From this brief sketch of the formation of the earth, its progress and processes, and from the hints which the volcanoes and earthquakes of to-day afford us, we may obtain some idea of the underworld that lies far beneath our feet. Not much, however, for we are ignorant of the actual conditions which exist towards the centre of the earth. But there seems a strong case for supposing that there is an outer solid crust of earth and an inner molten core of very great heat. CHAPTER IX THE CHILDHOOD OF THE EARTH Let us now sum up the various stages in the early growth of the earth, as most geologists believe them to have occurred. The first stage was that when the earth, shining like a star, existed as a fluid globe surrounded by heavy vapours of great thickness, which contained the future waters of the globe. Then began the second era of the earth's existence, when it was a hot solid globe--solid at any rate at the surface, and with a temperature of about 2500° F. The globe's atmosphere still contained all the waters of the earth. It contained all the carbonic acid gas which now exists in the limestones and in coal and other minerals containing carbon. It contained also all the oxygen since shut up in the rocks and in vegetation and in animal substances. Such an atmosphere was probably at least two hundred times as great as the atmosphere which now surrounds the earth. Then followed an epoch when great volcanic action set in. We have partially described it already. Just a thin solid crust was all that covered the molten interior of the globe. It was too thin always to contain the boiling liquid, and Titanic explosions, followed by enormous overflows of lava, continually broke up the crust. The pressure was relieved by these explosions, and gradually the earth would settle down again to its process of consolidation. Another explosion would follow; again a great flow of lava; and again the effects of the catastrophe would subside. After each explosion and outflow the earth's crust would grow a little thicker. All this time, and for long succeeding ages, the earth was attracting to itself, as a magnet attracts iron filings, all the small bodies which it encountered on its path round the Sun. These little rocks or masses of matter, some great and some small, would each add something to the size of the earth, and, by the shock of collision with it, something to the earth's heat--just as a bullet flattening itself against a target melts in the heat of collision. Just also as the bits of matter which we call "shooting-stars" are set on fire by friction as they rush into the earth's atmosphere. These meteorites, as they are called when they are comparatively small bodies, or "planetismals," as they are called when they are large, still exist. But the earth, in the millions on millions of years which it has been coursing round the Sun, has swept up all the large ones that are likely to fall into