EARTH S CRUST WASHINGTON. 



309 



Daly and others as to the existence of a basaltic substratum beneath 

 the dominantly granitic outer shell, that is brought about by " gravi- 

 tative adjustment." 



Balancing up these conflicting factors, I am inclined to place the 

 average density of the crust at about 2.75, at least for the uppermost 

 shell, while that of 2.80 would probably be nearer the truth for the 

 average of any considerable depth, such as 20 or more miles. In the 

 present state of our ignorance and the paucity of our data, however, 

 it would seem to be wisest to accept the figure given by the many 

 analyses available and assume a density of 2.77 as that of the earth's 

 crust. 



290. 



2.80 

 Average n 

 Density 



210 



Average 



Spec. Vol. 



+ 4.000 



+ 2.000 



Sea Level 



-4000 



-8000. 



-12000. 







Spec 'fie 

 1 Volur les 





.370. 



-3m 



.350 



.340. 



-16000 

 Fig. 3. — Elevations and densities of the continents. 



Geodesists have assumed a density of 2.67 for their studies of 

 isostasy, as will be noted elsewhere. They take into their calcula- 

 tions, however, only the extremely superficial layers, including such 

 strata as soil and light sedimentary deposits. As will be mentioned 

 later, I am inclined to think that this estimate is much too small and 

 that the basis of their calculations should be a considerable higher 

 density. That of 2.77, here assumed, or possibly better, 2.75, would 

 seem to be the best available under the circumstances. 



ROCK DENSITIES AND ELEVATIONS. 



With the analyses in Table IV are given the calculated densities 

 and the average elevations of the continents and the depths of the 

 ocean floors referred to sea level. The general relations are graphi- 

 cally expressed in figure 3. The lowest graph is that of elevations, 

 the uppermost is that of densities, while the intermediate one is that 

 of specific volumes, or reciprocals of the densities, which serves 



