Bowie — Gravity Anomalies and Geologic Formations. 237 



Art. XXI. — Some Relations between Gravity Anomalies 

 and the Geologic Formation in the United States ; by 

 William Bowie.* 



In the report of an investigation of the effect of topography 

 and isostatic compensation on the intensity of gravity, which will 

 soon appear as a Coast and Geodetic Survey publication, there 

 are shown certain relations between the signs of the gravity 

 anomalies and the geologic formations on which are located the 

 89 gravity stations considered. There are now available 35 

 additional stations, making 124 in all, and it is proposed in this 

 paper to give the results of an additional study of the relations 

 between the anomalies and the surface geologic formations, 

 which confirm the earlier unpublished conclusions. 



The anomaly of gravity, which is used in this paper, is the 

 difference between the observed gravity and the theoretical 

 gravity for the latitude of the station, corrected for elevation, 

 and for the effect of the topography and its isostatic compen- 

 sation. The isostatic compensation is assumed to be complete 

 under every separate portion of the earth's surface. If the 

 surface density at the station is 2*67, which was that used in 

 the computations, and the densities of the successive layers in 

 the earth's crust are normal (except as they may be affected by 

 the compensation), and if the isostatic compensation of the 

 topography is complete and uniformly distributed from the 

 surface to a depth of 114 kilometers, then the anomaly will be 

 zero. It is assumed that every part of the surface at a depth 

 of 114 kilometers below sea level has equal pressure at all 

 points and, consequently, is in a state of hydrostatic equili- 

 brium. An anomaly at a station, different from zero, shows a 

 departure from normal conditions. If the anomaly is positive, 

 the observed is greater than the theoretical gravity ; and the 

 reverse is the case for negative anomalies. 



The computations are based upon the theory of complete 

 compensation of the separate topographic features ; the com- 

 pensation being uniformly distributed to a depth of 114 kilo- 

 meters. It is reasonably certain that each topographic feature 

 is not completely compensated locally, and that the compen- 

 sation is not distributed uniformly to a depth of 114 kilometers. 

 But the gravity operations prove that the average condition 

 over areas of large extent approximate very closely that of 

 complete local isostatic compensation, uniformly distributed to 

 a depth of 114 kilometers. They show that the United States 



*Read before the Geological Society of America, at Washington, D. C, 

 December 30, 1911, by William Bowie, Inspector of Geodetic Work, U. S. 

 Coast and Geodetic Survey. 



