THE STRENGTH OF THE EARTH'S CRUST 547 



In view of this analysis of the data given in Fig. 12 and in Table 

 XXVIII, it is to be concluded that for this region even the larger 

 outstanding masses from Solution H, capable of exerting a notable 

 influence on the Fx component of the deflections to a distance 

 of 400 km. or more, appear to have their centers not deeper than 

 20-25 km. Their mass is consequently within the outer half or 

 even the other third of the zone of isostatic compensation as given 

 by Solution H. There is further no evidence of centrospheric 

 heterogeneity. 



OTHER INDICATIONS REGARDING DEPTH OF OUTSTANDING MASSES 



Deflections by linear or dike-like masses. — The resultants of the 

 deflection residuals are shown by broken lines in Fig. 12. They 

 show a tendency to converge toward centers. This is true in general 

 for the whole United States, as shown in Hayford's illustration. 

 This tendency to convergence indicates that the dominating out- 

 standing masses may usually be regarded in a first or second approxi- 

 mation as symmetrical with respect to a vertical axis. In contrast, 

 however, to this rule the residuals of Solution H in the vicinity of 

 Washington^ indicate an outstanding mass with a northeast-south- 

 west extension of at least 120 km., whereas the breadth is probably 

 not more than 20 km. This narrowness is shown by the limited 

 distance between the large residuals of opposite sign in a northwest- 

 southeast direction. The linear extension is shown by the parallel 

 rather than radial arrangement of the resultants. The mass gives 

 rise to large deflections for a distance of as great as 100 km. from 

 the sides, but its influence dies out somewhat beyond. 



It is clear from these relations that the assumption of a form of 

 the mass S3niimetrical about a vertical axis for the purpose of deter- 

 mining the depth of the center would not be justifiable. Other 

 assumptions which might be made in order to subject the mass to 

 mathematical investigation would be to consider it as a horizontal 

 prolate spheroid, or as a horizontal linear mass at a certain depth, 

 cylindriform, or as a vertical plate. The latter would be preferable. 

 For a quantitative solution of its dimensions and mass it would 

 be desirable to have some observations farther to the northwest. 



'Hayford, Supplementary Paper, illustration No. 4. 



