682 T. C. CHAMBERLIN 



With such a plethora of heat at the start as a molten earth implies 

 and with a new agency whose current production of heat would 

 seem to be excessively great if its prevalence were not construc- 

 tively minimized, it is not with regret that I feel absolved from the 

 task of finding a reconciliation between this venerable view and 

 the requirements of juvenile discoveries. 



The discussion of Professor Joly, 1 though not explicitly based 

 on the theory of a molten earth, is sympathetic with the general 

 tenets associated with such an earth, and his treatment may be 

 taken as offering the best approach to a reconciliation that seems 

 now possible. 



It is interesting to note, however, that when Professor Joly 

 reached the critical question of a possible mode by which the 

 surface concentration of radioactivity could have come about 

 {Radioactivity and Geology, 184) he turned to the accretion or 

 planetesimal hypothesis. While he indicated the central line of 

 action on % which the concentration might have been accomplished 

 he left without elucidation the line of reconciliation between the 

 heat gradient postulated by the planetesimal view and the 

 gradient he deduces from radioactivity. 



It is the chief purpose of this paper to set forth what seems 

 to me to be the true harmony between the new light shed by radio- 

 activity and the tenets of the planetesimal view as shaped by 

 me before the discovery of radioactivity and to show the 

 co-ordination of the planetesimal and radioactive agencies in jointly 

 leading to the results observed. To this end it is necessary to sketch 

 with some care the thermal features of the planetesimal view in 

 the form to which preference was given from the start so that it 

 may be clear just what part radioactivity plays in the assigned 

 co-operation. 



On the assumption that the earth grew up by the accession of 

 planetesimals, whatsoever heat arose from the condensation of the 

 nucleus about which the growth took place centered in the inner- 

 most parts and can affect present surface phenomena only by 

 transfer. The infalling matter that is supposed to have built up 

 the earth to its mature size must have generated much heat by 



1 Radioactivity and Geology, 154-82. 



