256 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



rial changes more or less gradually from granitic to something more 

 basic than a gabbro; (2) a shell extending to a depth of 1,600 km., 

 consisting of peridotite, that is, mainly of iron-magnesium silicates 

 and having a normal density 3.3 and a density at 1,600 km. of 4.35 ; 

 (3) a shell of pallasite reaching to 3,000 km. below the surface, in 

 which silicate rock is gradually replaced by metallic iron (or nickel- 

 iron) not yet completely segregated, the density in this shell chang- 

 ing gradually from 4.35 to 9.5; and (4) below this layer of pallasite 

 a central core of nickel-iron of nearly constant density — varying 

 from a little below to a little above 10. The existence of other lay- 



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Fio. 3. — The density of the earth at various depths according to the present estimate 

 (full-line curve). For comparison Goldschmidt's distribution (dotted lines), and the 

 density law of Laplace (broken line) are included 



ers or of other discontinuities is neither affirmed nor denied. The 

 proposed distribution of material merely attempts to harmonize 

 certain known facts regarding the mass and moment of inertia of 

 the earth, the velocities of earthquake waves, and the compressi- 

 bilities of rocks. It is put forth more as a basis for future specula- 

 tion than as a definite and final arrangement. 



The distribution here described is shown graphically in Figure 3 

 (full-line curve). At the boundary between the various zones the 

 corners are arbitrarily slightly rounded. This diagram also con- 

 tains, for comparison, a plot of Goldschmidt's distribution (dotted 

 lines), and the density according to Laplace's law (dashed line) 

 with surface density 2.7. 



