for the Study of Megadiastropliism. 



407 



Heat of Impact. — The efficacy of the heat generated by 

 impact of planet esimals is doubted though probably the 

 additive effect if the larger sized bodies were pelted into 

 a molten or nearly molten surface would be greater than 

 if pelted onto a solid surface. The heat problem is more 

 concerned with the rate of growth rather than with size 

 of planetesimals and their impact effect. 



Resume. — In the speculative field of geogenesis there 

 seems to be no direct or conclusive evidence which forbids 

 belief in a molten stage for the earth at approximately 

 its present size. To assume that the original earth 

 nucleus was about the same size as the present planet 

 and was, by hypothesis, molten seems just as reasonable 

 as to assume that the nucleus was small and grew slowly. 

 But any conclusion in this field is rendered indefinite by 

 some pure assumption with no evidence back of it. More 

 direct evidence is to be sought in a consideration of the 

 present physical state of the earth, previously discussed 

 in its recorded diastrophic history, and in the field rela- 

 tionships of the igneous rocks. As Daly concludes : 33 

 "As critical facts are slowly accumulated, the greatly 

 appealing planetesimal hypothesis of Chamberlin and his 

 co-workers may well serve as a foundation for geological 

 philosophy, though the subsidiary doctrine of essential 

 crystallinity for the earth throughout geological time be 

 not accepted." 



The Earth's Diastrophism. 



The evidence submitted by Chamberlin on the dias- 

 trophic^ capabilities of the earth as indicating a solid, 

 crystalline, and heterogeneous state throughout does not 

 seem conclusive and is susceptible of more than one inter- 

 pretation. 



A "gaseo-molten" earth is pictured by Chamberlin as 

 wasting its dynamic energy in maintaining fluidity leaving 

 "little possibility of shrinkage left except the meagre 

 amount that could arise from further cooling. ' ' 34 But is 

 not this "meagre amount" sufficient to have produced 

 the visible diastrophic record? To attempt to trace a 

 pre-Archean diastrophism far greater in amount than all 



33 R. A. Daly : Op. cit., p. 495. 



34 T. C. Chamberlin: Bull. Geol. Soc. Am., vol. 32, p. 199, 1921. 



