PART IV.

THE GEOLOGICAL RECORD OF THE

HISTORY OF THE EARTH.

CHAPTER XVI.

THE EARLIEST CONDITIONS OF THE GLOBE-THE
ARCHÆAN PERIODS.

THE foregoing chapters have dealt chiefly with the materials of which the crust of the earth consists, with the processes whereby these materials are produced or modified, and with the methods pursued by geologists in making their study of these materials and processes subservient to the elucidation of the history of the earth. The soils, rocks, and minerals beneath our feet, like the inscriptions and sculptures of a long-lost race of people, are in themselves full of interest, apart from the story which they chronicle; but it is when they are made to reveal the history of land and sea, and of life upon the earth, that they are put to their noblest use. The investigation of the various processes whereby geological changes are carried on at the present day is undoubtedly full of fascination for the student of nature; yet he is conscious that it gains enormously in interest when he reflects that in watching the geological operations of the present day he is brought face to face with the same instruments whereby the very framework of the continents has been piled up and sculptured into the present outlines of mountain, valley, and plain.

The highest aim of the geologist is to trace the history of the earth. All his researches, remote though they may seem from this aim, are linked together in the one great task of unravelling the successive mutations through which each area of the earth's surface has passed, and of discovering what successive races of plants and animals have appeared upon the globe. The investigation of facts and processes, to which the previous pages have been devoted, must accordingly be regarded as in one sense introductory to the highest branch of geological inquiry. We have now to apply the methods and principles already discussed to the elucidation of the history of our planet and its inhabitants. Within the limits of this volume only a mere outline of what has been ascertained regarding this history can be given. I shall arrange in chronological order the main phases through which the globe seems to have passed, and present such a general summary of the more important facts regarding each of them as may, I hope, convey an adequate outline of what is at present known regarding the successive periods of geological history.

As the primitive stages of mankind upon the earth and the early progress of every race fade into the obscurities of mythology and archæology, so the story of the primeval condition of our globe is lost in the dim light of remote ages, regarding which almost all that is known or can be surmised is furnished by the calculations and speculations of the astronomer. If the earth's history could only be traced out from evidence supplied by the planet itself, it could be followed no further back than the oldest portions of the earth now accessible to us. Yet there can be no doubt that the planet must have had a long history before the appearance of any of the solid portions now to be seen. That such was the case is made almost certain by the traces of a gradual evolution or development which astronomers have been led to recognise among the heavenly bodies. Our earth being only one of a number of planets revolving round the sun, the earliest stages of its separate existence must be studied in reference to the whole planetary system of which it forms a part. Thus, in compiling the earliest chapter of the history of the earth, the geologist turns for evidence to the researches of the astronomer among stars and nebulæ.

In recent years, more precise methods of inquiry, and, in particular, the application of the spectroscope to the study of the stars, have gone far to confirm the speculation known as the Nebular Hypothesis. According to this view, the orderly related series of heavenly bodies, which we call the Solar System, existed at one time, enormously remote from the present, as a Nebula-that is, a cloudy mass of matter, like one of those nebulous, faintly luminous clouds which can be seen in the heavens. This nebula probably extended at least as far as the outermost planetary member of the system is now removed from the sun. It may have.consisted entirely of incandescent gases or vapours, or of clouds of stones in rapid movement, like the stones that from time to time fall through our atmosphere as meteorites, and reach the surface of the earth. The collision of these stones moving with planetary velocity would dissipate them into vapour, as is perhaps the case in the faint luminous tails of comets. At all events, the materials of the nebula began to condense, and in so doing, threw off, or left behind, successive rings (like those around the planet Saturn), which, in obedience to the rotation of the parent nebula, began to rotate in one general plane around the gradually shrinking nucleus. As the process of condensation proceeded, these rings broke up, and their fragments rushed together with such force as not improbably to generate heat enough to dissipate them again into vapour. They eventually condensed into planets, sometimes with a further formation of rings, or with a disruption of these secondary rings, and the consequent formation of moons or satellites round the planets. The outer planets would thus be the oldest, and, on the whole, the coolest and least dense. Towards the centre of the nebula the heaviest elements might be expected to condense, and there the high temperature would longest continue. The sun is the remaining intensely hot nucleus of the original nebula, from which heat is still radiated to the furthest part of the system.

When a planetary ring broke up, and by the heat thereby generated was probably reduced to the state of vapour, its materials, as they cooled, would tend to arrange themselves in accordance with their respective densities, the heaviest in the centre, and the lightest outside. In process of time, as cooling and contraction advanced, the outer layers might grow.quite cold, while the inner nucleus of the planet might still be intensely hot. Such, in brief, is the well-known Nebular Hypothesis.

Now the present condition of our earth is very much what, according to this hypothesis or theory, it might be expected to be. On the outside comes the lightest layer or shell in the form of an Atmosphere, consisting of gases and vapours. Below this gaseous envelope which entirely surrounds the globe lies an inner envelope of water, the ocean, which covers about two-thirds of the earth's surface, and is likewise composed of gases. Underneath this watery covering, and rising above it in dry land, rests the solid part of the globe, which, so far as accessible to us, is composed of rocks twice to thrice the weight of pure water. But observations with the pendulum at various heights above the sea show that the attraction of the earth as a whole indicates that the globe probably has a density about five and a half times that of water. Hence we may infer that its inner nucleus not improbably consists of heavy materials, and may be metallic. There is thus evidence of an arrangement of the planet's materials in successive spherical shells, the lightest or least dense being on the outside, and the heaviest or densest in the centre.

Again, the outside of the earth is now quite cool; but abundant proof exists that at no great distance below the surface the temperature is high. Volcanoes, hot springs, and artificial borings all over the world testify to the abundant store of heat within the earth. Probably at a depth of not more than 20 miles from the surface the temperature is as high as the melting-point of any ordinary rock at the surface. By far the largest part of the planet, therefore, is hotter than molten iron. We need have no hesitation in admitting it to be highly probable that the earth was originally in the state of incandescent vapour, and that it has ever since that time been cooling and contracting. Its present shape affords strong presumption in favour of the opinion that the globe was once in a plastic condition. The flattening at the poles and bulging at the equator, or what is called the oblately spheroidal figure of the planet, is just the shape which a plastic mass would have assumed in