THE GENESIS OF THE EARTH.
The title of this work is intended to indicate precisely its nature. It consists of rough, broad sketches of the aspects of successive stages in the earth’s history, as disclosed by geology, and as they present themselves to observers at the present time. The last qualification is absolutely necessary, when dealing with a science whose goal to-day will be its starting point to-morrow, and in whose view every geological picture must have its light and shaded portions, its clear foreground and its dim distance, varying according to the lights cast on them by the progress of investigation, and according to the standpoint of the observer. In such pictures results only can be given, not the processes by which they have been obtained; and with all possible gradations of light and distance, it may be that the artist will bring into too distinct outline facts still only dimly perceived, or will give too little prominence to others which, should appear in bold relief. He must in this judge for himself; and if the writer’s impressions do not precisely correspond with those of others, he trusts that they will allow something for difference of vision and point of view.
The difficulty above referred to perhaps rises to its maximum in the present chapter. For how can any one paint chaos, or give form and filling to the formless void? Perhaps no word-picture of this period of the first phase of mundane history can ever equal the two negative touches of the inspired penman—“without form and void”—a world destitute of all its present order, and destitute of all that gives it life and animation. This it was, and not a complete and finished earth, that sprang at first from its Creator’s hand; and we must inquire in this first chapter what information science gives as to any such condition of the earth.
In the first place, the geological history of the earth plainly intimates a beginning, by utterly negativing the idea that “all things continue as they were from the creation of the world.” It traces back to their origin not only the animals and plants which at present live, but also their predecessors, through successive dynasties emerging in long procession from the depths of a primitive antiquity. Not only so; it assigns to their relative ages all the rocks of the earth’s crust, and all the plains and mountains built up of them. Thus, as we go back in geological time, we leave behind us, one by one, all the things with which we are familiar, and the inevitable conclusion gains on us that we must be approaching a beginning, though this may be veiled from us in clouds and thick darkness. How is it, then, that there are “Uniformitarians” in geology, and that it has been said that our science shows no traces of a beginning, no indications of an end? The question deserves consideration; but the answer is not difficult. In all the lapse of geological time there has been an absolute uniformity of natural law. The same grand machinery of force and matter has been in use throughout all the ages, working out the great plan. Yet the plan has been progressive and advancing, nevertheless. The uniformity has been in the methods, the results have presented a wondrous diversity and development. Again, geology, in its oldest periods, fails to reach the beginning of things. It shows us how course after course of the building has been laid, and how it has grown to completeness, but it contains as yet no record of the laying of the foundation-stones, still less of the quarry whence they were dug. Still the constant progress which we have seen points to a beginning which we have not seen; and the very uniformity of the process by which the edifice has been erected, implies a time when it had not been begun, and when its stones were still reposing in their native quarry.
What, then, is the oldest condition of the earth actually shown to us by geology,—that which prevailed in the Eozoic or Laurentian period, when the oldest rocks known, those constituting the foundation-stones of our present continents, were formed and laid in their places? With regard to physical conditions, it was a time when our existing continents were yet in the bosom of the waters, when the ocean was almost universal, yet when sediments were being deposited in it as at present, while there were also volcanic foci, vomiting forth molten matter from the earth’s hidden interior. Then, as now, the great physical agencies of water and fire were contending with one another for the mastery, doing and undoing, building up and breaking down. But is this all? Has the earth no earlier history? That it must have had, we may infer from many indications; but as to the nature of these earlier states, we can learn from conjecture and inference merely, and must have recourse to other witnesses than those rocky monuments which are the sure guides of the geologist.
One fact bearing on these questions which has long excited attention, is the observed increase in temperature in descending into deep mines, and in the water of deep artesian wells—an increase which may be stated in round numbers at one degree of heat of the centigrade thermometer for every 100 feet of depth from the surface. These observations apply of course to a very inconsiderable depth, and we have no certainty that this rate continues for any great distance towards the centre of the earth. If, however, We regard it as indicating the actual law of increase of temperature, it would result that the whole crust of the earth is a mere shell covering a molten mass of rocky matter. Thus a very slight step of imagination would carry us back to a time when this slender crust had not yet formed, and the earth rolled through space an incandescent globe, with all its water and other vaporisable matters in a gaseous state. Astronomical calculation has, however, shown that the earth, in its relation to the other heavenly bodies, obeys the laws of a rigid ball, and not of a fluid globe. Hence it has been inferred that its actual crust must be very thick, perhaps not less than 2,500 miles, and that its fluid portion must therefore be of smaller dimensions than has been inferred from the observed increase of temperature. Further, it seems to have been rendered probable, from the density of rocky matter in the solid and liquid states, that a molten globe would solidify at the centre as well as at the surface, and consequently that the earth must not only have a solid crust of great thickness, but also a solid nucleus, and that any liquid portions must be of the nature of a sheet or of detached masses intervening between these. On the other hand, it has recently been maintained that the calculations which are supposed to have established the great thickness of the crust, on the ground that the earth does not change its form in obedience to the attraction of the sun and moon, are based on a misconception, and that a molten globe with a thin crust would attain to such a state of equilibrium in this respect as not to be distinguishable from a solid planet. This view has been maintained by the French physicist, Delaunay, and for some time it made geologists suppose that, after all, the earth’s crust may be very thin. Sir William Thomson, however, and Archdeacon Pratt, have ably maintained the previous opinion, based on Hopkins' calculations; and it is now believed that we may rest upon this as representing the most probable condition of the interior of the earth at present. Another fact bearing on this point is the form of the earth, which is now actually a spheroid of rotation; that is, of such a shape as would result from the action of gravity and centrifugal force in the motion of a huge liquid drop rotating in the manner in which the earth rotates. Of course it may be said that the earth may have been made in that shape to fit it for its rotation; but science prefers to suppose that the form is the result of the forces acting on it. This consideration would of course corroborate the deductions from that just mentioned. Again, if we examine a map showing the distribution of volcanoes upon the earth, and trace these along the volcanic belt of Western America and Eastern Asia, and in the Pacific Islands, and in the isolated volcanic regions in other parts of the world; and if we add to these the multitude of volcanoes now extinct, we shall be convinced that the sources of internal heat, of which these are the vents, must be present almost everywhere under the earth’s crust. Lastly, if we consider the elevations and depressions which large portions of the crust of the earth have undergone in geological time, and the actual crumpling and folding of the crust visible in great mountain chains, we arrive at a similar conclusion, and also become convinced that the crust has been not too thick to admit of extensive fractures, flexures, and foldings. There are, however, it must be admitted, theories of volcanic action, strongly supported by the chemical nature of the materials ejected by modern volcanoes, which would refer all their phenomena to the softening, under the continued influence of heat and water, of materials within the crust of the earth rather than under it.[A] Still, the phenomena of volcanic action, and of elevation and subsidence, would, under any explanation, suppose intense heat, and therefore probably an original incandescent condition.
[A] Dr. T. Sterry Hunt, in Silliman’a Journal, 1870.
La Place long ago based a theory of the originally gaseous condition of the solar system on the relation of the planets to each other, and to the sun, on their planes of revolution, the direction of their revolution, and that of their satellites. On these grounds he inferred that the solar system had been formed out of a nebulous mass by the mutual attraction of its parts. This view was further strengthened by the discovery of nebulae, which it might be supposed were undergoing the same processes by which the solar system was produced. This nebular theory, as it was called, was long very popular. It was subsequently supposed to be damaged by the fact that some of the nebulæ which had been regarded as systems in progress of formation were found by improved telescopes to be really clusters of stars, and it was inferred that the others might be of like character. The spectroscope has, however, more recently shown that some nebulæ are actually gaseous; and it has even been attempted to demonstrate that they are probably undergoing change fitting them to become systems. This has served to revive the nebular hypothesis, which has been further strengthened by the known fact that the sun is still an incandescent globe surrounded by an immense luminous envelope of vapours rising from its nucleus and condensing at its surface. On the other hand, while the sun may be supposed, from its great magnitude, to remain intensely heated, and while it will not be appreciably less powerful for myriads of years, the moon seems to be a body which has had time to complete the whole history of geological change, and to become a dry, dead, and withered world, a type of what our earth would in process of time actually become.