Although coal is thus essentially a product of Carboniferous age, it must not be concluded that this mineral is found in no other geological formation. The conditions favourable for the deposition of beds of vegetable matter have prevailed again and again, at various periods of geological time and on different parts of the earth, although there is at present no distinct evidence that such a luxuriant growth of vegetation, combined with the other necessary conditions, has ever existed at any other period in the history of the globe. Thus in the very oldest rocks of Canada and the northern States of America, in strata which take us back to the dawn of geological history, there is found abundance of the mineral graphite, the substance from which black-lead pencils are made, which is almost pure carbon. Now most geologists admit that graphite represents the carbon which formed part of the woody tissue of plants that lived during those remote times, so that this mineral represents coal in the ultimate stage of carbonization. In some few instances true coal has been found converted into graphite in situ by the intrusion of veins of volcanic rock (basalt), so that the connection between the two minerals is more than a mere matter of surmise.

Then again we have coal of pre-Carboniferous age in the Old Red Sandstone of Scotland, this being of course younger in point of time than the graphite of the Archæan rocks. Coal of post-Carboniferous date is found in beds of Permian age in Bavaria, of Triassic age in Germany, in the Inferior Oolite of Yorkshire belonging to the Jurassic period, and in the Lower Cretaceous deposits of north-western Germany. Coming down to more recent geological periods, we have a coal seam of over thirty feet in thickness in the northern Tyrol of Eocene age; we have brown coal deposits of Oligocene age in Belgium and Austria, and, most remarkable of all, coal has been found of Miocene, that is, mid-Tertiary age, in the Arctic regions of Greenland within a few degrees of the North Pole. Thus the formation of coal appears to have been going on in one area or another ever since vegetable life appeared on the globe, and in the peat bogs, delta jungles, and mangrove swamps of the present time we may be said to have the deposition of potential coal deposits for future ages now going on.

Although in some parts of the world coal seams of pre-Carboniferous age often reach the dignity of workable thickness, the coal worked in this country is entirely of Carboniferous date. After the explanation of the mode of formation of coal which has been given, the phenomena presented by a section through any of our coal measures will be readily intelligible (see [Fig. 1]). We find seams of coal separated by beds of sandstone, limestone, or shale representing the encroachment of the sea and the deposition of marine or estuarine sediment over the beds of vegetable remains. The seams of coal, varying in thickness from a few inches to three or four feet, always rest on a bed of clay, known technically as the “underclay,” which represents the soil on which the plants originally grew. In some instances the seams of coal with their thin “partings” of clay reach an aggregate thickness of twenty to thirty feet. In many cases the very roots of the trees are found upright in a fossilized condition in the underclay, and can be traced upwards into the overlying coal beds; or the completely carbonized trunk is found erect in the position in which the tree lived and died (see [Fig. 2]).

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Fig. 1.—Section through Carboniferous strata showing seams of coal. Dislocations, or “faults,” so common in the Coal Measures, are shown at H, T, and F. Intrusions of igneous rock are shown at D. At B is shown the coalescence of two seams, and at N the local thinning of the seam. The vertical lines indicate the shafts of coal mines.

Fig. 2.—Section showing coal seams and upright trunks attached to roots in situ. A′, A″, A′″, beds of shale. B, coal seams. C, underclay. D, sandstone.