i9o6.] SEE— THE CAUSE OF EARTHQUAKES. 347 



With the approval of Dr. O. H. Tittman, superintendent, Mr. 

 J. F. Hayford has recently presented to the Washington Academy of 

 Sciences and published in the Proceedings for May i8, a valuable 

 summary of the results deduced from the geodetic operations in 

 the United States. He has discussed with care the long series of 

 deflections of the vertical as determined by Clarke's Standard 

 Spheroid of 1866, which has been found to fit best for the area 

 covered by the United States, and determined the depths at which 

 isostatic compensation takes place. By five solutions of the prob- 

 lem he finds the residuals least for a depth of 71 miles. Some of the 

 conclusions are stated thus : 



" The evidence shows clearly and decisively that the assumption of com- 

 plete 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 the sea level by the rigidity of the earth, but is, in the main, buoyed up, 

 floated, upon underlying material of deficient density. 



"The conclusions just stated were based upon the 507 residuals con- 

 sidered 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 con- 

 tradicts 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 overcompensation rather than undercompensation 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 observations 

 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 continent were maintained in its elevated position, 

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

 the earth." 



Mr. Hayford assumed that the compensation is uniformly dis- 

 tributed with respect to the depth, but he remarks that this is only a 

 convenient working hypothesis. Pendulum observations combined 

 with deflection observations, he thinks, may furnish the means of 

 detecting the distribution of compensation. Assuming that the 

 compensation all occurs within a stratum 10 miles thick, the bot- 

 tom of the stratum of isostatic compensation comes out 37 miles. 

 Mr. Hayford, however, prefers the depth of 71 miles, though he 

 adds that it rests upon an insecure foundation. He concurs with 



