THE STRENGTH OF THE EARTH'S CRUST 



227 



formations and plateaus of circumdenudation may not be found, 

 since it is clear that the anomaly from this cause may be much 

 more than neutralized by other causes. A large number of stations 

 covering broad areas would therefore be required adequately to 

 eliminate these other influences from the means. 



LARGE OUTSTANDING ANOMALIES NOT RELATED TO GEOLOGY OR 



TOPOGRAPHY 



In Fig. 5, of Part II, the anomalies are shown for all stations 

 in the United States. It is seen that they possess an areal grada- 

 tion in magnitude which permits the drawing of anomaly contours. 

 The excessive anomalies of both signs cover oval areas in various 

 parts of the country and show a common disregard of physio- 

 graphic provinces, structural provinces, and geologic formations. 

 Looking at Fig. 5, one cannot see in either the distribution of 

 anomalies or trends of contours a reflection of Atlantic Coastal 

 Plain, or Appalachian Mountains, or Mississippi Valley. 



Typical examples of the lack of necessary relation of the large 

 anomalies to geologic formations are seen in the following tabula- 

 tion: 



TABLE XVI 



The lack of relation of these anomalies to topography is equally 

 striking. It is clear then that internal conditions in the crust, not 

 expressed on its surface, must be the principal cause of these larger 

 departures from isostasy. The large anomalies show their relation- 

 ship to internal causes most clearly, but the smaller anomalies may 

 also by analogy be ascribed in part to such hidden causes. The 

 results, however, of surface activities — -circumdenudation, sedi- 

 mentation, tangential pressure, or extravasation — -must show in 

 large ratio over regions where the internal variations from uniform 

 density are small; but over the greater part of the United States 



