444 JOSEPH BARRELL 



distances from the epicenter. The curves of the deflection force 

 for these forms, especially the spheroidal forms, resemble somewhat 

 closely those given by deeper spheres of greater mass. If the out- 

 standing masses are in reality horizontally extended the interpre- 

 tation of the deflection residuals as due to spherical masses assigns 

 to their centers in consequence too great a depth. If, however, 

 the masses have the forms of vertical prolate spheroids or vertical 

 elongate cylinders, the interpretation as spheres will give too shallow 

 a depth. The ratios of the maximum anomalies to the maximum 

 deflections constitute a criterion to show whether the masses depart 

 from spheres by the spreading-out of their substance in a horizontal 

 plane or along a vertical axis. 



In Section B the outstanding masses are shown in some cases 

 to be horizontally extended in form and this is thought for the 

 larger masses to be a rather general relationship; that is, the verti- 

 cal thickness is much less than the length or breadth. Conse- 

 quently the interpretation as spheres gives maximum limits to the 

 depth. 



A general inspection of the geodetic data as well as a detailed 

 study of a certain test region shows that the smaller disturbing 

 masses have their centers in the outer third of the zone of com- 

 pensation; that is, within 40 km. of the surface. This result is 

 to be expected, since similar small masses at greater depth would 

 not exert a notable effect because their gravitative force varies 

 inversely with the square of the distance. But evidence of more 

 significance is found in regard to the larger centers of outstanding 

 mass not related to topography. These also are found, in so far 

 as they have been investigated, to be situated in the outer third 

 of the zone of compensation. Yet these masses are capable of 

 showing notable effects to distances of from 100 to 150 km. If 

 they were situated at any depth within the zone of compensation 

 they would, therefore, betray both their existence and their depth. 

 The greater departures from isostasy appear, therefore, to be really 

 absent from the deeper parts of the lithosphere. 



Centrospheric heterogeneity, if present, would require greater 

 masses in order to show surface effects. But no such effects are 

 noted. In so far as they may be existent, they are largely masked 



