546 



SCIENCE. 



[N. S. Vol. XX. No. 513. 



interest to the subject which it would other- 

 wise lack. 



Physical geology begins with the solar 

 nebula and the genesis of the earth-moon 

 system. The harmonies of the Solar sys- 

 tem compelled the immortal Kant and the 

 ever-living Laplace to seek the origin of 

 the planets, the sun and the other stars in 

 heterogeneous nebulas which they sup- 

 posed to have condensed about one or sev- 

 eral nuclei. Every attempt to devise an 

 essentially different, hypothesis has failed, 

 and every history of the globe which be- 

 gins after the birth of the planet is un- 

 satisfying. In the drama of the universe 

 there must have been pre-nebular scenes, 

 but of these we have as yet no inkling. 

 The nebular hypothesis, as its authors pro- 

 pounded it, explains the similarity in the 

 composition of the members of the solar 

 system which is indicated by the analysis 

 of meteorites and by the spectroscope, 

 though the facts thus revealed were un- 

 known to Kant and Laplace. It is also 

 compatible with and accounts for the 

 heterogeneity in the composition of the 

 earth manifested in the actual asymmetric 

 distribution of oceans, mountain ranges 

 and anomalies of gravitational force, as 

 well as in the curiously local occurrence of 

 certain ores (such as those of tin and 

 mercury) and in the predominance of cer- 

 tain alkalies among the rocks over wide 

 areas. 



This heterogeneity, however, is of a 

 small order of magnitude. The general 

 dependence of gravity on latitude, the 

 nearly spheroidal shape of the earth and 

 other phenomena show that the distribu- 

 tion of density is nearly symmetrical, Avhile 

 the divergence of the spheroid from the 

 figure characteristic of a fluid of the same 

 mean density and mass as the earth demon- 

 strates that the interior layers of equal 

 density are oblate. These and similar 

 facts are consistent with and are strong 



evidence for the hypothesis that the globe 

 has been fused at least to a considerable 

 depth from the growing surface of the 

 gathering nebulous mass. Nevertheless, 

 Houghton, and more recently Professor 

 Chamberlin, have supposed that the accre- 

 tion of nebulous matter was so slow that 

 the heat of impact did not suffice to pro- 

 duce fusion. The hypothesis of super- 

 ficial fusion is not incompatible with the 

 minor heterogeneity pointed out above; 

 for the laws of diffusion in viscous fluids 

 give proof that sensibly perfect homo- 

 geneity could not be produced even in 

 50,000,000 years throughout a body of 

 liquid originally heterogeneous and pos- 

 sessing a tenth of the mass of the earth. 

 On the other hand, there is no known 

 ground other than mere convenience for 

 supposing an original homogeneity either 

 of the nebula or of the earth. 



The problem of the distribution of 

 density in the earth is one of the most im- 

 portant in all geophysics. It is as signifi- 

 cant for geodesy and terrestrial magnetism 

 as for geology. That Laplace's empirical 

 law represents it approximately is generally 

 acknowledged, but it appears substantially 

 certain that this is merely an approxima- 

 tion without theoretical value. Only ex- 

 tended researches, however, can replace it 

 by one better founded. 



The solidity of the earth is now very 

 generally accepted, though Descartes 's 

 hypothesis of its fluidity, invented to 

 satisfy his erroneous theory of vortices, 

 died hard. Lord Kelvin showed from tidal 

 phenomena that the effective rigidity of the 

 earth is about that of a continuous globe 

 of steel. Professor Newcomb pointed out 

 that the Chandlerian nutation leads to the 

 same conclusion and an almost identical 

 value of the modulus of rigidity, and Pro- 

 fessor George H. Darwin demonstrated 

 that, if the earth is a viscous liquid, its 

 viscosity must be some 20,000 times as 



