EDITORIAL 669 



reaching the final, most reasonable, conclusion, as he saw it. 

 However, in judging alternatives, there are many opportunities 

 for divergence of opinion, and among those which are offered in 

 this particular case Barrell has seized upon the one which is most 

 significant in separating two distinct lines of geologic inference: 

 Did the earth grow up as a comparatively cold, solid globe, or did 

 it become molten and cool down from that condition? The 

 question turns upon the size of the planetesimals and their manner 

 of infall. We may best let Barrell state the alternatives by quoting 

 from a lecture delivered by him at Yale in November, 19 16: 



Under the terms of either nebular or planetesimal hypothesis a scattered 

 state of the planetary material is implied as a stage antecedent to the origin 

 of the planets. Was this growth of the planets geologically slow or rapid? 

 Did it take tens or hundreds of millions of years, or was it on the contrary 

 largely accomplished in tens or hundreds of thousands of years ? Was the 

 material largely in dust-like or molecular form or was it to a large extent in 

 nuclei of considerable size? From these different postulates very divergent 

 consequences may be traced in the formative stages of the earth; and finally 

 the present nature of the earth itself may speak in favor of one or the other of 

 these views. 



Chamberlin adopts the hypothesis that the stages of earth-growth were 

 very prolonged, even geologically speaking, and that the accretion was domi- 

 nantly of dust-like or molecular particles. According to him the buUding up of 

 the planets followed three stages: first, the direct condensation of the nuclear 

 knots of the spirals into liquid or soHd cores; second, the less direct collection 

 of the outer or orbital and satellitesimal matter; third, the still slower gathering 

 up of the planetesimal material scattered over the zone between adjacent 

 planets. This third factor in Chamberlin's view is regarded as very important, 

 and he believes this diffused matter contributed much of the earth's substance, 

 very slowly and in dust-like form. This is one of the critical points in the 

 details of the theory upon which turns much of the development of the following 

 argument. 



Chamberlin conceives the earth to have been built up as a solid body, not 

 to have been fluid or viscous at any time later than the early nuclear stage, 

 and to have begun to hold an ocean by the time it contained thirty or forty 

 per cent of the present mass. Such liquid rock as was generated by compres- 

 sion or radioactivity during earth-growth is regarded as having been kneaded 

 and squeezed to the surface, where it solidified approximately as fast as it was 

 formed. In earth-growth the denser planetesimal dust, he argues, tended to 

 be somewhat segregated into the primitive ocean basins, and served to 

 maintain in them, as the earth was built outward, a greater density than in 

 the elevated zones between, establishing thus a relation between density and 

 elevation. 



