Our British coal and a good deal of foreign coal is found in certain stratified rocks of the earth's crust known as "the Carboniferous System," about 12,000 ft. thick, consisting chiefly of very dense limestone. The "seams," or stratified beds of coal, occur in sandy rock known as the "Coal Measures," and vary in thickness from a mere film to 40 ft. Above the Carboniferous System are later deposits, some 14,000 ft. in thickness–the Permian, Triassic, Jurassic, Cretaceous, and Tertiary strata. Below them we find stratified deposits containing fossilized remains of plants and animals, to a depth of another 40,000 ft.: they are the Devonian, Silurian, and Cambrian "systems" or series of strata. Coal of a workable nature is found in many parts of the world in the beds or strata of later age than our Coal Measures–namely, those of Jurassic, Cretaceous, and Tertiary age.
Coal is so valuable and used in such vast quantities by modern man that, though procured at first from beds lying at or near the surface, it has been found remunerative to mine far into the depths of the earth's surface, where its existence is ascertained, in order to procure it. A depth of 4000 ft. is apparently the limit set to such mining by the increase of temperature in mines which penetrate to that extent below the surface. In 1905 the annual output of British coal-mines was in round numbers 230,000,000 tons. It is certain that there is a limit to this production, but not possible to calculate what that limit may be, owing to the uncertainty as to the future working of coal-fields as yet unexplored.
Such questions have been, and are being, considered by experts on behalf of the Government. A matter of interest of another kind is that in and associated with the coal seams of our Coal Measures, fossilized remains of peculiar fern-like trees, ferns, and other strange plants, and of very peculiar, extinct newt-like animals (as large as crocodiles) are found in great variety. The notion that the toads occasionally found embedded in the black mud of a coal-yard or even in a fractured lump of coal are survivals from the time–many millions of years past–when the plants and animals of the Coal Measure swamps were living, is a baseless fancy. The toads so found are of the kind or species now living on the earth–totally different from those whose bones occur in the Coal Measures, and the presence of such modern toads embedded in black slime, in coal-heaps in store-yards, or even in coal-scuttles, is only what may be expected to occur and does occur in damp quarries and other places where these familiar little beasts love to hide.