I9IS.] EARTH FROM THE VIEWPOINT OF GEOLOGY. 287 



which he found measurably compatible with his geodetic data. 

 From these he derived the respective depths of 37, 76, 109 and 179 

 miles as the horizons to which differences of density extended and 

 below which they vanished or became negligible. Now all these 

 depths are greater than had been assigned for probable differentia- 

 tion in the traditional molten earth. On the other hand, the highest 

 figure, 179 miles, was derived from a curve drawn specifically to 

 represent the probable distribution of densities in an earth of plan- 

 etesimal growth. The distribution represented by this highest 

 figure fits the geodetic data quite as well as either of the other as- 

 sumptions of distribution, though drawn on a strictly naturalistic basis 

 If it could be said that geodetic data demonstrate that the actual dif- 

 ferentiation of specific gravities has its sensible limits somewhere 

 between 37 and. 179 miles below the surface, such considerable depth 

 would distinctly favor an accretionary origin as against a molten 

 origin. But a conclusive determination is yet to be reached by geo- 

 detic inquiries. 



While it is possible, within the broad terms of the planetesimal 

 hypothesis, to suppose that the rate of accretion was so fast as to 

 give rise to a molten planet, such a result seems to me extremely 

 improbable under the actual conditions of the case. The growing 

 planet should have become capable of holding a considerable atmos- 

 phere by the time it attained one tenth of its present mass, i. e., 

 about the mass of Mars. After this the protective cushion of the 

 atmosphere should have greatly checked the plunge of the planetesi- 

 mals and largely dissipated them into dust in the upper atmosphere 

 where the inevitable heat of impact would be promptly radiated 

 away. The dust presumably floated long and came gently to earth, 

 so that, while the total heat generated by impact was large, the tem- 

 perature of the earth body was probable never very high during the 

 later stages of growth, and perhaps not at any stage of growth. 

 Following out as well as may be the probable rates and conditions 

 of growth, the most tenable concept of the state of the earth's in- 

 terior under the planetesimal hypothesis is as follows : 



The condition of the nuclear portion supposed to be formed from 

 one of the knots of the parent spiral nebula and constituting a minor 

 fraction of the mass of the earth, say thirty or forty per cent., is 



