THE STRENGTH OF THE EARTH'S CRUST 151 



A distribution suggested by Chamberlin, of compensation greatest 

 a little below the surface and diminishing to nothing at 178. 6 miles, 

 is also about as probable. Hayford therefore does not claim that 

 his geodetic studies determine with precision the nature or depth of 

 the distribution of compensation. The figure of 76 miles should 

 therefore be used always with this reservation. 



The residuals were classified into fourteen geographic groups. 

 The most probable depths of compensation indicated for the several 

 groups range from 66 to 305 km. According to Hayford, the 

 evidence from these groups is, however, so weak and conflicting that 

 he sees no indication that the depth of compensation is not constant 

 over the whole area investigated.^ He notes that, so far as the 

 evidence goes, it indicates the depth of compensation to be greater 

 in the eastern and central portions of the United States than in the 

 western portion.^ The subject is one which will be taken up later 

 in the discussion of geodetic results. 



In regard to the completeness of compensation, Hayford states: 



From the evidence it is safe to conclude that the isostatic compensation is 

 so nearly complete on an average that the deflections of the vertical are thereby 

 reduced to less than one-tenth of the mean values which they would have if no 

 isostatic compensation existed. One may properly characterize the isostatic 

 compensation as departing on an average less than one-tenth from completeness 

 or perfection. The average elevation of the United States above mean sea-level 

 being about 2,500 feet, this average departure of less than one-tenth part from 

 complete compensation corresponds to excesses or deficiencies of mass repre- 

 sented by a stratum only 250 feet (76 meters) thick on an average.^ 



It is not intended to assert that every minute topographic feature, such, for 

 example, as a hill covering a single square mUe, is separately compensated. It 

 is believed that the larger topographic features are compensated. It is an 

 interesting and important problem for future study to determine the maximum 

 size, in the horizontal sense, which a topographic feature may have and still not 

 have beneath it an approximation to complete isostatic compensation. It is 

 certain, from the results of this investigation, that the continent as a whole is 

 closely compensated, and that areas as large as states are also compensated. 

 It is the writer's belief that each area as large as one degree square is generally 

 largely compensated. The writer predicts that future investigations will show 

 that the maximum horizontal extent which a topographic feature may have and 

 still escape compensation is between i square mile and i square degree. This 

 prediction is based, in part, upon a consideration of the mechanics of the 

 problem.'' 



^ 1909, pp. 55-59- * 1909. P- 59- 



^1906, pp. 143, 146. ■• 1906, p. 169. 



