Again, the supposition of a central heat of the earth, considered as the effect of a more ancient state of its mass, appeared to indicate that its cooling must still be going on. But if this were so, the earth might contract, as most bodies do when they cool; and this contraction might lead to mechanical results, as the shortening of the day. Laplace satisfied himself, by reference to ancient astronomical records, that no such [555] alteration in the length of the day had taken place, even to the amount of one two-hundredth of a second; and thus, there was here no confirmation of the hypothesis of a primitive heat of the earth.
Though we find no evidence of the secular contraction of the earth in the observations with which astronomy deals, there are some geological facts which at first appear to point to the reality of a refrigeration within geological periods; as the existence of the remains of plants and shells of tropical climates, in the strata of countries which are now near to or within the frigid zones. These facts, however, have given rise to theories of the changes of climate, which we must consider separately.
But we may notice, as connected with the doctrine of central heat, the manner in which this hypothesis has been applied to explain volcanic and geological phenomena. It does not enter into my plan, to consider explanations in which this central heat is supposed to give rise to an expansive force,[69] without any distinct reference to known physical laws. But we may notice; as more likely to become useful materials of the science now before us, such speculations as those of Mr. Babbage; in which he combines the doctrine of central heat with other physical laws;[70] as, that solid rocks expand by being heated, but that clay contracts; that different rocks and strata conduct heat differently; that the earth radiates heat differently, or at different parts of its surface, according as it is covered with forests, with mountains, with deserts, or with water. These principles, applied to large masses, such as those which constitute the crust of the earth, might give rise to changes as great as any which geology discloses. For example: when the bed of a sea is covered by a thick deposit of new matter worn from the shores, the strata below the bed, being protected by a bad conductor of heat, will be heated, and, being heated, maybe expanded; or, as Sir J. Herschel has observed, may produce explosion by the conversion of their moisture into steam. Such speculations, when founded on real data and sound calculations, may hereafter be of material use in geology.
[69] Scrope On Volcanoes, p. 192.
[70] On the Temple of Serapis, 1834. See also Journal of the Royal Inst. vol. ii., quoted in Conyb. and Ph. p. xv. Lyell, B. ii. c. xix. p. 383, (4th ed.) on Expansion of Stone.
The doctrine of central heat and fluidity has been rejected by some eminent philosophers. Mr. Lyell’s reasons for this rejection belong [556] rather to Theoretical Geology; but I may here notice M. Poisson’s opinion. He does not assent to the conclusion of Fourier, that once the temperature increases in descending, there must be some primitive central heat. On the contrary, he considers that such an increase may arise from this;—that the earth, at some former period, passed (by the motion of the solar system in the universe,) through a portion of space which was warmer than the space in which it now revolves (by reason, it may be, of the heat of other stars to which it was then nearer). He supposes that, since such a period, the surface has cooled down by the influence of the surrounding circumstances; while the interior, for a certain unknown depth, retains the trace of the former elevation of temperature. But this assumption is not likely to expel the belief is the terrestrial origin of the subterraneous heat. For the supposition of such an inequality in the temperature of the different regions in which the solar system is placed at different times, is altogether arbitrary; and, if pushed to the amount to which it must be carried, in order to account for the phenomenon, is highly improbable.[71] The doctrine of central heat, on the other hand, (which need not be conceived as implying the universal fluidity of the mass,) is not only naturally suggested by the subterraneous increase of temperatures, but explains the spheroidal figure of the earth; and falls in with almost any theory which can be devised, of volcanoes, earthquakes, and great geological changes.
[71] For this hypothesis would make it necessary to suppose that the earth has, at some former period, derived from some other star or stars more heat than she now derives from the sun. But this would imply, as highly probable, that at some period some other star or stars must have produced also a mechanical effect upon the solar system, greater than the effect of the sun. Now such a past operation of forces, fitted to obliterate all order and symmetry, is quite inconsistent with the simple, regular, and symmetrical relation which the whole solar system, as far as Uranus, bears to the present central body.
Sect. 5.—Problems respecting Elevations and Crystalline Forces.
Other problems respecting the forces by which great masses of the earth’s crust have been displaced, have also been solved by various mathematicians. It has been maintained by Von Buch that there occur, in various places, craters of elevation; that is, mountain-masses resembling the craters of volcanoes, but really produced by an expansive force from below, bursting an aperture through horizontal strata, [557] and elevating them in a conical form. Against this doctrine, as exemplified in the most noted instances, strong arguments have been adduced by other geologists. Yet the protrusion of fused rock by subterraneous forces upon a large scale is not denied: and how far the examples of such operations may, in any cases, be termed craters of elevation, must be considered as a question not yet decided. On the supposition of the truth of Von Buch’s doctrine, M. de Beaumont has calculated the relations of position, the fissures, &c., which would arise. And Mr. Hopkins,[72] of Cambridge, has investigated in a much more general manner, upon mechanical principles, the laws of the elevations, fissures, faults, veins, and other phenomena which would result from an elevatory force, acting simultaneously at every point beneath extensive portions of the crust of the earth. An application of mathematical reasoning to the illustration of the phenomena of veins had before been made in Germany by Schmidt and Zimmerman.[73] The conclusion which Mr. Hopkins has obtained, respecting the two sets of fissures, at right angles to each other, which would in general be produced by such forces as he supposes, may suggest interesting points of examination respecting the geological phenomena of fissured districts.
[72] Trans. Camb. Phil. Soc. vol. vi. 1836.