THE THEORY OF ISOSTASY 623 



The theory of isostasy does not explain the apparently heterogene- 

 ous relation of uplift and subsidence to erosion and deposition. — Since 

 isostasy postulates an adjustment or flowage which is comparable 

 in speed to the process of erosion, a high area which is subject to 

 erosion should be further uplifted as erosion progresses and should 

 not be reduced to sea level until its deficient density is equalized 

 by erosion of the lighter material at the surface and restoration of 

 heavier material below. With a depth of compensation of 76 

 miles, the theory of isostasy would require greater continuous 

 uplifts than are known to exist. As a matter of fact, some areas 

 have been uplifted as erosion progressed and others have remained 

 stationary. In some cases erosion to a peneplain has been fol- 

 lowed by subsidence and in other cases by uplift. 



ALTERNATE HYPOTHESES TO ACCOUNT FOR HAYFORD'S GEODETIC 



RESULTS 



Erosion and deposition are assumed to be the principal cause 

 of disturbance of the equilibrium condition of isostasy. Since 

 deposition does not in general extend beyond the boundaries of the 

 continental shelves, the cause and effect of the type of deformation 

 postulated by isostasy would be confined to the continents proper. 

 So far, then as distributions of density are to be made a proof of 

 the theory of isostasy, the critical test is not in the density relation 

 of the continental masses as a whole compared to the ocean basins, 

 but in the completeness of compensation of the topographic features 

 of the continents. Now it has been shown in the preceding section 

 of this paper that, though there is very likely a complete compensa- 

 tion of the ocean defects of mass, yet it is a distinct possibility so 



one foot for every 2,$ feet of deposition Hayford neglects the fact that the irregularities 

 in the isothermal surfaces near the surface of the earth flatten out with depth. How- 

 ever, taking Hayford's estimate and assuming that an area of deposition was covered 

 by very shallow water and that the expansion due to heating took place all at one 

 time, the maximum uplift above sea level could not be more than one-thirty-third 

 of the thickness of the sediments deposited. Subsequent erosion would tend to 

 reduce this elevation and any further elevation caused by relief from eroded material 

 would certainly not more than equal the eroded layer. Hence, wherever the present 

 elevation above sea level is more than one-thirty-third the thickness of the last con- 

 formable sedimentary series, some other factor than expansion due to heating effect of 

 sedimentation must be sought to account for the uplift. 



