THE STRENGTH OF THE EARTH'S CRUST 303 



producing the greater disturbances have much smaller size. This 

 is especially striking in the case of the largest negative anomaly 

 in the United States, that at Seattle, only 50 miles from the large 

 positive anomaly at Olympia. The latter is surrounded on all 

 sides by negative anomalies as follows: 



DISTANCES FROM OLYMPIA, WASHINGTON 



Astoria, Ore 76 miles S.W. — . 013 dyne anomaly 



Heppner, Ore 195 " S.E. —.027 " " 



Skyhomish, Wash. . 84 " N.E. -.028 " 



Seattle, Wash 50 " N.N.E. -.093 " 



The excess of mass which exists in the vicinity of Olympia, above 

 that required for compensation under solution G, must therefore 

 be much less than 166.7 km. (102.5 miles) in radius. The same 

 is doubtless true of that excessive deficiency which exists at Seattle, 

 since the anomaly sinks to less than one-third the value at Sky- 

 homish only 45 miles east, and changes to a large positive anomaly 

 at Olympia, 50 miles south-southwest. 



The large positive mass in the vicinity of Olympia must dimin- 

 ish appreciably the effect of the still larger negative mass in the 

 vicinity of Seattle. The latter with the other surrounding negative 

 masses must diminish still more the anomaly due to the positive 

 mass at Olympia. Furthermore, it is highly improbable that the 

 observations at Seattle should happen to be made at the point of 

 really maximum anomaly. Let the very moderate assumption be 

 made then that the abnormal Seattle mass as a unit by itself 

 would give a maximum anomaly of — o.ioo dyne. It would 

 doubtless give more. Let limiting assumptions be made as to the 

 dimensions and density of this mass such that the actual volume 

 and density are quite probably embraced somewhere within these 

 hmits. Tables XXI^ and XXII show the results of such assump- 



^ Table XXI is readily derived from Table X, Part III. Take, for example, 

 the cylinder of radius 1,280 meters, depth of 1,000 feet, and density o. 267. Multiply 

 its dimensions by 30 and the volume of each unit portion will be increased by the cube 

 of 30. The attraction of each unit of mass on the given point will vary inversely 

 with the square of the distance and will therefore be diminished by the square of 30. 

 The anomaly will consequently increase directly with the dimensions, provided that 

 the density remains constant. This gives the basis for the calculations in column 2, 

 Table XXI. 



