932 H. K. JORDAN ON COAL-PEBBLES AND TKEIE DERIVATION. 



50. On Coal-pebbles and tliexr Derivation. 

 By H. K. Jordan, Esq., F.G.S. 



[Abridged.] 



Tee author commeEced 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 inter- 

 stratified 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, 104). 



The aufhor noticed the division of the coal-bearing strata of the 

 part of the Welsh coal-field east of the Yale 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 Yale 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 Sand- 

 stones, 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 sup- 

 posed 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 

 " Bock 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 tbe author concludes that this lesser 

 thickness of coal, and the disappearance of the shales, &c, arc duo 



