Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/64

Rh 50 GAL [COAL-FIELDS. merit of about 3000 feet on the south rise of the measures near Swansea; at Neath the thickness is reduced to about 1200 feet and in Monmouthshire to between 500 and 7W feet It contains all the free burning and bituminous coals of the Swansea and Neath districts, and the house- coals of Monmouthshire and the eastern districts, which latter contain 26 seams above 12 inches thick, making a total of about 100 feet of coal, an amount that increases westward to 82 seams and 182 feet. The Lower Pennant series averages from 1100 to 1500 feet between the faff Vale and Llanelly, but on the north side of the anticlinal thickens to 3000 feet. The average total of workable coal in seams which do not exceed 3 feet is 25 feet, among which are some fair steam coals, associated in places with black-band ironstone and good manufacturing and house hold coals, yielding slack suitable for coking, the most valuable among them being those of the Rhondda valley. The lowest or White Ash series contains the bulk of the valuable steam and iron making coals which have given the coal field its great reputation. It is about 500 feet thick on the eastern side, and about 1000 feet in the centre of the basin. The coals and accompanying ironstone are generally thicker and more abundant on the south than^on the north coast. The workable coals in this division amount to about 50 feet, in seams varying from 3 to 9 feet in thickness. The western extension into Pembrokeshire belongs to this part of the series ; it covers about 70 square miles, extending in a narrow east and west belt, varying from 2 to 6 miles in breadth from Tenby to St Bride s Bay. The measures are very much dis turbed, but are probably about 1500 feet, containing in the upper 1000 feet 8 seams of anthracite of about 18 feet total thickness. The total area of the coal-field is about 1000 square miles, of which amount about 153 square miles lie beneath the sea in Swansea and Carmarthen Bays. Only one square mile is covered by newer formations. According to the quantity of the coal produced, the area fs divided as follows : Bituminous coal district 410 square miles. Anthracite, ,, 410 ,, Intermediate, or Semi- Anthracite ...180 ,, The most valuable class of South Wales coals is the semi-anthracite or smokeless steam coal of the lower measures, which is in constant demand for the use of ocean steamers all over the world. It is principally ex ported from Cardiff, Neath, and Swansea. The configuration of the ground, owing to the deep north and south valleys of the Usk, Ebbw, Taff, Rhondda, and Neath Rivers, and the longitudinal anticlinal axis, renders the coals of comparatively easy access. The surface rises to a height of about 2000 feet above the sea-level, and in the valleys a greater vertical range is brought within working limits than is the case in any other coal-field of similai thickness. Forest of The Forest of Dean basin is an outlying portion of that Dean coal- O f South Wales, from which, as is shown by Ramsay, it has been separated by denudation. It is of triangular form, occupying an area of 34 square miles, between the Wye and the Severn estuary, with a total thickness of 2765 feet and 31 seams, together 42 feet thick, only 9 of which are above 2 feet in thickness. The depth from the surface to the bottom of the basin, in the centre, is about 2500 feet The lower beds of sandstone and the Carboniferous lime stone contain considerable quantities of brown hematite, ir irregular deposits, which is smelted in part on the spot anc partly exported to other districts. Owing to the symme trical basin-shaped form of the measures (Plate II. tig. 1) the coals have been worked from the surface downward along the outcrops of the seams, leaving large hollows fo he accumulation of water, which render the working of the ower ground difficult, on account of the great pumping- ower required to keep down the water flowing in from the Id shallow mines. North of the Malvern Hills a straggling patch of coal Severn neasures extends about 35 miles N. and S., from near Bailey Worcester to Newport in Shropshire. This is divisible into co wo nearly equal areas of triangular form. The southern &amp;gt;art is known as Forest of Wyre, and the northern as Cole- &amp;gt;rookdale. The former is unimportant, having a great hickness of measures which rest directly on the Devonian ocks, but scarcely any workable coal seams. The Cole- )rookdale measures rest upon the Upper Silurian rocks, are ibout 800 feet thick, with about 50 feet of coal in 18 earns, and many beds of nodular ironstone, which has iven the district a celebrity in the production of iron work, especially high-class castings. The eastern boundary s concealed by overlying Permian strata, and it was for merly supposed that the productive measures had been removed by denudation on this side ; but there is little loubt of their continuity towards South Staffordshire. To the westward of Colebrookdale are the two small fields of Leebotwood and Shrewsbury. These lie on the Silurian rocks. The exposed area of the former extends o 1 2 square miles ; that of the latter (which stretches in a crescent shape to the south and west of Shrewsbury) to L8. Both are partly hidden by Permian strata. The South Staffordshire coal-field extends about 22 miles S. Staf i fl&quot;. and S., from Rugeley to Halesowen, with a greatest * ire Creadth of about 1 miles from Wolverhampton to Oldbury. [t is entirely surrounded byNewRed Sandstone rocks, which in some places are faulted against the coal measures, render ing it difficult to decide upon the chances of a profitable ex tension beneath the visible boundaries. The coal measures rest upon the Upper Silurian rocks, which are exposed at several points within the area, especially at Dudley and the Wren s Nest. This district is remarkable as containing the thickest known coal seam in England, the Thick or Ten Yard Seam, which varies from 30 to 45 feet in thick ness in the neighbourhood of Dudley, but splits up north wards into several thinner seams in the northern or Cannock Chase district. There are 6 principal seams, with a total of from 57 to 70 feet in 1309 feet of measures. The field was formerly very productive of clay ironstone, but the supply has now considerably diminished. The coals are also subject to curious alterations in places, from the in trusion of igneous rock, especially in the Rowley Hills, near Dudley. The Warwickshire or Tamworth coal-field is a narrow strip of measures, with a maximum thickness of 3000 feet, extending about 12 miles in a N.W. and S.E. line from Coventry to Tamworth. It contains 5 seams, which are mainly worked for house coal and steam purposes. It is entirely surrounded by New Red Sandstone strata, ex cept for a short distance near Atherstone, where it is seen to rest upon the millstone grit, which is altered into quartzite by intrusive igneous rocks. The Leicestershire or Ashby coal-field is an irregular Leicesl patch of 30 square miles, on the east side of Charnwood *j^ c Forest, about midway between Leicester and Burton-on- Trent. It has 7 principal seams, and probably rests upon the mountain limestone, except at the eastern end, where it may lie upon the old slatey rocks of Charnwood Forest. Southward it extends under the New Red marl towards Leicester. In the centre is a patch of barren measures upon which the town of Ashby-de-la-Zouche stands, after which the coal-field is often named. The eastern side, wuich contains the mines of Whitwick, Snibston, and Cole- Orton. contains some igneous rocks apparently connected with those of Charnwood Forest, which are not seen on the