Geology and Natural History. 193 



3. The Figure of the Earth and Isostacy from Measurements 

 in the United States ; by John F. Hayford, Inspector of 

 Geodetic Work, and Chief, Computing Division Coast and Geodetic 

 Survey. Pp. 178, plates and figures 17. Washington, 1909. — 

 This report is one of great interest to geodesists and geologists, 

 for though the principal conclusions have been previously 

 published by Hayford, this is the first appearance of the complete 

 work. The author points out that earlier computations upon the 

 elements of the spheroid have regarded the deflections of the 

 vertical as accidental errors, an assumption which is evidently 

 untrue. By considering them as due to the known irregularities 

 in topography largely counterbalanced by the unknown irregu- 

 larities in subsurface densities, a solution is reached giving the 

 character of the latter, and by thus allowing for constant errors 

 attaining more correct and larger values for the dimensions of 

 the spheroid. By assuming in the solution the existence of 

 certain deficiencies of mass underlying elevated tracts, the weight 

 of the new determination of the terrestrial dimensions becomes 

 1*7 times that derived otherwise. This may be taken as a 

 mathematical demonstration of isostacy. Hayford furthermore 

 finds that isostatic adjustment is so nearly complete that the deflec- 

 tions of the vertical average are less than a tenth of what they 

 would be if due to topographic irregularities alone and the stress 

 differences in the crust are not more then one-twentieth what 

 they would be if isostacy did not prevail. Consequently the 

 United States is not maintained in its position above sea level by 

 terrestrial rigidity but is in the main buoyed up, floated, in each 

 of its parts, because it is composed of material of deficient but 

 irregular density. The solution further shows that the flotation 

 is not due to a lighter crust resting upon a fluid and denser sub- 

 stratum and that the isostatic compensation is approximately 

 satisfied within a hundred miles of the surface. This report 

 brings forth the results of a monumental labor and its author 

 and the organization which he represents are to be congratulated 

 upon its completion. The results will be most interesting if 

 gravity determinations are now made in order to throw further 

 light upon the variations in subsurface densities extending to the 

 depth at which isostatic compensation becomes complete. 



The reviewer would point out that the conclusion, that the 

 various physiographic provinces are now so closely compensated 

 that the unbalanced stresses in the earth are not more than a 

 twentieth as great as they would be if isostatic adjustment did 

 not prevail, is seemingly at variance with the geological evidence 

 that the crust is able to remain unwarped during long periods ot 

 time, permitting the wide development of base-leveled surfaces. 

 The reconciliation of these two well-founded conclusions of 

 modern geology is one of the larger problems awaiting solution 

 in the future. Although, as Willis has suggested, the present 

 epoch may be one of unusually complete isostatic adjustment, 

 how comes it that if so complete at present, at other times 

 the crust could for so long have resisted the stresses due to wide- 

 spread erosion ? j. b. 



