290 JOSEPH BARRELL 



whether the stresses represented by the incompleteness of isostasy 

 are carried by the rigidity of the outer crust, or are transferred 

 in some measure to the deeper body of the earth; third, the magni- 

 tudes of the stresses, measured in terms of loads, which are indi- 

 cated by the gravity anomalies and deflection residuals. 



It is found in answer that under the h3^othesis which forms 

 the basis of Hayford's work, that of uniform compensation, com- 

 plete at a given depth, there are indications, given by comparing 

 different areas, of a great range in the depth of the bottom. Under 

 an assumption which is probably nearer to nature — that is, the 

 hypothesis of a variable and gradually disappearing compensa- 

 tion — there is room for even a greater heterogeneity of the crust 

 and a greater Variability in the depth reached by the zone of 

 compensation. But, on the other hand, it is concluded that the 

 zone of compensation, as an outer rigid crust separated from the 

 rigid inner earth by an intervening zone of lowered rigidity, is a 

 reality in earth structure. The stresses due to the heterogeneities 

 of density and relief within and upon this crust appear to be borne 

 by the crust, not by the inner earth. Under the third subject it 

 appears, upon review of the evidence given by the deflection 

 residuals, that these may be interpreted so as to show departures 

 from equilibrium comparable to the results given by the gravity 

 anomalies, instead of the 250 feet which Hayford thought to exist. 

 The two independent lines of geodetic investigation are thus seen 

 to agree and it may be concluded with some confidence that the 

 individual isostatic regions of the United States are on the average 

 between 600 and 900 feet out of equilibrium. Evidence from other 

 parts of the world appears to show, furthermore, that a number of 

 regions exhibit greater departures from isostasy than those observed 

 within the United States. The strain imposed on the crust by the 

 Niger Delta, though large, is apparently not as large as some made 

 known by geodetic measurements. 



Thus from various directions of attack the crust is shown to be 

 an earth shell of high rigidity and consequently high elasticity. 

 Geodetic evidence justifies the view, brought forward by geologic 

 evidence, that the delta of the Niger is to be looked upon as sup- 

 ported by the strength of the crust. 



