Following the question of the composition of coal comes the question of its origin, of which, indeed, there is no longer any serious doubt. It is generally conceded that coal is a vegetable product, and there are excellent reasons for this belief. The fragments of which coal is composed have been greatly deformed by compression and decomposition. But when one of those fragments is made so thin that it will transmit light, and is then subjected to a powerful microscope, its vegetable structure may readily be distinguished; that is, the fragments are seen to be the fragments of plants. Immediately under every separate seam of coal there is a stratum of what is known as fire clay. It may, under the beds of softer coals, be of the consistency of clay; but under the coal seams of the harder varieties it is usually in the form of a slaty rock. This fire clay stratum is always present, and contains in great abundance the fossil impressions of roots and stems and twigs, showing that it was once the soil from which vegetation grew luxuriantly. It is common also to find fossil tree-stems lying mashed flat between the layers of black slate which form the roof of the coal mines, also the impressions of the leaves, nuts, and seeds which fell from these trees while they were living. In some beds of cannel coal whole trees have been found, with roots, branches, leaves, and seeds complete, and all converted into the same quality of coal by which they were surrounded. In short, the strata of the coal measures everywhere are full of the fossil impressions of plants, of great variety both in kind and size. If a piece of wood be subjected to heat and great pressure, a substance is obtained which strongly resembles mineral coal.