to the erosive action to which he ascribes the origin of the coal-pebbles.
When these "coal-conglomerates" are found without any coal-seam immediately beneath them, the author infers that the seams from which they were derived have been entirely broken up, and converted into pebbles and fine detritus, the characters of the pebbles and the friability of the material of which they are composed being, he thinks, incompatible with the notion of their having been transported from a distance.
The greater frequency of coal-pebbles in the south than in the north and east parts of the coal-field he attributes to the occurrence of stronger water-currents, probably resulting from more rapid subsidence, in the former—in support of which he instances the increase which takes place in the thickness of certain strata in a south and west direction.
With regard to the occurrence of a pebble of cannel-coal at Penclawdd above a seam of bituminous coal, as recorded by Sir W. Logan, the author remarks that several seams of coal in South Wales have for their upper layer a thin bed of cannel-coal; and he infers that the Penclawdd seam was of this kind, and that its original superficial layer of cannel-coal was broken up and eroded as above suggested.
As an example of the effects of a water-current, the author stated that at a colliery in the Forest of Dean the "Coleford-Hill-Delf " seam, which had a thickness of about 5 feet, was in one part of the colliery found to be entirely "washed out," the sandstone roof resting directly upon the underclay. In an adjoining area the coal was from 8 to 12 feet thick; and the author was of opinion that the coal, after its formation, but before the deposition of the roofing sandstone, had been entirely removed from one locality and piled up in the other by the action of a strong current of water. The thick coal, although unusually friable, was free from any admixture of roof-material.
From the preceding considerations the author infers that, previous to the deposition of the roof-material, the coal was to a great extent consolidated, although perhaps only partially indurated; and he points out further that the hardness of coal is not, as sometimes supposed, dependent upon pressure, as hard and friable coal may be found in contiguous beds of the same seam, and the highest seams in the South-Wales Coal-field yield harder coal than others 3000 feet lower in the series.
Discussion.
Mr. Moggridge stated that he was well acquainted with the district referred to, and had always thought that these pebbles resulted from beds broken up and much rolled. There is a bed of good cannel-coal in the district, four miles from and at a higher elevation than Penclawdd, so that pebbles from it might have been deposited at the latter place. Thus we should not have far to go for the source of the pebbles.
