Since the crust of the earth was so thin and yielding, it wrinkled up as the earth cooled, much as the skin of an apple wrinkles when the apple dries. This brought some of the strata of coal to the surface, and after a while people discovered that it would burn. If a vein of coal cropped out on a man's farm, he broke some of it up with his pickaxe, shoveled it into his wheelbarrow, and wheeled it home. After a while hundreds of thousands of people wanted coal; and now it had to be mined. In some places the coal stratum was horizontal and cropped out on the side of a hill, so that a level road could be dug straight into it. In other places the coal was so near the surface that it could be quarried under the open sky, just as granite is quarried. Generally, however, if you wish to visit a coal mine, you go to a shaft, a square, black well sometimes deeper than the height of three or four ordinary church steeples. You get into the "cage," a great steel box, and are lowered down, down, down. At last the cage stops and you are at the bottom of the mine. The miners' faces, hands, overalls, are all black with coal dust. They wear tiny lamps on their caps, and as they come near the walls of coal, it sparkles as it catches the light. Here and there hangs an electric lamp. It is doing its best to give out light, but its glass is thick with coal dust. The low roof is held up by stout wooden timbers and pillars of coal. A long passageway stretches off into a blacker darkness than you ever dreamed of. Suddenly there is a blaze of red light far down the passage, a roar, a medley of all sorts of noises,--the rattling of chains, the clattering of couplings, the shouts of men, the crash of coal falling into the bins. It is a locomotive dragging its line of cars loaded with coal. In a few minutes it rushes back with empty cars to have them refilled. All along this passageway are "rooms," that is, chambers which have been made by digging out the coal. Above them is a vast amount of earth and rock, sometimes hundreds of feet in thickness. There is always danger that the roof will cave in, and so the rooms are not made large, and great pillars of coal are left to hold up the roof. Not many years ago the miner used to do all the work with his muscles; now machines do most of it. The miner then had to lie down on his side near the wall of coal in his "room" and cut into it, close to the floor, as far as his pickaxe would reach. Then he bored a hole into the top of the coal, pushed in a cartridge, thrust in a slender squib, lighted it, and ran for his life. The cartridge exploded, and perhaps a ton or two of coal fell. The miner's helper shoveled this into a car and pushed it out of the room to join the long string of cars. [Illustration: HOW A COAL MINE LOOKS ABOVEGROUND All that shows on the surface is the machinery shed where the various engines work to keep the air fresh, and bring up the miners and the coal.]