THE INTERIOR OF THE EARTH 



which constitutes two-thirds of the surface has a 

 density of only 1 . ; proves that the inner mass of 

 the earth beneath the crust is composed of much 

 heavier substances than the crust itself. After 

 making due allowance for all possible increase of 

 density by compression from the superincumbent ex- 

 terior, it necessitates the view that the interior is 

 formed by the heavier metals, with possibly some of 

 their sulphides. The fallen aerolites by their analysis 

 support this view, since most of them consist of me- 

 tallic iron united with small percentages of nickel, 

 cobalt and other metals, showing the absence of 

 oxygen in their former state. Native or metallic 

 iron is never found on or in the earth, excepting very 

 rarely in the form of small metallic grains in Ba- 

 salt ; the latter a product of ejections of lava, or fused 

 volcanic matter, from fissures in the rocks of the 

 earth's surface. This condition of the heavier metals 

 of the earth's interior is what might be expected from 

 the theory of the earth's evolution, the high tempera- 

 ture of the molten mass dissociating the elements, and, 

 as the molecules cooled, permitting the denser and more 

 condensible atoms to become the centre of the form- 

 ing mass. After the earth became cool enough to 

 allow the present conditions of chemical affinities to 

 exist, the metals were excluded by their own mass 

 from all but external surface contact with oxygen, 

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