A Manual of Elementary Geology Part 47

The next group which we meet with in the descending order is the Carboniferous, commonly called "The Coal;" because it contains many beds of that mineral, in a more or less pure state, interstratified with sandstones, shales, and limestones. The coal itself, even in Great Britain and Belgium, where it is most abundant, const.i.tutes but an insignificant portion of the whole ma.s.s. In the north of England, for example, the thickness of the coal-bearing strata has been estimated at 3000 feet, while the various coal-seams, 20 or 30 in number, do not in the aggregate exceed 60 feet.[308-A]

The carboniferous formation comprises two very distinct members: 1st, that usually called the Coal-measures, of mixed freshwater, terrestrial, and marine origin, often including seams of coal; 2dly, that named in England the Mountain or Carboniferous limestone, of purely marine origin, and containing corals, sh.e.l.ls, and encrinites.

In the south-western part of our island, in Somersets.h.i.+re and South Wales, the three divisions usually spoken of by English geologists are:

1. Coal-measures { Strata of shale, sandstone, and grit, with { occasional seams of coal, from 600 to 12,000 { feet thick.

2. Millstone grit { A coa.r.s.e quartzose sandstone pa.s.sing into a { conglomerate, sometimes used for millstones, with { beds of shale; usually devoid of coal; { occasionally above 600 feet thick.

3. Mountain or } A calcareous rock containing marine sh.e.l.ls and Carboniferous } corals; devoid of coal; thickness variable, limestone } sometimes 900 feet.

The millstone grit may be considered as one of the coal sandstones of coa.r.s.er texture than usual, with some accompanying shales, in which coal plants are occasionally found. In the north of England some bands of limestone, with pectens, oysters, and other marine sh.e.l.ls, occur in this grit, just as in the regular coal-measures, and even a few seams of coal. I shall treat, therefore, of the whole group, as consisting of two divisions only, the Coal-measures and Mountain Limestone. The latter is found in the southern British coal-fields, at the base of the system, or immediately in contact with the subjacent Old Red Sandstone; but as we proceed northwards to Yorks.h.i.+re and Northumberland it begins to alternate with true coal-measures, the two deposits forming together a series of strata about 1000 feet in thickness. To this mixed formation succeeds the great ma.s.s of genuine mountain limestone.[309-A] Farther north, in the Fifes.h.i.+re coal-field in Scotland, we observe a still wider departure from the type of the south of England, or a more complete intercalation of dense ma.s.ses of marine limestones with sandstones, and shales containing coal.

COAL-MEASURES.

In South Wales the coal-measures have been ascertained by actual measurement to attain the extraordinary thickness of 12,000 feet, the beds throughout, with the exception of the coal itself, appearing to have been formed in water of moderate depth, during a slow but perhaps intermittent depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.

In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten princ.i.p.al ma.s.ses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by ma.s.ses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of them, which has two or three layers of clay interposed, attaining 9 feet.[309-B] At other points in the same coal-field the shales predominate over the sandstones. The horizontal extent of some seams of coal is much greater than that of others, but they all present one characteristic feature, in having, each of them, what is called its _underclay_. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called firestone, because it can be made into bricks which stand the fire of a furnace. They vary in thickness from 6 inches to more than 10 feet; and Mr. Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the one hundred seams of coal met with in their coal-field. They are said to form the _floor_ on which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.

All of them, as Mr. Logan pointed out, are characterized by inclosing a peculiar species of fossil vegetable called _Stigmaria_, to the exclusion of other plants. It was also observed that, while in the overlying shales or "roof" of the coal, ferns and trunks of trees abound without any _Stigmariae_, and are flattened and compressed, those singular plants in the underclays always retain their natural forms, branching freely, and sending out their slender leaves, as they were formerly styled, through the mud in all directions. Several species of _Stigmaria_ had long been known to botanists, and described by them, before their position under each seam of coal was pointed out. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, and which were finally enveloped in the same mud.

CARBONIFEROUS FLORA.

These statements will suffice to convince the reader that we cannot arrive at a satisfactory theory of the origin of coal till we understand the true nature of _Stigmaria_; and in order to explain what is now known of this plant, and of others which have contributed by their decay to produce coal, it will be necessary to offer a brief preliminary sketch of the whole carboniferous flora, an a.s.semblage of fossil plants, with which we are better acquainted than with any other which flourished antecedently to the tertiary epoch. It should also be remarked that Goppert has ascertained that the remains of every family of plants scattered through the coal-measures are sometimes met with in the pure coal itself, a fact which adds greatly to the geological interest attached to this flora.

_Ferns._--The number of species of carboniferous plants. .h.i.therto described amounts, according to M. Ad. Brongniart, to about 500. These may perhaps be a fragment only of the entire flora, but they are enough to show that the state of the vegetable world was then extremely different from that now established. We are struck at the first glance with the similarity of many of the ferns to those now living, and the dissimilarity of almost all the other fossils except the coniferae. Among the ferns, as in the case of _Pecopteris_ for example (fig. 351.), it is not always easy to decide whether they should be referred to different genera from those established for the cla.s.sification of living species; whereas, in regard to most of the other contemporary tribes, with the exception of the coniferae, it is often difficult to guess the family, or even the cla.s.s, to which they belong. The ferns of the carboniferous period are generally without organs of fructification, but in some specimens these are well preserved. In the general absence of such characters, they have been divided into genera, distinguished chiefly by the branching of the fronds, and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group called _Caulopteris_, by Lindley, and the _Psaronius_ of the upper or newest coal-measures, before alluded to (p. 307.).

[Ill.u.s.tration: Fig. 351. _Pecopteris lonchitica._ (Foss. Flo. 153.)]

[Ill.u.s.tration: Fig. 352. Sphenopteris. (Foss. Flo. 101.)

_a._ _Sphenopteris crenata._ _b._ The same, magnified.]

[Ill.u.s.tration: Fig. 353. _Caulopteris primaeva_, Lindley.]

All the recent tree-ferns belong to one tribe (_Polypodiaceae_), and to a small number only of genera in that tribe, in which the surface of the trunk is marked with scars, or cicatrices, left after the fall of the fronds. These scars resemble those of _Caulopteris_ (see fig. 353.). No less than 250 ferns have already been obtained from the coal strata; and even if we make some reduction on the ground of varieties which have been mistaken, in the absence of their fructification, for species, still the result is singular, because the whole of Europe affords at present no more than 50 indigenous species.

[3 Ill.u.s.trations: Living tree-ferns of different genera. (Ad. Brong.)

Fig. 354. Tree-fern from Isle of Bourbon.

Fig. 355. _Cyathea glauca_, Mauritius.

Fig. 356. Tree fern from Brazil.]

[3 Ill.u.s.trations: _Lepidodendron Sternbergii._ Coal-measures, near Newcastle.

Fig. 357. Branching trunk, 49 feet long, supposed to have belonged to _L.

Sternbergii_. (Foss. Flo. 203.)

Fig. 358. Branching stem with bark and leaves of _L. Sternbergii_.

(Foss. Flo. 4.)

Fig. 359. Portion of same nearer the root; natural size. (Ibid.)]

_Lepidodendra._--These fossils belong to the family of _Lycopodiums_, yet most of them grew to the size of large trees. The annexed figures represent a large fossil _Lepidodendron_, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification.

Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude. The living club-mosses, of which there are about 200 species, are abundant in tropical climates, where one species is sometimes met with attaining a height of 3 feet. They usually creep on the ground, but some stand erect, as the _L.

densum_, from New Zealand (fig. 360.).

[Ill.u.s.tration: Fig. 360. Lycopodium.

_a._ _Lycopodium densum_; banks of R. Thames, New Zealand.

_b._ branch, natural size.

_c._ part of same, magnified.]

In the carboniferous strata of Coalbrook Dale, and in many other coal-fields, elongated cylindrical bodies, called fossil cones, named by M. Adolphe Brongniart _Lepidostrobus_, are met with. (See fig. 361.) They often form the nucleus of concretionary b.a.l.l.s of clay-iron-stone, and are well preserved, exhibiting a conical axis, around which a great quant.i.ty of scales were compactly imbricated. The opinion of M.

Brongniart is now generally adopted, that the _Lepidostrobus_ is the fruit of _Lepidodendron_.

[Ill.u.s.tration: Fig. 361. _Lepidostrobus ornatus_, Brong.; half nat. size. Shrops.h.i.+re.]

[Ill.u.s.tration: Fig. 362. _Calamites cannaeformis_, Schlot. (Foss. Flo. 79.) Lower end with rootlets.]

[Ill.u.s.tration: Fig. 363. _Calamites Suckowii_, Brong.; natural size. Common in coal throughout Europe.]

_Equisetaceae._--To this family belong two species of the genus _Equiset.i.tes_, allied to the living "horse-tail" which now grows in marshy grounds. Other species, which have jointed stems, depart more widely from _Equisetum_, but are yet of a.n.a.logous organization. They differed from it princ.i.p.ally in being furnished with a thin bark, which is represented in the stem of _C. Suckowii_ (fig. 363.), in which it will be seen that the striped external pattern does not agree with that left on the stone where the bark is stripped off; so that if the two impressions were seen separately, they might be mistaken for two distinct species.

The tallest living "horse-tails" are only 2 or 3 feet high in Europe, and even in tropical climates only attain, as in the case of _Equisetum giganteum_, discovered by Humboldt and Bonpland, in South America, a height of about 5 feet, the stem being an inch in diameter. Several of the Calamites of the coal acquired the height and dimensions of small trees.

[Ill.u.s.tration: Fig. 364. _Asterophyllites foliosa._ (Foss. Flo. 25.) Coal-measures, Newcastle.]

_Asterophyllites._--In this family, M. Brongniart includes several genera, and among them _Calamodendron_, _Asterophyllites_, and _Annularia_. The graceful plant, represented in the annexed figure, is supposed to be the branch of a shrub called _Calamodendron_, a new genus, divided off by Brongniart from the _Calamites_ of former authors. Its pith and medullary rays seem to show that it was dicotyledonous, and it appears to have been allied, by the nature of its tissue, to the gymnogens, or, still more, to the _Sigillaria_, which will next be mentioned.

_Sigillaria._--A large portion of the trees of the carboniferous period belonged to this genus, of which about thirty-five species are known. The structure, both internal and external, was very peculiar, and, with reference to existing types, very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, which they resemble in the scalariform texture of their vessels, and, in some degree, in the form of the cicatrices left by the base of the leafstalks which have fallen off (see fig. 365.). But with these points of a.n.a.logy to cryptogamia, they combine an internal organization much resembling that of cycads, and some of them are ascertained to have had long linear leaves, quite unlike those of ferns. They grew to a great height, from 30 to 60, or even 70 feet, with regular cylindrical stems, and without branches, although some species were dichotomous towards the top. Their fluted trunks, from 1 to 5 feet in diameter, appear to have decayed rapidly in the interior, so as to become hollow, when standing; when, therefore, they were thrown prostrate on the mud, they were squeezed down and flattened. Hence, we find the bark of the two opposite sides (now converted into bright s.h.i.+ning coal) to const.i.tute two horizontal layers, one upon the other, half an inch, or an inch, in thickness. These same trunks, when they are placed obliquely or vertically to the planes of stratification, retain their original rounded form, and are uncompressed, the cylinder of bark having been filled with sand, which now affords a cast of the interior.

[Ill.u.s.tration: Fig. 365. _Sigillaria laevigata_, Brong.]

_Stigmaria._--This fossil, the importance of which has already been pointed out, was formerly conjectured to be an aquatic plant. It is now ascertained to be the root of _Sigillaria_. The connection of the roots with the stem, previously suspected, on botanical grounds, by Brongniart, was first proved, by actual contact, in the Lancas.h.i.+re coal-field, by Mr. Binney. The fact has lately been shown, even more distinctly, by Mr. Richard Brown, in his description of the _Stigmariae_ occurring in the underclays of the coal-seams of the Island of Cape Breton, in Nova Scotia.

[Ill.u.s.tration: Fig. 366. Stigmaria attached to a trunk of _Sigillaria_.[315-A]]

In a specimen of one of these, represented in the annexed figure (fig.

366.), the spread of the roots was 16 feet, and some of them sent out rootlets, in all directions, into the surrounding clay.

The manner of attachment of the fibres to the stem resembles that of a ball and socket joint, the base of each rootlet being concave, and fitting on to a tubercle (see figs. 367 and 368.). Rows of these tubercles are arranged spirally round each root, which have always a medullary cavity and woody texture, much resembling that of _Sigillaria_, the structure of the vessels being, like it, scalariform.