274 W. BOWIE THEORY OF ISOSTASY 



I shall attempt to show what has been accomplished by the isostatic 

 investigations. I shall also outline some conclusions of a geological na- 

 ture which seem to be justified. 



ExTEXT OF ISOSTATIC INVESTIGATIONS 



The geodetic data on which the investigations of isostasy have been 

 based are confined largely to the areas of the United States. Canada, and 

 India. Investigations are now under way by geodetic organizations in 

 other countries, and it is hoped that the results may soon appear, in order 

 that we may learn whether isostasy obtains in those countries. At pres- 

 ent, it may be said we are extrapolating in regard to the extent to which 

 the condition of isostasy obtains throughout the world. In the first place, 

 less than three-tenths of the surface of the earth is land area, and the 

 United States, India, and the part of Canada where gravity observations 

 have been made comprise only about 10 or 12 per cent of the land surface. 

 It will be seen, therefore, that our investigations in isostasy represent 

 only about 3 per cent of the entire surface of the earth. 



While it is true that the isostatic investigations have been confined to 

 rather limited areas, we have within the areas in question a great variety 

 of topography and of geological formations. 



Relatiox of Gravity Axomalies to Topography 



I think it can safely be said that there is no relation whatever between 

 the character of the topography and the residual deflections of the ver- 

 tical or the gravity anomalies. This means that the theory of isostasy. 

 as applied in the investigations, is applicable to areas having various 

 elevations. Some of the deflection and gravity stations are at sealevel. 

 with no elevation ; others are on the highest mountains, while many are 

 on the intermediate plains and plateaus. When we plot the gravity 

 anomalies and draw curves of equal anomaly, it is impossible from these 

 curves to detect the high and low ground. This condition is in striking 

 contrast to that which obtains with the gravity anomalies based on the 

 rigid-earth theory. When the earth's crust is considered to be rigid, with 

 masses above sealevel as extra loads on the earth's crust, and the defi- 

 ciency of mass in the ocean an underload, we get anomalies which are 

 very much larger than those by the isostatic theory. The anomalies on 

 the theory of a rigid earth are due to the isostatic compensation which is 

 ignored in the reductions. With a gravity map drawn from these anom- 

 alies, it is possible to detect the areas where the ground is mountainous 

 and where it is flat. 



