EARTH'S INTERIOR ADAMS AND WILLIAMSON 255 



due to a gradually increasing amount of metallic iron mixed with 

 the siliceous rock. The normal tendency for pressure, and hence 

 depth, to increase the velocity is thus offset by the admixture, in 

 gradually increasing amount, of iron (or nickel-iron). 



The material in this region may be thought of as resembling cer- 

 tain meteorites consisting of a heterogeneous mixture of silicates and 

 metallic iron, which is called pallasite. The lower limit of this zone 

 of incomplete segregation is thought to lie at about 3,000 km. depth 

 where the velocity shows distinct evidence of falling off. 



CENTRAL METALLIC CORE 



The remaining part of the earth consists, beyond reasonable doubt, 

 mainly of iron or nickel-iron with density appropriate to the con- 

 ditions of pressure (and of temperature) existing in the central 

 region. This density should increase toward the center, but by a 

 relatively small amount. 



Now, if we assume (a) that the density in the surface layer 

 varies linearly with depth from 2.7 to some chosen density p B at the 

 top of the basic layer, (6) that in this basic layer the density change 

 can be calculated by interpolation between the two curves of figure 

 2, (c) that in the pallasite layer the density changes linearly with 

 depth (the simplest assumption), and (d) that in the central core 

 the density changes parabolically, 31 the fact that the distribution 

 must satisfy the known mass and moment of inertia of the whole 

 earth allows us to solve two simultaneous equations and find the 

 density distribution in the pallasite layer and in the central core. 

 If this calculation be carried out for various values of p 6 , it is found 

 that p a must be close to 3.35 in order to yield a reasonable density 

 variation in the central core. The value 3.45 demands that in the 

 core the density decrease with depth. On the other hand, the value 

 3.25 leads to an unreasonably high density at the center. For this 

 reason the density at the top of the basic layer has been taken as 

 3.35, corresponding, as stated above, to a normal density 3.3 and to a 

 density 4.35 at 1,600 km. The density of the iron would then be 

 9.5 at 3,000 km. and 10.7 at the center. 



These various considerations lead us to adopt more or less tenta- 

 tively, the following arrangement of material within the earth: 

 (1) An outer layer 60 km. (about 35 miles) thick 32 in which the mate- 



a That is, according to the relation : p = ky + for 2 , fci and k 2 being constants. This is the 

 simplest assumption compatible with the necessary condition that the rate of change of 

 pressure with distance is zero at the center. 



M This applies more particularly , to continental areas. In the Atlantic Ocean, and 

 especially in the Pacific Ocean, the thickness of the outer layer is probably much less than 

 60 km. 



