DIASTROPHISM AND THE FORMATIVE PROCESSES 677 



evidences are regarded as among the most cogent that bear upon 

 the duration of the early history of the earth. 



Qualified and defined as thus specified, the period of efifective 

 planetesimal infall subsequent to the nuclear stage is made to 

 range from 600,000,000 years on the old geological scale, to 2,400, 

 000,000 years on what seems to me to be the more probable radio- 

 active scale. 



These assignments of time may impress some readers as very 

 long, but the question is to be asked anew, are they longer than 

 the biological evidence requires? We shall soon inquire whether 

 they are any longer than the mechanics of the case warrant. But 

 before passing on, it is to be noted that the interpretation of 

 biological evolution should no longer suffer from duress due to 

 supposed limitations of time, such as were vigorously urged during 

 the last half of the last century by advocates of the contractional 

 theory of the sun's heat and of other physical tenets which were 

 really less well grounded than the biological and geological inter- 

 pretations. This alien stress is now not only lifted, but a new 

 theoretical urgency of precisely opposite import has taken its 

 place, a seemingly imperative need to find a source of heat for the 

 maintenance of the stars of such potency as will enable them to 

 serve their indicated functions in the protracted history of star 

 clusters and our stellar galaxy. For this, a stellar longevity of 

 the order of ten billion years, or some such great period, seems to 

 be required. Short of trespassing on some such time allowance as 

 that, biology and geology cannot be said to be necessarily restricted 

 for lack of solar endurance. The seeming demand of biological 

 and geological evidences for a total earth age of three or four 

 billion years need not be thought extravagant or unreasonable, if 

 either class of evidence is found to really require it. 



2, The intimations of the planetesimal mechanism. — Let us now 

 turn to the planetesimal mechanism to see what may be its most 

 probable time requirements. Neglecting planetesimals of high and 

 unusual orbital range, a fair and at the same time conservative 

 working approximation to the extent of that portion of the planet- 

 esimal field which was tributary to the earth, may be made by 



