Bowie — Gravity Anomalies and Geologic Formations. 239 



two formations is so small that no definite conclusions can be 

 safely drawn from them. It is not necessary to consider the 

 unclassified anomalies. 



It may appear obvious that this relation between the oldest 

 and newest formations and the anomalies is what should have 

 been expected inasmuch as the densities of the oldest rocks 

 are, on the average, greater and those of the recent rocks 

 lesser than the average surface density, 2*67, used in the com- 

 putations. But it will appear, on reflection, that after all these 

 cannot be mere surface phenomena. 



Let it be assumed that the pressure at the depth of 114 kilo- 

 meters under a Precambrian station is normal ; that is, the crust 

 is in a state of perfect isostasy, and that the average anomaly 

 with regard to sign of 4- '016 is caused by an excess in density of 

 the geologic formation just under the station. Then if the for- 

 mation considered extends 1,300 meters out from the station and 

 1,000 feet below it, this material would have an increase in den- 

 sity of P47. With the same radius but with a depth of 5,000 

 feet, the material would have an increase in density of 0*48. 

 With 10,000 feet, there would be an increase of *40. 



If it is assumed that the formation considered extends 19 

 kilometers around the station and 1,000 feet down, then an 

 increase in density of 1*37 would be necessary to cause an 

 anomaly of +'016. With a depth of 10,000 feet, the increase 

 would be -13. 



The maximum anomaly in the Precambrian formation is 

 + •054 dyne, therefore the increases in density above the normal 

 would be three and one-half times those previously stated. If 

 this formation is assumed to extend 19 kilometers from the 

 station and to a depth of 10,000 feet, the increase in density 

 would have to be - 45 greater than the normal. 



The mean with regard to sign of the anomalies in the Ceno- 

 zoic formation is —'008, and the changes in density necessary 

 to cause this anomaly are one-half the size and of the opposite 

 sign of the changes necessary to cause the mean Precambrian 

 anomaly with regard to sign of -f*016. The maximum anom- 

 aly in the Cenozoic is —'091, and to cause this a decrease in 

 density of *80 is necessary in the material extending 19 kilo- 

 meters from the station and to a depth of 10,000 feet. 



Now let it be assumed that there is normal pressure under 

 a station at the depth of 114 kilometers, and the density of the 

 surface geology is also normal, and that an anomaly is caused 

 by an excess of density in a stratum of material 5,000 feet in 

 thickness, 10,000 feet below the station, which extends horizon- 

 tally 19 kilometers from the station. Under these conditions 

 an increase of *32 in the normal density would be necessary to 

 cause an anomaly of +*016. 



