234 JOSEPH BARRELL 



compared to previous measurements, are yet very scattered for 

 the precise application of these tests and can at best give but 

 qualitative results. It is thought, nevertheless, that the general 

 nature of the answer is determinative. 



GRAVITY ANOMALIES CAUSED LARGELY BY REGIONAL DEPARTURES 



FROM ISOSTASY 



The first question is: To what degree do the areas of excess 

 (or deficiency) of mass as indicated by gravity anomalies coincide 

 with areas of excess (or deficiency) as shown by the deflection 

 residuals ? In Fig. 5^ there are indicated a number of ovals shown 

 in dot-and-dash outHne and marked + or — . These are the 

 definitely bounded areas of excess or deficiency of mass indicated 

 by the deflection residuals. The entire surface of the crust must 

 be constituted of such areas, but only a few are surrounded by 

 sufficient observations to permit a boundary to be drawn at present. 

 Even this boundary must not be regarded as sharply definite. 

 Beside these ovals there are shown in illustrations 5 and 6, Hayford, 

 1909, areas of residuals characterized by like sign, referred to in 

 the present paper as "areas of grouped residuals." They are not 

 definitely bounded on all sides and are not shown in Fig. 5 of this 

 article. The areas of grouped residuals show the intercepts across 

 areas of like sign, but at least two intercepts at an angle to each 

 other are necessary to define well the limits of the area of which 

 they are a part. As the deflection stations are situated largely 

 in lines or zones across the country and not surrounding the areas 

 of like sign, it is seen why the boundaries of relatively few areas 

 are well determined. In so far, however, as the relations of the 

 areas of positive and negative anomaly to positive and negative 

 deflections of the vertical are apparent, Hayford and Bowie state: 

 ''The gravity anomalies corroborate the evidence given by the 

 deflections. In no important case are the anomalies and deflections 

 contradictory. "- 



It is seen by inspection of the illustrations by Hayford, and 

 also by the discussion in Part II of this article, that the areas of 



I P. 153, Part II. 



^ Hayford and Bowie, 191 2, p. 112. 



