BEARINGS OF RADIOACTIVITY ON GEOLOGY 683 



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. 1 In the original shaping of the planetesimal hypothe- 

 sis (before the discovery of radioactivity) the main source of inter- 

 nal 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 rearrangment and recom- 

 bination 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 

 -began to be sensible only 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 generated 

 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 here by alternative 

 hypotheses. The extrapolation of the curve found under experi- 

 mental pressures is of course entitled to precedence and this alter- 

 native was used as the basis of the first approximation to the heat 

 curve of the earth's interior. For the other factors, such as specific 

 heat, necessarily taken into account in the computation, assump- 

 tions as near to known facts as possible were made. On these 

 assumptions it was found that the heat generated between the 

 surface and the center of the earth may be represented by a curve 



1 Chamberlin and Salisbury, Geology, I, 533. 



