676 JOSEPH BARRELL 



Hayford, and among them this question. Regarding the possi- 

 bility of folding by means of isostatic undertow, Lewis concludes: 



Now, according to the theory of isostasy, compensation would be essen- 

 tially 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 questionable 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. ^ 



In regard to this criticism by Lewis concerning the cause of 

 folding, Hayford states in reply: 



On pp. 621-22 Mr. Lewis sets forth the argument that there is much 

 geological evidence of horizontal movements in the outside portions of the 

 earth, especially in the form of folding, that the controlling movements of 

 isostasy are assumed to be vertical and hence cannot account for folding, and 

 that the horizontal movement or undertow concerned in isostatic readjustment 

 must be below the depth of compensation and hence so far below the surface 

 as to be very ineffective in producing folding. 



There are two fatal defects in this argument as applied to controverting 

 anything that Hayford believes or has written. 



First, Hayford has already indicated clearly his belief that the undertow 

 concerned in isostatic readjustment is above, not below, the depth of compensa- 

 tion. In both the figures published in his Minneapolis address the undertow 

 is clearly indicated as being above the depth of compensation and it is also 

 so indicated in the corresponding text. As Hayford puts the undertow com- 

 paratively near the surface, where it is conceded that it would be effective in 

 producing folding, the existence of extensive folding is a confirmation, not a 

 contradiction, of his theory of the manner in which isostatic readjustment 

 takes place. It is certainly not fair to hold Hayford responsible, either directly 

 or by inference, for any theory which someone else may believe which involves 

 an undertow situated entirely below the depth of compensation. Mr. Lewis 

 apparently believes such a theory. 



Second, the movements which produce isostatic readjustment are neces- 

 sarily horizontal, not vertical. If two adjacent columns of the same horizontal 

 cross-section extending from the surface to the depth of compensation have 

 different masses the readjustment to perfect compensation must involve a 

 transfer of mass out of one column, or into the other, or from one to the other. 

 In any case the transfer must be a horizontal movement. Hayford has already 

 shown in print more than once that he understands that vertical movement 

 alone does not produce isostatic readjustment. Moreover, a careful reading 



^ Op. cit., p. 622. 



