400 Jones — Revieiv of Chamberlin's Groundwork 



There is some disagreement in the interpretation of 

 the records of transmitted vibrations which penetrate 

 depths between the surface of demarkation as between 

 outer shell and inner homogeneous material, and a depth 

 of about half the earth's radius. Knott 13 in his work, 

 and Davison 14 accepts his conclusions, shows an increas- 

 ing rate of velocity for both phases of waves down to 

 depths of 1,400 kilometers (.22R. R=earth's radius) for 

 the secondary and 1,600 kilometers (.25R) for the primary 

 waves. Slight decreases in the rates of velocity increase 

 are indicated at a depth of about 750 kilometers. At 

 these greater depths the velocities reach their maximum 

 and from these points down slowly decline. Oldham on 

 the other hand, both in his paper of 1906 15 and in his 

 discussion of 1918, 16 concludes that there is a steady 

 increase of the rate of transmission, with no marked break 

 in regularity down to a depth of about one-half the 

 earth's radius. The interpretations by Knott and Davi- 

 son would imply a breakdown of the earth's rigidity and 

 elasticity at .22R and .25R, respectively, while Oldham's 

 interpretation would place the breakdown of these states 

 at greater depths. 



All these authorities, however, agree that below a depth 

 of 3.825 kilometers (.6R) the earth body is non-rigid. 

 Below this depth the secondary waves are either blotted 

 out or, according to Oldham 17 " are no longer rep- 

 resented in their typical form, but are replaced by a 

 record of different character, probably not due to any 

 form of wave, which has followed the direct path from 

 the origin, and markedly delayed from the time at which 

 they should have arrived.". 



The conclusions to be drawn from these facts are 

 obvious. On them rests the conclusion of Oldham 18 that 

 the material in the central nucleus has a very low degree 

 of rigidity even against stresses of only a few seconds 



13 C. G. Knott: The Propagation of Earthquake Waves through the Earth, 

 Proc. Royal Soc. Edinburgh, vol. 39, pp. 157-208, 1919. 



11 C. Davidson : Op. tit., p. 157. 



15 R. D. Oldham : The Constitution of the Interior of the Earth as revealed 

 by Earthquakes, Geol. Soc. London, Quart. Jour., vol. 62, pp. 135-174, 1906. 



10 R. D. Oldham : The Constitution of the Earth 's Interior. Nature, vol. 

 102, pp. 235-236, 1918. 



17 Op. tit., p. 235. 



18 Op. tit., p. 235. 



