EARTH'S CRUST WASHINGTON. 319 



The calculation of rock densities from their chemical composi- 

 tion is discussed and the average chemical compositions and densities 

 of the continental masses and oceanic floors are given. It is shown 

 by these that the average densities of the continents, ocean floors, 

 and various smaller regions of the earth stand in inverse relation to 

 their elevations. The bearing of this relation of average density and 

 elevation on the theory of isostasy is pointed out, and it is shown that 

 the data presented are confirmative of the theory. 



