It is interesting to know that the case supposed here has actually been realized in the delta of the Ganges. Some years ago an Artesian well was attempted to be made near Calcutta, and the auger was sunk to a depth of 481 feet.[71] The material passed through consisted of sand, clay, and nodules of argillaceous limestone, and at various depths, from 50 to 380 feet, several seams of decaying wood and peat were found, along with bones of various animals, such as deer and fresh-water tortoises, and fragments of lacustrine shells. Each of these vegetable layers evidently formed at one time a forest-covered swamp like those of the surrounding delta at the present day; and hence it follows, that during the accumulation of the Gangetic delta, the ground in that locality must have undergone a depression of more than 300 feet, and that this sinking has been interrupted by slight elevations, or by periods when the ground remained stationary, so as to admit of a dense and prolonged growth of vegetation, at successive intervals, upon the swampy flats and shifting islands. The general appearance of these old forests is pretty well shown by the mangrove swamps along the mouths of the river. These trees flourish in dense jungles on the banks, and extend even below high water mark, being covered in places by shell fish. So that were these maritime parts of the delta inundated by the ocean, and buried beneath a mass of mud and silt, the peaty layer that would be formed would display trunks of trees still occupying their original erect position, and spreading out their roots in the clay below, exactly as the sigillaria is found to do in the coal-seams of the carboniferous rocks, while clustered round the carbonized stems, or scattered among the decayed leaves and branches, there might be detected limpets and barnacles (as lingulæ and pectens occur in the coal-seams), showing, by their mode of occurrence, that they lived and died upon the spot.

[71] See Lyell's Principles, p. 280.

If my reader will now suppose this sand of the Indian river to be hardened into sandstone, the mud in like manner compressed into shale, and the peat beds chemically altered into coal, can he fail to perceive the striking analogy between the section thus displayed and those already given from the Mid-Lothian and Cape Breton coal-fields? The differences between the ancient and modern strata are not in kind but in degree. The Scottish series reaches to more than six times the thickness of the Indian one, and the coal-seams in the one exceed in individual thickness the peat-beds in the other. We must remember, however, that the climate of Hindustan is not remarkably favourable to the accumulation of vegetable matter, the heat being so great that the plants decay almost as rapidly as they grow. And it should likewise be borne in mind, that were the conditions of subsidence and of the gradual accumulation of sedimentary matter to continue even in the same ratio as heretofore, the Ganges might, in the course of ages, heap up a series of stratified sands, clays, and peat-beds, many thousand feet in thickness, and many thousand square miles in extent, rivalling, or perhaps surpassing in depth, the largest coal-field in the world. The parallelism between this delta and an ordinary coal-field holds singularly close, not merely as regards the nature of the stratified deposits. The alluvial plain of Bengal has undergone a process of subsidence to an unknown depth, whereby successive areas of terrestrial vegetation have been carried down to be entombed beneath fluviatile sand and mud. It is likewise subject to the more sudden operation of earthquakes, whereby large tracts of country become permanently altered, and changes are effected on the direction, rapidity, and detritus of the streams. It is, moreover, liable to wide-spread inroads of the sea, which sometimes covers cultivated districts to a depth of several feet, laying waste the fields and destroying the inhabitants. These and other features help us to understand the origin of such vast masses of sedimentary strata as those of our coal-fields, where terrestrial, fluviatile, and marine remains alternate in rapid sequence, or sometimes occur together.

The origin of the constant succession of coal seams, sandstones, and shales, of the Edge series may be thus accounted for. The area of Mid-Lothian formed part of a great delta, which, like that of the Ganges, was undergoing a gradual subsidence during the Carboniferous era. The rate of this movement probably varied at different times, and might even be occasionally interrupted by short periods of elevation. When the ever-increasing accumulations of silt brought down by the river reached or nearly reached the surface of the water, they would become the site of wide tracts of swampy vegetation that flourished for hundreds or thousands of years. Eventually, however, these jungles, invaded by the changing currents of the river, were buried beneath a thick deposit of fluviatile sediment, or more probably the vegetation might become unable to keep pace with an accelerated rate of submergence, and the forests would then be tranquilly carried down beneath the water, and soon covered over with sand and mud. The detrital matter might in like manner continue to be deposited over the sunk forest for many years, perhaps centuries, until the muddy bottom again reached the surface, and once more waved green with sigillariæ, calamites, and lepidodendra. Another long interval might here elapse, in which a thick bed of vegetable matter might accumulate, much after the manner of the formation of peat among the bogs and mosses of our own country. The periodical inundations of the river probably gave rise to wide marshes and lagoons, often tenanted by lacustrine shells, and thickly overgrown with aquatic vegetation. The decaying plants decomposed the red ochreous matter with which the water was charged, and re-deposited it among the mud and rotting leaves at the bottom as a carbonate of iron. Such ferruginous accumulations, often entombing fern-stems and other plants, with scales and teeth of ganoidal fishes, sometimes conulariæ and lingulæ, and, in certain localities, whole acres and miles of fresh-water shells, are known now as our clay-band and black-band ironstones. We can easily conceive that, in shallower parts of the lagoons, a dense growth of marshy plants might spring up, preventing any deposition of iron, and when the whole came to be covered over with later accumulations of sand or mud, the deeper parts of the old lake would be covered with a seam of ironstone, and the shallower portions would display a bed of coal. In some such way we may account for the frequent passage of ironstone into coal, and coal into ironstone in many of our coal-fields. If undisturbed by the ever changing currents of the river, these wide expanses of marsh and lake might continue for many long years, the constant evaporation being counterbalanced by continual supplies of water from the main stream. Eventually, however, owing perhaps to another period of more rapid submergence, the water gained the ascendency, and once more rolled over prostrate stems and matted thickets of ferns, that sank slowly down beneath a deepening sheet of sand and mud. Often, too, the sea must have flooded, perhaps for years, the flat delta-lands, carrying with it its own productions, such as the lingulæ and cardiniæ, which we find among the coal seams. And thus the process went on during the long ages of the Carboniferous system. Forest after forest spread its continuous mantle of green athwart the low swampy lands of that old delta, and each in succession foundered amid the muddy waters, now of the ocean and now of the river, that strewed over its site a rich detritus which went to form the soil of new jungles and forests.

The Edge series measures from 800 to 900 feet in depth, so that the depression must have been carried on till the forest that once grew nearly on the sea-level had sunk 800 feet below it This process was undoubtedly a very slow and tranquil one. Yet geologists used to regard these frequent changes of sedimentary matter as so many proofs of repeated catastrophic submergences, when the ocean came rolling over the land, prostrating forests, uprooting the hugest trees, and leaving the scattered bones and scales of fishes amid vast accumulations of mud and sand, where but lately there had bloomed a luxuriant vegetation. But the sober and diligent student of geologic fact will read in these rocks no such record of cataclysms. He will see in them evidences of the same gradual and sure operation which marks the processes of Nature at the present day. He will note how during a tranquil and probably imperceptible submergence of the river-bottom, forest after forest sprang up, flourished perhaps for ages, and eventually settled down beneath the waters of the river and sometimes of the ocean, amid ever increasing accumulations of mud and sand. Musing on these ancient changes he will be lost in wonder at the immense duration of the period during which they were in progress; and he will try in some measure to realize the features of their scenery. He will picture the delta with its ever-varying islets and sand-banks, its lakes and submerged forests, its leafless trunks peering above the water and sticking along the shoaling mud, and its crowded jungles that cover every drier spot. He will cast his eyes to where the delta opens out into the ocean, and mark how the waves encroach upon the mud-banks, cutting away what the river has piled up, and washing the roots of gigantic trees that wave their green coronal of fronds above, and overshadow the rippling of the green sea below. He will try to thread the windings of the stately river through brakes of ferns and calamites, and banks richly hung with tree-ferns and sigillariæ, and then upward through dark shaggy pine-woods, silent and gloomy, with the water creeping lazily through the shade or dashing in white cascades over dripping rocks, and onward still, far away among the distant hills till the fountainhead of the great stream is reached, gushing from the splintered sides of some lone rock, or pouring perchance out of the glimmering caverns of some massive glacier high amid the regions of perpetual snow.

Many attempts have been made to estimate the amount of time which some of our coal-fields may have required for their accumulation. But so large a number of conjectural elements must necessarily enter into such calculations, that the results come to be of very doubtful value. By estimating the amount of sediment annually transported by such rivers as the Ganges or Mississippi, we may ascertain how long a mass of similar sedimentary strata would take to form under similar conditions. And if our calculation had to do merely with such detrital accumulations, we might hope to arrive at some approach to accuracy. But besides these sedimentary strata, the formation of which must have been wholly analogous to that of similar deposits at the present day, we have to deal with the problems suggested by the coal-seams. We know nothing of the climate of the Carboniferous period save what may be conjectured from the analogy of existing climates; and in a question regarding the accumulation of decaying vegetable matter climate is a subject of the first importance. We are ignorant, too, of the rate of growth peculiar to the carboniferous flora; and even if we hold that it was probably rapid, the process of decay may have been equally speedy, and so a forest might go on shooting up fresh trees as the old ones rotted away, yet at the end of a thousand years there might be a scarcely greater thickness of vegetable matter on the ground than at the commencement. A seam of coal two feet thick might thus represent, say the accumulation of a hundred years, and another of exactly the same thickness might stand as the accumulation of a thousand years. Until we know more of the vegetation and climate of the coal period, the thickness of a coal-seam can hardly be held as a certain guide to the lapse of time required for its formation.

For the sake of illustration, let me take the following fragment of a coal-measure section:—

Shale,20feet.
Coal,4"
Fire-clay,6"
Sandstone,40"

Beginning at the bottom, we may compute the period of the forty feet of sandstone variously, according to the river selected as the type of a transporting agent. Tried by the standard of the Nile, all other conditions being similar, such a deposit would require perhaps not less than 14,000 years; by that of the Mississippi, 5000; and by that of the Ganges, nearly 2000.[72] We come, then, to the superincumbent fire-clay and coal, representing an ancient soil and the forest that grew on it. The occurrence of these seams shows us that the river-bed had become a swampy tract clothed with vegetation; but who shall say how long it may have continued so? Like the sunk country of the Mississippi, it may have been submerged, and to some extent cut off from the sediment-transporting channels of the river, and thus, as a vast lake, have nourished a prolific growth of marshy and aquatic plants. If the temperature resembled that of our own country, the growth of peaty matter, other circumstances being favourable, might be comparatively rapid. If, however, as seems probable, the climate were more warm and humid, giving rise to a more luxuriant vegetation, and at the same time to a more rapid decay, a long interval might have elapsed without adding materially to the thickness of the vegetable accumulations, and the eventual entombment of peaty matter sufficient to consolidate into four feet of coal, might be owing in some measure to the submergence of the swamp beneath the waters of the river, whereby a quantity of detrital matter was deposited that arrested the process of putrefaction, and entombed the thickly matted plants which were growing on the spot at the time. Hence, until we know more of the conditions under which vegetation may accumulate at river-mouths in such a climate as the coal plants are conjectured to have enjoyed, calculations of the amount of time required for the formation of a great series of coal-bearing strata must be regarded as premature. In the present instance, we can but affirm that the growth of the four-foot coal-seam probably occupied many long years, even at the most rapid rate of accumulation known to us. The forest-covered swamp on which the plants grew was eventually invaded by muddy detritus brought down by the river; and during another period of indefinite extent—five hundred years or five thousand years—fine mud continued to settle down over the foundered forest, hardening eventually into twenty feet of shale.

[72] Some observers have pointed to the occurrence of vertical and inclined trunks of trees in the Carboniferous sandstones, and deduced therefrom what has seemed to them a triumphant argument in favour of the rapidity wherewith our coal-fields must have formed. A foundered tree, they say, sank with its heavy-laden roots among the sand at the bottom, its stem pointing up into the water like the snags of the Mississippi, so that the sand must have come rapidly down to entomb the whole before it had time to decay, and thus thirty or forty feet of sediment must have been deposited in a few years, perhaps even months. But this is somewhat like a begging of the question. We have yet to learn how long a water-logged trunk will resist decomposition.