In the days when the earth’s crust had formed but was still unstable, the process of cooling not having gone far enough, there would not be the mountains which now characterise it. These came when the earth contracted and crumpled up along certain well defined lines, which are now represented by the three great mountain chains of the world.
Building Up the Earth
Finally, a heavier rainfall would result from a more active atmospheric circulation, creating larger rivers, and thus, at the beginning, all those denuding agents which are engaged in wearing the land down into the sea would be working at a more rapid pace. Correspondingly, all the agents which are occupied in building up deposits of sediments would have extended their operations over a wider area, laying down a foundation broad and deep.
On the other hand, the contraction of the earth, due to the loss of its energy of rotation as well as of its internal heat, would also have proceeded more rapidly, new land would have emerged from the sea, old lands would have been submerged beneath it far less slowly than at the present day; ruptures of the crust, accompanied by earthquakes and volcanic action, would have been more frequent and thus, by the more rapid loss of its intrinsic energy, the renovation of the earth would have kept pace with its accelerated destruction.
One effect of the contraction of the earth which has manifested itself in even late geological times is the crumpling up of the terrestrial crust into the sharp folds of mountain chains; but at the beginning this crumpling must have been far more universal and energetic. In this connection it is interesting to observe that the most ancient rocks known to us—the Archæan—never present themselves under any other form than as intensely plicated masses. They originally consisted of lava flows and volcanic ashes, of ancient sediments and limestones, into which subterranean masses of granite and other molten, deep-seated rocks have been injected; but under the intense pressures to which they were subjected after their formation they and the invading granite have entirely lost their original character, and have been metamorphosed into gneisses, schists, and marble, all sharply and closely folded together. In any given district the direction of their folding is maintained with wonderful constancy over great distances. There is no succeeding system of rocks that has been so completely transformed, so universally plicated, as this ancient Archæan complex.
In later times we can pass from stratum to stratum of the sedimentary series and read their history almost as we turn over the pages of a book; in the Archæan all are kneaded together into a state of such desperate entanglement as to defy the powers of human ingenuity to unravel them. Thus the line of demarcation between the Archæan and subsequent sedimentary systems is the sharpest and most absolute that is known to us in the history of the earth. It marks the close of our planet’s infancy, the several events of which have passed into oblivion as profound as that of our own forgetfulness of our earliest days. Later events, on the other hand, are recorded in the stratified series with a faithfulness which increases as we approach existing times.
How We Know These Wonders
A history without dates must seem very unsatisfactory to a historian, and the question will naturally arise whether we can assign any definite time to the various critical events recorded in the evolution of the earth. At present we can only make more or less plausible estimates. Thus, from a consideration of the thickness of the sedimentary crust, and the rate at which sediments are now being deposited, it has been asserted that the interval which separates us from the close of the Archæan era may amount to about twenty-six millions of years. Professor Joly, basing his argument on the undoubted fact that the ocean derives the greater part of its salt from the dissolved material contributed to it by rivers, comes to the conclusion that the ocean first came into existence about one hundred millions of years ago. As regards the birth of the moon, Sir George Darwin has given a minimum limit of fifty-four millions of years, but he adds that it may have taken place many hundreds of millions of years before this. Lord Kelvin has attempted to determine the time which has elapsed since the earth first acquired a solid crust. If we only knew the rate at which the earth is cooling we might calculate back to this time with some assurance of certainty, always, however, on the assumption that the earth is simply a hot body cooling like any other hot body—such, say, as a red-hot cannonball. But a few years ago it began to be seriously suspected that this assumption was a very doubtful one, for a new element—radium—was discovered in 1898, which possesses the remarkable property of spontaneously liberating heat, and this not in small quantities, but at an astonishing rate. One gramme of radium, for example, gives out enough heat in one hour to raise the temperature of one gramme of water to boiling point; hour after hour, year in, year out, this wonderful substance is setting free the energy it contains, and will continue to do so until, some thousands of years hence, it has exhausted its store. If this element should happen to exist in sufficient quantity within the earth, then the earth could not be said to be cooling just like a piece of hot iron, and the increase of temperature we experience as we descend towards the interior of the earth might possibly be due to the heat set free from radium. Indeed, the argument is not confined to the earth; it may apply also to the sun, and much of the heat we derive from that luminary may be provided by bursting atoms of radium. This was pointed out by Sir George Darwin and Professor Joly in 1903.