240 Bowie — Gravity Anomalies and Geologic Formations. 



It is seen that, under the assumptions stated, large changes 

 in the normal densities are necessary, even in thick strata, to 

 account for anomalies of average size. On the other hand, 

 only small changes in density are necessary if it is assumed that 

 the anomalies are caused by a departure from the state of per- 

 fect isostasy in the earth's crust. An anomaly of +*016 would 

 be caused by a change of *02 from normal density in the 

 material of a column 19 kilometers in radius and 120 kilo- 

 meters in depth. The largest anomaly, "091, would be caused 

 by a change in density of only *12. 



If a column with a radius of 167 kilometers and depth of 

 114 kilometers be considered, then an increase in density of 

 only "005 is necessary to produce an anomaly of +'016 and a 

 decrease of only *03 in normal density is necessary to cause the 

 Seattle anomaly of —'091. 



In the same sized column the average Cenozoic anomaly 

 with regard to sign, +'008, would be caused by a decrease in 

 density of only *002. 



It appears to be improbable that the average sized anomalies 

 found in certain geologic formations are the result of greater 

 or smaller densities of the surface materials than that used in 

 the reductions, nor is it likely that the cause is an abnormal 

 density in a stratum of moderate thickness in the upper portion 

 of the crust. Such changes in density no doubt have some 

 effect in causing the anomalies, but it appears to be probable 

 that the principal cause of all except the smaller anomalies is an 

 actual departure from the condition of perfect isostasy in the 

 crust of the earth in the vicinity of the station. 



It is extremely interesting that the gravity anomalies, though 

 very small, show a relation to the geologic formation at the 

 earth's surface. 



Coast and Geodetic Survey, 

 Washington, D. C. 



