2i8 JOSEPH BARRELL 



mean departure from isostatic compensation amounting to 600 ft. ; 

 given more exactly by Bowie as 630 ft. 



It is seen from the quoted statement that the authors accept, 

 first, as one alternative a very widespread regional net excess (or 

 deficiency) of mass uniformly distributed in depth; or, second, a 

 somewhat broad regional distribution but confined to the outer 

 part of the zone of compensation; or, third, some combination of 

 the two assumptions. 



The first assumption would throw a real strain upon the bottom 

 of the zone of compensation and signifies regional compensation to 

 limits very far beyond those stated elsewhere by the authors. It 

 is therefore inconsistent from that standpoint, but gives a smaller 

 vertical load and consequently a smaller vertical departure from 

 the level giving isostatic equilibrium than would a more limited 

 area. If, for example, it be assumed that the radius of the zone 

 limiting regional compensation is 58.8km., which is about the 

 maximum limit for regional compensation which Hayford allows 

 elsewhere; then it may be computed that for uniform distribution 

 of the excess (or deficiency) of mass to a depth of 114 km., a mass 

 equivalent to 100 ft. of density 2.67 corresponds to an anomaly 

 of but 0.0013 dyne instead of 0.0030. This would, for a mean 

 anomaly of 0.018, signify an average departure over the United 

 States of 1,380 ft. from the level giving isostatic equilibrium, instead 

 of 600 ft. 



The second assumption, that the excess (or deficiency) is in the 

 outer part of the crust, gives also a much higher anomaly for a 

 unit mass than would an equally permissible assumption that the 

 excesses or deficiencies occurred at various levels and on the average 

 were at a depth of one-third or one-half of the zone of compensation. 

 The relationship of anomalies to geologic formations, to be dis- 

 cussed later, shows certain variations in density in the outer crust, 

 but the greater parts of the anomalies are not due to this cause. 

 From the previous discussion on the limits of regional compensa- 

 tion it would seem that, on the assumption that the excesses or 

 deficiencies of mass are on the whole uniformly distributed, 0.0024 

 would be an appropriate figure to use as the mean anomaly for 

 unit thickness of mass. The highest anomalies, however, are 



