502 T. C. CHAMBERLIN 



planetesimal accretion was in progress. The earth core was then 

 youngest and least compressed, and so probably least rigid and 

 most susceptible to the influence of differential stresses. I have 

 elsewhere shown that the changes in rotation were probably 

 oscillatory about a medial rate in conformity to a law of equilib- 

 rium.'^ They may be regarded therefore as commanding influences 

 both in respect to power and to the times and modes of application. 



The elevated poles and depressed equator of the rhythmical tidal 

 deformations were transverse to the elevated equator and depressed 

 poles of the rotational changes, and this transverse attitude no 

 doubt lent facility to the kneading action which their rhythmical 

 periods brought to bear on the interior of the earth. 



These co-operating agencies thus brought to bear on the whole 

 interior of the solidifying earth a rhythmical series of differential 

 stresses, most intense in the deeper parts and less intense toward 

 the surface, and so admirably fitted to force the mobile and the 

 lighter material toward the surface and to favor readjustments 

 that brought about increased density and rigidity and probably 

 also increased elasticity. It seems to me probable that this com- 

 bination of strong mechanical stresses at distant intervals working 

 with much gentler and more rapid rhythmical stresses has been the 

 master-factor in controlling the secular reorganization of the 

 earth's interior, a gradual reorganization which I think has been in 

 progress from the time solidification began down to the present 

 day. The normal result, as I see it, would be a general gradation 

 of concentrative effects from surface to center — taking form in 

 appropriate gradations of density, rigidity, and elasticity, also 

 graded from surface to center. The results of our comparative 

 study of the earth, moon. Mars, and Venus tally perfectly with 

 this view and make it theoretically logical and consistent. The 

 steadily increasing density from smaller body to larger body, in 

 spite of the high probability that the smaller bodies inherited the 

 heavier molecules, points very definitely to reorganization under 

 the influence of compression. The oscillating differential stresses, 

 greater below than above, seem peculiarly well suited to aid in 

 working out the graded adjustments. 



' The Origin of the Earth (1916), pp. 95-110, 172-79. 



