THE TERRESTRIAL DISTRIBUTION OF RADIUM 29 



is a remarkable decrease in their rates of propagation, a fact 

 which indicates a high degree of resistance to compression, and 

 therefore a marked change in constitution. 



Oldham ! originally suggested a thickness of about 20 miles 

 for the outer shell of heterogeneous and fractured rocks which 

 surrounds the homogeneous zones of the interior, and which 

 we may conveniently describe as the earth's crust. He has 

 now 2 halved his former estimate and gives 10 miles as a 

 probable value. The late Prof. Milne 3 in 1906 deduced a 

 thickness of 30 miles from the data then at his disposal. 

 These varying figures may mean that the thickness itself 

 varies from place to place, but in any case they are certainly 

 of the right order. Some recent experiments by Adams 4 have 

 demonstrated that under the conditions of pressure and 

 temperature believed to obtain in the earth's crust, empty 

 cavities may exist to a depth of at least 11 miles. King 5 

 has similarly shown that small cavities will remain open at 

 all depths up to about 20 miles, provided that the temperature 

 is not excessive. We may therefore conclude that the earth's 

 crust as defined above is at least 10 miles thick and may be 

 more than 20. 



According to Wiechert, 6 the earth is built up essentially of 

 two strongly contrasted zones separated somewhat sharply by 

 a surface of discontinuity at a depth of about 950 miles. An 

 inner core of mean density 7*8-8o is postulated, this being 

 surrounded by a rocky mantle having a mean density of about 

 3*4. Oldham finds a surface of discontinuity beneath the 

 Pacific at a depth of 1,000 miles, 7 which is in close agreement 

 with Wiechert's results, but for the main body of the earth 

 he gives 2,000-2,400 miles as the probable depth. The whole 

 subject, however, is still in its infancy, and the discrepancies 

 suggest that the problem is less simple than these pioneer solu- 

 tions would indicate. That this is undoubtedly the case is 

 shown by more recent work. K. Zoeppritz, L. Geyer, and 

 B. Gutenberg have made an exhaustive study of the earth- 



1 Q.J.G.S., vol. lxii. p. 456, 1906. 



2 Nature, August 21, 19 13, p. 635. 



3 Milne, Bakerian Lecture, Proc. Roy. Soc, A., vol. lxxvii. p. 365, 1906. 



4 Journ. Geo?., vol. xx. p. 97, 1912. 



5 Ibid. vol. xx. p. 119, 1912. 



6 Deutsche Rundschau, pp. 376-94, 1907. 



7 QJ-G.S., vol. lxiii. p. 344, 1907. 



