26 R. D. Oldham — T/te Interior of the Earth. 



form. At long distances, on the contrary, the commencement is 

 more gradual; there is no well-marked maximum, but two or more 

 succeeding each other, and the record bears the impress of being due 

 to the successive arrival of more than one group of waves, just the 

 appearance, in fact, which would be anticipated from Dr. Walker's 

 interpretation. Taking all this into consideration it is not possible 

 to accept the supposed second phase at distances beyond 120° as 

 being identical in character with the feature which is so well marked 

 at lesser distances, and in these very long distance records nothing 

 can be l'ecognized which may be identified as the arrival of 

 condensational waves travelling by, a direct path from the origin; 

 if present they are much reduced in intensity and delayed in arrival. 

 Hence we may conclude that the wave paths which penetrate deeper 

 than the outer limits of a central nucleus, extending to something 

 less than half the radius of the earth from the centre, encounter 

 a material which is devoid of rigidity even against stresses of only 

 a few seconds' duration. 



A similar conclusion seems to have been reached by Mr. Harold 

 Jeffreys, if I understand him aright, as the result of a mathematical 

 investigation of the viscosity of the earth, 1 based on tidal 

 deformation and the periodic variation of latitude, so that we 

 have two entirely independent lines of research leading to the 

 same conclusion. 



To sum up, we have found that the interior of the earth is divided 

 into three distinct regions, characterized by differences in the 

 physical condition of the material. They are : — 



1. An outer crust, of matter which is solid in every sense usually 

 attaching to the word. Its thickness is comparatively insignificant, 

 being little more than half a hundredth of the radius. At its lower 

 limit this passes rapidly into 



2. A shell of about half to three-fifths of the radius in depth, 

 consisting of matter to which neither the term solid nor fluid can be 

 applied without introducing a connotation which is contradictory to 

 some of its properties, for while highly rigid as against stresses of 

 short duration, or even of duration measured by years, it is capable 

 of indefinite yielding to stresses of small amount if of secular duration. 

 At its lower limit this passes somewhat rapidly, but more gradually 

 than at the outer limit, into 



3. A central nucleus consisting of matter having little or no 

 rigidity, even against stresses of very short duration. Here the 

 material may be described as fluid, whether liquid or gas, without 

 introducing any contradictory connotation. 



In composition, as distinct from constitution, the earth appears to 

 consist of two parts ; a central portion mainly metallic and principally 

 iron, extending to somewhere between three-quarters and four-fifths 

 of the radius, and an outer envelope composed of stony material. 



Geologically, we have a twofold division, into the outer crust 

 composed of material more or less similar, in composition and 



1 Monthly Notices of R.A.S., May, 1917, p. 454. 



