THE THEORY OF ISOSTASY 625 



uplift. In this case also the deformation would tend to localize 

 where the resistance to uplift were least, in other words, in the 

 lighter segments. 



The type of deformation suggested here is perfectly distinct 

 from that postulated by isostasy. The theory of isostasy supposes 

 that light areas are high because the strength of the material below 

 is not sufficient to support segments different in weight. The 

 possibility suggested in the preceding paragraphs is that high 

 areas are light because the great deforming forces of the earth 

 follow the path of least resistance. 



The automatic compensation of uplifts and subsidences due to 

 expansion and contraction. — It is possible that some uplifts and sub- 

 sidences are due to expansion and contraction of the underlying 

 material. Such deformations are not a distinctive assumption 

 of the theory of isostasy. Changes of volume may be due to 

 changes of temperature or pressure which in turn may be due to 

 a variety of causes. Any changes of elevation caused by expansion 

 or contraction will be automatically compensated since the weight 

 does not change. This would be another factor tending to produce 

 compensation which does not involve the type of deformation 

 postulated in isostasy. 



SECTION IV. SUMMARY OF CONCLUSIONS 



Isostasy is a theory of earth movements based on the assump- 

 tion that the lighter portions of the earth are elevated in propor- 

 tion to their defect of density because the earth is not strong 

 enough to support segments of different weights. The principal 

 support for the theory is the geodetic work of Hayford from which 

 it was concluded that the excesses of mass at the surface are com- 

 pletely compensated for by defects of density below, said defects 

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



It is believed that Hayford made an error in determining the 

 degree of completeness of compensation which invalidates his 

 conclusions, for he assumed complete compensation in calculating 

 the depth and then used this depth to calculate the degree of com- 

 pleteness. Hence, instead of the single possibility of a practically 

 complete compensation, there are, so far as has been shown from 



