64 ILLINOIS ACADEMY OF SCIENCE. 



and he was led to this final statement : "If we said that the richer 

 part of the crust must be between 9 and 15 kilometers deep, we 

 cannot be far from the truth. This appears to be the best we can 

 do on our present knowledge."^ It is to be noted that these 

 deductions are reached on the supposition that all the internal 

 heat given out arises from radioactivity ; no margin is left for 

 any original heat or for secular heat from any other source. On 

 the other hand, the computations seem to take no account of loss 

 of heat by means of igneous extrusions. 



These remarkable deductions raise two questions of radical 

 import : 



(1) If supplies of heat are generated currently by radioactivity 

 in such abundance that it is necessary to put these severe limits 

 on the distribution of radioactive substances, must we abandon 

 entirely all further consideration of supposed supplies handed 

 down from a white hot earth or from any other form of the 

 primitive earth? 



(2) Is there among the internal processes previously postu- 

 lated any that provides a way in which such a concentration at 

 the surface might naturally have taken place, or must we find a 

 new geological process to fit the new thermal difficulty ? 



The rigor of the dilemma is softened somewhat by noting that 

 the deductions of Strutt, Joly, and their colleagues are based 

 simply on comparisons between the heat-generating power of 

 radioactive substances in the crust and the conductive power of 

 the crust. The functions of igneous extrusion as a mode of trans- 

 fer of internal heat do not seem to be taken into account. This is 

 not unnatural since the heat carried out by extrusive matter and 

 by waters heated by igneous intrusions has not usually been 

 regarded as an important factor in reducing the high temperature 

 inherited by the earth under the older view. But the movement of 

 igneous matter and of waters and gases heated by it has been 

 made to play an essential part in the working concepts that have 

 been based on the planetesimal hypothesis. There will be occa- 

 sion to return to this critical difference of view. 



When the apparent excess of thermal riches arising from the 

 new source was first realized an escape from the dilemma raised 

 by it was sought in the natural supposition that the disintegration 

 of uranium and thorium was restrained by pressure in the depths 

 of the earth, and that, though present there, their activity was 



•Loc. cit, p. 183. 



