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The South Staffordshire Coalfield/Stigmaria...

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Note on the Stigmaria Beds of the South Staffordshire Coalfield. By Sir Henry de la Beche, C.B.. F.R.S., &c.

The coal-field of South Staffordshire forms no exception to the other coal-fields of Great Britain as regards the occurrence, in certain of its beds, of the peculiar fossil roots known as Stigmaria, in their relative places of growth. These vegetable remains, long considered as the stems of a distinct fossil plant, are now known, chiefly through the researches of Mr. Binney[1] in England, and of Mr. Brown[2][3] in Nova Scotia, to be the roots of the fossil genus Sigillaria, or of some other of the like kind of plants.

Mr. Steinhauer would appear (in 1818) to have been the first to have observed the mode of occurrence of Stigmaria in certain beds of the coal measures, in a manner pointing to their growth in the bed where they are thus found. Speaking of the rootlets or fibres, as he terms them, diverging from the main Stigmaria root, he remarks that "on examining ​the projecting ends of some trunks (of Stigmaria) which lay horizontally in a bed of clay, extending along the southern bank of the rivulet which separates the townships of Putsey and Tong, and which is exposed in several places, it excited no little surprise to find traces of these fibres proceeding from the centre cylinder in rays through the stratum in every direction, to the distance of twenty feet." He further inferred that these fibres or rootlets "belong to the trunks in question, and, consequently, that the vegetable grew in its present horizontal position at a time that the stratum was in a state capable of supporting its vegetation, and shot out its fibres in every direction through the then yielding mud."[4][5]

Though the evidence on this head is common to all the coal-fields of Great Britain, and can be so readily obtained in many localities, the subject did not engage much attention until Mr. William Edmond Logan,[6] examining the coal measures of Glamorganshire and Carmarthenshire, was not only enabled to confirm the views of Mr. Steinhauer, but also to ascertain (in 1833) the important fact that all the coal-beds of that district reposed on such beds. When the Geological Survey entered upon the examination of the Coal-measures in the vicinity of Swansea in 1837. Mr. Logan pointed out the Stigmaria beds as constantly beneath the coal.[7] In verifying the beautiful maps and sections of the western portion of the South Welsh coal-field, which, with a generous love for the advancement of knowledge. Mr. Logan presented to the Geological Survey, and which were subsequently published by it, abundant opportunities were afforded for ascertaining the truth of this view, one still further confirmed, as might have been anticipated, by the general examination of the whole coal-field of that portion of Great Britain. The Survey in its progress among many other coalfields of the country, has always observed similar facts, and hitherto, including South Staffordshire, the connexion between coal and Stigmaria beds has, with few exceptions, and many of these doubtful, been found constant.

Having had occasion to visit nearly all the Coal-measure districts of Great Britain, from those of Scotland on the north to those of Somersetshire on the south, inclusive, I twice visited the South Staffordshire coal-field, and in all the cases where opportunities were afforded for examining the beds beneath the coal, found Stigmaria in them occurring as in their relative places of growth. Dr. Joseph Hooker in 1847, (then botanist to the Geological Survey,[8]) having directed his attention at that time especially to the structure of Stigmaria, and having had occasion, therefore, to examine into the subject with much detail, found these Stigmaria beds common;[9] Mr. Dawes, of Southwick House near Birmingham, who has given so much attention to the fossil botany ​of the district, considers that all the coal-beds in it are based on Stigmaria beds.[10]

In studying these beds care is needed in seeing that the rootlets of the Stigmaria really diverge from the main root, and permeate the bed in which they may be discovered, in the manner in which the roots of the water lily (as pointed out by Dr. Hooker) and those of many other aquatic plants permeate the silt beneath still waters. Portions of the main Stigmaria roots and even of the rootlets are to be found in the other beds, washed out of their soils (Stigmaria beds) and drifted like the stems of Sigillaria to which they belonged. From not distinguishing between the mode of occurrence of the Stigmaria and their rootlets in the different beds, the not unfrequent mistake has arisen of considering the remains of Stigmaria as so diffused throughout the various coal-measures that no conclusion can be drawn respecting their mode of growth in place.

In South Staffordshire, as in the other Coal-measure districts of Great Britain, the composition of the inorganic portions of the Stigmaria beds varies considerably, as, indeed, might be anticipated from its detrital mode of accumulation. At the same time the number of instances in which the mineral matter of these beds is of a character to afford good materials for fire-bricks, and is hence known as fire-clay, is somewhat remarkable. By reference to the Vertical Sections of the Geological Survey, Sheets 16, 17, and 18 (all relating to South Staffordshire), the numerous instances of fire-clays beneath the coal will be at once seen; and so far as my experience has extended the fire-clays so situated contain the Stigmaria roots, with every appearance of growth in place. At times it requires careful observation to detect the Stigmaria roots in the beds containing them, as well beneath the beds of coal, sufficiently important to have names assigned them, as beneath mere seams of an inch or two in thickness. As will be readily understood, cven all traces of a coal above a Stigmaria bed may be absent, either from the carbonaceous matter having been removed by the stream or current of water which deposited new matter, such as sand, above it, or from the conditions not having been so far advanced as to permit the Stigmaria bed or soil to be coated over with such carbonaceous matter.[11]

Seeing the general occurrence of the Stigmaria beds beneath those of coal, a proper appreciation of them may become, as it has been already found in some districts, practically useful in tracing the outcrop of beds, especially where the crop of a coal itself may be uncertain, and the thickness of a Stigmaria bed may be considerable, though the latter necessarily, seeing the respective origin of the two, bears no relation to that of any coal which it may support, or be intermingled with.[12] When ​"fire-clays" contain the Stigmaria roots, properly permeating their ancient soil, there is usually a little difficulty in practically tracing their crop, but when the mineral matter of these beds takes the more ordinary form of a sandstone or arenaceous shale, the case is different. Then an examination of the mode of occurrence of the Stigmaria roots themselves is essential, especially when the lithological character of the bed may be liable to change, and in some coal districts we have been enabled to trace the same Stigmaria bed from the ordinary condition of a fire-clay to a rock in which the arenaceous matter greatly prevailed.[13]

  1. ↑ Mr. Binney's observations respecting the trees discovered at St. Helen's. Lancashire, were made known to the meeting of the British Association for the Advancement of Science, in June 1845, and his description of the Sigillaria terminating in Stigmaria, found at Duckinfield, seven miles east of Manchester, was read before the Geological Society of London, in April 1846, and published in their Quarterly Journal, with figures, vol. ii, p. 390.
  2. ↑ Mr. Brown read a paper before the Geological Society, on the same day (22nd April 1852), with Mr. Binney's last-mentioned paper. It is also published in that Society's Journal, vol, ii., and is entitled "On a Group of Erect Fossil Trees, in the Sydney Coal-field at Cape Breton." He gives figures of an upright Sigillaria stem with branching Stigmaria roots.
  3. ↑ Brown, Richard On a Group of Erect Fossil Trees in the Sydney Coal-Field of Cape Breton. Quarterly Journal of the Geological Society, 2, 393-396, 1 February 1846, doi:10.1144/GSL.JGS.1846.002.01-02.54 (Wikisource contributor note)
  4. ↑ American Philosophical Transactions, new series, vol. i.
  5. ↑ Steinhauer, Henry. 1818. On fossil reliquia of unknown vegetables in the coal strata. Transactions of the American Philosophical Society, new series, 1: 265-297, pls. 4-7. (Wikisource contributor note)
  6. ↑ Now Sir W. E. Logan. Director of the Geological Survey of Canada.
  7. ↑ It was not until February 1840 that Mr. Logan communicated his knowledge of this circumstance to the Geological Society of London, in a paper entitled "On the Character of the Beds of Clay immediately below the Coal-Seams of South Wales, and on the Occurrence of Boulders of Coal in the Pennant Grit of that District." This memoir was published in the Transactions of that Society, vol. vi., 1842.
  8. ↑ An appointment held by Dr. Hooker until his Botanical Mission, for the Government, to India, towards the end of 1847. The results of Dr. Hooker's researches are published in the Memoirs of the Geological Survey, vol, ii, part 2, p. 431, in a paper entitled "On some Peculiarities in the Structure of Stigmaria."
  9. ↑ Subsequently Professor A. Ramsay (Local Director of the Geological Survey of Great Britain) and Mr. Warington Smyth (Mining Geologist to the Geological Survey of the United Kingdom) examined the Stigmaria beds in their official visits to the district.
  10. ↑ Dawes, MSS.
  11. ↑ In some coal districts, as, for example, in South Wales, the carbonaceous matter, which formed the coal, has been sometimes entirely removed, and even channels cut in the supporting Stigmaria beds, by the water which bore, and allowed the deposit of, the sand or silt now forming sandstones or arenaceous shales, covering the eroded and non-eroded parts: such erosion having taken place when the coaly matter was unconsolidated. An excellent example of the erosion of coal into channels, like those amid some peat bogs, occurs in the Forest of Dean, where it is known as "the Horse." This "Horse," with its tributaries, named "Lows," will be found well described by the late distinguished colliery viewer, Mr. John Buddle, in the Transactions of the Geological Society of London, new series, vol, vi. 213. The Memoir is accompanied by an illustrative plan and section.
  12. ↑ The intermingling of the Stigmaria and coal beds or seams has often led to the supposition, especially when a Stigmaria bed beneath any coal worked was rarely touched or examined, that these beds occurred quite as much above coal beds as beneath them, the roof of a given working being formed of a Stigmaria bed. Illustrative instances of the interstratification of Stigmaria with coal beds in South Wales, will be found in the Memoirs of the Geological Society, vol, i, p. 153.
  13. ↑ The consolidation of Stigmaria beds is occasionally very considerable, even approaching that of quartz rock. Good examples of Stigmaria beds thus consolidated may be seen on the coast (at Lilliput) near Swansea, where from the infiltration of silica, two beds, each supporting small beds of coal, and penetrated by Stigmaria roots and rootlets in their relative places of growth, have become hard quartz rocks.
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