THE STRENGTH OF THE EARTH'S CRUST 163 



for ascertaining the limits of the areas of like sign. Such a belt 

 extends across the United States between lat. 38° and 39°. 



The following adjustments in groups seem, however, fair to 

 make, considering the lack of exact accuracy in any one station. 

 At Cincinnati is a station showing 'small residuals opposite in sign 

 to the stations on each side. If this is overlooked, three small 

 groups become one of average size. In central Kansas a small 

 minus area depending on a single observation may be likewise 

 omitted. In western Colorado several small areas depending each 

 upon two observations had their number diminished by one. The 

 same was done in California. This gave 14 areas extending over 

 2,580 miles, a mean individual intercept of 184 miles. If 16 areas 

 be taken, a mean value is derived of 161 miles. More weight, it is 

 thought, is to be attached to the determination of 184 miles, and 

 this is supported by the 180 miles shown by the north-south chain 

 of stations. 



The areas of like sign are between centers of excess and defect of 

 mass. They are not, therefore, coincident with the areas of excess 

 and defect, but in discussing the average size of areas, the one may 

 be used as a measure of the other. 



It may be concluded, therefore, that the deflections of the verti- 

 cal show areas with departures from isostatic equilibrium in one 

 direction and these areas average about 180 miles, 290 km., in mean 

 intercept. The mean diameters of the areas of like sign are pre- 

 sumably somewhat greater. This would make the mean radius of 

 areas of regional compensation, as indicated by similarity of sign 

 among residuals, at least 166.7 ^^^i. — the radius of the outer limit 

 of zone O used in the discussion of the gravity anomalies. 



If we turn now to the anomalies shown by the determinations of 

 gravity, Fig. 5, adapted from Bowie, shows their segregation into 

 areas of like sign. The mean value without regard to sign for all 

 stations excluding Seattle is 0.018 dyne per gram. Including the 

 two Seattle stations the mean is 0.020 dyne. Between the con- 

 tours for —0.020 and -I-0.020 lie tracts where the anomalies are 

 within the mean limits. The areas of exceptionally large anomalies 

 are above those limits. It is only these which form on this illus- 

 tration well-defined inclosed areas, but even these are far from 



