DIASTROPHISM AND THE FORMATIVE PROCESSES 68i 



might revolve in harmony indefinitely, as do the earth and moon. 

 It was this concurrency of movement that made the evolution of 

 a planetary system highly endowed with moment of momentum a 

 possibility. Therein lies the soul of the planetesimal theory. But 

 evolution under these conditions requires great lapses of time. 



But lest this be over stressed, it is to be noted that the sub- 

 parallelism of orbits and the subequality of speeds gave greater 

 effect to the mutual attractions of the earth-core and the planet- 

 esimals, and so tended either to bring them together or else into 

 harmoniously adjusted orbits, such as those of the earth and its 

 satellite. Compared with the much more familiar gaseous and 

 meteoritic types, the fundamental tendency of a planetesimal 

 system is not so much direct concentration as concurrent revolution, 

 though, in so far as the nuclei are competent, they gather in the 

 smaller bodies. It seems clear, therefore, that the time required 

 for collecting the planetesimals would be some multiple of that 

 assigned in the preceding case. It is not clear just how large it 

 would be, but if taken at three or four, the total time requirement 

 would equal the maximum estimate of the biological requirement. 

 In the nature of the case, it should not be less, for life-evolution 

 could not proceed until a solid core was formed and the rate of 

 infalling planetesimals permitted a congenial temperature. At any 

 rate, however large may be the latitude for different numerical 

 estimates of the total time and rate of planetesimal infall, it is 

 altogether clear that a precipitate ingathering is incompatible with 

 the mechanics of the planetesimal system. 



THE RATES OF PLANETESIMAL INFALL 



a) The infall of normal planetesimals. — We have already found 

 reasons for thinking that the planetesimals were usually small, as 

 their name implies, and have chosen one-fiftieth of a pound as a 

 working figure. We have also chosen one-third of the total mass 

 of the earth as the amount of material already in the earth-core 

 and two-thirds as the amount still in the form of planetesimals at 

 the beginning of the specifi,ed period of infall. 



The mass of the present earth is 6 X lo'' tons. There would 

 then be 4X10''* planetesimals of the specified mass to be gathered 



