Nature and Bearings of Isostasy. 289 



rium with the conical oceanic sectors, thus giving a form 

 of isostasy in which the compensation extends downward 

 more than a thousand miles. To balance the elevation 

 of the continents above the ocean basins Chamberlin 

 states that the oceanic sectors would have to possess 

 only about one-fifth of one per cent, greater density. 1 

 Chamberlin recognizes the essential difference between 

 this kind of isostasy and that which rests upon a com- 

 pensation restricted to an outer shell of the earth, mostly 

 within an outer fiftieth of the radius. The limitations 

 of differences of density in depth are not regarded by 

 him as having any trustworthy basis, and he states that 

 this view is supported by competent mathematical inves- 

 tigation, the results of which will appear in time. 2 



A final settlement of this question must wait until such 

 full investigations as Chamberlin promises have been 

 published and have been tried out against the opposing 

 views which have been here adopted. We have found 

 occasion to differ from Hayf ord in many particulars, but 

 have modified his hypothesis in detail and in degree 

 rather than in its central idea. The least-square solu- 

 tions seem to point mathematically and conclusively to 

 a concentration of the isostatic compensation in an outer 

 shell, though Hayford himself notes that the form of 

 hypothesis which restricts it to a uniform distribution 

 to a uniform depth of 122 kilometers has no certainty 

 over some other forms of distribution of compensation. 

 But to satisfy the least-square solutions all hypotheses 

 must restrict the compensation to an outer zone. Vari- 

 ous other and independent lines of investigation, such 

 as the yielding under the Pleistocene ice loads and the 

 determination of the curve of strength, support the geo- 

 detic evidence that isostatic compensation is largely 

 limited to the outer fiftieth of the earth's radius. There- 

 fore, until Chamberlin has shown the error in these 

 demonstrations and has published in detail the mathe- 

 matical basis which supports his own view, we are justi- 

 fied in holding to the probability of the hypothesis here 

 set forth, — that an outer shell of strength and hetero- 

 geneous density rests upon a yielding shell of weakness. 

 There has not been published as yet any contrary demon- 



1 Chamberlin and Salisbury, Geology, Vol. II, pp, 110, 111, 123, 1906. 

 Chamberlin, The Origin of the Earth, pp. 159-225, 1916. 



2 T. C. Chamberlin, Isostasy in the Light of the Planetesimal Theory, this 

 Journal, vol. 42, p. 371, 1916. 



