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Were we to incorporate in this work the facts, theories, and speculations which have been discussed at different times on the coal subject, we should Occupy at least an entire volume. There are many excellent treatises embracing these topics, which the reader, if seeking more information, may consult to advantage. The few notes we add here are inserted with little regard to classification.

The discovery, in 1839, during the progress of excavating a part of the route of the Manchester and Bolton Railway, within the limits of the Lancashire coal-field, of numerous fossil trees of the family sigillaria, standing in a vertical position, with their roots embedded in a thin coal seam, gave rise at the time to much discussion. Mr. Hawkshaw described these trees in two communications to the Geological Society. These trunks were wholly enveloped by a coating of friable coal, varying from one-quarter to three-quarters of an inch in thickness. Their internal casts consisted of shale, traversed beneath the place of the bark by irregular longitudinal flutings, less than one-quarter of an inch broad and about two inches apart.

Mr. Bowman communicated a paper on the same subject. He is opposed to the drift theory in accounting for coal beds, because they would have been intermixed with more earthy matter than is now proved to be the case in coal; and because they could not have maintained that singular uniformity of thickness and character throughout so many square miles, and such extensive areas that we find prevails in the coal measures; as an instance of which the author cites the thin seam below the Gannister or Rabbit coal, which extends in a linear direction thirty-five miles. It is much more rational to suppose, that the coal has been formed from plants which grew on the areas now occupied by the seams; that each successive race of vegetation was gradually submerged beneath the level of the water, and was covered up with sediment, which accumulated till it formed another dry surface for the growth of another series of trees and plants, and that these submergences and accumulations took place as many times as there are seams of coal within the confines of each basin.

Mr. Bowman proceeds to the examination of the phenomena presented by the fossil trees discovered in the railroad excavations above referred to by Mr. Hawkshaw. He describes, generally, the markings on the internal casts of the trees. The only indications of scars which he could find, his practised eye recognized to be those of a sigillaria.

From a careful consideration of the phenomena presented by the fossils, the author is convinced that they stand where they originally flourished; that they were not succulent, but dicotyledonous, hard-wooded, forest trees; and that their gigantic roots were manifestly adapted for taking firm hold of the soil; and, in conjunction with the swollen base of the trunks, to support a solid tree of large dimensions, with a spreading top.

With reference to fossil trees in general, and especially to those near Manchester, Mr. Bowman proceeds to show; 1st, that they were solid, hard-wooded, timber trees, in opposition to the common opinion that they were soft or hollow; 2d, that they originally grew and died where they have been found, and consequently were not drifted from distant lands; and 3d, that they became hollow by the decay of their wood from natural causes, similar to those still in operation in tropical countries, and were afterwards filled with inorganic matter, precipitated from water.

trees.

The author states his reasons for believing that these were solid timber In soft monocotyledonous trees, their stems never expand laterally, * Proceedings Geol. Soc. Lon., Vol. III. p. 139; and 269, 1840.

but are as thick when only a few years old, and a foot high, as when they attain the height of sixty or one hundred feet. Their roots, also, instead of being massive and forking, generally present a dense assemblage of straight, succulent fibres, like those of an onion or a hyacinth.

Mr. Bowman then combats the view generally entertained, that fossil stems, with perpendicular furrows, as in the sigillariæ, were succulent or hollow plants. He showed by specimens of recent dicotyledonous wood from New Zealand, that, both upon the bark and on the naked wood, longitudinal ribs and furrows, as regular as those on sigillariæ, were displayed; proving, therefore, that these characters are not incompatible with a dicotyledonous structure. By sliced and polished specimens of the bark of one of these fossil trees, he showed evidence of coniferous structure, proving, also, further, their dicotyledonous character. We note this decision with the more particularity, since M. Brongniart at the same time had asserted that "no wood of dicotyledonous plants, properly speaking, have been found in the coal-fields,"* but has since materially changed his views on that point. The roots of these trees are fixed in what is now a seam of coal nine inches thick. Mr. Bowman infers that one hundred years must be the minimum of time which would be required for the production of the vege table matter out of which the nine inches of coal were produced; and he estimates that the thickness of the solid coal is equal to about one-third that of the vegetable matter out of which it was produced.t

An instance very similar to this was detailed by Mr. Witham, in a communication to the Philosophical Magazine, entitled, "On the vegetation of the first period of the world, during the deposit of the transition and coal series." The author illustrates by a diagram the fossil stems of sigillaria, which occur beneath the main seam in the great Newcastle coal-field, at one hundred and fifty yards beneath the surface.

The fossil plants stand erect in the sandstone, their roots being imbedded in the ten inch seam of coal below. "These stems, [as shown in the figure,] are truncated after passing through the sandstone, and are lost in the main coal seam; leaving room to believe that they may have formed part of this combustible mass or bed." The saginariæ, the stigmariæ, and the calamites, he observes, do not appear to have been sufficiently strong to have resisted the force of a current of water, but are placed horizontally.

Position occupied by Sigillaria.-The trunks of these trees are found both in the floor and the roof of coal seams; their position commonly being the upper part of the coal and the lower part of its roof. The sigillariæ are arranged by M. A. Brongniart among the conifera; by Dr. Lindley under the name of caulopteris, and by Count Sternberg as syringodendrons. Some discussion and much new light have arisen, and it seems nearly settled that the numerous tribe of sigillariæ are to be removed altogether from the arborescent ferns to the dicotyledon family. M. Brongniart has been able to take the measurement of one of these stems, which was horizontally extended to the distance of more than forty feet; but has rarely had opportunity to examine their height, their general form, and their mode of termination, on a large scale, in the mines.

In Pennsylvania we have had some favourable opportunities of observing and illustrating the position of enormous trunks in the anthracite mines. The Transactions of the American Philosophical Society contain a memoir

* Histoire des Vegetaux Fossiles.

+ Proceedings Geol. Soc. London, Vol. III. p. 270.

Phil. Mag. January, 1830.

on the fossil stems of large trees belonging to the family of sigillaria, which occur both in the roof and floor of a coal seam in Dauphin county.* They consist of several species of these trees, which are displayed in a very interesting manner upon the nearly vertical walls of the vein for several hundred feet in length.

The Floor-As usual in Pennsylvania, the "bottom slate" consists of indurated clay and shale, more or less laminated. This lamination, it may be observed, is principally due to the flattened sheets of enormous sigillaria. Very few of these compressed trunks are of a less diameter than two feet; many of them are three feet; several are four and four feet and a half wide, and one specimen is at least five feet broad in its flattened diameter. More than a hundred of these are exhibited in the drawing which illustrates the paper referred to. The coal seam had not at the time commenced to be worked; and as its position was approaching to vertical, the gallery of exploration was conducted longitudinally along it, having the floor on the right hand and the roof on the left. Consequently, although several hundred feet in length of walls were exposed on either side, the height denuded was comparatively limited, and afforded little chance for determining the length of the trunks. In no instance was the area of excavation sufficiently extensive to exhibit either extremity of these gigantic stems, notwithstanding that many of them are inclined in such a position as to be exposed for thirty, forty, and fifty feet of their length, without much apparent diminution or tapering upwards, and are perfectly straight.

The Roof-This is the north or hanging wall of the vein, and consists of coarse siliceous conglomerate of white quartz pebbles. Between it and the coal, and embossed, as it were, upon the surface of the pudding stone, is a very thin coating of clay slate, and an extraordinary assemblage of prostrated trunks of sigillariæ. In diameter they are much smaller than those of the species which form the floor. Instead of being straight like them, these are bent or curved, and some of them appear to be dichotomous, and to possess the characters of S. elegans. Such is the scale, as regards height, of these trees, that the extent of cleared space was, as in the floor, inadequate to elucidate their entire developement at any point or in any instance.

One specimen, although laid bare for a length of more than fifty feet, showed no signs of either termination, and looked as if it might have extended thirty or more feet further. Another exhibited sixty-five feet in length, of a flexuous stem, which, apparently, extended at least thirty feet beyond. A third, the most interesting of the group, showed at its base what obscurely seemed to be the root. Near this base the stem was about two feet and a half in diameter. Forty feet up the trunk it measured two feet broad, and continued in about this rate of diminution as far as it was traced. Seventy feet in length of this specimen occur above the level of the floor of the gallery. It was followed, by direction of the author, several feet further, below the floor, and in all was perhaps from eighty to one hundred feet high when growing; but of this, and of the character of that superior termination, we have no present knowledge. It was covered with a bark of anthracite, about half an inch to three quarters or more thick. The interior cast consisted of shale or fire clay.

On applying to this interesting illustration of the ancient flora, Mr. Logan's views as to the universal presence of the stigmariæ in the argillaceous floors of coal seams, and of their absence in the roofs, it was found that in this

* Memoir by Mr. Richard C. Taylor, in Trans. Amer. Phil. Soc., Vol. IX. part II. 1845.

instance, where a surface of seven or eight thousand feet had been recently denuded, stigmariæ were rare. Only two well defined specimens, but of small size, were observed. One of these was seen in the roof above the coal; the other in the floor, below it: but detached leaves were abundant in the lower shale. Six other species of fossil plants were observed in the roof, and seventeen species in the floor. As usual in the coal seams of this country, a remarkable contrast appears in the condition of the roof and floor. While the appearance of the floor attested the state of tranquillity under which the mud of the ancient surface had accumulated, and the pressure that had flattened those enormous stems of sigillariæ upon which the coal appears to be based, the roof, on the contrary, exhibits the usual indication of violent action of the waters, in the rolled fragments of subjacent rocks, and in the prostration and drifting of gigantic trees, such as we have described above.t

A few of these prostrate trees are very imperfectly represented, as regards scale and details, in the following figure, which has been reduced from a very elaborate drawing.

For further details of fossil vegetation in the formations between the carboniferous series and diluvial accumulations, the reader is referred to the occasional notices under the heads England, France, Austria, Prussia, &c.

USUAL POSITION OF STIGMARIÆ, IN THE FLOORS OF COAL BEDS.

The existence of beds of Stigmaria, in the slate and fire clay which so generally form the strata, upon which coal seams repose, has been pointed out by various persons; in particular by Mr. Logan, who found it to hold good in the coal-fields of both the European and American continents. It is due to earlier observers, however to state that this fact had long since been noticed by Mr. Martin, Dr. McCulloch, and others, including numberless working miners.

In a communication to the British Association by Mr. Binney, in 1842, it appears that the workmen in the principal coal-fields in England, more especially that of Lancashire, regard the presence of stigmaria as a favourable evidence of the vicinity of coal.

All the floors, with the exception of one rock floor, in the Lancashire region, from the thin coal seams in the Ardwick limestone, to the two seams in the Millstone Grit, a thickness of near sixteen hundred yards, contain Stigmaria ficoides. All the fifteen floors of the Manchester coal-field contain them; and at least sixty-nine beds in the middle and lower divisions of the Lancashire field.

He adds, [a fact we greatly doubt,] that, in all instances of true floors, the stigmaria occurs without any intermixture of other plants.

Sir Henry T. De la Beche corroborated the former portion of this statement as regards Glamorganshire, Somersetshire, Yorkshire, Scotland, and Ireland; and said that he had never seen a workable coal bed which did not bear out Mr. Binney's conclusions.

Proceedings American Philosophical Society, Vol. III. p. 149. + Trans. Amer. Phil. Soc., Vol. IX. part II. 1845.

Mr. Logan showed that below every regular seam of coal, in South Wales, [and nearly 100 are known to exist there,] is constantly found a bed of clay, so well known to the collier, that he considers it an essential accompaniment of the coal; and only where it ceases, does he give up his expectation of finding coal.

These beds are most strongly marked by containing innumerable specimens of Stigmaria ficoides. The stems of this plant, which are usually of considerable length, are said, by Mr. Logan, to lie always parallel to the plane of the bed, and nearer to the top than to the bottom. Portions of the stem of the Stigmaria are found in other parts of the coal measures, but it is only in the underclay that the fibrous processes are attached to the stem, or are associated with it. The same rule appears to hold good in the coal formation of Nova Scotia, New Brunswick, and the United States.

With regard to the specific plant whose remains have chiefly contributed to form our coal seams, different views have been advocated at times, by naturalists. Without assenting to the doctrine, "that each bed of coal is an ancient Stigmaria bog,† we think that many other plants united to make up the mass, and that the predominant character of these may, in great measure, be inferred from an inspection of the shales, slates, clays, and sandstones, which occur in close contact with the coal itself. In Pennsylvania we have had abundant opportunity of observing coal seams, whose roofs and floors were crowded with sigillaria, and showed but rarely the traces of Stigmaria, or of those forms which are now ascertained to be the roots of the Sigillaria itself. In other cases, the prevailing plants of the shales, on which the coal rested, appeared to be Stigmariæ; while the roof contained chiefly Sigillariæ, and Lycopodiaceae. On the whole, we were at one time quite inclined to adopt the view of M. Brongniart, that the mass of coal vegetation was more likely to have been derived from Sigillariæ than from Stigmaria. The great number of leaves, he observes, which the Sigillares bore, along their whole length, and which evidently were disarticulated, and had fallen to the then surface of the earth, announce a life of some duration, and a growth which required a considerable lapse of time.‡

One thing appears to be pretty certain,-that the coal-fields exhibit alternate intervals of repose and of energetic action by currents of water-in other words, of a series of epochs of dry land and of inundation. These evidences testify that, after long periods which favoured the quiet growth and accumulation of masses of vegetable matter, they were abruptly terminated; and that this state of things was succeeded by overwhelming currents, which prostrated the forests of Sigillariæ or arborescent ferns; rooted in the ancient surface, and covered them with a debacle derived from older formations, and which we now recognize under the term conglomerates.

During an investigation of the coal beds of Dauphin county, in Pennsylvania, we had ample means of observing, at leisure, these facts: and it was seen, that while the floor of every coal seam consisted of shale, its roof, in the majority of cases, consisted of pudding-stone, whose lower side was impressed and embossed with enormous casts of prostrated Sigillariæ.

Before quitting the subject of coal vegetation, or rather that of the Sigillariæ and Stigmariæ, whose exuviæ are considered mainly to form our coal seams, the progress of discovery in regard to the real nature of those plants,

* Proceedings Geol. Soc. London, Vol. iii. p. 275.

+ Proceedings of the American Philosophical Society, May, 1843, p. 182.

Brongniart, Histoire des Vegetaux Fossiles. For details of fossil vegetation in Great Britain, see England.

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