THE STRENGTH OF THE EARTH'S CRUST 675 



who have not themselves made a critical study of the problems of 

 isostasy. The arguments which the writer advanced in 1909 

 against this hypothesis were published under a title which appar- 

 ently did not call attention to them. The style of Hayford's reply 

 to Lewis is crushing and conveys the impression that Lewis has been 

 completely refuted. It is because of these reasons that the sub- 

 ject calls here for fuller development. 



In his Minneapolis address Hayford outlines a theory of the 

 principles of diastrophism which turns upon his conclusion that 

 isostasy is so nearly complete that areas of even limited size average 

 only 250 feet from the level of isostatic equilibrium. He assumes 

 chemical and physical changes to be induced in the crust by 

 the changing load due to erosion and sedimentation. These he 

 thinks are superimposed upon the effects of nearly continuous 

 vertical movements of isostatic readjustment. The vertical move- 

 ments in turn produce a lateral undertow which is given as a cause 

 of localized heating and folding. Apparently this is regarded as 

 a complete mechanism of deformation since the author raises the 

 query : 



Is it at all certain that under the influence of such actions the geological 

 record at the earth's surface at the end of fifty to one hundred million years 

 would be appreciably less complicated than the geologic record which is actually 

 before us ? I think that it would be fully as complicated as the actual record.^ 



This theory of folding as the result of subcrustal undertow is 

 illustrated by means of two diagrams. In Fig. i , the zone of viscous 

 flow from the sinking toward the rising area is placed in the lower 

 quarter of the zone of isostatic compensation. In Fig. 2 it is shown 

 in the middle of that zone, dying out both above and below. 

 Apparently then, as shown by these two different conceptions, the 

 author cited was guided by no definite theory, based upon the 

 mechanics of materials, as to the factors which would determine 

 the depth of this zone of undertow and its relations to the zone of 

 compensation. 



Harmon Lewis in his paper on the "Theory of Isostasy" has 

 discussed various aspects of the isostatic theory as developed by 



' Op. cit., p. 206. 



