Status of the Theory of Isostasy. 329 



able areas, however, would have a differential elevation 

 of 3 miles. 



Take two areas of the crust with a difference in ele- 

 vation of 3 miles, the higher having a density of 2-72 

 for a depth of 3 miles. What difference in crustal den- 

 sities will be required to support these two areas in iso- 

 static equilibrium? The answers may be tabulated as 

 follows : 



Relations of isostasy to density. 



Distribution of Difference in densities to support Maximum suboceanic 



compensation, relief of 3 miles with density, subcontinental 



Fig. 5 density 2*72 being taken as 2'80 



A 0-81, from 27 to 37 miles 3-61 



B 0115, from to 703 miles 2-91 



C 0-15, at surface 2-95 



D 0-165, at surface 2-96 



0014, at 100 miles 2-81 



As the effects of temperature and pressure may be 

 taken as roughly the same at equal depths under conti- 

 nents and oceans, those factors influencing density may 

 be neglected. 



It is seen that compensation confined to a 10 mile 

 stratum requires an excess of density to the oceanic por- 

 tion of the stratum 0-81 greater than the density of the 

 continental portion where the differential relief is 3 miles. 

 At depth of 27 to 37 miles the subcontinental crust cannot 

 be less dense for surface conditions than 2-80. It may 

 be 3-00. This would give 3-61 or 3-81 for the suboceanic 

 rock, types unknown at the surface of the earth. As 

 igneous activity brings up material from great depths, 

 this is wholly improbable. The geodetic evidence ruled 

 out the possibility of variable density serving to maintain 

 isostasy in a 10 mile stratum close to the surface, or at a 

 depth of 60 miles, by showing that its center of gravity 

 would have to be not far from 32 miles deep. The 

 geologic evidence based on known densities excludes this 

 form of isostatic compensation, or even compensation 

 restricted to a 20 mile stratum at any depth. Thus the 

 argument from densities shows that solutions giving such 

 a stratum are imaginary. So far as the known range 

 of densities gives testimony, B, C, and D, figure 5, are 

 readily possible, but A is impossible. 



Let attention be given next to the real or imaginary 

 nature of curve D. 



