438 WILLIAM BOWIE 



It would not be a very difficult matter to draw curves represent- 

 ing different methods of distribution of compensation which would 

 have effective depths of the compensation equivalent to those shown 

 above for uniform distribution. If we have the effective center of 

 compensation about what it is for the uniform distribution, then, 

 under any method of distribution of compensation, the greater 

 portion of the compensation would be between the sea-level surface 

 and about ioo km. 



It may be concluded from a study of the gravity data and also 

 of the deflection of the vertical data that there is no geodetic evi- 

 dence which favors any particular method of vertical distribution of 

 compensation. Anyone is therefore free to use a method of dis- 

 tribution which best serves his purpose or which may fit the par- 

 ticular theory he may hold in regard to the constitution of the 

 earth's lithosphere. But, in order to secure results which are as 

 accordant as those given in the latest report of the Survey, the effec- 

 tive depth of compensation must be between 30 and 50 km. 



It was noticed early in the investigations of gravity and isostasy 

 that there were apparently some relations between the gravity 

 anomalies and the densities of the materials at the surface of the 

 earth close to the station. This subject was treated briefly in 

 Special Publications Nos. 10 and 12, which gave the results of the 

 earlier investigations of gravity and isostasy. With the additional 

 material available from other countries as well as from the United 

 States for the most recent investigations of gravity and isostasy, 

 these relations between the gravity anomaly and the surface density 

 are shown to be stronger. 



In the United States the stations on the dense rock which 

 belongs to the pre-Cambrian formation have anomalies which tend 

 strongly to be positive. This is an indication that under the station 

 the material of this formation extends to a considerable depth where 

 the gravity anomaly is large. 



It was found that the stations on the pre-Cambrian formation 

 in Canada did not have the tendency to be positive that was shown 

 in the United States. This may be due to the extensive areas 

 covered by this formation in Canada. As was stated above, the 

 attraction of a disk of material of indefinite extent is independent 



