Then, life still being absent from the earth, the Oceanic Era began. The waters condensed into an ocean over the earth, or else collected in some great oceanic depression. Lands presently emerged from it. It was a hot ocean, steaming no doubt, for its temperature was perhaps about 500° F. Some one may ask—Why, then, did it not steam away into clouds? The answer is that the atmosphere was still very heavy in that past era, probably still exerting a pressure as much as fifty times as great as to-day. The pressure of the atmosphere at the earth's surface to-day is usually about fifteen pounds to the square inch. In such circumstances water boils rapidly away at the temperature of 212° F. But if the water be taken up to the top of Mont Blanc, where the air pressure is less than that at the sea-level (or if, which amounts to the same thing, we reduce the pressure on the water's surface by placing it under the receiver of an air-pump and partially exhaust the air), it will boil at a lower temperature than this. If, on the other hand, we increase the pressure on the surface of the water by any means, such, for example, as by placing it in a chamber of compressed air, the water can be heated to a higher temperature without boiling away. In the bygone era of which we are speaking the pressure of the atmosphere on the water's surface was 700 lb. or 800 lb. to the square inch; and therefore it could be heated up to a high temperature without evaporating rapidly.
Another thing began to happen in those days. All bodies in space attract one another; the Sun attracts its planets; the planets attract the Sun and their satellites; and the satellites in their turn attract the planets. Ages before the earth had a moon these forces were at work. But the attractions of solid bodies for one another do not bring about any very perceptible alterations in their shapes; though if the bodies are spinning they effect slow changes in their speed of rotation. It is different when the bodies are liquid, or if they have liquid surfaces. Then the attractions of a sun or a moon on a planet begin to draw up the waters of the planet and produce tides. The attraction of the earth would produce tides on the Moon if an ocean existed there; and, it is suspected, do produce something resembling tides on the present surface of the Sun.[8] As soon, therefore, as oceans appeared on the earth the waters began to ebb and flow in tides. (Another consequence of this constant ebb and flow was that the friction of these movements began to diminish the speed of the earth's rotation—just as a string that was placed round the circumference of a spinning-top would, if constantly pulled backwards and forwards, gradually help to slow down the top.) Then oceanic waves and currents would begin to eat a way into the land that was on their borders, or which was emerging from their depths. Rivers would begin to arise, and they would carry on the work of erosion. Other causes tending to break up the rocks would be the gases in the air—the excessive quantities of carbonic acid and oxygen would be active chemical agents in this work. Before the close of this era the limestones and iron carbonates began to form; sediments arose in the lifeless oceans, and thus began the first formation of those sedimentary rocks and strata which have been dealt with in the earlier chapters.
[8] A paper read by Mr. E. W. Maunder before the Royal Astronomical Society in 1907 gave reasons for believing that the earth has perceptible effects on the movements of sun-spots.
After the lifeless era began the age when the lowest forms of life came into existence. The initial stage was perhaps the Era of the First Plants, Algæ, and later still aquatic fungi or bacteria. This began when the general temperature of the ocean may have been as high as 150° F. (some water plants can now live in waters up to and above 180° F.). Limestones began to form from the secretions of plants, and deposits of silica from silica secretions. Also where the conditions were favourable there were large sedimentary deposits and accumulations. In the second part of this æon the earth, still continuing to cool, and going down in temperature to 115°, gave opportunity to animal life. At the end of this era the general temperature of the earth and its oceans was as low as 90° F. The first animal life had begun to appear; its activity greatly increased under what were favourable conditions for it. This increase of animal life had its effect on the earth's crust. We have already spoken of the formation of limestones from the bodies of sea animals. This was going on in those ages millions of years ago before any of the higher forms of life had appeared on the earth, and though it was not going on so rapidly, still it must be remembered that at some point of the world's history the oceans were of greater extent than now, and consequently the deposits of lime and the accumulations of sediment were more widespread. The sedimentary rocks grew faster and faster, especially on the floors of the oceans.
It will be understood by those who have read the foregoing two chapters closely that the "igneous" or fire-born rocks must lie underneath the sedimentary ones. But that is only true in general terms, for a double reason. In the first place, owing to the inestimable forces which for millions of years were still continually effective below the earth's crust, the igneous rocks over and over again were able to burst their way through the slow-forming sediment of other rocks laid down above them. In the second place, the igneous rocks, owing to their composition and superior hardness, were much less worn by wind and weather than the less compact "sedimentary rocks," and these remained, showing themselves at the surface as coast-lines of oceans and in mountain ranges, after the sandstones and shales and limestones had disappeared.
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The Crater of an Extinct Volcano
This is the entrance to the long extinct volcano of Red Mountain, Arizona.