694 WILLIAM HERBERT HOBBS 



The writer shares with many others a natural pride in this notable 

 achievement of American research, which has attracted attention 

 by reason both of the large scale of its operations and of its thorough 

 and painstaking execution. With the conclusions concerning the 

 figure of the earth this paper is not especially concerned. It is 

 with reference to Hayford's theoretical deductions within the realm 

 of geophysics and geology upon the basis of Pratt's hypothesis of 

 compensation that the writer would offer suggestions, particularly 

 concerning the fundamental assumptions of which no account is 

 taken in Hayford's papers. In his later article in Science — a 

 presidential address — geologists are informed by Dr. Hayford that 

 they have no recourse but to accept his conclusion. We quote: 



Within the past ten years geodetic observations have furnished positive 

 proof that a close approximation to the condition called isostasy exists in the 

 earth and comparatively near the surface [p. 199]. 



The geodetic observations show that the isostatic compensation under the 

 United States is nearly complete. It is not merely a compensation of the con- 

 tinent as a whole, it is a compensation of the separate, large, topographic 

 features of the continent [p. 200]. 



The compensation may properly be characterized as departing from com- 

 pleteness only one-tenth on an average [p. 201]. 



Elsewhere in the article over- or under-compensation is stated 

 not to exceed that of a mass of rock strata 250 feet in thickness, 

 on an average, and that the limiting depth of compensation is 

 122 km. (76 miles). 



Printing Office, 1905), pp. 535-40; "The geodetic Evidence of Isostasy," Proc. Wash. 

 Acad. Sci., VIII (1906), 25-40; "The Earth a Failing Structure," Bull. Phil. Soc. 

 Wash., XV (1907), 57-74; "The Figure of the Earth and Isostasy from Measurements 

 in the United States," Dept. Com. and Labor, Coast and Geod. Surv. (Washington, 1909), 

 pp. 1-178, maps; "Supplementary Investigation in 1909 of the Figure of the Earth 

 and Isostasy," ibid. (Washington, 1910), pp. 1-80, maps; (with William Bowie) "The 

 Effect of Topography and Isostatic Compensation upon the Intensity of Gravity," 

 ibid., Special Publication No. 10 (1912), pp. 1-132, maps; William Bowie, "Effect of 

 Topography and Isostatic Compensation upon the Intensity of Gravity" (2d paper), 

 ibid., Special Publication No. 12 (1912), pp. 1-28, maps; "The Relations of Isostasy 

 to Geodosy, Geophysics, and Geology," Science, XXXIII (191 1), 199-208; "Isostasy, 

 Rejoinder to the Article by Harmon Lewis," Jour. Geol., XX (1912), 562-78. An 

 outline of Hayford's studies from a very sympathetic standpoint is G. K. Gilbert's 

 "Interpretation of Anomalies of Gravity," Prof. Paper No. 85 C, U.S. Geol. Surv., 

 1913, pp. 29-37, PI- 4- 



