2i6 JOSEPH BARRELL 



1° F. for 60 ft. in depth, gives an aggregate expansion of 3 . 6 per 

 cent to the outer 76 miles. Using 6,500,000 as the modulus of 

 cubic compressibility of the average rock in pound-inch units^ 

 gives a total compression of 3 . 7 per cent to the outer 76 miles due 

 to pressure; that is, the volume effects of heat and pressure prac- 

 tically offset each other within the zone of isostatic compensation. 

 Therefore 2 . 80 appears to be the lowest mean figure which should 

 be taken. The use of 2.67 as a mean figure requires for isostatic 

 equilibrium a density of but 2 . 60 extending to a depth of 76 miles 

 under land 3 km. high, a figure lower than the specific gravity of 

 granite. 



INTERPRETATION OF ANOMALIES IN TERMS OF MASS AND DEPTH 



Suppose that the zone of isostatic compensation is not ot 

 uniform density under any one station, but contains masses of 

 variable density irregularly distributed. Let these masses be of 

 considerable thickness and area as compared to the depth of the 

 zone of compensation. Suppose that the topography is so adjusted 

 to the aggregate density that the pressures are everywhere equal 

 at the bottom of the zone of compensation. Abnormally light 

 masses would then have to be balanced by abnormally heavy 

 masses in the same column. There would still be deflections of 

 the vertical and anomalies of gravity because gravitation varies 

 inversely with the square of the distance, the upper and adjacent 

 masses of abnormal density affecting the station more than those 

 more distant ones of opposite abnormality lying vertically below 

 the upper. The residuals from deflection and gravity measure- 

 ments would under such an arrangement measure strains within 

 the outer crust but not upon its bottom. The strains, if produced 

 by abnormalities in the upper parts of the crust, would further be 

 proportionately smaller and yet give rise to residuals of a certain 

 magnitude than if produced by abnormalities in the lower parts 

 of the crust. This aspect of the problem must be investigated 

 before any final significance regarding the strength of the crust 

 can be attached to the grouping of residuals discussed under the 



I F. D. Adams and E. G. Coker, An Investigation into the Elastic Constants of 

 Rocks, More Especially ivith Reference to Cubic Compressibility, 1906, p. 67. 



