TRANSACTIONS OF SECTION C. 619 



5. Coal-Jield of South Wales: its extent, thickness, and constituent strata. — The 

 valuable coal-field of South Wales (estimated by some to occupy 640,000 acres, 

 and stated by others to be 906 square miles in extent) forms an irregular oval 

 basin or trough lying E. and W. (about fifty-six miles long, from Pontypool to 

 Caermarthen Bay), with a narrow extension westward beyond Caermarthen Bay, 

 through Pembrokeshire, to St. Bride's Bay (about seventeen miles long). The 

 greatest width is about sixteen miles. In 1881 there were 662 collieries at work 

 (see Hull, 1881). The strata of the whole area have been much undulated and 

 broken ; on the south they dip at an angle of 45°, and at about 12° on the north. 

 Great faults, approximately north and south, alter the levels from forty to a 

 hundred fathoms ; they are generally filled with clay ; but one, near Swansea, 

 many fathoms wide, is filled with fragments of the broken strata. (Trimmer.) 



A strong anticline once passed along the middle of the trough (E. and W.), 

 with its complemental synclines, one on each side. These have been somewhat 

 shifted (the eastern moiety towards the S.W., and the other to the N.E.) by a 

 great dbWqxxe fault coincident with the valley of the Neath. Except at Swansea 

 and Caermarthen Bays, the outcrops of the lowest part of the series of strata, of 

 irregular width, are continuous around the coal-field. About seven unequal 

 patches of the upper measures have been preserved from denudation in the 

 syncli7ies. Two of these areas are in the eastern moiety ; both long, but the 

 southern syncline retains only a narrow and interrupted series of patches.' The 

 Ebbw, the Sirhowy, the Rhymney, and the upper part of the Afon cross the 

 former ; the Tafl" and the Rhondda, with their branches, cross both synclines ; 

 and the Ely and Ogwr cross the lower syncline. The respective valleys give 

 local opportunities for opening certain beds, and afl'ord facilities for roads and 

 railways from the hills to the sea-coast. In the western moiety there are five 

 circumscribed areas of the upper measures in the two synclines ; the united 

 Amman and Llwchwr River runs between four of them, and the Tawe between 

 two and across one (just north of Swansea). The favourable position for mining 

 some of the measures is due to the local angle of dip in the synclinal strata ; 

 and, indeed, without the anticlinal arrangement some of the coals could never 

 have been reached even by deep mines." 



Mr. Etheridge (in his new edition of Phillips's ' Manual of Geology,' 1885, p. 

 238) mentions that in the southern part of the western moiety the Coal-measures 

 have a thickness of 11,000 feet ; and that on the northern side of the anticlinal axis 

 there they are 7,000 feet thick, and that near Britton Ferry, in the middle, they 

 diminish to 4,800 feet on that side. 



Taking the whole basin or trough, it may be roundly said that in the north- 

 east side the coals are mainly coking or partly bituminous ; to the west and north- 

 west they are anthracitic ; and in the south bituminous or gaseous ; and more 

 especially that ' in the Aberdare area the coals are very free-burning, but at the 

 same time smokeless ; hence their importance for steam purposes, especially the 

 Aberdare four-foot steam-coal.' ^ 



The physical features and structural condition of the South-Welsh coal-field, 

 also the occurrence of fossils, were succinctly treated of by G. P. Bevan in the 

 ' Geologist,' vol. iii., 1860, pp. 90-99. 



The order and thickness of the strata belonging to the coal-field of South 

 Wales as given in Geikie's ' Textbook of Geology,' 2nd edit., 1885, p. 742, are (for 

 Glamorganshire) : — 



Feet. 

 Upper series : sandstones, shales, &c., with 26 



coal-seams, more than .... 3,400 



Pennant grit : hard, thick-bedded sandstones, 



and 15 coal-seams 3,246 



Lower series : shales, ironstones, and 34 coal- 

 seams 450 to 850 



Millstone-grit. 



' ' See Hull's The Coalfields, &c., 4th edit., pp. 88, &;c., and map. 



» lUd., Chap. I. ' Etheridge, 1885, p. 238. 



