286 CHAMBERLIN— THE INTERIOR OF THE [April 24, 



bilities of the case seem to point strongly to wide variation in nature 

 due to selective solution or differential fusion. The liquefying 

 action that brings magmas into being, under this view, is presum- 

 ably controlled by the same chemical and physical principles as the 

 solidifying phases of the same cycle. The logical presumption is 

 that at all stages of a magma's career from its inception through 

 its growth, climax and decline to its final solidification, selective 

 action will be in progress more or less and that no stage will be en- 

 titled to be regarded as original or parental in a special sense, such 

 a sense for example as might be appropriate if the lava were the 

 residue of an inherited original state and were merely differentiated 

 by fractional crystallization as it passed toward solidification. 



While these contrasted views of the history of magmas are 

 naturally connected with views of the genesis of the earth, they are 

 not limited to this relation. They are inherent in the very rela- 

 tions of solid and liquid matter and have a more or less important 

 place irrespective of the earth's genesis. 



An element of no small importance to a revised concept of the 

 interior of the earth has arisen from geodetic studies on the distribu- 

 tion of densities within the earth. As the geodetic point of view 

 is to be presented by its foremost exponent, Dr. Hayford, it is per- 

 missible for me merely to refer to certain geologic bearings. 



On the assumption that the earth was once in a molten state, the 

 inference is unavoidable that a perfect state of isostatic equilibrium 

 was originally assumed by the surface, and that its configuration 

 was at first strictly spheroidal. The material must have been ar- 

 ranged in concentric layers according to specific gravity and each 

 layer should have had the same density at every point. All such 

 reliefs of the earth's surface, and all such differences of specific 

 gravity in the same horizon as have since arisen, must have been 

 superinduced upon this originally perfect isostatic surface. With 

 good reason therefore these inequalities have heretofore been sup- 

 posed to be relatively shallow. On the hypothesis that the earth 

 grew up by heterogeneous accretions, it is an equally natural in- 

 ference that differences of specific gravity extend to great depths. 

 In an endeavor to find out the bearings of geodetic data on the dis- 

 tribution of densities, Dr. Hayford tested four assumptions, all of 



