Opinions concerning the internal constitution of the earth differ very radically and only within the last few years has evidence begun to accumulate which permits the drawing of certain inferences with a considerable degree of probability.

Many hypotheses as to the condition of the earth's interior have been proposed, of which the following are the most important: (1) That the earth is a molten globe, covered only by a relatively thin crust. (2) That it is substantially a solid body. (3) That the interior passes gradually from a solid crust to a gaseous core, heated beyond the critical temperature and yet under ruch enormous pressure that the core is as rigid as a solid body, but still a gas in molecular condition. According to this theory, the temperature of the earth at the centre is about 180,000° F. and the pressure 3,000,000 atmospheres. (4) That it has a very large solid nucleus surrounded by a layer of fused material, upon which the crust floats in equilibrium.

In the present imperfect state of knowledge, it is not possible to decide definitely between these conflicting hypotheses, but, as mentioned above, evidence has been obtained which seems to point clearly to certain conclusions.

(1) The first, or "thin crust" hypothesis, is now almost entirely abandoned, for there is really no evidence in its favour and very much against it. The velocity and character of the earthquake waves which traverse the mass of the globe and the astronomical relations of the earth as a planet, especially the tidal phenomena, are strongly opposed to this view.

(2) That the earth is substantially a solid body, is the opinion held at present by many geologists and astronomers. In support of it may be cited the astronomical evidence just mentioned, and the earthquake waves, the speed of which requires a medium more rigid than steel, while the very transmission of the transverse or distortional waves would seem to require a solid medium.

(3) Between the second and third hypotheses the distinction is one not easy to explain in an elementary manner, and there are many modifications of the latter. According to Arrhenius, "the rigidity of the earth is greater rather than less than that of steel, but the interior forms an extremely viscous mass, with qualities somewhat like those of asphalt at a low temperature, of pitch, sealing-wax and glass." These bodies behave under forces of deformation, which act quickly or with constantly changing direction, like solids; but under slow, long-continued pressures, acting in a constant direction, they behave like fluids. Observations and records of very distant earthquakes show that when the path of the mass-waves penetrates to a depth of more than three-fifths of the earth's radius, the transverse waves of distortion are either extinguished or greatly retarded. This points to a change in the character of the medium and decidedly supports the notion of a gaseous core postulated by this hypothesis.

(4) The fourth hypothesis, which assumes the presence of a fused layer between the crust and the solid nucleus, with gradual transitions from one to the other, is believed to avoid the astronomical objections to a molten globe, as well as certain geological difficulties in accepting the hypothesis of an entirely solid earth. The earthquake observations, so frequently cited, are decidedly opposed to the belief that a layer of actually fused matter can exist at a moderate depth below the surface.

It is thus probable that below the superficial crust, only a few miles in thickness, the great mass of the earth is composed of very dense material, which transmits elastic waves like a very perfectly elastic solid, and yet is so highly heated and under such enormous pressure that it is potentially fused and liquefies upon sufficient release of pressure, and yields plastically to slow, long-continued stresses which act in a constant direction. Furthermore, there is evidence that a core, two-fifths of the earth's diameter and composed of matter in a different state of aggregation, which may be gaseous, occupies the centre.

In this connection something should be said concerning the important theory of isostasy, which may be thus defined: "The earth is composed of heterogeneous material which varies considerably in density. If this heterogeneous material were so arranged that its density at any point depended simply upon the depth of that point below the surface, ... a state of equilibrium would exist, and there would be no tendency toward a rearrangement of masses.

"If the heterogeneous material composing the earth were not arranged in this manner at the outset, the stresses produced by gravity would tend to bring about such an arrangement; but as the material is not a perfect fluid, . . . the rearrangement will be imperfect. . . . The excess of material represented by that portion of the continent which is above sea-level will be compensated for by a defect of density in the underlying material. The continents will be floated, so to speak, because they are composed of relatively light material; and, similarly, the floor of the ocean will, on this supposed earth, be depressed, because it is composed of unusually dense material. This particular condition of approximate equilibrium has been given the name isostasy." (Tittmann and Hayford).

The recent very extensive and exact operations of the United States Coast Survey have brought strong confirmation of the theory of isostasy. "The United States is not maintained in its position above sea-level by the rigidity of the earth, but is, in the main, buoyed up, floated, by material of deficient density." (Tittmann and Hayford).

It should be noted that isostasy, being a condition of approximate equilibrium, is conservative in tendency and does not explain the active movements of elevation and depression of the crust.

The whole subject of the temperatures and physical state of the earth's interior has been complicated and obscured by the discovery of radio-activity, and already some very far-reaching inferences have been drawn from the distribution of radio-active substances in the rocks. At present, however, it would be premature to give any extended discussion of this problem.