of the district, considers that all the coal-beds in it are based on Stigmaria beds.[1]
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.[2]
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.[3] When
- ↑ Dawes, MSS.
- ↑ 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.
- ↑ 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.
