THE STRENGTH OF THE EARTH'S CRUST 233 



same column are rare. Furthermore, it has been shown under 

 the topic "The Variable Rate of Compensation upon Gravity 

 Anomalies" that a variable distribution of balanced densities 

 has more effect if in areas of between 100 and 200 km. radius and 

 has but little effect on anomalies if the balanced densities extend 

 over much larger areas. 



As to the aggregate effects produced upon Fh, giving deflection 

 residuals, by these sheets I and III: If the sheets have like sign 

 the deflection force, as shown in Fig. 7, will die out somewhat 

 gradually and extend to considerable distances. If they have 

 unlike sign the deflection force will fall off sharply between 0.25 

 and 1. 00. If, however, the abnormalities of density should dis- 

 appear gradually, that is, if the sheets did not terminate sharply 

 at o, this rate of falling off would be slower. Reversals of sign 

 of the deflection residuals would require areal, not vertical, irregu- 

 larities of mass. They could not take place as an effect of dis- 

 tance from a single mass or of two masses of unlike sign and vertically 

 over each other. Where sharp reversals of sign take place in the 

 deflection residuals the presence of areally contiguous areas of 

 unlike departures in mass is shown. A mere difference in magni- 

 tude of excess of mass but of the same sign may, however, produce 

 changes in the sign of the deflection residuals. In the irregular 

 areal distribution of abnormal masses not balanced by being over 

 each other, the deflection areas of Hke sign would thus tend to be 

 smaller than the anomaly areas of Hke sign. A gradual fading-out 

 of the deflection residuals would be the mark of gradual fading-out 

 of the abnormal mass or the increasing influence of distant masses. 



Various special combinations of three or more masses could at 

 any one point simulate the relations indicated, but such special 

 relations would not be of common occurrence and could not give 

 a generality of relation of this sort. 



There have thus been drawn up a set of criteria by which 

 balanced irregularities within the zone of compensation may be 

 distinguished from regional departures from isostasy. It remains 

 to apply those to the areal distribution of gravity anomahes and 

 deflection residuals as given by Hayford and Bowie. It must be 

 recognized, however, that the stations, although numerous as 



