280 W. BOWIE THEORY OF ISOSTASY 



There has been diversity of opinion as to the size of the topographic 

 feature which might escape compensation. This subject has been much 

 confused by the fact that there has not been made a clear distinction be- 

 tween the horizontal extent of the topographic feature which may escape 

 compensation and the cross-section of the column of the isostatic shell 

 which may be in equilibrium independently. 



A test was recently made of the mass which might escape compensation 

 and the results have appeared in one of the papers of the speaker. This 

 test shows that for stations at high elevations the compensation of the 

 surrounding topography can not be ignored to any great extent without 

 having a very marked effect on the gravity anomalies. 



When the compensation of the topography for a disk with a radius of 

 about 18 miles and 3,000 feet in thickness was ignored, the gravity 

 anomalies became much larger than they were when all the topography 

 was considered to be compensated, and 37 of the anomalies at the 42 sta- 

 tions used had negative signs. When the area of the disk was given a 

 radius of about 36 miles, the anomalies were still further increased in 

 size and all of them except one had the negative sign. 



The conclusion from this test would seem to be that a disk of material 

 3,000 feet in thickness and 18 miles in radius is largely compensated. 

 Considering the mass above sealevel forming a continent, this amount of 

 topography is very small. 



The compensation of this disk 3,000 feet in thickness and 18 miles in 

 radius is probably not directly under the material, but is distributed 

 somewhat regionally. 



After a consideration of this test, made to show the size of the topo- 

 graphic feature which may not be compensated, we are led to the con- 

 clusion that a mountain system certainly does not escape compensation. 



It should be recognized that the mountains and the oceans are not com- 

 pensated by a deficiency of density under the land and an excess of density 

 under the oceans, but that the landmasses compensate the deficiency of 

 density in the isostatic shell below them and the deficiency of mass in the 

 oceans compensates the excess density below the oceans. The landmasses 

 and deficiency in the oceans are the effect rather than the cause of the 

 isostatic compensation. 



Mountain Formation by vertical versus horizontal Movements 



If we accept isostasy as substantially true, and it would seem to be very 

 logical that we should do so after considering the geodetic evidence and 



Some geologic conclusions from geodetic data. Proceedings of National Academy of 

 Sciences, vol. 7. no. 1, January. 1921, pp. 23-28. 



