VOLUME XXV NUMBER 2 



THE 



JOURNAL OF GEOLOGY 



FEBRUARY-MARCH 19 17 



ON THE HYPOTHESIS OF ISOSTASY 



W. D. MacMILLAN 



University of Chicago 



The splendid papers by Hayford 1 and jointly by Hayford and 

 Bowie 2 have brought the subject of isostasy into the foreground for 

 discussion by geologists and others who may be interested. These 

 papers have taken the subject out of a field of more or less vague 

 conjecture, and by subjecting it to a very careful quantitative 

 examination have shown very clearly that isostasy in some form 

 can be accepted as a reality. 



To be sure, they have not proved the reality of isostasy, for in 

 the mathematical sense no physical hypothesis can be proven. 

 But they have formulated precise hypotheses of isostasy and have 

 shown that a vast mass of observational data covering the United 

 States is very much better satisfied by theories which include their 

 hypotheses than by the usual gravitational theory which excludes 

 the hypothesis of isostasy. 



Four distinct laws of isostasy have been discussed in these 

 papers, viz., (a) uniform compensation, (b) uniformly decreasing 



1 John F. Hayford, The Figure of the Earth and Isostasy from Measurements in 

 the United States, Publications of the U.S. Coast and Geodetic Survey, 1909. 



2 John F. Hayford and William Bowie, The Effect of Topography and Isostatic 

 Compensation upon the Intensity of Gravity, Publications of the U.S. Coast and Geo- 

 detic Survey, 191 2. 



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