As coal seams have been discovered as far to the north as Greenland, Melville Island, and Spitzbergen, where now no trees will grow, it has been inferred that, in the primeval ages which witnessed their birth, a tropical climate must have reigned over the whole surface of the earth; but the vegetation of arborescent ferns does not necessarily imply a very warm climate, as such plants are found to flourish in New Zealand, together with many conifers and club-mosses, so that a forest in that temperate country may make a nearer approach to the carboniferous vegetation than any other now existing on the globe. So much is certain, that a very different distribution of sea and land must in those times have mitigated the severity of the Arctic winter, or, perhaps, as Professor Oswald Heer conjectures, our solar system may then have rolled through a space more densely clustered with stars, whose radiant heat gave to our earth the advantage of a mild climate, even at the poles.
The space of time required for the formation of the coal-fields is as immeasurable as the distance that separates us from Sirius. We know by experience how thin the sheet of humus is which the annual leaf-fall of our trees, or the yearly decay of our moor-plants, leaves behind, and how many decenniums must elapse before one single inch of solid mould is formed. But there are many coal strata eight, ten, or even forty or fifty feet thick; and if we consider besides the mighty pressure of the superincumbent rocks which store them in the smallest compass, we cannot possibly doubt that one such stratum must have required thousands of years for its formation. Our wonder increases when we reflect that in many coal-measures (the series of beds intimately associated with the seams of coal) no less than a hundred thick and thin seams of coal alternate with layers of sandstone and shale, so that the reckoning would swell to millions were we able to fathom the ages of their successive growth.
It may well be asked how such vast masses of vegetable origin, which required the sun’s light for their formation, came thus to be incased in stone thousands of feet beneath the surface of the earth? More than one theory has been advanced to solve this difficult problem, which can hardly be explained in any other manner than by a general, slowly progressing subsidence of the humid lowlands, alternating with periods of rest. Fancy a wide delta land, similar to Egypt or the Netherlands, covered with luxuriant forests, whose spoils, accumulating where they fall, form in the course of centuries a thick stratum of vegetable matter. This land then sinks, suddenly or gradually, under water, many a fathom deep, and remains there perhaps for ten thousand years, till a vast deposit is formed of sandstone and shale, brought down from the highlands by the rivers that come rolling from the interior, the pressure of which, aided by water, converts the stratum of wood into coal. By this deposit the bottom is gradually filled up, and the bay again converted into marsh or meadow, upon which again vegetation flourishes for a thousand years till the materials of a second bed of coal are collected. A third submergence takes place, rocky strata are again deposited, the water again shoals into land capable of bearing plants, a third period of forests commences, and continues till the mass of vegetable matter destined to form a new bed of coal is accumulated. It is unnecessary to pursue the series any further; let it suffice to say that in this manner coal followed upon sand, or sand upon coal, till in the carboniferous basin of Nova Scotia, for instance, a vertical subsidence of three miles was gradually filled up by the waste swept down from the higher lands, or by the accumulation of vegetable matter.
Great as are these changes of level, they do not indicate any more considerable or violent perturbations than those which take place at the present day, either from earthquakes or from slow oscillations of the soil. Large areas in the Pacific and elsewhere are known to be actually subsiding at the rate of three or four feet in the century, and when measured on the scale of geological time, the depression which sunk the first carboniferous forests of South Wales or Nova Scotia to the depth of ten or twenty thousand feet, probably proceeded at the same slow rate. Adding to these vast epochs of gradual subsidence the long periods of rest which intervened between them, it is perhaps no exaggeration to affirm that several millions of years may have been required for the formation of a coal-field such as that of Saarbrücken or South Wales. The fossil remains inclosed in the various layers of the carboniferous beds alone suffice to prove the immensity of time required for their accumulation, for the species of ferns or lycopods imbedded in the lower seams of a coal-field are found gradually to disappear in the higher ones, while new species are continually appearing on rising in the series, until, finally, the plants of the older seams have completely made way for newer forms. Thus the coal formation has, during the vast ages of its growth, changed more than once the aspect of its flora, and the plants which flourished in its youth had long since disappeared from the earth when it approached its end.
COAL-BEDS RENDERED AVAILABLE BY ELEVATION.
a b c, shafts. A B C, coal-beds.
Although all coal-fields must have originally been formed in horizontal or slightly undulating situations, yet in many cases they have undergone enormous derangements or dislocations from subsequent terrestrial changes; and to this circumstance is mainly due their utility to man. Had they been permitted to remain in their primitive geological position, we probably should never have enjoyed the benefit of the coal, because it would have been too deep for our reach. We might have known it to be there, but it would have been beyond our power to pierce a mile or two into the earth. But, by a wonderful and merciful providence, the oscillations to which the earth-rind is subject, have frequently upheaved them enormously out of their original positions; and the elevated portions having often been denuded by water, large patches of coal have thus been rendered available to man.
The various subterranean changes which have acted upon the coal-fields during the course of unnumbered ages have not only raised or sunk, but frequently dislocated, contorted, ruptured, or broken them up in a most extraordinary manner. In the coal-field near Mons, in Belgium, for instance, a vast lateral pressure has curved the strata again and again, and even folded them four or five times into zigzag bendings, so that on sinking a shaft the same continuous layer of coal is cut through several times.
SECTION OF COAL-FIELD SOUTH OF MALMESBURY.