204 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



De?isity. — Surface density by estimation of its known com- 

 ponents cannot be far from 2.67. A gradual increase with 

 pressure due to recoalescence, and compression and inclusion of 

 gases, due also to the closing of cavities, might be expected. 

 This gradual increase is supported by seismic investigations. 



This topic and many others have been elucidated by the 

 recent papers of the Chamberlins in Jour, of Geol. Vol. 29-30. 



Adjustability. — The most important result of our studies is the 

 establishment of the fact that adjustment throughout the crust 

 to external forces or to internal stresses is almost perfect. We 

 have seen that the variation of latitude occurs because the equat- 

 orial bulge persists in shifting with the shift of axis. But back of 

 this wehave evidence of adjustmentin the shape of the earth itself, 

 which is very nearly the true form of an ellipsoid of revolution. 

 That it varies at all from this form is evidence, equally good, 

 perhaps better, of its plasticity. Its apparent inequalities are 

 the result of its adjustment of heterogeneous density to the 

 equilibrium of the ellipsoid of revolution. (See note.) We 

 believe that the whole problem of volcanism, mountain-making, 

 continent and ocean-basin formations, and all diastrophism are 

 but phases of the problem of maintaining adjustment to this 

 form of the earth. That this adjustment is nearly perfect, hav- 

 ing an average departure of only 171 meters of rock of 2.67 

 specific gravity in the United States, has been proven beyond 

 the stage of hypothesis. Even the maximum anomaly or lack 

 of adjustment found in the United States corresponded to a 

 stratum only 1,000 meters thick or a deficiency of pressure of 

 278 kilograms per square cm. or less than one-fifth the crushing 

 strength of granite. This is an extreme and unusual anomaly. 

 That this adjustment has been demonstrated in a broad way 

 in regard to ocean basins and continental masses is beyond 

 doubt. 



The fact that there is essentially no relation to loading and 

 unloading in the gravity anomalies would indicate rather rigid 

 adjustment. The quick submergence of volcanic islands having 

 a positive anomaly is also good evidence. There is no doubt that 

 the area of deficient density through Nevada, part of Colorado, 

 Arizona, and Southern California is in a stage of uplift. 



Note — By equilibrium is ineant an equilibrium of density, essentially what is 

 implied in the term "isostasy." 



