224 T. C. CHAMBERLIN— THE GREATER EARTH 



The Spheroid or anomalous Effects on seismic Waves 



By far the largest part of the earth's material transmits earthquake 

 waves directly through the interior along curves that are related to 

 chords, with sufficient fidelity to preserve the distinctness of the first and 

 second sets of preliminary waves and thus render the record interpret- 

 able ; but in the center of the earth there is a spheroid, embracing perhaps 

 a fifteenth of its volume and a fifth of its mass, which in some way affects 

 the transmission, so that the record is illegible. The line A. E. S. W. of 

 Diagram IV outlines approximately this central spheroid. Neither the 

 precise nature of the disturbing effect nor the cause of it has yet been 

 satisfactorily determined. One of the more familiar suggestions is that 

 this central spheroid is liquid; another that it is metallic. The first of 

 these is naturally losing plausibility in proportion as evidence increases 

 that this central portion, like the rest of the earth-body, is highly rigid. 

 The metallic theory fits only very clumsily the growing evidence that the 

 interior elasticity and rigidity rise faster than density, not only from the 

 surface to the borders of this spheroid, as now well demonstrated, but all 

 the way to the center, since this seems to be required to meet the mean 

 rigidity and elasticity of the earth as a whole. 



The Core of tpie Earth 



Approaching the problem of this seismic anomaly from the cosmolog- 

 ical side, the hypothesis that this central spheroid is simply the primitive 

 core of the earth formed by the solidification of the nebular nucleus postu- 

 lated by the planetesimal hypothesis seems an easy and natural solution. 

 If the original nucleus concentrated itself along the gaseo-molten line of 

 descent, in the modified way sketched in my recent discussion,^ it would 

 take on a more or less concentric strjicture similar to that logically as- 

 signed the whole earth by the old masters under the gaseo-molten hy- 

 pothesis. The concentric layers, though homogeneous in themselves, 

 would differ one from another in elasticity and density, and so they 

 would be likely to divert the seismic waves by refraction sufficiently to 

 seriously confuse the record. If this view shall find direct support as the 

 evidence grows and skill in interpretation increases, it will aid in settling 

 the as yet open question of the mass of the original earth nucleus. The 

 spheroid of anomalous transmission is of the order of one-fifth of the total 

 mass of the earth. This is not an improbable value for the nucleus, 

 judged from quite independent considerations. 



" Diastrophism and the formative processes. XII. Tlie physical phases of the planetary 

 nuclei during their formative states. Jour, of Geol., vol. xxviii, 1920, pp. 473-504. 



