THE 



JOURNAL OF GEOLOGY 



NOVEMBER-DECEMBER, igu 



THE BEARINGS OF RADIOACTIVITY ON GEOLOGY 



T. C. CHAMBERLIN 



University of Chicago 



To the geologist the center of interest in the phenomena of 

 radioactivity lies in the spontaneous evolution of heat attending 

 atomic disintegration. This interest is the more piquant because 

 the source of the internal heat of the earth is one of the oldest of 

 its problems and the discovery of radioactivity brings into the 

 study an unexpected element. During the last century there 

 was a rather general consensus of opinion that the earth's internal 

 heat was derived from the condensation of the nebula from which 

 the earth was then commonly supposed to have taken its origin. 

 This nebula was usually regarded either as a gaseous body or as 

 a quasi-gaseous meteoritic swarm, and in either case its condensa- 

 tion was thought to have given rise to intense heat. The primi- 

 tive gaseous or quasi-gaseous earth-mass was held to have passed 

 later into a molten globe, and the subsequent incrusting of this 

 to have entrapped in the interior the heat supply of subsequent 

 ages. This older view was still in general possession of the field 

 when the apparition of radioactivity forced a new line of thought. 

 But there was also an alternative view built on the belief that the 

 earth grew up gradually by the slow accession of discrete orbital 

 matter in distinction from the direct condensation of a gaseous 

 or quasi-gaseous mass. In this view, the internal heat arose 

 mainly from the self-compression of the earth-mass as it grew. 



Vol. XIX, No. 8 673 



