H. K. JORDAN ON COAL-PEBBLES AND THEIR DERIVATION. 933 



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 com- 

 posed 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 sub- 

 sidence, 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 Pen- 

 clawdd 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 dis- 

 trict 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. 



Q.J.G.S. No. 132. 3p 



