406 Jones — Revieiv of Chamberlin's Groundwork 



The earth's sphere of control is approximately 700,000 

 by 900,000 miles in cross-section area. For the nucleus 

 it would be smaller but still large. It would seem evident 

 that any smaller body passing within these limits would 

 either be entrapped or thrown into a satellic orbit unless 

 velocity gave to it a sufficient momentum to carry it 

 through. But by hypothesis the planetesimals were 

 "moving in the same general direction, at somewhat 

 similar speeds." 30 Eelative velocities were thus materi- 

 ally reduced. It would seem that the orbital arrange- 

 ments then would favor a quite rapid entrapment. The 

 task of the nucleus is thus materially reduced but even 

 approximate determination of growth rate is speculative 

 since the size of the nucleus is also speculative. There 

 is no direct evidence, however, to show that the growth 

 may not have been quite rapid. 



Size of Planetesimals. — As Barrell has pointed out 31 

 the only direct evidence we have as to size of planetes- 

 imals is to be sought in the asteroids. These bodies range 

 in size and numbers from a few over several hundred 

 miles in diameter to many ten miles in diameter and 

 probably many more of still smaller size. The lunar pits 

 offer only questionable evidence, for the cause of these 

 features is not known. But if, as Daly points out, 32 the 

 pits are due to impact, then they constitute the record 

 of the last infall since they are not veneered over by 

 finer material. 



If there was any great amount of interplanetary dis- 

 persed material doubtless much of it, or most of it, was 

 finely divided, but there is no evidence to show that there 

 were not, and there is some evidence to show that there 

 were, numerous bodies of sizes up to some hundreds of 

 miles in diameter. These larger bodies would avoid 

 entrapment the longest owing to their great momentum. 

 The infall may very possibly have been of a larger pro- 

 portion of finer material at first, with only occasional 

 larger bodies, and finally of a greater proportion of 

 larger bodies. 



30 T. C. Chamberlin ; Ibid., p. 40. 



31 Joseph Barrell and Others : The Evolution of the Earth. Yale Univ. 

 Press, New Haven, Ct., 1920, p. 26. 



32 E. A. Daly : The Planetesimal Hypothesis in Eelation to the Earth, 

 Scientific Monthly, May, 1920, p. 489. 



