422 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 19 
changes very little—contrary to what might be expected of a homo- 
geneous material under a constantly increasing pressure. It may 
be argued that the effect of temperature in this region may decrease 
the elastic constants and hence also the velocity. But on any hypoth- 
esis the temperature is not increasing rapidly as far down as this, 
and moreover it seems improbable that increasing temperature would 
decrease both the rigidity and the bulk modulus by the right amount 
so that the two velocities would remain so nearly constant. 
In recent times Goldschmidt™ has postulated an arrangement of 
the matter within the Earth as follows: (1) an outer silicate layer 
120 km thick and of density 2.8; (2) a layer of dense silicates (eclogite) 
extending to 1200 km depth with density varying from 3.6 to 4.0; 
(3) an intermediate zone of sulfides and oxides of density 5.6 and 
extending to 2900 km; and (4) a central core of nickel-iron having a 
density .about 8. The average density of this arrangement is very 
close to the accepted value, and the moment of inertia although 3 per 
cent too low can be considered in fair agreement. Zoeppritz, Geiger 
and Gutenberg,’ and Mohorovi¢ié,!® and others, have adduced evi- 
dence in favor of the existence of various shells or layers in the Earth. 
For lack of space we can not discuss them here, but will pass on to a 
statement of the distribution here proposed. 
PROPOSED DENSITY DISTRIBUTION 
Outermost layer. The average density of the igneous rocks!’ at 
the surface is about 2.8. Allowing for a small amount of sedimentary 
rock let us take the surface density as 2.7. The density and basicity 
of the rocks must increase with depth, the increase being gradual 
but not necessarily regular. Probably the outer 10 to 20 km has the 
average composition of a granite or a granodiorite. From the seismo- 
graphic records of the Oppau explosion Wrinch and Jeffreys'’ found the 
velocity of the longitudinal waves to be 5.4 km/sec which agrees well 
with 5.6, the velocity in typical granite at moderate pressures as 
determined by Adams and Williamson" from the elastic constants of 
the rock. Theoretically the surface velocity can be obtained from the 
initial slope of the ordinary time-distance curve, but on account of 
144V. M. Goldschmidt. Z. Elektrochem, 28: 411. 1922. 
16 Nachr. Kgl. Ges. Wiss. Gottingen. 1912, p. 121. 
16 Beitr. z. Geophysik 18. 
17H. S. Washington. Bull. Geol. Soc. Am. 33: 388. 1922. 
18 Dorothy Wrinch and Harold Jeffreys. Roy. Astr. Soc., M. N., Geophys. Suppl. 1: 
15-22. 1923. 
19 On. ctt., p. 520. 
