igoS.] THE PHYSICS OF THE EARTH. 197 



As the rigidity increases so rapidly towards the center of the 

 earth, flow ought not to take place at those depths; and the absence 

 of any evidence of deep seated movements among the ruins wrought 

 by geological time in turn supports the theory of rigidity depending 

 on the pressure. 



Darwin's hypothesis of homogeneity is only a rough approxima- 

 tion to the truth, ''and Laplace's law would no doubt give a much 

 more exact representation of the density and the resulting stress- 

 difference in the earth. But this suggested change of data would not 

 greatly modify the general conclusions already stated. 



§ 20. The Theory of Iso'stacy. — A more important dift'erence 

 might arise from the theory of isostacy, the applicability of which 

 to the earth seems to be becoming better established by recent re- 

 searches. In this view the crustal inequalities seen at the surface 

 are compensated for by lighter or greater densities beneath, accord- 

 ing as the crust is elevated or depressed, so that for a certain thick- 

 ness of crust equal blocks have equal mass, however unequal the 

 level of the blocks at the surface. 



The recent investigations by the U. S. Coast Survey indicate 

 that the depth of complete compensation for the United States and 

 outlying stations is about yi miles. Xo doubt a depth of something 

 like this extent would hold true for the entire globe. If this view 

 be admissible, it will follow that all inequalities of the crust cease 

 to be effective at depths greater than 71 miles, and no stress-differ- 

 ences depending on plateaus and mountains would exist in the globe 

 except in the layers just beneath the crust. There would thus be no 

 stresses in the deep interior depending on the weight of continents 

 and mountains. 



This theory of isostacy is confirmed by the theory of mountain 

 formation developed in the paper on the " Cause of Earthquakes," 

 which shows clearly that these elevated ridges are underlaid by 

 material lighter than the average rock of the crust. On the one 

 hand, therefore, if stress-differences exist deep down, no move- 

 ment can take place, owing to rigidity ; on the other, if the theory of 

 isostacy be admissible, no stress-differences can exist except in the 

 outer layers of the globe, within 71 miles of the surface. 



IVe conchide therefore that in no case could movements occur 



