32 HAYFORD 



large. In the former solution there are but 19 per cent, of the 

 residuals greater than 5" and the maximum residual is 16", 

 whereas in the latter 66 per cent, of the residuals are greater 

 than 5" and the maximum residual is 44". In the former solu- 

 tion the average residual is 3".i and the latter 8". 8. 



The evidence shows clearly and decisively that the assump- 

 tion of complete isostatic compensation within the depth of 71 

 miles is a comparatively close approximation to the truth, that 

 the assumption of extreme rigidity is far from the truth — that 

 the United States is not maintained in its position above sea 

 level by the rigidity of the earth, but is, in the main, bouyed 

 up, floated, upon underlying material of deficient density. 



The conclusions just stated were based upon the 507 residuals 

 considered as one group. The residuals have been examined 

 in separate groups of 25, each group covering a small region. 

 Not a single group of 25 contradicts the conclusion just stated. 



It is certain that for the United States and adjacent regions, 

 including oceans, the isostatic compensation is more than two- 

 thirds complete — perhaps much more. 



The departure from perfect compensation may be, in some 

 regions, in the direction of over-compensation rather than 

 under-compensation but in either case the departure from perfect 

 compensation is less than one-third. 



In terms of stresses, it is safe to say that these geodetic ob- 

 servations prove that the actual stresses in and about the United 

 States have been so reduced by isostatic adjustment that they 

 are less than one-tenth as great as they would be if the con- 

 tinent were maintained in its elevated position, and the ocean 

 floor maintained in its depressed position, by the rigidity of the 

 earth. 



In order to secure the greatest possible accuracy in deriving 

 the figure of the earth it is necessary to determine as accurately 

 as possible the depth at which the isostatic compensation occurs. 

 This is also of great importance on account of its bearing on the 

 greater problems of geology. With what degree of accuracy 

 does this geodetic investigation fix the depth of compensation? 



When all the evidence from the solutions for depths infinity, 

 101 miles, 75 miles, 71 miles, and zero, is also taken into ac- 



