446 JOSEPH BARRELL 



SECTION A 

 DEVELOPMENT OF CRITERIA FOR SPHEROIDAL MASSES 



Separation of lithospheric from centrospheric outstanding masses. — • 

 Let the zone of compensation be regarded as the boundary of the 

 lithosphere. At its bottom consider to exist a zone in which that 

 lateral flowage takes place which is necessary for movements 

 of isostatic readjustment and the maintenance through geologic 

 time of a condition of more or less complete isostasy. Below it 

 is the inner and more rigid core of the earth, the centrosphere. 

 Let those excesses or defects of density above the zone of isostatic 

 flow which are not in accord with the isostatic compensation of the 

 topography be designated for convenience as lithospheric outstand- 

 ing masses. Let all heterogeneities of density within any earth 

 shell below the zone of isostatic flow be called centrospheric out- 

 standing masses. 



In his recent paper on the ''Interpretation of Anomalies of 

 Gravity,"' Gilbert calls attention to the fact that if abnormalities 

 of density exist below the zone of compensation they will pro- 

 duce anomalies of gravity without these signifying real departures 

 from isostasy. This is a very necessary addition to the theory 

 of the cause of gravity anomahes and deflection residuals. As a 

 test, Gilbert has calculated the influence of a right cyhnder with 

 vertical axis, of density ±0.025, with height and radius each equal 

 to 122 km., whose upper surface is at a depth of 122 km., thus 

 reaching up to the bottom of the zone of compensation as given by 

 Solution H. Such a cylinder would give a maximum anomaly of 

 =1=0.023 dyne at the epicenter, a quantity of the same order of 

 magnitude as the mean anomaly for the United States, 0.018 or 

 0.020 dyne. 



In the application of this test to the earth it would appear, 

 however, that two things should be noted. First, to account for 

 the mean anomaly of 0.020 dyne the centrospheric masses would 

 have to be several times as great as this cylinder, even for this depth 

 of 122 km. to the top surface, since the maximum value of the 

 anomaly occurs at the epicenter of the mass, and for a cylinder of 



' Professional Paper 85C, U.S. Geol. Survey, 1913, pp. 35, 36. 



