EARTH'S INTERIOR ADAMS AND WILLIAMSON 251 



PREVIOUS THEORIES OF DENSITY DISTRIBUTION IN THE EARTH 



Laplace's distribution, already mentioned, should perhaps best be 

 regarded as an empirical relation connecting density with depth, 

 and should not be taken to imply anything concerning the cause of 

 the increased density. The law of Laplace has been criticized be- 

 cause it requires too low a surface density in order to yield the cor- 

 rect value for the moment of inertia. Darwin 16 suggested a differ- 

 ent density law with a surface density of 3.7. He held that the 

 ordinary rocks on the outside of the earth were a mere shell, to 

 be considered separately, and that the density immediately beneath 

 should be taken as the starting point. 



The earlier pictures of the earth's interior involved the tacit 

 assumption that the composition of the deeper parts was the same, 

 or practically the same, as at the surface. According to this view, 

 the earth was a huge ball of granite, chemically homogeneous, 

 although possibly molten at the center. But following what might 

 be called the granitic era of geophysics, the belief arose that the 

 center of the earth might be quite unlike the rocks which we find 

 at the surface and in our shallow excavations. More than 50 years 

 ago Dana 16 discussed the possibility of the earth being made up 

 of a central iron core surrounded by silicate rock. Later, 

 Weichert 17 elaborated this hypothesis and postulated a core of 

 density 8.4 within a stony shell 1,500 km. thick and of density 3.4. 

 His arrangement 18 fits both the mass and moment of inertia of 

 the earth very well, and the transition point from rock to metal at 

 1,500 km. is in fair agreement with the sudden change of direction 

 of the curve of earthquake velocities shown in Figure 1; but it 

 takes no account of the density due to compression, and fails to 

 explain why there should not be an actual discontinuity at the transi- 

 tion point. At moderate pressures . the velocity in basic rocks is 

 notably higher than in iron, 10 and at very high pressures this differ- 

 ence will probably increase rather than decrease. Moreover, as may 

 be seen in Figure 1, the velocity beyond 1,600 km. changes very 

 little — contrary to what might be expected of a homogeneous mate- 



"G. H. Darwin. Proc. Roy. Soc. 1883. 

 19 J. D. Dana. Manual of Geology. 1873. 



17 Nachr. Kgl. Ges. Wiss. Gottingen. 1897, p. 221. Phys. Z. 11, 294. 1910. 



18 It may be noted that on the assumption of a core and a shell each of uniform density 

 the radius and density of the core may be calculated from the known mass and moment 

 of inertia and an assumed outer density by the two equations : 



Pi — Pi" x* (pi — p|) 

 Pm — Pl= I* (pi — Ol) 



in which p a is the mean density, pm is the density of a homogeneous sphere of moment 

 of inertia equal to that of the earth, pi is the density of the core, pi that of the shell, 

 and x the ratio of the radius of the core to that of the earth. Thus, if the density of the 

 outer layer is 3.00, its thickness must be 1,300 km. and the density of the core is 8.03 ; 

 and if the outer density is 3.40, the thickness of the shell would be 1,600 km. and the 

 central density 8.45. 



19 See Table 2. 



