THE STRENGTH OF THE EARTH'S CRUST 677 



of his Minneapolis address will certainly show that he believes that the total 

 amount of material moved horizontally during isostatic readjustment, and 

 especially the total number of ton-miles of such movement, is vastly in excess 

 of the corresponding quantities concerned in the vertical components of the 

 movement which takes place. Hence the folding and other abundant evidence 

 of past horizontal movements observed by geologists confirm Hayford's 

 hypothesis as to the manner in which isostatic readjustment takes place, 

 instead of conflicting with it as Mr. Lewis' article would lead one to think.^ 



The present writer, however, believes with Mr. Lewis in the 

 theory that an undertow must be essentially below the zone of 

 compensation and is incapable of producing surficial folding. The 

 reasons have been given in part in the consideration of the stress- 

 relations, as they would exist under the hypothesis of extreme 

 isostasy. But there are other reasons why the subject should be 

 discussed in further detail. One reason is that, if Lewis is right 

 on this point and Hayford wrong, it is desirable that this should 

 be made clear, in justice to Mr. Lewis as well as to the subject. 

 The other reason is that here in reaching a conclusion we can 

 advantageously pursue a method of exclusion. By showing that 

 isostatic undertow cannot take place within the zone of compen- 

 sation, for various reasons besides those discussed in the stress 

 diagrams, we reach the conclusion that it must take place in a 

 level below that zone. Furthermore, by noting the conditions 

 which would hinder lateral flowage we may arrive at a conclusion 

 as to those which must exist to greater or less degree in order to 

 permit it. 



Objections against undertow in the zone of compensation. — The 

 pressures which occur during a state of isostasy and after the 

 destruction of that condition have been discussed. It was seen 

 that the pressures making for the undertow necessary to restore 

 isostasy were greatest at the bottom, but, more especially, below 

 the bottom of the zone of compensation. The possibility remains 

 to be considered, however, that perhaps the distribution of the 

 rigidity of the crust more than offsets the distribution of pressures. 

 Suppose the middle of the zone of compensation should be very 

 weak and the crust at and below the bottom be very strong. Then, 



^ Op. ciL, pp. 573, 574. 



