164 JOSEPH BARRELL 



regular in outline. The areas showing positive anomahes of more 

 than 0.020 dyne were estimated roughly to average 130 by 240 

 miles across, a mean diameter of 175 miles. The areas showing 

 negative anomalies of more than 0.020 dyne were found to average 

 roughly about 190 by 250 miles, a mean diameter of 220 miles. The 

 long narrow connections were neglected in making this estimate. 

 Unit areas of more than mean anomaly may therefore be taken to 

 average about 200 miles or 320 km. in diameter. The mean radius 

 is therefore approximately that of the outer limits of zone O, 

 166.7 km. 



The figures, although they correspond fairly closely to those 

 derived from the deflections of the vertical, cannot in reality be very 

 well compared, since these are areas selected because the anomaly 

 rises above a certain magnitude; the others represent, on the con- 

 trary, a succession of contiguous areas between centers of excess 

 and defect in mass without reference to magnitude. Apparently 

 some influence blurs out the limitations of areas of small gravity 

 anomaly. This will be discussed in a later part. 



Now assume for the moment that isostatic compensation is uni- 

 form to> the bottom of the zone, as postulated by the hypothesis; 

 that is, that the residuals and anomalies are due to excesses or 

 defects of mass which are uniformly distributed. Then, over any 

 one area of excess or deficiency of mass, the deflections around it 

 and anomalies within it signify a departure from compensation in 

 one direction. This is a regional departure. If the strength of the 

 crust was so small that it was able to support notable departures 

 from compensation over areas of only one square degree or less, 

 then these large unit areas could not exist. A vertical warping up 

 or down would immediately take place until the broad region as a 

 whole lay so close to complete compensation that its surface 

 irregularities became subdivided into subordinate positive and 

 negative areas of the limiting size. The simi of the excesses and 

 defects of mass would approach zero in broad areas containing 

 many unit departures. It would seem, therefore, that the geodetic 

 results shown in Fig. 5 , instead of indicating local compensation to 

 limits of less than one square degree, show on the contrary a ready 



