260 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1&23 



from the surface to the center of the earth is some 4,000 miles, of which hardly 

 more than a mile has been actually explored. What properties or what sub- 

 stances even a few more miles might reveal is a matter for inference alone, 

 inference for the most part with a very inadequate if not an insecure founda- 

 tion of fact. 



The physical properties and the chemical composition of the in- 

 terior of the earth are not yet amenable to precise measurement. 

 But, starting from recent determinations of the compressibility of 

 rocks, in conjunction with the known velocities at which earthquake 

 shocks are transmitted through the earth, it is possible to draw im- 

 portant conclusions concerning the nature of the material far within 

 the earth. For example, we can estimate the pressure at a given 

 depth, and we can tell approximately the amount by which the 

 density of rocks is raised by these enormous pressures. We know 

 definitely that rocks can not be squeezed into a volume so small that 

 their density would be sufficient to account for the high density of 

 the earth as a whole. Beyond reasonable doubt, the earth has a 

 metallic core of iron, or nickel-iron, the diameter of this core being 

 about one-half that of the earth. Bordering the iron core, more- 

 over, there is a fringe of mixed iron and silicate, and surrounding 

 this is a silicate shell. We can determine with fair accuracy the 

 rigidity of the material at a given depth within the earth, and can 

 assure ourselves that, except near the surface, the rigidity is con- 

 siderably greater than that of steel. Finally, as to chemical com- 

 position, the earth, except for the surface layer — which in compari- 

 son with the whole earth is of negligible volume — is composed mainly 

 of metallic iron and iron-magnesium silicates. Accordingly, the 

 earth consists almost entirely of four elements: Iron, magnesium, 

 silicon, and oxygen. 



