THE STRENGTH OF THE EARTH'S CRUST 549 



approximations, not as a conclusion. The anomalies can be 

 accounted for just as readily by an assumption of much shallower 

 depths. The outstanding masses would then possess marked thin- 

 ness in comparison with their breadth. They would be hori- 

 zontally extended masses or the algebraic sum of many masses; in 

 either case they could lie within a quarter of the depth indicated by 

 the assumption of spherical form. The mere measurement of the 

 mean distance from epicenter to half value of Fv is therefore wholly 

 indeterminate except as regards the limits of regional compensation. 

 In some respects, however, the present map does give suggestions. 

 Let the attention be turned to these individual features. 



A line of stations extends along the margin of the Coastal Plain 

 from Washington, D.C., to Hoboken, New Jersey. The anomalies 

 at the stations and their distances apart as measured on the map 

 are as follows: 



No. 22. Washington, D.C., -I-0.039 dyne 



58 km. 

 No. 23. Baltimore, Md., — o.oii 



138 km. 

 No. 24. Philadelphia, Pa., 4-0.022 



61 km. 

 No. 25. Princeton, N.J. , —0.019 



69 km. 

 No. 26. Hoboken, N.J. , -|-o. 024 



This line of stations extends in the direction of the trend of the 

 foundation rocks, yet the sign is reversed at every station, showing 

 marked heterogeneity even in the direction of the strike. The 

 average anomaly without regard to sign is 0.023, ^ little larger than 

 the average for the whole United States. The average with 

 respect to sign is +0.011. As there is only one station for each 

 of the positive and negative masses the positions and magnitudes 

 of the real maxima and the curves of changing anomaly are unknown, 

 Masses in the upper half of the zone of compensation could produce 

 these effects at these horizontal distances with but little mutual 

 neutralization. Masses below the zone of compensation would, 

 however, have to be very great, not only because the force decreases 

 inversely with the square of the distance, but because masses of oppo- 

 site sign whose centers are more than 120 km. deep and situated but 



