the motive power, of deformation. 



