Before telling you what its parts are, I ought to say where many of them have been found, and how they are still being looked for. They are found _upon_ the ground, _under_ it, in caves, in rivers, and in the sea. Since railways have been in use a great many tunnels have been made, as well as very deep cuttings through hills, and some of these are several miles long. In this way we have come to know something of the Earth below the surface. Some of these tunnels are bored right through high hills and even mountains, and the cuttings are deep enough to hide high houses if they were put into them. While digging these the workmen have found many of the parts of our puzzle, which are the bones of animals, and fishes, and shells, and even smaller things—such as insects. These could not possibly have been put there by anyone, because they were many, many yards below the surface, and, until they were dug up, nobody imagined that they could be there. Many other things besides have been dug out of these places, but nearer the surface, such as weapons and tools made of flint, and stone, and bone, and metal, and pieces of rough crockery, and various ornaments, all of which must at some time or other have been made and used by people very like ourselves. In digging canals, too, the same kinds of things have been found, and some caves are almost filled up with them. We have other means, too, of knowing what is under the surface of the ground we walk upon. Many of the coal-mines are so deep that the Tower of London, or St. Paul’s Cathedral, or York Minster, or even the Pyramids of Egypt could be buried in them! In digging these the workmen have had to go through a great quantity of earth, sometimes chalk, sand, or gravel, or clay or limestone, layer upon layer, placed, like a pile of books of different kinds and different thicknesses, one upon the other, until they have come to the coal. In these different layers of earth parts of the puzzle have been found, and we shall see by-and-by what parts have been found in the coal itself. Then again, when deep mines are made to get the metals, iron and gold and silver, these layers of earth have to be dug through; and when the beautiful kinds of stone, like marble and limestone, are wanted, they must be dug out of the sides of the hills, and in doing this still more pieces of the puzzle come to hand. But there are other places where Nature herself seems to have shown us some of them without the trouble of searching for them. In many parts of the World, by the sea, and on the banks of rivers, there are cliffs hundreds of feet high, like the chalk cliffs at Dover and Ramsgate, and the sandy cliffs at Folkestone and on the south coast of Devonshire. These cliffs have been cut into by the sea very gradually, and a kind of wall has been left, and from the sides of the cliffs great numbers of the pieces of the puzzle, bones, shells, &c., have been collected and taken away to museums. But the little we