ON THE HYPOTHESIS OF ISOSTASY 



109 



elevation is twice as great as under a 500-foot elevation on the one 

 hypothesis and only twenty-nineteenths times as great on the other. 

 This indicates that the sea-level basis overaccentuates the impor- 

 tance of changes of level in the topography of the continents. 



TABLE I 



If the density in the earth's crust actually varies in the manner 

 supposed by Hayford (using the sea-level basis), one would expect 

 regions over which the compensation was effected to be smaller 

 than if the density varies according to a true isostasy (i.e., the 

 9,000-feet-below-the-sea basis), since the changes in density are 

 relatively greater in the first case than in the second. Hayford 

 has attempted to determine the sizes of these areas of compensation, 

 but the quantities to be considered were so small that success was 

 scarcely to be expected; indeed, they are "frequently less than the 

 errors of observation and computation" and, possibly, also less 

 than the effects of local irregularities of density. The evidence, 

 though inconclusive, leaned faintly toward rather small areas of 

 compensation, and Hayford is of the opinion that these areas are 

 between a square mile and a square degree. 1 If the present writer 

 is correct in assuming that a true isostasy must be based upon a 

 level 9,000 feet below the sea, then the evidence published by 



1 It is certain from the results of this investigation that the continent as a whole 

 is closely compensated, and that areas as large as states are also closely compensated. 

 It is the writer's belief that each area as large as one square degree is generally largely 

 compensated. The writer predicts that future investigations will show that the 

 maximum horizontal extent which a topographic feature may have and still escape 

 compensation is between one square mile and one square degree. This prediction 

 is based, in part, upon a consideration of the mechanics of the problem. — Hayford 

 and Bowie, The Effects of Topography and Isostatic Compensation upon the Intensity 

 of Gravity, p. 102. 



