302 JOSEPH BARRELL 



MAXIMUM LOADS INDICATED BY ANOMALIES 



Hayford and Bowie consider that 0.0030 dyne of anomaly may 

 be regarded as equivalent to 100 ft. of rock possessing a density of 

 2 . 67. From the previous considerations it would seem that this is 

 probably too high for a mean figure, but may apply to certain areas, 

 especially those with extremely broad boundaries. In other regions 

 0.003 "^^y be far too high, since it is shown under the topic "Vari- 

 able or Constant Depth of Compensation" that in certain parts of 

 the United States the depth of the zone of compensation probably 

 goes notably deeper than in other parts and the density may be 

 distributed either nearly uniformly or with considerable irregularity. 

 The greatest depth of compensation indicated for any region is 305 

 km. A unit thickness of mass uniformly distributed to this depth 

 and to a radius of 166.7 km. would give but 0.0014 of anomaly 

 instead of o . 0024 as given by a depth of 1 14 km. , or o . 0030 as taken 

 by Hayford and Bowie. For general use 0.0024 dyne is perhaps 

 the best value, corresponding to a uniform distribution of a unit 

 excess or defect of mass to a depth of 114 km. and to a radial dis- 

 tance of 166.7km. For the mean anomaly of 0.018 this would 

 give 750 ft. of elevation as the mean departure of the surface of the 

 United States above or below the position giving isostatic equi- 

 librium, instead of 600, or more exactly, 630 ft. as taken by Bowie. 

 The largest known anomaly in the United States is at Seattle, 

 —0.093. This corresponds to a defect in mass equivalent to a 

 stratum 4,000 ft. thick if the divisor is 0.0024, a stratum 3,200 

 ft. thick if the divisor is 0.0030. At Olympia, but 50 miles or 80 

 km. distant, the anomaly is +0.033, corresponding to excesses of 

 mass of 1,375 or 1,100 ft., according to the divisor. The difference 

 of regional load between Olympia and Seattle becomes 5,375 or 

 4.300 ft. 



But these relations of unit thickness of mass to the gravity 

 anomaly are based on the assumption that the excess or deficiency 

 of mass extends to as great a radial distance as 166.7 ^^i. radius. 

 This minimizes the thicknesses or densities needed to account for 

 the anomalies above what would be required for a more local 

 concentration of mass. But an inspection of the distribution of 

 gravity and deflection residuals shows that in many cases the masses 



