lxXXviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I909, 



space, with the other the internal constitution of our planet, and 

 in each case the message to be deciphered is brought to us by 

 elastic waves. 



Already the obscure signs of a subdivision into parts, to which 

 Milne and Oldham have called attention, are beginning to acquire 

 greater definition. As a result of bis latest seismological investi- 

 gations, Wiechert * concludes that the earth consists of a metallic 

 kernel with a density slightly over 8, surrounded by a stony crust, 

 having a mean density of 3*4 and a thickness of 1500 kilometres ; 

 he regards this as a striking confirmation of a view to which he had 

 been led by other considerations so long ago as 1896. The crust 

 itself he regards as divided into two sheets by a more yielding 

 intermediate layer, which occurs at a depth of from 100 to 200 

 kilometres. 



Geological Time. — It is but a few years since the discovery 

 of a long unsuspected source of radiate energy existing within the 

 earth's crust immensely widened the possible limits of geological 

 time. 2 The quantitative experiments of Mr. Strutt lead us to 

 suppose that the liberation of this energy does not continue very 

 far below the surface ; it would seem to cease at a depth of about 

 40 miles ; the change of properties conjectured by Milne to occur 

 at this depth and the more yielding layer inferred by "Wiechert may 

 eventually be found to stand in some connexion with this limit. 



Much constructive work based on the properties of radium is now 

 in progress. 3 Mr. Strutt 4 in particular is engaged in an endeavour 

 to apply certain of its transformations to the determination of a 

 chronological scale. One of the products of the disintegration of 

 radium is helium, which is evolved at a uniform rate, and thus 

 affords a means of measuring time. Let us suppose that the radium 

 which enters into the substance of a mineral was introduced con- 

 temporaneously with its formation, and that the helium subsequently 

 generated has been unable, under natural conditions, to escape. If, 



1 E. Wiechert, ' Die Erdbebenforschung, ihre Hilfsmittel & ihre Eesultate 

 fur die Geophysik' Physikalische Zeitschrift, 1908, pp. 36-47. 



2 J. Joly, ' Eadium & the Geological Age of the Earth ' Nature, vol. lxviii 

 (1903) p. 526 ; E. J. Strutt, ' On the Distribution of Eadium in the Earth's 

 Crust, & on the Earth's Internal Heat ' Proc. Eoy. Soc. ser. A. vol. Ixxvii 

 (1896) p. 472. 



3 J. Joly, ' Uranium & Geology,' Presidential Address to Section C, Brit. 

 Assoc. ' Nature ' vol. lxxviii (1898) p. 456. 



4 E. J. Strutt, Proc. Eoy. Soc. ser. A. vol. lxxxi (1908) p. 272. 



