694 T. C. CHAM BERLIN 



of radioactive substance. When this was in the state of a mechani- 

 cal sediment it was chiefly deposited on the borders of the basins; 

 when it was in solution it mixed with the waters of the oceans and 

 was later largely concentrated in the oceanic precipitates. Thus 

 the prolonged process of denudation cut away the radioactively 

 richer part of the shield and added it to the undenuded crust of 

 the continental borders and the oceanic basins, thinning the one 

 and thickening the other in a special radioactive sense. Besides 

 this the lower crust in the denuded area was lifted relatively toward 

 the cold surface, while in the depositional area it was relatively 

 depressed beneath a growing radioactive mantle. 



The rise of the denuded embossments of the crust was attended 

 by elastic expansion of the whole sector of the earth beneath, since 

 the gravitative pressure was lessened throughout. A lowering 

 of the melting-points indeed attended this and doubtless a change 

 also of the mutual-solution conditions, but this was anticipated 

 by the elastic expansion and its instantaneous cooling effects, a 

 point usually overlooked. 



In addition to this immediate expansional effect, it is held by 

 some geologists, with whom I am glad to associate myself, that 

 the protruding portions of the continents tend to lateral creep and 

 that this carries with it tensional effects as well as some further 

 elastic expansion. At the same time, the penetration of surface- 

 water is promoted and this aids effectively in carrying off the heat 

 of the outer crust. It may be observed that while meteoric cir- 

 culation penetrates to considerable depths beneath land surfaces 

 there is little reason to think that there is any effective circulation 

 to appreciable depths in the ocean beds. 



One further agency is believed to co-operate with these at a 

 lower horizon but this can be touched only with reserve as it 

 involves joint studies yet in progress upon which I do not feel at 

 liberty to draw further than may be necessary merely to indicate 

 their bearing on this particular problem. 1 In a previous part of 



1 The studies are common to my son, Rollin T. Chamberlin, and myself and in 

 the particular here applicable the junior partner is the leader in pursuance of lines of 

 inquiry growing out of his studies on "The Appalachian Folds of Central Pennsylvania," 

 Journal of Geology, XVIII, No. 3 (April-May, 1910). 



