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).

Larger Image

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.

Owing to the chemical and mechanical forces to which the original vegetable deposit has been subjected, the organic structure of coal has for the most part been lost. Occasionally, however, portions of leaves, stems, and the structure of woody fibre can be detected, and thin sections often show the presence of spore-cases of club-mosses in such numbers that certain kinds of coal appear to be entirely composed of such remains. But although coal itself now furnishes but little direct evidence of its vegetable origin, the interstratified clays, shales, and other deposits often abound with fossilized plant remains in every state of preservation, from the most delicate fern frond to the prostrate tree trunk many yards in length. It is from such evidence that our knowledge of the Carboniferous flora has been chiefly derived.

Now this carbonized vegetation of a past age, the history of which has been briefly sketched in the foregoing pages, is one of the chief sources of our industrial supremacy as a nation. We use it as fuel for generating the steam which drives our engines, or for the production of heat wherever heat is wanted. In metallurgical operations we consume enormous quantities of coal for extracting metals from their ores, this consumption being especially great in the case of iron smelting. For this last operation some kinds of raw coal are unsuitable, and such coal is converted into coke before being used in the blast furnace. The fact that the iron ore and the coal occur in the same district is another cause of our high rank as a manufacturing nation.

It has often been a matter of wonder that iron ore and the material essential for extracting the metal from it should be found associated together, but it is most likely that this combination of circumstances, which has been so fortunate for our industrial prosperity, is not a mere matter of accident, but the result of cause and effect. It is, in fact, probable that the iron ore owes its origin to the reduction and precipitation of iron compounds by the decomposing vegetation of the Carboniferous period, and this would account for the occurrence of the bands of ironstone in the same deposits with the coal. In former times, when the area in the south-east of England known as the Weald was thickly wooded, the towns and villages of this district were the chief centres of the iron manufacture. The ore, which was of a different kind to that found in the coal-fields, was smelted by means of the charcoal obtained from the wood of the Wealden forests, and the manufacture lingered on in Kent, Sussex, and Surrey till late in the last century, the railings round St. Paul’s, London, being made from the last of the Sussex iron. When the northern coal-fields came to be extensively worked, and ironstone was found so conveniently at hand, the Wealden iron manufacture declined, and in many places in the district we now find disused furnaces and heaps of buried slag as the last witnesses of an extinct industry.

From coal we not only get mechanical work when we burn it to generate heat under a steam boiler, but we also get chemical work out of it when we employ it to reduce a metallic ore, or when we make use of it as a source of carbon in the manufacture of certain chemical products, such as the alkalies. We have therefore in coal a substance which supplies us with the power of doing work, either mechanical, chemical, or some other form, and anything which does this is said to be a source of energy. It is a familiar doctrine of modern science that energy, like matter, is indestructible. The different forms of energy can be converted into one another, such, for example, as chemical energy into heat or electricity, heat into mechanical work or electricity, electricity into heat, and so forth, but the relationship