288 CHAMBERLIN— THE INTERIOR OF THE [April 24, 



left indeterminate by present lack of knowledge of the physical state 

 of the knots of spiral nebulae. If these are gaseous — which is ren- 

 dered doubtful by their lack of strict sphericity — the nucleus was 

 doubtless originally molten. If the constituents of the knot were 

 held in orbital relations, their aggregation might have been slow 

 enough to permit a solid state of even this portion. The matter 

 added to the nucleus as planetesimal dust, or as planetesimals re- 

 duced in mass and speed by the atmosphere, probably retained its 

 solid condition, with negligible exceptions, throughout the process 

 of accretion except as selected portions passed into the liquid state 

 and became subject to extrusive action. An intimate heterogeneity 

 naturally prevailed throughout the whole mass so aggregated. A 

 selective process, however, probably brought in the heavier matter 

 faster and earlier than the lighter matter, for the magnetism of the 

 earth should have aided gravity in gathering in the magnetic metals 

 while the inelastic planetesimals, predominantly the heavy basic 

 ones, when in collision destroyed the opposing components of their 

 motions and hence -yielded to the earth's gravity sooner than the 

 more elastic ones. Relatively high specific gravity in the material 

 of the deep interior is thought to have arisen at the outset and to 

 have been increased by the selective vulcanism that came into action 

 as growth proceeded. Special emphasis is laid on the selective 

 nature of vulcanism under this hypothesis. The intimate mixture 

 of planetesimals and planesesimal dust gave rise to a multitude of 

 minute contacts between particles of different chemical and physical 

 properties and hence there arose wide differences in the solution 

 points. As the temperature in the growing planet rose, the more 

 soluble portions passed into the liquid state by stages long before 

 the remaining larger portion reached the temperature of solution. 

 In a stressed globe certain of whose stresses are more intense to- 

 ward the center than toward the surface, the solutions worked in 

 the direction of least resistance, for them generally outwards, car- 

 rying heat of liquefaction and leaving the less soluble larger portion 

 behind with temperatures inadequate for further liquefaction until 

 there was a renewed accession of heat. The mechanism thus auto- 

 matically tended to remove the most soluble constituents by progres- 

 sive stages, while it tended to preserve the solid condition of the 



