This account, as given by geologists, of the origin of the mountain limestone, is rendered not only probable, but almost certain, by the manner in which we find these little insects, the coral-builders, constructing their piles of masonry at the present day. For example, certain species of polyps, of solitary habits, work alone, each rearing a single stem or stalk, from which others project; then more stems are produced, until, upon the completion of the whole, there results one of those beautiful arborescent structures so much prized as ornaments for cabinets and drawing-rooms. Some, again, attach themselves to the loose stones, upon which they form their little tree or flower-top; others adhere to the solid rock, from which there springs a stony vegetation, rivaling often, in variety, luxuriance, and brilliancy, the most showy vegetable productions of tropical climes. But a certain class are gregarious, and will only work in company. Myriads of these inhabit the Pacific, constructing entire islands, and throwing up mighty barriers of rock, and threading over vast areas of the sea with inosculating lines of coral reef. The calcareous accumulation, known as the Great Barrier Reef, extends for about a thousand miles in length, by about thirty in mean breadth, filling up, with its various reticulations, the whole intermediate space betwixt the coast of Australia and Bristow Island, off the coast of New Guinea. The works of these minute creatures thus occupy an area which may be roughly estimated at thirty thousand square miles; the different branches forming compartments of variable extent, which are divided into linear, outer, and inner reefs, and embracing within their ample folds the entire spoils of ocean living or floating in these parts.
The mountain limestone of our own country, formed in like manner on the sea-bottom of corallines, has a wide geographical range, extending from the bay of St. Andrew’s on the north, to the extremity of Wales on the south; passing into Ireland, where it is elevated into long ridges, or occupies the mountain-slopes; and forming outliers or extended barriers in all the southern counties of Scotland, and in the greater portion of the northern, the middle, and the south-western districts of England. These were the coral reefs of an ocean now raised into dry land, divided, too, into outer and inner compartments, or arranged into systems of lines and branches, which diverged from or inosculated with each other. Nor does the resemblance between the recent and the more ancient formations stop here, but extends to the structure of the deposits, lithologically considered, the mechanical, sub-crystalline, and crystalline texture being exhibited in both sets of rocks. Thus, in the examination of Heron Island, the coral beds, one to two feet thick, are found to have a tendency to split into slabs, and joints are observed to cross each other at right angles, parallel to the dip and strike, respectively, giving to the still living coral rock the jointings, cleavage, and stratification of the greater palæozoic deposits. Naturalists divide these polyps into existing and extinct races. But whether extinct and specifically different, or otherwise, they are creatures of a family, possessed of the same habits and performing the same operations, now as of old; and if, as geologists say, millions of ages have elapsed between the actings of the first and last generations, our admiration will be only all the more unbounded by thus witnessing the harmony of creation through indefinite time, and the accuracy of the Book which contains the record of it.
The coal itself, as now universally admitted, is of vegetable origin. Under the microscope, in the most compact specimens, the tissues by which all the coal plants are more or less distinguished can be distinctly traced. Chemically considered, its vegetable origin is equally well established. Carbon constitutes the principal ingredient of the mineral the quality of which enters most abundantly into the composition of vegetables. One theory of its formation is, that vegetable matter, carried to the sea or extensive lakes, has undergone a process of decomposition, by which, while some of its principles may have escaped or been evolved in new combinations, the carbon, with a portion of the hydrogen, has remained; this, mixed with more or less earthy matter, has in its soft state been consolidated by the force of aggregation simply, or by compression from the superincumbent strata, and the action of a higher degree of temperature than now exists. Others are of opinion that coal is the altered residuum of trees and smaller plants that have grown on the spot where we now find them—that the forests were submerged and covered by detrital matter, which was upraised to form a foundation and a soil for another forest, to be in its turn submerged and converted into coal—and that thus the alternations which the vertical section of a coal-field exhibits are to be accounted for. The former views are maintained by Sir R. Murchison and other eminent geologists. The latter have been adopted by Sir Charles Lyell, in consequence mainly of the arrangements and structure observed in the remarkable coal-field of Nova Scotia, where he states that there is a range of perpendicular cliffs in the Bay of Fundy, composed of regular coal-measures, inclined at an angle between twenty-four and thirty degrees, whose united thickness is between four and five miles. By neither theory, perhaps, nor by any other yet advanced, is it possible to reconcile all the appearances which that singular compound, a coal-field exhibits—the various changes which the vegetable matter has undergone to convert it into lignite, jet, common coal, cannel coal, and anthracite, two or more of these varieties often occurring in the same coal-measures—in one quarter the clearest indications that the sea has let in its floods and mingled its spoils with those of the land, and in another quarter, through fourteen thousand feet, for example, of the drift accumulations in Nova Scotia, that there is not a trace even of any substance of a marine character, all appearing to have been deposited in fresh water. But while no explanation yet given of the phenomena can be regarded as satisfactory, while Nature withholds much, and ever will, of the wonderful processes through which she attains her ends, the vegetable source of the product cannot be questioned; nay, the origin of coal from the extinct forests, from the trees and plants of a former age, is so very probable, that some beds sound like wood under the beat of the hammer; and large areas, when thin slices are placed under the microscope, are found in every portion to retain the woody-fibrous structure.
III. The Botanical Characters of the flora of the coal period form of themselves an interesting subject of study, and suggest some very important considerations as to the history and purpose of the formation. These will be best understood by a reference to the structure and habits of plants in general. Those of the coal, it will thence be seen, belong exclusively to one or two families,—as ferns, palms, and coniferæ,—which seem to have grown in every soil, and to have been adapted to every climate.
The most general divisions of existing plants are into the vasculares and cellulares. The former kinds all bear flowers, possess a system of spiral vessels, and are termed phonogamous. The latter, on the other hand, are flowerless, have no spiral vessels, and are denominated cryptogamous.
Another extensive subdivision of plants proceeds upon their anatomical structure, and the laws which regulate their mode of growth. Thus one class, it has been observed, increase in bulk by additional increments to the outside of all the parts which compose the plant, as the roots, stems, and branches; another, by additions to the inside of all these members: and for this reason the former are called exogenous, and the latter endogenous. In the one case the new or youngest growth is always exterior to the old; and if thus left unprotected, it will be readily admitted that the growth of all such plants would be greatly and constantly endangered by atmospheric as well as innumerable other causes. The remedy provided by nature against this, is a covering of the substance called bark, which is folded round the entire exterior, stem and branches, of the whole exogens, and within which the newly-formed tissue is all safely deposited. No plant, on the other hand, whose growth is from within, needs any such protection, and accordingly none of them—as all the grasses, corns, canes, and fungi—are possessed of bark, or any analogous membrane. The bark is an ephemeral substance, which lasts only for a year, and has annually to be renewed.
The additions to all exogenous plants are indicated in the stem or trunk, by concentric lines or circles. In the center there is a cellular substance called pith. When you take, therefore, a cross section of the trunk of this class, the structure and parts will be arranged thus—bark on the outside, pith in the center, and between these, concentric deposits of woody matter, and all connected into a solid mass by plates of comb-like tissue, radiating from the interior to the circumference, and termed medullary rays. A structure like this, so closely and firmly united, and filled up through all its parts, was surely intended for endurance; and yet out of this class of the vegetable tribes, nature has selected few of her carboniferous models. The plants of the period, as yet detected, are composed chiefly of cellular tissue, mixed up with the substance of the stem, and without pith, medullary rays, concentric woody deposits, or the binding ligament of bark. The hardy oak and tall slender cane may be taken as examples of the two modes of structure—the former allied to existing, the latter to extinct families.
Another ground of distinction among plants consists in the leaves or flattened expansions, from which they derive all their grace and symmetry. This is farther connected with the seed and rudimentary organs, and gives rise to the division into cotyledonous and acotyledonous plants. The non-flowering or cryptogamous are all of the latter kind. The flowering or phonogamous not only belong to the former, but are again subdivided into monocotyledonous or dicotyledonous, according as their seed-vessels are possessed of one or of two lobes. Where there are two lobes the expansion of the germ upon bursting from the ground terminates in two imperfect leaves, by which the botanist can at once determine the class to which it belongs. The corns and grasses have single cotyledons, from one extremity of which descend the roots, and from the other the stem springs up, terminated with a single leaf.
The leaves perform important functions in all those orders of plants with which they are connected, and serve as interesting guides in fossil botany, which seldom derives any assistance from the more destructible and “fleeting flower.” The leaves of plants consist of a complicated net-work of vessels, filled up in the interstices by cellular tissue, and covered over with a thin epidermis or skin. Those belonging to the monocotyledonous sub-class are traversed by a number of parallel veins, while dicotyledonous leaves are divided into regular compartments, some of which upon withering display the most perfect and beautiful system of reticulation, rivaling in delicacy of texture the wing of the gossamer. Leaves which outlast the season, as in evergreens, are termed non-deciduous, and are covered or interwoven with a thin crust of silex, which at once serves to protect and communicate to these ornamental shrubs their bright enameled appearance. The grasses possess this property, and some of them can elaborate in their joints crystals of considerable magnitude. The leaves of ferns are called fronds, and differ from true leaves in bearing the reproductive organs on the surface, while the slightest inspection of their form and mode of expansion readily distinguishes them from all others. Fronds, properly so termed, originate in the stem and are part of it; there is no distinct line of demarkation between them: stem, leaf, and spori, or seed; are all as one body; and thus, as being of one piece, these membranous organs have been quaintly likened to a garment without a seam.
From this brief description it will be seen that all plants and trees arrange themselves under two great classes, namely, the soft and spongy, or the hard and fibrous-woody structure. The remains of such as have been detected in the carboniferous rocks belong almost exclusively to the former class, the cryptogamiæ and endogenæ, while of the three hundred and upward of fossil species which have been described and figured, not more than ten, and some of these still of doubtful characters, can be regarded as of exogenous and true woody growth. Ferns, mosses, palms, and gigantic succulent plants, now all allied to those of tropical climates, constitute the vast preponderancy of the fossil flora of the age in question. Are we to infer from this that the other families and tribes which at present so abundantly cover the earth were not then in existence? The botanist can now refer to his catalogue of eighty to a hundred thousand species of existing plants, growing in the different regions of the globe, and of widely distinguished habits and forms; and were few or none of these in being then? We possess not, as yet, sufficient data for the solution of this very interesting problem, although in the progress of geological discovery, every year is adding to the list, and giving us a more extended acquaintance with the vegetable products of the coal period. An important experiment recently made by Professor Lindley would seem to favor the probability that a far more numerically abundant flora had then existed. One hundred and seventy plants were thrown into a vessel containing fresh water, and among them were species belonging to all the natural orders of which the flora of the coal-measures consists, and also to other natural orders which it might have been expected would be found associated with them. In the course of two years, one hundred and twenty-one species had disappeared, being entirely decomposed, and of the fifty which remained, the most perfect specimens were those of coniferous plants, ferns, palms, lycopodiaceæ, and the like—the families, all of them, most allied to those preserved in the coal-measures.