6 2 o HARMON LEWIS 



as to the nature of this compensation. None of the possible distri- 

 butions of compensation have been eliminated by Hayford's 

 geodetic work; in fact, so far as the geodetic work is concerned, 

 Hayford has shown that four different distributions of compensa- 

 tion are equally probable. 



The present possibilities for isostatic compensation may be 

 grouped with considerable certainty under three heads: first, there 

 is the possibility of complete compensation at a depth in the 

 neighborhood of 60 to 150 miles depending on the distribution of 

 compensation ; second, there is the possibility of an over-compensa- 

 tion at a greater depth for land areas with probably complete com- 

 pensation for ocean areas; and third, there is the possibility of 

 under-compensation at a shallow depth for land areas with com- 

 plete or over-compensation for ocean areas. 



SECTION III. THE THEORY OF ISOSTASY 

 INTRODUCTORY 



In any theory of earth movements it is recognized that the 

 earth is a failing structure in the sense that it has been and is 

 being permanently deformed under the ultimately controlling 

 force of gravity. It is not therefore the essential idea of the theory 

 of isostasy that the earth as a whole is a failing structure; but the 

 characteristic of the theory is the type of deformation which it 

 postulates. This may not be the critical point of isostasy as it 

 was originally conceived, or as conceived today by everyone; but 

 it is the point which has been elaborated by the supporters of the 

 theory and which is of first importance to the geologist; and it 

 will therefore serve as a basis of criticism in this paper. 



Type of deformation postulated in the theory of isostasy. — The 

 controlling movements of the earth's crust are vertical movements 

 of the various segments in response to changes of weight produced 

 by erosion and deposition. 



These vertical movements are brought about by flowage beneath 

 the surface from areas of deposition to areas of erosion or, in general, 

 from areas of excessive weight to areas deficient in weight. 



This flowage beneath the surface is comparable in speed to the 

 process of erosion and is started under stress-differences so small 



