We now come to an argument of a practical kind which appeals to common sense and common experience. Let us suppose that a person wholly unacquainted with the art of manufacturing paper, were to enter a paper-mill when the workmen are away, and the process of manufacture for a time suspended. At first sight he would probably find it difficult to persuade himself, that the piles of clean white paper, which attract his notice at one end of the building, are produced from the heaps of filthy rags which he sees accumulated at the other. But if he be a sagacious observer, he will soon find evidence to convince him that this is really the case. For he will perceive, upon close examination, that the self-same material is exhibited in every intermediate state of progress from one extreme to the other. First, there is the great chest with its numerous compartments, in which the rags are seen carefully sorted, according to their various degrees of quality and texture. Next comes the fulling-mill, where they are washed and bleached. Then the revolving cylinder, furnished on the exterior surface with sharp blades or cutters; and the vat in which it moves is filled with the rags, which now assume the form of a thin liquid pulp. Advancing still further he will see this pulp evenly spread out upon a wire-gauze frame, and now at last it is beginning to exhibit some likeness to the form and substance of paper. Further on it is seen pressed and dried; and last of all cut into sheets and laid aside in lofty piles. Now it seems to us that we are placed in somewhat of the same position, as regards the manufacture of Coal. We cannot observe the process actually going on; for though, in this process, the work is never suspended, the workmen never at rest, yet extending as it does over a space of many centuries, it is too slow to be sensible; and besides it is conducted in great part beneath the surface of the Earth. Nevertheless, we can trace the progress of change through each intermediate stage of the transition, from one extreme to the other,--from the primeval swamps and forests through the numerous varieties of the Peat and Lignite to the richest beds of pure Coal. First, then, we have the great forest-covered swamps, like those which now occupy the valley and delta of the Mississippi. They are composed in many cases of pure vegetable matter without any intermixture of earthy sediment. A dense growth of reeds, and shrubs, and herbage of every kind, covers the whole surface of the land, mixed up with the decaying leaves and prostrate trunks of forest-trees. Sir Charles Lyell mentions a very remarkable fact observed in the swamps of Louisiana. During an unusually hot season, when any part of a swamp is dried up, if the surface be set on fire, a pit is burned into the ground many feet deep, in fact, as far down as the fire can descend without meeting water; and it is then found that scarcely any residuum or earthy matter is left.[62]