GQ ILLINOIS ACADEMY OF SCIENCE. 



tion 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 aflfect 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 its 

 impacts, but as the infall is held to have been slow and as this heat 

 was superficial, it may be assumed that it was largely radiated 

 away before it became so deeply buried as to be permanently 

 retained, and so the most of the heat of impact may be regarded 

 as negligible.^ ^ In the original shaping of the planetesimal 

 hypothesis (before the discovery of radioactivity) the main 

 source of internal heat was made to spring from the compression 

 which the deeper parts of the earth underwent by the increase 

 of its mass as the planet grew to maturity. This chief source was 

 supposed to be abetted by heat springing from the rearrangement 

 and recombination of molecules within the mass as time went on. 

 Changes in the distribution of the heat after it was developed were 

 supposed to follow by means of conduction and especially by the 

 transfer of hot fluid matter carrying latent heat. 



It is important to the present discussion to note that the heat 

 generated by pressure did not affect the outer part and that it 

 only began to be sensible when those depths were reached at 

 which the rocks suffered appreciable compression from the weight 

 of the rock-mass above them. Thus the heat gradient so gen- 

 erated would rise only slowly in the outer part of the earth and 

 faster in a systematic way toward the center for a considerable 

 depth, if the compressibility of the rocks remained uniform to 

 indefinite depths. If the compressibility fell off as compactness 

 increased the rate of thermal rise toward the center would have 

 been slower. Compressibility at the surface seems to be nearly 

 proportional to pressure, but the compressibility of rocks after 

 they have been compacted by such pressures as are attained at 

 considerable depths is unknown, and it is necessary to proceed 



i^Geology, Vol. I, p. 533, Chamberlin and Salisbury. 



