COAL MEASURES. 1 95 



The following general succession of the Coal-series has been 

 given by De la Beche : ^ — 



Upper (Penllergare) Series. — Sandstones, shales, and 26 coal-seams, more 

 than 3400 feet in thickness. 



Middle Series (Pennant Grit). — Hard and thick-bedded sandstones, with 

 15 coal-seams, 3246 feet in thickness. Cockshoot rock at base. 



Lozver (or IV/iife Ask) Series. — Principally shales, ricli in ironstone, 34 coal- 

 seams, 450 feet thick near Pantypool, to 850 feet near Merthyr 

 Tydfil. (Ironstone Series.) 



Millstone Grit and Gower Series. (See p. 167.) 



The Lower Coal-measures form the iron-bearing strata at 

 Merthyr Tydfil and the Taff Vale. The clay-ironstone is an impure 

 carbonate of iron, which yields from 50 to 80 per cent, of ore. 

 Fortunately the strata are rolled into two or more anticlinal axes, 

 one of which, running through the neighbourhood of Maesteg, 

 brings the Lower Coal-measures near the surface over a much 

 larger area than would otherwise have been the case. 



The total thickness of all the coal-seams taken together has 

 been estimated at 120 feet: they vary in thickness from two to 

 nine feet. Peacock Coal has been met with in the Mynydd 

 Ystwyn Vein, in Monmouthshire. 



The junction of the Lower Coal-measures and Pennant Grit is 

 marked near Maesteg, east of Neath, by two or three beds of 

 quartz-rock, known as the " Cockshoot Rock." 



A remarkable feature in connection with the coal is, that while 

 bituminous in the neighbourhood of Swansea, and to the east of 

 Neath, it becomes anthracitic westward and northward. The 

 changes, which appear to be gradual, are chiefly due to a loss 

 of oxygen and hydrogen, and it is considered clear that similar 

 changes must be going on where carbonic acid gas and carburetted- 

 hydrogen are given off. 



That the change from bituminous coal to anthracite might be 

 connected with igneous eruption has been suggested from the fact 

 that in Pembrokeshire eruptive rocks occur in proximity to the 

 Coal-measures ; but Prof. L. C. Miall has pointed out that coal 

 altered by contact with igneous rocks does not form anthracite, but 

 "cinder coal," or "soot coal." According to his experiments, 

 coal loses its volatile constituents at ordinary temperatures, and 

 this is facilitated by disturbance of the strata.- Mr. E. Wethered 

 attributes the change in the coals to the changes in the sedimentary 

 strata accompanying them, which must have had a considerable 

 effect on the vegetable mass.^ In any case, by the changes under- 

 gone, fuel of great variety, and suitable for numerous purposes, has 

 been furnished. In Pembrokeshire the beds are much dislocated 

 and contorted. Culm is exposed at Landshipping.* 



^ Mem. Geol. Survey, vol. i. p. 197. 



^ Proc. Geol. and Polytech. Soc. W. Riding (2), i. 22. See also Murchison, 

 Proc. G. S. ii. 227 ; and E. T. Hardman, Journ. R. G. S. Ireland (2), iv. 200. 

 » G. Mag. 1 88 1, p. 470. 

 * See Murchison, Silurian System, p. 371 ; Siluria, edit. $, p. 290. 



