Geology 



considered to have formed the nuclei of the planets, 

 which have since increased in size by the gathering in 

 of the smaller scattered fragments through the course of 

 the ages. Chamberlin considers that the gathering in 

 of the planetesimals, as he designates these tiny planets, 

 is still in progress, and regards the meteorites which 

 enter our atmosphere at the rate of many hundreds a 

 day as belated planetesimals. 



This planetesimal hypothesis does not necessitate the 

 occurrence of extremely high temperatures in the Earth's 

 interior, though doubtless much heat would be evolved 

 during the compression of the original nucleus, as layer 

 upon layer of planetesimal matter was added to its 

 surface. 



The distribution of temperature in the outer part of 

 the solid Earth would render it certain that exceedingly 

 high temperatures must occur near the centre were it 

 not for recent discoveries which make it unnecessary to 

 believe that the Earth-heat is merely the residuum of an 

 original supply imparted to it as the result of central 

 compression or otherwise. 



The knowledge that has been added to our store by 

 the discovery that radium and certain other elements 

 which act in a similar manner are continually producing 

 heat, and that these elements are present in many ordinary 

 rock materials, justifies us in concluding that here is a 

 considerable source of heat which must profoundly affect 

 the underground temperature gradient, and invalidate 

 any calculations as to internal temperatures, based on 

 the supposition that the present distribution of tempera- 

 ture is due to the simple cooling of a heated Earth. 



An examination of specimens of many different kinds 



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