108 W. D. MacMILLAN 



regions of sufficiently deficient' density would eventually appear 

 above the surface of the waters of the ocean, while the regions of 

 greatest density would form the great depths of the basins into 

 which the the waters of the ocean were gathered. Those regions 

 which have neither excess nor deficiency of density would still 

 remain about 9,000 feet below the sea-level except in so far as, 

 through rigidity, they partook of the movement of neighboring 

 regions. A theory of isostasy comprehending the entire earth in 

 its grasp should therefore be based upon a level 9,000 feet below 

 the surface of the ocean. Regions which lie above this level are 

 deficient in density and regions which lie below this level are exces- 

 sive in density. Clearly, it would not do to regard the sea-level, 

 which from an isostatic point of view is an accidental level, as the 

 dividing surface. Indeed, if the differentiation of density were so 

 small that the elevated regions were all below the sea-level, we 

 should be compelled to conclude that the density was everywhere 

 excessive — -a reductio ad absurdum. 



But the sea-level is precisely the surface which Hayford has 

 chosen, and, as a consequence, to all those regions which lie between 

 the sea-level and 9,000 feet thereunder he has ascribed an excess 

 of density instead of a deficiency. Since approximately three- 

 quarters of the earth's surface lies under the ocean at an average 

 depth of approximately 15,000 feet, and since under Hayford's 

 hypotheses too great density is ascribed to this region, it would 

 seem that his hypothesis has had the effect of raising the mean 

 surface density for the entire earth from 2 . 67 to a somewhat higher 

 figure. This alone might not be of any great importance, since 

 at best the figure 2.67 is somewhat uncertain, but a hypothesis 

 which is based upon sea-level gives horizontal changes of defect of 

 density over the continents which are relatively too great, as is 

 shown in Table I, which is based upon the hypothesis of uniform 

 compensation. 



Thus, with the sea-level basis the defect in density under a 

 region which has an altitude of 6,000 feet is six times as great as 

 the defect in density under a region which has an elevation of 1,000 

 feet, while with a true, isostatic theory the defect would be only 

 one and one-half times as much. The defect under a 1,000-foot 



