Status of the Theory of Isostasy. 311 



the sea level is an unessential surface, valuable only as 

 a datum plane for measuring elevations. 



California, for example, is bordered on one side by 

 the Pacific Ocean basin, reaching depths of over 4,000 

 meters. At about an equal distance on the other side 

 the Sierras rise to over 3,000 meters. The great mass 

 of the Cordilleras lies beyond. It is this larger relief 

 which is the chief cause of the computed topographic 

 deflection, as shown for Point Arena, California. 19 If 

 for group 10, Southern California, the mean effective 

 differential relief is taken as 6,000 meters, the ratio of 

 •06 corresponds to a departure from isostasy of 360 

 meters, 1,180 feet. This is very different from the 

 interpretation just quoted from Hayf ord. 



Having noted in 1914 the error due to interpreting 

 the residuals by the use of a sea-level datum, the writer 

 was not looking for another error in the same paragraph. 

 It is evident, however, upon further inspection, that 

 still another of fundamental importance exists which is 

 brought out by comparison with the following table : 



"Investigation in 1909 of the Figure of the Earth and Isostasy. 20 



Mean values of residuals without regard to sign, for different 

 depths of compensation. 



s 



olution B. 



Solution E. 



Solution H. 



Solution G. 



Solution A. 



Group No isostasy 



162-2 km. 



120-9 km. 



113-7 km. 



0-0 km. 



United States 



9-52 



2-92 



2-91 



2-98 



n 



3-88 



1 



12-73 



2-69 



2-76 



2-80 



2-99 



2 



8-14 



216 



2-25 



2-27 



3-64 



3 



4-72 



3-36 



3-36 



3-36 



3-43 



4 



4-67 



2-52 



2-48 



2-48 



2-88 



5 



4-66 



3-71 



3-73 



3-74 



4-22 



6 



4-89 



2-39 



2-40 



2-40 



2-50 



7 



15-70 



2-15 



2-17 



2-20 



4-64 



8 



10-31 



3-92 



3-57 



3-51 



4-19 



9 



10-67 



2-73 



2-93 



2-96 



4-62 



10 



1609 



3-99 



3-91 



3-91 



6-48 



11 



8-42 



2-84 



2-84 



2-85 



3-05 



12 



11-82 



209 



2-17 



2-19 



2-78 



13 



16-12 



3-25 



3-23 



4-30 



4-62 



14 



10-34 



3-64 



3-37 



3-33 



5-13" 



Solution B is the hypothesis of infinite depth to com- 

 pensation, that is, the density is assumed horizontally 

 uniform in the crust giving no isostasy. Solution A 



19 See J. F. Hayf ord, The figure of the earth and isostasy from measure- 

 ments in the United States, 32, 1909. 



20 J. F. Hayf ord, Supplementary investigation, 56, 1910. 



