50. On Coal-pebbles and their Derivation.
By H. K. Jordan, Esq., F.G.S.
[Abridged.]
The author commenced by referring to the discovery by the late Sir William Logan, as early as 1840, of a pebble of cannel coal in a layer of indurated clay at Penclawdd, and of coal-pebbles interstratified with the Pennant Sandstones in considerable abundance (Proc. Geol. Soc. vol. iii. p. 276). The late Sir Henry de la Beche also discovered coal-pebbles in several areas of the coal-field both in Monmouthshire and Glamorganshire (Mem. Geol. Surv. vol. i. pp. 193, 194).
The author noticed the division of the coal-bearing strata of the part of the Welsh coal-field east of the Vale of Neath into two groups. The higher, consisting of the sandstones known as the Pennant Grit, with comparatively few coal-seams, varies from 3000 feet in thickness at the Vale of Neath to 900 feet at its eastern outcrop near Pontypool; the lower, consisting chiefly of argillaceous shales with numerous seams of coal and ironstone, is about 1600 feet thick to the lowest workable coal-seam. According to his observations, coal-pebbles are not found in the shales of the lower group, but they occur towards the base of the Pennant Sandstones, sometimes associated with granite pebbles.
Sir W. Logan's pebble of cannel-coal was found in a layer of indurated clay overlying a seam of ordinary coal; and it was supposed to have been derived from a coal-seam about 2000 feet lower in the series. This view the author regards as untenable, as it would imply the elevation and denudation of the lower scams in some adjoining area of the original coal-field before the deposition of the higher Coal-measures—an assumption which is negatived by the structure of the district. His own opinion is that the pebbles are derived either from the seam of coal above which they are found, or from the destruction by erosion of a seam of coal which once existed approximately in the position where they are found, the erosion in either case being effected by the strong water-currents which distributed the grains of sand and other material upon the coal-seam.
The author instanced the sandstone forming the roof of the "Rock Fawr" seam near Bridgend as containing in some areas very large quantities of detrital coal and coal-pebbles up to the size of a hen's egg; and these pebbles, which consist, of the same coal as that in the seam below, have their angles but slightly rounded. In some seams the coal is overlain by thin beds of shale, coal, and fire-clay, upon which the roof of the seam reposes; but in places these overlying beds disappear, sometimes from large areas, bringing the sandstone roof into direct contact with the coal, which is generally thinner at these parts; and the author concludes that this lesser thickness of coal, and the disappearance of the shales, &c, are due
