COSMICAL GEOLOGY.
Counical geology, or cosmogony, brings out certain harmonies in the solar system, namely, that the orbits of all the important planets are practically in same .plane and nearly, though, not quite, circular; that they all re volve in the same direction in their orbits; that most.of theist rotate in the same direction on their axes; and that these peculiarities of movement are shared by most of the satellites. These harmonies have led to the widespread belief among astronomers in a common origin for all the bodies of the solar system. Many theories of origin have been advanced, but only two are considered of sufficiently serious im portance to be outlined here. See AsraoNowv; LAPLACE; NEBULA; SOLAR SYSTEM; etc.
Nebular Hypothesis.— This theory, known' also as the Laplacian hypothesis, has had more widespread acceptance than any other. It postulates a slowly rotating gaseops nebula, very tenuous, with its centre at the site of the present sun and so expanded that it filled all the space now occupied by the solar system. The nebula was supposed to have cooled and shrunk, the• rapidity of rotation increasing with the cooling. Centrifugal force caused a bulging at the equator; with the final separation of an equatorial ring which further cooled and con tracted until it broke and condensed into a planet. Several such occurrences in turn gave the several planets while the remaining portion of the patent nebula constitutes the sun. In a similar manner the planets gave off satellites. The molten earth is then supposed to have under gone progressive cooling, resulting in the for mation of a solid crust. Certain mechanical
difficulties inherent in the theory, but much too abstruse to be considered here, finally led Chamberlin and Moulton to the formulation of the following. theory.
Planetesimal brief the elements of this hypothesis are as follows: There are known to be in the heavens a lair number of spiral nebulae consisting of a main or central nebular mass and two curved or spiral arms in which are smaller nuclei or knots of varying sizes within a nebular haze. Each separate solid particle of matter is considered as a planetesimal (little planet) and the parti cles in such a nebula are supposed to have finally gathered together by overtake collisions while pursuing slightly different orbits. The central core' became the sun, larger knots became planets and smaller knots became satellites, by gradual infall of solid planetesi mals.
Under the nebular hypothesis the earth, once a molten mass, underwent progressive cooling until a rigid crust formed, perhaps over a still molten interior. It was not until this crust be came rigid and at least moderately cool, that oceans could forin and the ordinary geological processes go on. Under the planetesimal hypothe sis, the earth was never molten, probably never very hot. As soon as it grew to sufficient size to have the necessary gravitative attraction, an atmosphere gathered, oceans formed, and the agencies of gradation began operation. The history of 'our earlier geologic eras thus would differ greatly in the minds of adherents of these two opposing hypotheses. See COSMOG ONY.