1 62 JOSEPH BARRELL 



difference in the anomalies in this case is so nearly constant as to 

 give an indeterminate answer. In the absence of more detailed 

 statements by Hayford and Bowie the reason why the anomaly at 

 the Grand Canyon station 69 reaches a larger negative value for 

 regional compensation to 166.7 km. than for more limited com- 

 pensation is not evident. The usual rule is that the progressive 

 change in the anomaly for stations below the regional level for 

 successive assumptions of wider regional compensation is by 

 increments with a plus sign. Here, on the contrary, the change in 

 the limits from zone M to zone O involves a minus increment of 

 o.oio in the anomaly. The cause of this reversal of sign, which 

 the writer does not understand, seems in this case to be the cause of 

 the indeterminate result. 



Another line of evidence as to the effective limits over which 

 the rigidity of the earth may extend is derived from a study of the 

 grouping of the deflections of the vertical shown in illustrations 

 2,3,5,6, Hayford, 1909, and the lines of equal anomaly for the new 

 method of reduction, illustration No. 2, Bowie, 191 2, the latter 

 giving the basis for Fig. 5 of this article. 



The test by areas of grouped residuals. — Illustration No. 5, 

 Hayford, 1909, shows the grouping of the residuals of solution H for 

 the north and south components of the deflections. An area with 

 a plus sign corresponds to an excess of density to the south, or 

 deficiency to the north. An area with a minus sign corresponds to 

 a deficiency of density to the south, or excess to the north. A 

 north-south chain of stations is therefore best for ascertaining the 

 limits of the areas of north-south deflection of like sign. Such a 

 belt extends across the United States between long. 97° and 98°, 

 showing 9 areas covering 1,620 miles. The mean intercept is 

 therefore 180 miles. This mean intercept must be somewhat less 

 than the mean diameter. 



Illustration No. 6, Hayford, 1909, shows the grouping of the 

 residuals of solution H for the east and west components of the 

 deflections. An area with a plus sign corresponds to an excess of 

 density to the east, or deficiency to the west. An area with a 

 minus sign corresponds to a deficiency of density to the east, or 

 excess to the west. An east-west chain of stations is therefore best 



