282 W. BOWIE THEORY OF ISOSTASY 



There is much distortion of the material of the earth's crust during 

 the isostatic adjustment of an area of active erosion. In order to main- 

 tain equilibrium in any particular area that is undergoing erosion, ma- 

 terial equal in mass to that eroded must be brought into the lower part 

 of the column. It is probable that the material brought into the column 

 is as much as 10 per cent heavier than that eroded from the surface. 

 Let us assume that the difference is exactly 10 per cent. Then as a cer- 

 tain thickness of material is eroded from the surface, the elevation will 

 be lowered by about 10 per cent of the thickness of the disk that has been 

 removed. It is j:>ossible that a mountain area may have 30,000 or 40,000 

 feet of material eroded before the area is baseleveled. 



The reason that the magma is exposed in the older mountains is that, 

 as erosion takes place, the mountain area is maintained approximately 

 in its elevation, thus permitting long-continued erosion of the sediment- 

 ary material from the area. 



During the process of uplift of the material of the column under the 

 eroded area due to the isostatic adjustment, there will be distortion of the 

 material which is sufficiently plastic to yield under the vertical stresses 

 acting on them. There will be fracture of material close to the surface 

 that is so weak as not to be able to resist the movement and at the same 

 time not sufficiently plastic to yield without fracturing. 



It is probable that many earthquakes are due to the process of isostatic 

 adjustment, which tends to maintain the elevation of a mountain area as 

 erosion continues and which causes the earth block under an area of 

 deposition to subside. This point was discussed by Reid some time ago 

 in his article entitled "Isostasy and mountain ranges." 7 



The geodetic evidence is sufficient to justify us in eliminating regional 

 horizontal thrusts as the cause of the uplift of the mountain area, and the 

 geodetic data are also such as to make us feel that the downwarping or 

 subsidence of an area that is being sedimented can not be due to regional 

 horizontal thrusts. The evidence would appear to justify our holding 

 that mountains are caused by an actual decrease in the density in the 

 column under them, and that the elevations of mountains, after erosion 

 begins, are approximately maintained by the isostatic adjustment which 

 brings material into the column under the mountain area. The subsi- 

 dence of a surface in an area of sedimentation is probably due both to 

 the load of sediments and to an actual contraction of the material of the 

 column. 



If we have been successful in eliminating regional horizontal thrusts 

 as the cause of the uplifting of mountain areas, then we must seek for 



Proceedings of American Philosophical Society, 1911, pp. 444-451. 



