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PHYSICAL GEOLOGY 



perature so high as to overcome the effect of the pressure. It is evident for this reason 

 that it would be necessary to go deeper to find the fluid interior than if the pressure of 

 the overlying rocks was slight. At a depth of 50 miles a temperature of 3500 F., 

 though sufficient to melt almost any rock at the surface, might not be high enough to 

 overcome the enormous weight of the overlying rocks, and since a still greater pressure 

 is encountered at the increased depth necessary to obtain a higher temperature, it is 

 evident that the melting point, for this cause alone, might never be reached. • 



(c) Rigidity of the Earth. — Two further objections to this theory have caused its 

 abandonment by scientists. The first of these is that the earth is not pulled out of 

 shape by the attraction of the moon and sun, as would be the case if it were substantially 

 a molten globe. On the contrary it is shown to be more rigid than glass or steel. 

 The second objection consists in the fact (Milne) that the velocity and character of 

 earthquake waves (p. 283) suffer an abrupt change at a depth of about 30 miles, being 

 transmitted at a more rapid rate below this level than on the crust, showing that the 

 nucleus is more rigid than the overlying rocks. 



(2) Solid Interior. — The second theory, as has already been indicated, is that 

 the earth is substantially a solid because of the increasing conductivity and pressure. 



(3) Gaseous Center. — Upon the assumption that below a depth of 190 miles 

 the temperature of the earth is at the critical temperature of all substances (the tem- 

 perature above which a substance can exist only in a gaseous state), it is held that the 

 solid crust passes into a liquid zone, which in turn passes gradually to a gaseous magma. 

 (Arrhenius.) The gaseous magma is potentially but not actually a fluid with a tem- 

 perature above the fusion point of all substances. The rigidity of the earth, according 

 to this theory, may nevertheless be greater rather than less than that of steel. 



(4) Radioactivity and a Solid Center. — A fourth theory is in direct contradiction 

 to the preceding and holds that the temperature of the interior is derived from the 

 heat given off by the radioactive minerals of the earth's crust. According to this theory 

 a radioactive crust, 30 to 45 miles thick, supplies all of the heat for the interior of the 

 earth, and below a depth of about 45 miles the earth has a temperature of only about 

 1500 C. (Strutt.) So many elements of doubt enter into the above theory that it 

 should merely be considered as suggestive, although all theories must take into 

 account the enormous amount of heat generated in this way. 



(5) Subcrust Theory. — Another theory which has been generally abandoned 

 holds that between the solid crust and the solid center is a fused or semifused layer. 



Summary. — Any hypothesis of the constitution of the earth's 

 interior which is in accord with the known facts must hold (1) that 

 the earth is a globe which increases in density from the surface 

 toward the center; (2) that the temperature of the interior is in- 

 tensely hot, perhaps 20,000° C. at the center, or even higher; (3) that 

 the rigidity of the earth as a whole is greater than that of steel. 



REFERENCES FOR STRUCTURE OF THE EARTH 



Chamberlin and Salisbury, — Geology, Vol. I, 2d ed., pp. 486-589. 



Geikie, J., — Structural and Field Geology, 3d ed., 1912. 



Leith, C. K., — Structural Geology, 1913. 



Reid, H. F., — Bull. Geol. Soc. America, Vol. 24, 1913, pp. 163, 186. 



