demonstration. He states,* that in Westphalia "is a fine"grained micaceous rock, occasionally silicious, in which no "fossils, except fragments of plants, have been detected. This "sandstone seems to occupy the place of the lower shale of the "South of England, of the yellow sandstone of Ireland, and of "the lower coaly sandstone of Scotland; for, like them, it is "immediately covered by the carboniferous limestone, or its "equivalent, often in a state of chert or petro-silex, with Posi"donomya Becheri and Goniatites crenistria. The rock which "next succeeds is, as before said, the distinct equivalent of the "British millstone grit; like which, it underlies the great pro"ductive coal-fields." The South Wales coal-field seems to have extended originally much further northt; and in a western direction probably was united with the coal-fields of Ireland. Later experience seems to confirm more fully still the connection of the British carboniferous formations with each other, and with those referred to on the continent. On a short examination of this and the Devonian series in Westphalia and Belgium a few months since, we noticed an iron-ore of marked character, found in the lower limestone shales in those districts, and largely used at the different ironworks. It was quite different from any known in this country. On returning home, after a few weeks careful search, the same ore was found in the same geological position in South Wales. (Description, page 217.) Mr. Warington Smyth, tracing the lowest bed of workable coal above the "Farewell Rocks" in Yorkshire, Derbyshire, Lancashire, and other districts, under which an indurated siliceous clay or rock of marked character, penetrated by roots of stigmaria, is always found, and known there by the local term of "Ganister" Rock, on examination finds the same bed in the South Wales coalfield. The same persistency of this bed has been also long observed in the South Wales coalfield, but never before connected with the northern coalfields. The coal above it is well known in this coalfield under different names. In the ironstone shales above this coal, Mr. Salter also finds fossil remains of like characters as found in the same position in the Coalbrookdale coalfield. Below this a series of beds also is found, which, although very thin at the eastern part of the coalfield, as we proceed to the west become of much importance, and yield large quantities of * Siluria, p. 376 et seq., where this important and interesting subject is fully dealt with. † Mem. Geol. Surv. Gt. Eritain, vol. i. p. 314 et seq. It is known at the different works as follows: 1211112 49066 ironstone. The "Rosser Veins" are those now most worked.* Below the "Farewell Rock," as we proceed westward, are also found thin seams of coal and deposits of ironstone. These have not yet been carefully explored, but they are very persistent, and, so far as at present known, abound more than the higher measures with fossil remains of early life. The breaking-up and denudation of this vast coal field has been most ably commented upon by Sir H. De la Beche, Sir R. Murchison, Professor Ramsay, and others, and can only be referred to here as preliminary to a more detailed description of the iron ores.† In the great change at the termination of the Carboniferous period this trough or basin, as before shown, was formed and protected to a great extent from denudation, and now forms the South Wales coal field. The extent of this is estimated by Mr. Vivian, M.P. at 640,000 acres, and over all this area the clay ironstones we hereafter describe are interstratified with the coal. We have on the north the Silurian and Old Red Sandstones; on the south of the coal basin the Old Red Sandstones, overlaid, in patches and unconformably, by the Permian series. The importance and extent of the iron trade of this district may be illustrated pecuniarily by the fact that one firm alone pays at the rate of nearly 3,000l. per day in wages. The principal manufacture in iron is railway bars; but every variety of iron,‡ and more than two thirds of all the tinned plates made in the kingdom are produced here. The main dependence of the iron works has hitherto been upon the clay ironstones of the coal measures (argillaceous carbonates of iron); as these are, however, becoming more expensive to raise as they recede from the outcrop, other resources in the district are lately attracting attention. We will classify the iron ores of South Wales in this memoir as follows; viz. 1st. The Clay Ironstones found in the Coal-measures. *Section from the "Rough Pin Coal" to the " Farewell Rock" at Rhymney, made by Mr. R. Bedlington:- Argillaceous shale ft. in. ft. in. Argillaceous shale 4 Argillaceous shale Soft shale (holing) 0 64 0 Lower Rosser Vein (average) 0 Argillaceous shale De la Beche, Mem. Geol. Surv. Gt. Brit. vol. i. p. 157.; Murchison, Siluria; Ramsay, Mem. Geol. Surv. Gt. Britain, vol. i. p. 297 et seq. A list of these works may be found in the Mineral Statistics of the Geological Survey by Mr. Robert Hunt. The Lias, the Old Red Sandstone, and the Silurian series have hitherto been so imperfectly examined on this point as to give very insufficient data; but it is very probable, from the observations made, that iron ores may be found in these. 1st. The Clay Ironstones found in the Coal-measures. Observation and experience show us that the Coal-measures of South Wales must be divided into two series, which we may call here the upper and lower series. These are separated over the whole coal field by thick strata of hard silicious rock, sometimes passing into a silicious conglomerate, known locally under the name of the " Cockshute" or "White Rocks." The hardness of these rocks, and the water they yield, are always formidable hindrances in sinking through them. The lower series are often called the "iron-bearing measures," and in these the great bulk of the iron and coal beds are found. The coal in this series is bituminous at the eastern outcrop, and gradually changes westward, until, after passing the great dyke or fault in the Vale of Neath, it becomes anthracite. The upper series contains few iron ores. The coals in this are every where bituminous, even in the localities where anthracite coal exists below, as in the Swansea district, &c. The rocks in which these upper iron and coal seams are imbedded are mostly micaceous sandstones, known locally as the "Pennant Rocks." The upper or Pennant series occupies only the higher strata of the coal basin. (See Fig. 1, partly made from No. 11, Horizontal Section of the Geological Survey of Great Britain.) * Geological Survey, Map No. 36. The lower or iron-bearing measures extend over the whole coalfield, and the principal beds of ironstones are found in the lowest strata. The ironstones are on the average richer in yield of iron on the east, but they increase in thickness as we go westward, and at the same time become poorer.* At the eastern end of the basin, which is the great ironmaking district, the south outcrop dips very rapidly, often at an angle of 30 to 45 degrees, whilst on the north outcrop the dip is only from 5 to 7 degrees. This has enabled a large extent of work to be won by "levels" or galleries opening from the valleys, and at first induced the establishment of works there. Many of these levels are now working out, and sinkings have to be made to the "deep." This makes little difference in the cost of the coal, but it is generally admitted that the cost of the "mine" or ironstone is seriously increased: the shales become harder in depth, and adhere more tenaciously to the mine, and larger stocks must be kept to "weather" (i.e. to allow the shale to separate from the mine by the action of the atmosphere). The highest part of the basin in which ironstone profitably workable has yet been found is a bed in the form of "black band,"† occurring on top of the Mynyddysllwyn seam of coal. The principal workings opened on this are by Messrs. Latch and Co. in the Rhymney Valley. The bed is irregular, sometimes several feet in thickness, and of the same general character as the "black bands" of Scotland, occurring in small basins, and often running out altogether. The next bed of ironstone occurs over the coal known on the north outcrop as the "Old Man's" coal, and Gŵr-hyd Coal, Dowlais, on the south outcrop as the Charcoal Seam at Abercarn, and the Rock Vein at Risca. This is also a "black band," and occurs, where worked at Abercarn, under the same conditions as the Mynyddysllwyn bed. (See Analysis, No. XXIII.) A considerable quantity has been raised, and it works well in the furnace. These two irregular beds are the only workable seams yet found in the upper series. The lower series (or "iron-bearing measures") contain so many beds of ironstone alternating with the coals and shales, that we can only attempt here to describe those now used in actual working for the manufacture of iron; and as these are not persistent over the whole coal field, and a very large portion of this coalfield is yet quite untried ground, this description * Following these lower beds of ironstones into Ireland, we find great deposits. In Limerick the cliffs on the coast near the mouth of the Shannon are mostly the lower ironstone beds, and from the beach millions of tons may be gathered, prepared by the wash of the Atlantic; the average yield of iron is, however, only 19 5 per cent. from specimens we have taken in situ. This term is applied to the argillaceous ironstones which contain sufficient carbonaceous matter to calcine themselves when fired, without any addition of fuel. |