EARTH'S CRUST — WASHINGTON. 295 



being a nucleus or core of iron-nickel and possibly other heavy 

 metals, above this a zone of heavy silicate rocks, and at the surface 

 the lighter silicate rocks of the "crust," but presumably passing 

 gradually one into the other, without sharp borders. 



Wiechert and Knott have recently shown, through a study of 

 the propagation of earthquake waves, that there is a change in the 

 material, or in the physical properties of the material, at a depth 

 of about 0.5 of the earth's radius. Still more recently, by labora- 

 tory measurements of the compressibility of rocks, as well as by 

 the study of earthquake waves, Adams and Williamson 22 of the 

 Carnegie Geophysical Laboratory, have shown that the much greater 

 density of the interior of the earth can not be accounted for by the 

 compressibility of the materials, whether rocks or metals. They are 

 also led to the conclusion that, while there is segregation of heavier 

 material toward the center, the change is continuous, and not dis- 

 continuous, as is held by Wiechert and Knott. 



Following the views of Adams and Williamson, and accepting 

 a lower zone of nickel-iron beneath the silicate " crust," I would 

 suggest here the idea that the central core, the real nucleus, of 

 the earth is composed of the metallogenic elements, that is, the 

 elements or metals of highest atomic weight, either as "native" 

 metals, or possibly in the form of selenides, tellurides, arsenides, 

 antimonides, bromides, and iodides. Above this would be the nickel- 

 iron zone, and above this the silicate crust. 



We can not here discuss this suggestion in all its rather complex 

 aspects. But the somewhat intermediate chemical character of the 

 metals of the iron group, with manganese and chromium, is in 

 accord wth the hypothesis, differing as they do from the other 

 petrogenic elements in their occurrence as sulphides and arsenides, 

 in which they resemble the heavier metallogenic elements. Iron is 

 the fourth most abundant element, and if the position of the nickel- 

 iron zone, or a zone of alloy mixed with silicate rocks, were com- 

 paratively near the surface, this would be expected. The occur- 

 rence of iron-bearing basalts at the surface (met with in Greenland, 

 Russia, Spain, and elsewhere) is also in line with this supposition. 



Again, as on this supposition the true metallogenic elements are 

 most deeply buried, their relative scarcity at the surface is readily 

 understandable. Forming the nuclear core, not only would their 

 total volume be relatively small, but it would also be difficult for 

 them to find their way, even as vaporized or soluble compounds, 

 from the great depths to the surface. The generally low melting 

 points of the ore minerals is also in line with the opinion of Adams 



22 1 must express my thanks to my colleagues for permission to mention briefly here some 

 of their conclusions which have not yet been published, 



