DIASTROPHISM AND THE FORMATIVE PROCESSES 675 



THE TIME OVER WHICH THE INGATHERING OF 

 PLANETESIMALS WAS SPREAD 



It is obvious that the time over which the infall of planet- 

 esimals was spread is an essential factor in determining whether 

 the heat of their infall would melt the earth surface or not. And 

 so if there are any naturalistic evidences bearing on this point, they 

 should be brought under consideration at once so that they may 

 serve as guides or tests where assumptions have to be made in 

 trying to deduce the period of infall from the mechanics of the 

 case. Successful study of earth history has been found to rest 

 much more largely on naturalistic considerations than on deduc- 

 tion, especially when the premises involve so much that is assump- 

 tive. An approach along naturalistic lines may be found in biologic 

 evolution combined with geologic chronology. 



I. The intimations of biologic evolution. — It seems to be the con- 

 sensus of opinion among those best fitted to judge that the portion 

 of life-evolution that has taken place since the faunas and floras 

 of the early Paleozoic offered a fair criterion for judgment, is of 

 the order of one-tenth of the total life-evolution, or some such pro- 

 portion. This proportion will therefore be made the basis of the 

 time-scale used in the following discussion. It will be easy to 

 modify the results of the computations to suit any other proportion 

 that may be thought nearer the reality. I do not think that any 

 other proportion which is tenable will change the general tenor of 

 the conclusions, so far as these bear on the melting effects of 

 planetesimal infall. 



Two geologic time-scales are now in use, an older one built on 

 estimates of the present rates of geological progress, and a newer 

 one built on radioactive processes. For myself I regard the latter 

 as much the more trustworthy. The former seems to me to need 

 radical correction (i) for the exceptional speed of present denuda- 

 tion due to the stripping of very large portions of the surface of 

 its native protection, and (2) for the exceptional speed induced by 

 the present high relief of the surface brought about by recent 

 dias trophism.^ But let us use both scales. Those who prefer 



^ See Article VIII of this series, "The Quantitative Element in Circumcontinental 

 Growth," this Journal, Vol. XXII (1914), pp. 516-26. 



