266 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



ISOSTASY AS A WORKING HYPOTHESIS 



The recognition of heterogeneity as a characteristic of the earth re- 

 quires that one should postulate either extreme rigidity or a tendency 

 toward isostatic equilibrium, to account for the inequalities of surface 

 which the earth presents. The working hypothesis on which this dis- 

 cussion is based assumes isostasy in the terms in which it was stated by 

 Grilbert. 



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

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

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

 heterogeneous as to density." ^® 



Since Gilbert wrote that interpretation of the hypothesis of isostasy, 

 which he and Dutton had developed in friendly discussion and of which 

 Dutton has become the recognized author, the extraordinarily elaborate 

 and precise mathem^atical studies of Hayford have demonstrated the 

 truth of the general theory, but have at the same time cast it in a rigid 

 special form which, as it appears to me, is inconsistent with the variety 

 and variable character of geologic facts. 



It seems extremely improbable that isostatic compensation is every- 

 where within the United States nearly complete, as is computed by Hay- 

 ford and Bowie, it being demonstrated that the processes of degradation 

 and aggradation, and also those of molten intrusion and extrusion, tend 

 constantly to modify the conditions of equilibrium. A coincidence of 

 nearly complete equilibrium within narrow limits would be an accidental 

 and temporary state on which no general theory of the stresses in the 

 earth's crust could be based. This view was stated some years ago,^^ and 

 I do not find that the doubt raised by geologic reasoning has yet been 

 answered by geodetic observation and computation. 



In the meantime Adams' researches have demonstrated the very great 

 strength of the crust, and have rendered more difficult the explanation 

 of isostatic adjustments. 



We are thus led to reconsider the fundamental assumptions of the 

 Hayford concept of isostasy, and there are two which appear to be incon- 

 sistent with geologic facts. The first relates to the distribution of density 

 in any vertical column. The second is the assumption of hydrostatic 

 pressures at and below the level of compensation. 



The distribution of density assumed by Hayford postulates uniform 



i» G. K. Gilbert : Strength of the earth's crust. Bull. Geol. Soc. Am., vol. 1, 1890, 

 25. 



20 Bailey Willis: What is terra firma? Smiths. Kept. 1910, pp. 391-406 (402-403). 



