622 HARMON LEWIS 



a tendency for the earth to take this arrangement in the sense that 

 the stress-differences are on an average tending in this direction. 

 It would seem, therefore, as a general proposition, that where the 

 material of the earth is weak, the tendency would be more toward 

 equalization of densities laterally than toward lateral differentiation 

 of densities such as implied by isostatic compensation. Even 

 though the stress-differences due to lateral variations in density 

 were not sufficient to deform the rock so as to equalize the densities, 

 a flowage from beneath an area of deposition to an area of erosion 

 would certainly tend to produce a distribution of density within 

 the zone of flowage itself which would have no relation to topog- 

 raphy. It appears, therefore, that there could be very little 

 isostatic compensation in a zone where yielding occurs as readily 

 as postulated by isostasy. 



Now, according to the theory of isostasy, compensation would 

 be essentially complete, and if compensation is complete the depth 

 of compensation as determined by Hayford's geodetic work would 

 be as great as 60 miles. Hence, the undertow postulated by 

 isostasy would exist chiefly below 60 miles. It is decidedly ques- 

 tionable that an undertow even much nearer to the surface than 

 60 miles would cause the observed folding in the upper few miles 

 of the crust. 



The theory of isostasy cannot account for the general uplift of sedi- 

 ments without folding. — If the isostatic compensation is complete 

 any deposition of material should cause a sinking of the under- 

 lying segment. Isostasy could not therefore account for the fact 

 that horizontal sedimentary rocks are found far above sea level 

 unless a lowering of the sea level were supposed; but this possi- 

 bility can generally be dismissed because the relative change in 

 sea level is not registered in all parts of the world. 1 



1 In discussing isostatic adjustments (see Science, February 10, 19 n) Hayford 

 suggests that some uplifts are due to expansion and contraction caused by heating and 

 cooling of sedimentation and erosion. These deformations, however, are not a distinc- 

 tive assumption of the theory of isostasy at least as the theory is conceived in this 

 paper; but were suggested to account for certain geological phenomena which the 

 theory of isostasy could not explain. 



At any rate, expansion due to heating effect of sediments is entirely inadequate 

 to account for known uplifts. In making his estimate that the vertical expansion is 



