ISOSTATIC GRAVITY ANOMALIES 



275 



Relation of isostatic Gravity Anomalies to geologic Formation 



While there is no apparent relation between the character of the topog- 

 raphy and the isostatic anomalies, there is a decided relation between 

 certain geologic formations and the isostatic gravity anomalies. It was 

 early found in the investigations of gravity and isostasy that there ap- 

 peared to be definite relations between the gravity anomalies and the 

 Precambrian and the Cenozoic formations. This relation, first noticed 

 at the beginning of the investigations of gravity and isostasy by Hayford 

 and the speaker, has proved to exist to a very marked degree. 



There are now in the United States, Canada, and India 131 gravity 

 stations located on the Cenozoic formation. The average value of the 

 anomaly, without regard to sign, is 0.024 dyne, while the average for all 

 of the stations located in the United States, Canada, and India is 0.021 

 dyne. The mean anomaly, with regard to sign, is — 0.015 dyne, while 

 for all of the gravity stations in the areas in question the mean with re- 

 gard to sign is — 0.006 dyne. Of the Cenozoic stations under consider- 

 ation, 87 have negative anomalies and only 42 have positive anomalies. 



With regard to the Precambrian stations, we have 38 in the United 

 States, Canada, and India. The mean anomaly without regard to sign is 

 0.022 dyne, while the mean with regard to sign is -(-0.006 dyne. Of the 

 38 Precambrian stations, 27 have positive anomalies and only 11 have 

 negative anomalies. It was thought for some time, especially by the op- 

 ponents of the theory of isostasy, that the presence of the large anomalies 

 on the Cenozoic and Precambrian formations were indications of the 

 strength of the earth's crust, and that they indicated the magnitude of 

 the departures from a perfect isostatic state. 



The speaker, in his investigations, reached the conclusion that the Pre- 

 cambrian and the Cenozoic gravity anomalies did not indicate a departure 

 from the isostatic condition, but that the persistence in sign is an indica- 

 tion of the effect of the light material of the Cenozoic formation close to 

 the station, or of the extra heavy material of the Precambrian formation 

 close to the station, and that probably the columns under these formations 

 are normal in mass, the light material or heavy material being compen- 

 sated by material heavier or lighter than normal and lower in the isostatic 

 shell. 3 



This view has gained favor during the last few years and has received 

 very strong indorsement from Sir Sidney Burrard, former Surveyor- 



3 William Bowie : Investigations of gravity and isostasy. U. S. Coast and Geodetic 

 Survey Special Publication No. 40, p. 70. 



William Bowie : Our present knowledge of isostasy from geodetic evidence. Journal 

 of Geology, July-August, 1917, pp. 422-445. 



