The mechanical change which the coal has experienced consists in the flattening and hardening effect of the immense pressure of thousands of feet of superincumbent rock, which has crashed together the cell-walls of the vegetable matter, and reduced what was originally a pulpy mass of cellular tissue to the condition of a hard laminated rock. To understand this, perhaps the simplest way is to compare under the microscope a transverse section of recent pine-wood with a similar section of a pine trunk compressed into brown coal or jet. In the one the tissue appears as a series of meshes with thin woody walls and comparatively wide cavities for the transmission of the sap. In the other the walls of the cells have been forced into direct contact, and in some cases have altogether lost their separate forms, and have been consolidated into a perfectly compact structureless mass. With regard to its mode of occurrence, coal is found in beds ranging in vertical thickness from less then an inch to more then thirty feet, and of wide horizontal extent. Many such beds usually occur in the thickness of the coal formation, or "coal measures," as the miners call it, separated from each other by beds of sandstone and compressed clay or shale. Very often the coal occurs in groups of several beds, somewhat close to each other and separated from other groups by "barren measures" of considerable thickness. In examining a bed of coal, where it is exposed in a cutting or shore cliff, we nearly always find that the bed below it, or the "underclay," as it is termed by miners, is a sort of fossil soil, filled with roots and rootlets. On this rests the coal, which, when we examine it closely, is found to consist of successive thin layers of hard coal of different qualities as to lustre and purity, and with intervening laminae of a dusty fibrous substance, like charcoal, called "mother coal" by miners, and sometimes mineral charcoal. Thin partings of dark shale also occur, and these usually present marks and impressions of the stems and leaves of plants. Above the coal is its "roof" of hardened clay or sandstone, and this generally holds great quantities of remains of plants, and sometimes large stumps of trees with their bark converted into coal, and the hollow once occupied with wood filled with sandstone, while their roots spread over the surface of the coal. Such fossil forests of erect stumps are also found at various levels in the coal measures, resting directly on under-clays without any coals. A bed of coal would thus appear to be a fossil bog or swamp.