232 AT^NUAL KEPOET SMITHSONIAN INSTirUTION, 1916. 



the order named, such considerable depths would distinctly favor an 

 accretionary origin as against a molten origin. But the determina- 

 tion is inconclusive. 



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 atmos- 

 phere should have greatly checked the plunge of the planetesimals 

 and thus have largely dissipated them into dust in the upper atmos- 

 phere 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 mean 

 temperature of the earth body was probably never above the local 

 solution or fusion point of the more refractory material during the 

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

 lowing out as well as may be the probable rates and conditions of 

 growth, the most tenable concept of the state of the earth's interior 

 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 30 or 40 per cent, is left in- 

 determinate by present lack of knowledge of the physical state of 

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

 doubtful by their lack of strict sphericity — the nucleus was doubt- 

 less originally molten. If the constituents of the laiot 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 reduced in 

 mass and speed by the atmosphere, probably retained its solid con- 

 dition, with negligible exceptions, throughout the process of ac- 

 cretion^ except as selected portions passed into the liquid state and 

 became subject to extrusive action. An intimate heterogeneity nat- 

 urally prevailed throughout the whole mass so aggregated. A se- 

 lective 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 thus thought to have arisen at the outset and to 

 have been increased by the selective vulcanism that came into action 



