624 HARMON LEWIS 



far as the geodetic evidence is concerned that the compensation 

 of the topographic features of the continent is decidedly incomplete. 

 The theory of isostasy can not therefore be considered as established 

 by the geodetic work of Hayford. Furthermore, the probability 

 that isostasy exists is lessened by the fact that an incomplete com- 

 pensation can be very plausibly explained without involving the 

 conceptions of isostasy. 



The tendency of lateral compression to produce iso static compensa- 

 tion. — In the folding and overthrust faulting of rocks there is abun- 

 dant evidence of lateral compression. It has already been shown 

 that this folding is probably not caused by an undertow such as 

 isostasy supposes to be set in motion by erosion and deposition. 

 The compression indicated by folding may be due to shrinkage 

 of the earth ; it may be due to squeezing of the continental segments 

 by the oceanic segments; or it may be due to other causes; but 

 whatever the cause may be, it is certain that it has produced great 

 uplifts. Suppose that the continent is composed of portions of differ- 

 ent densities, but that the stress-differences set up by these differ- 

 ences in density are not sufficient to cause a deformation of the 

 material and a consequent uplift of the lighter masses. If this were 

 the case, it would seem reasonable to believe that there would be a 

 tendency for the effects of lateral compression to localize in the 

 lighter segments since there is always a tendency for lighter seg- 

 ments to move up even though the stress-differences tending in this 

 direction are not sufficient to produce an actual movement. Other 

 things being equal, folding would probably tend to localize in sedi- 

 mentary rocks since the parallel bedding planes allow slipping to 

 take place readily. Here again there might be a tendency toward 

 isostatic compensation since sedimentary rocks on an average 

 are lighter than igneous rocks. There are undoubtedly other 

 factors which determine the place of folding, but it is entirely 

 possible that uplifts by folding are incompletely compensated. 



A compensation of areas which have been uplifted without 

 folding may be accounted for in a similar way. It is possible that 

 lateral forces similar in magnitude to those forces which produce 

 folding at the surface, but localized at greater depth should cause 

 a deformation which is registered at the surface simply as a general 



