[3] Geology: Earth History, p. 127. Chamberlin and Salisbury.
Suppose, however, they cannot flow, that there is no room for them to flow, and that the pressure is not merely thirteen or fifteen tons to the square inch, as it would be at depths between five and six miles, but a hundred times that amount, as it might be between five and six hundred miles down. What would happen then? We can only imagine what does happen by stating what does not happen. It used to be supposed as late as half a century ago that the earth consisted of a crust of hard rocks perhaps thirty to fifty miles in thickness, and that below this crust the whole earth was a mass of red-hot or white-hot molten stuff with flaming gases mixed with it. If that were the case it would explain a good deal of what we see around us. It would explain the volcanoes, for instance, which belch out fire and lava and ashes and molten rock, and sometimes great fragments of rock. Perhaps some of our readers may remember the great eruption of Mount Pelée, which took place in Martinique some years ago. At one stage of the eruption a great obelisk of rock a thousand feet high was pushed upwards out of the crater, and eventually sank back again. It came out of the depths of the earth. It was like a vent-peg plugging some boiling mass below. Similarly we might suppose that all volcanoes were vent-holes for the tremendous commotion of boiling fiery rocks below the earth's surface. The only thing we can urge is that they do not seem big enough for the purpose, if the earth were indeed all molten except for a thin crust—thirty miles thick. For that would leave a molten ocean more than 7900 miles across any way it was measured: 7900 miles deep, 7900 miles broad, 7900 miles long, if we take the diameter of the earth to be 8000 miles. We all know what great tides the Moon and Sun by their attraction raise in the earth's outer ocean of water. Think what tides they would raise in this inner ocean of molten rock and metal. The earth's crust would not be able to hold such tides in. The molten stuff would be always breaking through the flimsy thirty miles of outer solid rock as if it were egg-shell. Twice a day there would be outbreaks of lava vast enough to submerge continents.
No, that will not do. We will not confuse our readers by telling them all the theories that have been formed, but will only state what the late Lord Kelvin believed, and most of the present generation of geologists believe. It is that the heat of the earth's crust continues to increase only for a certain distance of the way down, and that owing to pressure the earth is solid (though very hot except towards the surface) for two thousand miles down. There remains a thickness of another four thousand miles on either side of the earth's centre to be considered. That might be molten, but the pressure would be so great that it would behave as if it were a solid. We know the earth cannot be solid all through because it does not weigh enough. The earth cannot, of course, be weighed in any scales, but there are methods of weighing it nevertheless. One of two methods is by seeing how strongly it attracts bodies to itself. But these things belong rather to the romance of astronomy than to that of geology. We need only trouble ourselves at present about the results.
One word more about the deep interior of the earth. Dr. J. J. See, an American astronomer, has found how heavy and how hard the earth is, taken as a whole. He finds that if it were built from surface to surface of hardened steel it would be just about as heavy and as hard—or as rigid. The steel would be like that used for the armour-plate of battleships. Dr. See is not prepared, however, to discard the idea that the earth has a large fluid interior. If it were fluid, yet it would be subjected to such enormous pressure by its own weight, that if there were a moderately thick earth-crust, its tidal surgings would be so "cabin'd, cribbed, confined," that they would be comparatively ineffectual. We must not run away with the idea (against which Dr. See specially warns us), that there is any free circulation of currents within the fluid interior. The rigidity produced by pressure (or weight) is too great for that. Indeed, this pressure is so great that, as another scientific authority, Professor Arrhenius, has pointed out, the matter at the core of the earth might even be gaseous; and yet would be so compressed by pressure that it would possess a rigidity equal to the hardest steel. The earth may be partly solid, partly liquid, partly gaseous, but for all practical purposes Professor See would have us regard it as a solid sphere having an average hardness and weight and "rigidity" greater than that of ordinary steel.
We are still some way off an explanation of how the many igneous rocks which were and are being "boiled up" in some inner molten cauldron came to the surface; but the better to understand that we must ask our readers to carry their imagination back to the very beginning of the world when it was "without form and void."
CHAPTER VIII
THE EARTH AT ITS BEGINNING
If we look up at the sky with the eye of knowledge we can read in the celestial objects with which it is strewn something of the history of our earth. We can only read it dimly even with the aid of the greatest telescopes, and it is quite possible that in some respects we may read it wrongly. Let us, however, consider what the eye and the telescope will reveal to us. The eye will see the Sun—a great ball, into which the earth might sink without greatly altering the Sun's appearance, and surrounded with flaming gases hotter than the hottest furnace man has ever been able to contrive. In that heat every solid thing on the earth would melt and be turned into vapour. The eye will also perceive the Moon—another ball, much smaller than the earth, surrounded by no gases at all, having as far as can be seen no water; and being so cold during its long nights that all gases and liquids of which we know would be frozen solid there.
The eye can also see a myriad of stars of varying brightness, but for the most part only thus distinguishable. If the telescope be now called in to aid, the eye will, however, be able to discern differences and distinctions in the stars. It will see that some are balls like the Earth, the Sun, and the Moon. If these balls are studied attentively we shall discover that one of them, Jupiter, is a great deal hotter than the earth, though a great deal cooler than the Sun; and that another of them, Mars, is a great deal colder than the earth, but a great deal warmer than the Moon. Perhaps we might now begin to surmise that the Sun coming first, Jupiter next, the Earth next, Mars next, and the Moon last, were all like stages in the history of one of these balls; and that, for example, any one ball began by being as hot as the Sun, and ended, after passing through stages like Jupiter, the Earth, and Mars, in being as cold and lifeless as the Moon.
But if one had a very good telescope, and could examine those more distant specks of light which we call stars, we perhaps could spy a little earlier into the history of these great balls. For example, among the blazing lights of the heavens—the stars which we know to be suns—there are others which are not balls at all. There are the Pleiades, for instance, of whom the Prophet Amos wrote, "Seek Him that maketh the seven stars and Orion" (Amos v. 8). The seven stars (in the Authorised Version rendered Pleiades),[4] when seen through a great telescope, are caught in a mesh or a veil of something that may be starry matter, but of the exact nature of which we are uncertain. In other parts of the sky there are great masses of this starry mist; and to these bright patches astronomers have given the name of "nebulæ." The most wonderful of them all is the great nebula in Orion;[5] and one of the most beautiful is the great spiral nebula of Andromeda. These objects are not only wonderful and beautiful; they also give us a hint as to what might have been the earliest state of our earth, and of the Sun itself in those almost inconceivably distant ages before order took the place of chaos.