As we trace the history of mankind back to very ancient times, we find that the records become more and more scanty and less intelligible, until history fades into myth and tradition. Of a still earlier age we have not even a tradition; it is prehistoric. Similarly, among the geological records the earliest are in a state of such excessive confusion that they are exceedingly difficult to understand, and between different observers there are radical differences of opinion both as to the facts and as to their interpretation. Furthermore, there must have been an inconceiv-ably long time earlier than the most ancient recorded periods, as to which conjecture and inference are the only resource. In these difficult straits astronomy offers valuable assistance to the baffled geologist. The Nebular Hypothesis is a scheme of the development of the solar system which is very generally accepted by astronomers, in some form, as essentially true.

The term nebular hypothesis is usually, though not with exactness, limited to one particular form, according to which the place of the present solar system was originally occupied by a vast rotating nebula, a mass of intensely heated vapour, or possibly clouds of meteorites, extending beyond the orbit of the outermost planet. As the nebula cooled by radiation, it contracted, leaving behind it successive rings, like those of the planet Saturn, but on a vastly larger scale. The rings kept up the rotation imparted by the nebula, and all of them lay in nearly the same plane. Unequal contraction in various parts of each revolving ring caused it to break up and gather by mutual attraction into masses. If these rings were composed of relatively small solid masses, like meteorites, or if they had solidified by condensation of the vapours, the heat generated by the collisions, as the broken ring was gathered into a mass, would suffice to raise the temperature and liquefy or vapourize the mass. By revolution the nebulous masses would assume a spheroidal shape and become planets.

The central mass of the original nebula forms the sun, which is still in an intensely heated, incandescent state.

Another form of the nebular hypothesis, called for the sake of distinction the Planetesimal Hypothesis, has recently been proposed by Professor Chamberlin. This postulates, as the beginning of the solar system, a spiral nebula, "and that the matter of this parent nebula was in a finely divided solid or liquid state before aggregation. ... It regards the knots of the nebula as the nuclei of the future planets, and the nebulous haze as matter to be added to these nuclei to form the planets. It assumes that both the knots and the particles of the nebulous haze moved about the central mass in elliptical orbits of considerable, but not excessive, eccentricity. ... It deduces a relatively slow growth of the earth, with a rising internal temperature developed in the central parts and creeping outward." (Chamberlin and Salisbury).

This is not the place to discuss the evidence for an astronomical speculation, but it is clear that the hypothesis regarding the development of the solar system which we adopt must condition our views as to the early unrecorded stages of the earth's history. From the strictly geological standpoint the most important difference between the Nebular and the Planetesimal Hypotheses is that, according to the former, the earth has passed through a gaseous and a molten stage, and therefore must have formed a crust by solidification, while the latter leads to the conclusion that the earth has been solid from the beginning, and consequently never formed a crust of solidification.