248 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1923 



For the two assumed surface densities the average density below 

 3,400 km. would be 15 and 20, respectively — obviously much too 

 high to be reached by any reasonable extrapolation of the density 

 curves. The high central density demanded by the comparatively 

 low density shown in Figure 2 may be considered a consequence of 

 the fact that the core of radius 3,000 km., has only one-ninth of the 

 volume of the earth, whereas 0.3 to 0.4 of the mass remains to be 

 accounted for. 



It is therefore impossible to explain the high density of the earth 

 on the basis of compressibility alone. The dense interior can not 



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Fig. 2. — For two Initial densities, 3.0 and 3.5, these curves show the change of density 

 due to compression alone. The values are obtained from the variation of compressi- 

 bility, which In turn is determined from the earthquake velocity-depth curve 



consist of ordinary rocks compressed to a small volume. We must, 

 as a consequence, fall back upon the only reasonable alternative, 

 namely, the presence of a heavier material, presumably some metal, 

 which, to judge from its abundance in the earth's crust, in 

 meteorites, and in the sun, is probably iron. We thus arrive at the 

 conclusion accepted by the majority of geophysicists, but, in addi- 

 tion, we have here (1) a quantitative estimate of the increase of 

 density due to compression alone, and (2) direct evidence of the 

 presence in the interior of the earth of a dense material such as iron. 



