BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



VOL. 33, PP. 371-374 JUNE 30, 1922 



ROLE OF ISOSTATIC STRESS ' 



BY BAILEY WILLIS 



{Presented by title before the Society December 29, 1921) 



CONTENTS 



Page 



Gilbert's versus Hayf ord's concept of isostasy 371 



Curve of isostatic stress 372 



Incompetence of isostatic stress to produce motion in the lithosphere 372 



Conclusion : Isostasy can not produce, but may direct, earth movements, 

 whose cause is to be sought in the effects of the periodic activity of in- 

 ternal heat 374 



Gilbert's versus Hayford's Concept of Isostasy 



"Mountains, mountain ranges, and valleys equivalent to mountains exist 

 generally in virtue of the rigidity of the earth's crust ; continents, continental 

 plateaus, and ocean basins exist in virtue of isostatic equilibrium in a crust 

 heterogeneous as to density." — G. E. Gilbei't, 1S90. 



The relation between rigidity and equilibrium in the lithosphere was 

 thus stated by Gilbert thirty-odd years ago, and seems to me still to have 

 been most philosophically and accurately stated. Opposed to this view, 

 or rather supplementing it by refinements of extraordinary precision, is 

 the conclusion reached by Hayford and Bowie, to the effect that com- 

 plete isostatic compensation exists generally within the depth of 71 miles 

 and for areas of a square degree of extent or even less. I have elsewhere 2 

 stated my reasons for not being able to accept the conclusions based on 

 the mathematical analysis of gravity determinations, and will here only 

 repeat that I question the assumption of distribution of density postu- 

 lated by Hayford, and also the assumption of hydrostatic pressures at 

 and below the level of compensation. If these two assumptions be erro- 

 neous, the mathematical structure built on them can not stand. 



1 Manuscript received by the Secretary of the Society April 3, 1922. 

 This pa.Der is one of a series composing' a symposium on isostasy. 



2 Bailey Willis : Discoidal structure of the lithosphere. Bull. Geol. Soc. Am., vol. 31. 

 1920, pp. 266-269. 



(371) 



