THE THEORY OF ISOSTASY 621 



as to require that all segments of the earth of given area are essen- 

 tially equal in weight. 



This flowage may be accompanied by folding in the border 

 zones of the segments. 



Deformation of this kind is not restricted to the major segments, 

 the continents and oceans, but is the type of movement which 

 takes place between the smaller units of the continents. 



Lines of criticism ofisostasy. — In criticizing the theory of isostasy 

 two main lines of argument will be followed. First, the type of 

 deformation postulated by isostasy can not account for certain 

 facts. Second, Hayford's geodetic results can be accounted for 

 without supposing the type of deformation postulated by isostasy. 



FACTS NOT ACCOUNTED FOR BY THE TYPE OF DEFORMATION POSTU- 

 LATED IN THE THEORY OF ISOSTASY 



The degree to which isostasy must be discarded depends on the 

 importance of the phenomena which it will not explain. 



The theory of isostasy as conceived in this paper does not adequately 

 account for the folding of rocks of the earth's crust. — Folding is evi- 

 dence of lateral forces of enormous magnitude. On the other hand 

 the controlling movements of isostasy are assumed to be vertical 

 movements. However, it has been suggested by Hayford that 

 folding would be caused by the undertow from an area of deposition 

 to an area of erosion : 



Horizontal compressive stresses in the material near the surface above the 

 undertow are necessarily caused by the undertow. For the undertow neces- 

 sarily tends to carry the surface along with it and so pushes this surface material 

 against that in the region of erosion, see Fig. 2. These stresses tend to produce 

 a crumpling, crushing and bending of the surface strata accompanied by 

 increase of elevation of the surface. The increase of elevation of the surface 

 so produced will tend to be greatest in the neutral region or near the edge of 

 the region of erosion, not under the region of rapid erosion nor under the region 

 of rapid deposition. 1 



This undertow must exist chiefly below the depth of compensa- 

 tion. If the earth were a perfect fluid the materials of different 

 densities would, if not diffusible, arrange themselves in concentric 

 shells with the heavier material toward the center. There is always 



1 Science, February 10, 1911, p. 205. 



