THE STRENGTH OF THE EARTH'S CRUST 297 



as great for the groups with deep compensation as for the United 

 States as a whole. That is, the groups with deep compensation, 

 instead of showing a leaning toward solution B show on the con- 

 trary more definitely that it is not true. The hypothesis of uniform 

 compensation complete at a certain depth appears to be more nearly 

 true for regions with deep compensation than for shallow com- 

 pensation. This does not mean, however, a lesser rigidity of the 

 crust for the regions with deep compensation, their high capacity 

 to carry strain being shown by the large gravity anomalies which 

 are found in places within them. 



There seems to be no evidence, however, that the zone of 

 diminished rigidity is sharply bounded or is marked by real liquidity. 

 It is doubtless due to the gradual rise of temperature with depth, 

 overcoming within a certain zone the influence of the increasing 

 pressure. Seismologic and tidal evidences show, furthermore, that 

 under stresses of relatively brief duration the earth acts as a unit 

 and as an elastic rigid body. The physical condition of the zone of 

 low rigidity may approach that of a highly viscous fluid, the time 

 element thus entering within these limits as a fundamental factor. 

 This zone is incapable of bearing pronounced strains for long 

 periods in the manner of the zone above. In geologic operations 

 it thus serves to separate the mode of expression of forces gen- 

 erated below from those originating above this level. The former 

 give rise to the great compressive movements in the outer zone, 

 the latter to the vertical movements not determined by tangential 

 compression. 



INTERPRETATION OF DEFLECTION RESIDUALS IN TERMS OF MASSES 



On p. 59, paper of 1909, Hayford shows that the actual deflec- 

 tions of the vertical average only one-tenth of what they would be 

 if the continent and the portions of the ocean basins which were 

 included in the calculations were both underlain by matter of the 

 same density and the relief sustained wholly by the rigidity of the 

 crust. The effect of the topography calculated on this assumption 

 — that the density is uniform and the larger as well as the smaller 

 features are sustained by rigidity — -gives what is known as the 

 topographic deflections. These, as stated above, average ten times 



