Dr. Lindley. Brongniart.

I. Thallogens { Cryptogamous Amphigens, } Lichens, Sea-weeds, Fungi.

{ or Cellular Cryptogams }

II. Acrogens Cryptogamous Acrogens { Club-mosses, Equiseta, Ferns, { Lycopods, Lepidodendra.

III. Gymnogens Dicotyledonous Gymnosperms Conifers and Cycads.

{ Compositae, Leguminosae, Umbel- IV. Exogens Dicotyledonous Angiosperms { liferae, Cruciferae, Heaths.

{ All European except Conifers.

V. Endogens Monocotyledons { Palms, Lilies, Aloes, Rushes, { Gra.s.ses.

Calamites are among the most abundant fossil plants of the Carboniferous period, and occur also in the Devonian. They are preserved as striated, jointed, cylindrical, or compressed stems, with fluted channels or furrows at their sides, and sometimes surrounded by a bituminous coating, the remains of a cortical integument. They were originally hollow, but the cavity is usually filled up with a substance into which they themselves have been converted. They were divided into joints or segments, and when broken across at their articulations they show a number of striae, originating in the furrows of the sides, and turning inwards towards the centre of the stem. It is not known whether this structure was connected with an imperfect diaphragm stretched across the hollow of the stem at each joint, or merely represented the ends of woody plates of which the solid part of the stem is composed. Their extremities have been discovered to taper gradually to a point, as represented in _C. cannaeformis_ (Fig. 64), or to end abruptly, the intervals becoming shorter and smaller. The obtuse point is now found to be the root. Calamites are regarded as Equisetaceous plants; later botanists consider that they belong to an extinct family of plants.

_Sigillariae_ are the most abundant of all plants in the coal formation, and were those princ.i.p.ally concerned in the acc.u.mulation of the mineral fuel of the Coal-measures. Not a mine is opened, nor a heap of shale thrown out, but there occur fragments of its stem, marked externally with small rounded impressions, and in the centre slight tubercles, with a quincuncial arrangement. From the tubercles arise long ribbon-shaped bodies, which have been traced in some instances to the length of twenty feet.

[Ill.u.s.tration: Fig. 63.--Sphenophyllum restored.]

In the family of the Sigillarias we have already presented the bark of _S. laevigata_, at page 138; on page 157 we give a drawing of the bark of _S. reniformis_, one-third the natural size (Fig. 65).

In the family of the Asterophyllites, the leaf of _A. foliosa_ (Fig.

66); and the foliage of _Annularia orifolia_ (Fig. 67) are remarkable.

In addition to these, we present, in Fig. 63, a restoration of one of these Asterophyllites, the _Sphenophyllum_, after M. Eugene Deslongchamps. This herbaceous tree, like the Calamites, would present the appearance of an immense asparagus, twenty-five to thirty feet high.

It is represented here with its branches and _fronds_, which bear some resemblance to the leaves of the ginkgo. The bud, as represented in the figure, is terminal, and not axillary, as in some of the Calamites.

[Ill.u.s.tration: Fig. 64.--Calamites cannaeformis. One-third natural size.]

If, during the Coal-period, the vegetable kingdom had reached its maximum, the animal kingdom, on the contrary, was poorly represented.

Some remains have been found, both in America and Germany, consisting of portions of the skeleton and the impressions of the footsteps of a Reptile, which has received the name of Archegosaurus. In Fig. 68 is represented the head and neck of _Archegosaurus minor_, found in 1847 in the coal-basin of Saarbruck between Strasbourg and Treves. Among the animals of this period we find a few Fishes, a.n.a.logous to those of the Devonian formation. These are the _Holoptychius_ and _Megalichthys_, having jaw-bones armed with enormous teeth. Scales of _Pygopterus_ have been found in the Northumberland Coal-shale at Newsham Colliery, and also in the Staffordshire Coal-shale. Some winged insects would probably join this slender group of living beings. It may then be said with truth that the immense forests and marshy plains, crowded with trees, shrubs, and herbaceous plants, which formed on the innumerable isles of the period a thick and tufted sward, were almost dest.i.tute of animals.

[Ill.u.s.tration: XI.--Ideal view of a marshy forest of the Coal Period.]

[Ill.u.s.tration: Fig. 65.--Sigillaria reniformis.]

[Ill.u.s.tration: Fig. 66.--Asterophyllites foliosa.]

On the opposite page (PL. XI.) M. Riou has attempted, under the directions of M. Deslongchamps, to reproduce the aspect of Nature during the period. A marsh and forest of the Coal-period are here represented, with a short and thick vegetation, a sort of gra.s.s composed of herbaceous Fern and mare's-tail. Several trees of forest-height raise their heads above this lacustrine vegetation.

On the left are seen the naked trunk of a _Lepidodendron_ and a _Sigillaria_, an arborescent Fern rising between the two trunks. At the foot of these great trees an herbaceous Fern and a _Stigmaria_ appear, whose long ramification of roots, provided with reproductive spores, extend to the water. On the right is the naked trunk of another _Sigillaria_, a tree whose foliage is altogether unknown, a _Sphenophyllum_, and a _Conifer_. It is difficult to describe with precision the species of this last family, the impressions of which are, nevertheless, very abundant in the Coal-measures.

[Ill.u.s.tration: Fig. 67.--Annularia orifolia.]

[Ill.u.s.tration: Fig. 68.--Head and neck of Archegosaurus minor.]

In front of this group we see two trunks broken and overthrown. These are a _Lepidodendron_ and _Sigillaria_, mingling with a heap of vegetable debris in course of decomposition, from which a rich humus will be formed, upon which new generations of plants will soon develop themselves. Some herbaceous Ferns and buds of _Calamites_ rise out of the waters of the marsh.

A few Fishes belonging to the period swim on the surface of the water, and the aquatic reptile _Archegosaurus_ shows its long and pointed head--the only part of the animal which has. .h.i.therto been discovered (Fig. 68). A _Stigmaria_ extends its roots into the water, and the pretty _Asterophyllites_, with its finely-cut stems, rises above it in the foreground.

A forest, composed of _Lepidodendra_ and _Calamites_, forms the background to the picture.

FORMATION OF BEDS OF COAL.

Coal, as we have said, is only the result of a partial decomposition of the plants which covered the earth during a geological period of immense duration. No one, now, has any doubt that this is its origin. In coal-mines it is not unusual to find fragments of the very plants whose trunks and leaves characterise the Coal-measures, or Carboniferous era.

Immense trunks of trees have also been met with in the middle of a seam of coal. In the coal-mines of Treuil,[44] at St. Etienne, for instance, vertical trunks of fossil trees, resembling bamboos or large Equiseta, are not only mixed with the coal, but stand erect, traversing the overlying beds of micaceous sandstone in the manner represented in the engraving, which has been reproduced from a drawing by M. Ad. Brongniart (Fig. 69).

[44] "Elements of Geology," p. 480.

In England it is the same; entire trees are found lying across the coal-beds. Sir Charles Lyell tells us[45] that in Parkfield Colliery, South Staffordshire, there was discovered in 1854, upon a surface of about a quarter of an acre, a bed of coal which has furnished as many as seventy-three stumps of trees with their roots attached, some of the former measuring more than eight feet in circ.u.mference; their roots formed part of a seam of coal ten inches thick, resting on a layer of clay two inches thick, under which was a second forest resting on a band of coal from two to five feet thick. Underneath this, again, was a third forest, with large stumps of _Lepidodendra_, _Calamites_, and other trees.[46]

[45] Ibid, p. 479.

[46] Ibid, p. 479.

In the lofty cliffs of the South Joggins, in the Bay of Fundy, in Nova Scotia, Sir Charles Lyell found in one portion of the coal-field 1,500 feet thick, as many as sixty-eight different surfaces, presenting evident traces of as many old soils of forests, where the trunks of the trees were still furnished with roots.[47]

[47] Ibid, p. 483.

We will endeavour to establish here the true geological origin of coal, in order that no doubt may exist in the minds of our readers on a subject of such importance. In order to explain the presence of coal in the depths of the earth, there are only two possible hypotheses. This vegetable debris may either result from the burying of plants brought from afar and transported by river or maritime currents, forming immense rafts, which may have grounded in different places and been covered subsequently by sedimentary deposits; or the trees may have grown on the spot where they perished, and where they are now found. Let us examine each of these theories.

[Ill.u.s.tration: Fig. 69.--Treuil coal-mine, at St. Etienne.]

Can the coal-beds result from the transport by water, and burial underground, of immense rafts formed of the trunks of trees? The hypothesis has against it the enormous height which must be conceded to the raft, in order to form coal-seams as thick as some of those which are worked in our collieries. If we take into consideration the specific gravity of wood, and the amount of carbon it contains, we find that the coal-deposits can only be about seven-hundredths of the volume of the original wood and other vegetable materials from which they are formed.

If we take into account, besides, the numerous voids necessarily arising from the loose packing of the materials forming the supposed raft, as compared with the compactness of coal, this may fairly be reduced to five-hundredths. A bed of coal, for instance, sixteen feet thick, would have required a raft 310 feet high for its formation. These acc.u.mulations of wood could never have arranged themselves with sufficient regularity to form those well-stratified coal-beds, maintaining a uniform thickness over many miles, and that are seen in most coal-fields to lie one above another in succession, separated by beds of sandstone or shale. And even admitting the possibility of a slow and gradual acc.u.mulation of vegetable debris, like that which reaches the mouth of a river, would not the plants in that case be buried in great quant.i.ties of mud and earth? Now, in most of our coal-beds the proportion of earthy matter does not exceed fifteen per cent. of the entire ma.s.s. If we bear in mind, finally, the remarkable parallelism existing in the stratification of the coal-formation, and the state of preservation in which the impressions of the most delicate vegetable forms are discovered, it will, we think, be proved to demonstration, that those coal-seams have been formed in perfect tranquillity. We are, then, forced to the conclusion that coal results from the mineralisation of plants which has taken place on the spot; that is to say, in the very place where the plants lived and died.

It was suggested long ago by Bakewell, from the occurrence of the same peculiar kind of fireclay under each bed of coal, that it was the soil proper for the production of those plants from which coal has been formed.[48]

[48] "Introduction to Geology," by Robert Bakewell, 5th ed., p. 179.

1838.

It has, also, been pointed out by Sir William Logan, as the result of his observations in the South Wales coal-field, and afterwards by Sir Henry De la Beche, and subsequently confirmed by the observations of Sir Charles Lyell in America, that not only in this country, but in the coal-fields of Nova Scotia, the United States, &c., every layer of true coal is co-extensive with and invariably underlaid by a marked stratum of arenaceous clay of greater or less thickness, which, from its position relatively to the coal has been long known to coal-miners, among other terms, by the name of _under-clay_.

The clay-beds, "which vary in thickness from a few inches to more than ten feet, are penetrated in all directions by a confused and tangled collection of the roots and leaves, as they may be, of the _Stigmaria ficoides_, these being frequently traceable to the main stem (_Sigillaria_), which varies in diameter from about two inches to half a foot. The main stems are noticed as occurring nearer the top than the bottom of the bed, as usually of considerable length, the leaves or roots radiating from them in a tortuous irregular course to considerable distances, and as so mingled with the under-clay that it is not possible to cut out a cubic foot of it which does not contain portions of the plant." (Logan "On the Characters of the Beds of Clay immediately below the Coal-seams of South Wales," Geol. Transactions, Second Series, vol.

vi., pp. 491-2. An account of these beds had previously been published by Mr. Logan in the Annual Report of the Royal Inst.i.tution of South Wales for 1839.)

From the circ.u.mstance of the main stem of the Sigillaria, of which the _Stigmaria ficoides_ have been traced to be merely a continuation, it was inferred by the above-mentioned authors, and has subsequently been generally recognised as probably the truth, that the roots found in the underclay are merely those of the plant (_Sigillaria_), the stem of which is met with in the overlying coal-beds--in fact, that the _Stigmaria ficoides_ is only the root of the _Sigillaria_, and not a distinct plant, as was once supposed to be the case.

This being granted, it is a natural inference to suppose that the present indurated under-clay is only another condition of that soft, silty soil, or of that finely levigated muddy sediment--most likely of still and shallow water--in which the vegetation grew, the remains of which were afterwards carbonised and converted into coal.[49]

[49] For the opinions respecting the _Stigmaria ficoides_, see a Memoir on "The Formation of the Rocks in South Wales and South-Western England," by Sir Henry T. De la Beche, F.R.S., in the "Memoirs of the Geological Survey of Great Britain," vol. i., p. 149.

In order thoroughly to comprehend the phenomena of the transformation into coal of the forests and of the herbaceous plants which filled the marshes and swamps of the ancient world, there is another consideration to be presented. During the coal-period, the terrestrial crust was subjected to alternate movements of elevation and depression of the internal liquid ma.s.s, under the impulse of the solar and lunar attractions to which they would be subject, as our seas are now, giving rise to a sort of subterranean tide, operating at intervals, more or less widely apart, upon the weaker parts of the crust, and producing considerable subsidences of the ground. It might, perhaps, happen that, in consequence of a subsidence produced in such a manner, the vegetation of the coal-period would be submerged, and the shrubs and plants which covered the surface of the earth would finally become buried under water. After this submergence new forests sprung up in the same place.

Owing to another submergence, the second forests were depressed in their turn, and again covered by water. It is probably by a series of repet.i.tions of this double phenomenon--this submergence of whole regions of forest, and the development upon the same site of new growths of vegetation--that the enormous acc.u.mulations of semi-decomposed plants, which const.i.tute the Coal-measures, have been formed in a long series of ages.

But, has coal been produced from the larger plants only--for example, from the great forest-trees of the period, such as the Lepidodendra, Sigillariae, Calamites, and Sphenophylla? That is scarcely probable, for many coal-deposits contain no vestiges of the great trees of the period, but only of Ferns and other herbaceous plants of small size. It is, therefore, presumable that the larger vegetation has been almost unconnected with the formation of coal, or, at least, that it has played a minor part in its production. In all probability there existed in the coal-period, as at the present time, two distinct kinds of vegetation: one formed of lofty forest-trees, growing on the higher grounds; the other, herbaceous and aquatic plants, growing on marshy plains. It is the latter kind of vegetation, probably, which has mostly furnished the material for the coal; in the same way that marsh-plants have, during historic times and up to the present day, supplied our existing peat, which may be regarded as a sort of contemporaneous incipient coal.

To what modification has the vegetation of the ancient world been subjected to attain that carbonised state, which const.i.tutes coal? The submerged plants would, at first, be a light, spongy ma.s.s, in all respects resembling the peat-moss of our moors and marshes. While under water, and afterwards, when covered with sediment, these vegetable ma.s.ses underwent a partial decomposition--a moist, putrefactive fermentation, accompanied by the production of much carburetted hydrogen and carbonic acid gas. In this way, the hydrogen escaping in the form of carburetted hydrogen, and the oxygen in the form of carbonic acid gas, the carbon became more concentrated, and coal was ultimately formed.

This emission of carburetted hydrogen gas would, probably, continue after the peat-beds were buried beneath the strata which were deposited and acc.u.mulated upon them. The mere weight and pressure of the superinc.u.mbent ma.s.s, continued at an increasing ratio during a long series of ages, have given to the coal its density and compact state.