Although at the present time no one at all conversant with geology doubts the vegetable origin of Coal, the period is not distant when many eminent philosophers were sceptical on this point; and the truth in this, as in most other questions In natural philosophy, was established with difficulty. The experiments and observations of the late Dr. Macculloch mainly contributed to solve the problem as to the vegetable origin of this substance; and that eminent geologist successfully traced the transition of vegetable matter from peat-wood, brown coal, lignite, and jet, to coal, anthracite, graphite, and plumbago. Nor must the important labours of Mr. Parkinson in this field of research be forgotten. The first volume of the "Organic Remains of a former World," which treats of vegetable fossils, contains much original and valuable information on the transmutation of vegetable matter, by bituminous fermentation, into the various mineral substances in which its original nature and structure are altogether changed and obliterated; and that work may still be consulted with advantage by the student. But though the vegetable origin of all coal will not admit of question, yet evidence of the original structure of the plants or trees whence it was derived is not always attainable. The most perfect coal seems to have undergone a complete liquefaction, and if any portions of the vegetable tissue remain, they appear as if imbedded in a pure bituminous mass. The slaty coal generally preserves traces of cellular or vascular tissue, and the spiral vessels and dotted cells of coniferous trees may often be detected by the microscope. In many instances the cells are filled with an amber-coloured resinous substance; in others the organization is so well preserved, that on the surface of a block of coal cracked by heat, the vascular tissue, and the dotted glands, may be observed. Some beds of coal appear to be wholly composed of minute leaves or disintegrated foliage; for if a mass recently extracted from the mine be split asunder, the exposed surfaces are found covered with delicate laminæ of carbonized leaves and fibres matted together; and flake after flake may be peeled off through a thickness of many inches, and the same structure be apparent. Rarely are any large trunks or branches observable in the beds of coal; but the general appearance of the carboniferous mass is that of an immense deposit of delicate foliage shed and accumulated in a forest, and consolidated by great pressure while undergoing that peculiar process by which vegetable matter is converted into carbon.