THE THEORY OF ISOSTASY 60$ 



the rather startling conclusions were reached that the excesses 

 of mass composing the continents and mountains are completely 

 compensated by deficiency of density below and that this deficiency 

 of density extends to a depth of something like 60 to 150 miles. 

 These conclusions lent strong support to the theory of isostasy. 

 Considering the completeness of the density compensation, there 

 seemed to be no escape from the conclusion that readjustments 

 of the nature postulated by isostasy are continually taking place. 



With the theory of isostasy apparently on such a firm basis, 

 Hayford and others have elaborated the conceptions of earth move- 

 ments involved in the theory, but these further inferences have not 

 met the approval of many geologists. 



The possibility that Hayford had made an error in his geodetic 

 work suggested an investigation which is the basis of this paper. 

 The attempt has been made, first, to examine Hayford 's geodetic 

 work apart from any inferences which may have been drawn from 

 it, and second, to examine the theory of isostasy with reference 

 to inferences from geodetic evidence and also on general grounds. 

 This paper has accordingly been divided into two main parts 

 entitled "The Geodetic Work of John F. Hayford" and "The 

 Theory of Isostasy." A "Summary of Conclusions" is given at 

 the end. 



It seemed highly desirable in connection with the criticism of 

 Hayford' s geodetic work that several of the terms employed 

 should be defined. 



DEFINITIONS 



111 1 so static compensation'' is the compensation of the excess of 

 matter at the surface {continents) by the defect of density below, and 

 of the surface defect of matter {oceans) by excess of density below." 1 

 "Isostatic compensation" will also be referred to simply as "com- 

 pensation" and an area or segment of the earth will be spoken of 

 as "compensated" if there is isostatic compensation of the excess 

 or defect of matter over that area or at the surface of the given 

 segment. From the above definition it follows that there will be, 

 in general, a density difference between an average sea level segment 



1 The Figure of the Earth and Isostasy, U.S.C. and G.S., 190Q, p. 67. 



