I907.] AND CONTRACTION OF THE EARTH. 207 



by molecular yielding towards the nucleus, which furnished the heat 

 that caused the expansion. 



3. Now to burst the crust would be doing external work. Also 

 since the crust is very strong and its substratum deep, the movement 

 of these superincumbent layers would be very difficult, and involve 

 the expenditure of much energy. The easiest mode of adjusting 

 the tension therefore would be by the molecular movement in the 

 outer shells towards the inner shells of the nucleus, which have 

 shrunk by loss of heat, and in such an adjustment the crust would 

 not be disturbed. 



4. No surface disturbances traceable to deep nuclear movements 

 have occurred within the historical period, and geological evidence 

 shows the fixity of the continents throughout all geological time. 

 Therefore we may hold that earthquakes arising in the superficial 

 layers are the only causes which have been active in shaping the 

 topography of the surface since the moon was separated from the 

 globe. 



5. Since the effect of radial shrinkage in 100 million years is 

 shown to be very small, different estimates ranging from 0.26 to 

 1. 16 miles (cf. Chamberlin and Salisbury's " Geology," Vol. I., p. 

 573), it seems absolutely certain that progressive diffusion of heat 

 deep down in the earth could produce no sensible effect at the 

 surface. 



Speaking of these effects of secular cooling, Chamberlin and 

 Salisbury remark that " they are exceedingly small. Unless there 

 is a very serious error in the estimated rate of thermal loss, or in 

 the coefficients of expansion, cooling would seem to be a very inade- 

 quate cause for the shrinkage which the mountain foldings, over- 

 thrust faults, and other deformations imply. This inadequacy has 

 been strongly urged by Fisher^ and by Button.- In view of the ap- 

 parent incompetency of external loss of heat, the possibilities of dis- 

 tortion from other causes invite consideration." 



Thus the opinion here expressed is favorable to the views devel- 

 oped in the present investigation. Can it then be that the present 

 negation of deep-seated movement since the earliest geological time 

 is without significance in the argument against the theory of the 



' " Physics of the Earth's Crust," Chap. VIII. 

 'Penn. Monthly, Philadelphia, May, 1876. 



PROC. AMER. PHIL. SOC, XLVI. l86 O, PRINTED SEPTEMBER 3, I907. 



