PRESIDKNTIAL ADDRESS- -SECTION B. 2/ 



place would naturally lead us to suppose that at one time they 

 had a more uniform distribution. 



Professor Strutt concludes that a distribution of radium. 

 equal to that observed in the surface rocks, down U) a depth 

 of 45 miles, would supply sufficient heat to account for the 

 observed temperature gradient of the crust of the earth, while 

 Joly's estimate for the thickness of the radio-active crust is 

 approximately 20 miles. These views, however, apparently 

 take no account of heat derived from other sources such as 

 movements within the crust, chemical changes, etc. ; nor of loss 

 of heat by volcanic action. As further supporting the idea of 

 the great influence of radio-activity on the internal temperature 

 of the crust, it is of interest to note that Joly's investigation 

 into the radio-activity of the Simplon and St. Gothard rocks 

 Ijrought out a remarkable correspondence between the estimated 

 radium content and the oliserved temperature gradient, the 

 higher gradient corresponding with the higher radium content, 

 whereas in the case of the lower gradient the rocks examined 

 showed a decided fall in the amount of radium present. 



Let us now see whether the more generally accepted views 

 as to the constitution of the inner earth support the idea of a 

 comparatively thin radio-active crust or not. Although there 

 are some who hold contrary opinions, it is certainly widely 

 accepted that the earth is solid throughout, and consists of a 

 dense metallic core probably approximating in composition to 

 that of the heavier class of meteorites, which consist almost 

 entirely of nickel-iron, surrounded by more stoney material, 

 showing, on the whole, a gradual decrease in density towards 

 an outer crust. The arguments from specific gravity and pres- 

 . sure support this view, and the more recent hypothesis of 

 Chamberlin as to the origin of the earth involves the building 

 u]) of an essentially solid globe, this being eft'ected, according to 

 his view, by gradual aggregation and accretion from minute 

 bodies, or planetesimals. constituting portions of a spiral nebula. 



The results so far obtained by various workers from investi- 

 gations into the phenomena of earthquake waves unfortunate]}- 

 difi^er somewhat amongst themselves. But, in any case, they 

 demonstrate that the interior of the earth is very much denser 

 and more rigid than the crust, owing to the much higher velocity 

 at which the earthquake waves travel through it than through 

 the material forming the crust. Professor Milne has calculated 

 that the change in velocity due to change in the condition of 

 the interior commences at a depth of about 30 miles. R. D. 

 Oldham also found a marked difl^erence between the crust and 

 the interior, but he divides the interior into two zones, the inner 

 being of unknown composition owing to a decrease that he 

 claims to have observed in the velocity of certain of the waves 

 as they traverse the central core. Further, from the results 

 of recent observations carried out in Germany, it is concluded 

 that beneath the outer crust there are four zones, the innermost 



