COAL-MEASURES. The Coal-Measures consist of a series of shales, grits, sandstones, and ironstones, characterized by the abundance of coalseams and the general absence of limestones. The strata usually occur in what are termed 'Basins:' that is, in synclinal areas or troughs: these basins, however, are not the areas of deposition, but are caused by disturbances and by denudation of the Coal-Measures, which in many tracts were no doubt formerly connected. The total thickness of the Coal-Measures varies. It is seldom, perhaps never, that we obtain the full thickness, because great denudation has in nearly all cases affected the strata in some instances, however, the Permian beds are said to rest conformably upon them. In South Wales the total thickness of the series has been reckoned at from 10,000 to 12,000 feet. Estimating the increase of sediment at 2 feet a century, and admitting with Mr. C. Maclaren that it might take 1,000 years to form a bed of coal one yard in thickness, Prof. Hull has calculated that the deposits forming the South Wales Coal-field might have been accumulated in 640,000 years. COAL.-Coal is not, strictly speaking, a mineral, being of organic origin; but it is nevertheless frequently classed as such amongst the Hydrocarbons. It is composed of vegetable matter, which through chemical change and pressure, as well as from original decomposition, has lost much of its structure. Amongst the conspicuous kinds of plants which helped to form it, are Ferns, Equisetaceæ, Giant Club Mosses, and Conifers. Sir William Logan first pointed out (upwards of 30 years ago) that every coal-seam in the Great South Wales Coalfield rested upon a bed of clay, called 'underclay,'' which was 1 In Yorkshire this is generally called 'spavin;' and in Lancashire, warrant' or 'seat earth.' (Green.) penetrated by the roots of Stigmaria, and evidently formed the soil upon which the plants originally grew. This fact has been found to be generally applicable in all our coalfields. The physical and palæontological evidence prove that the Coal-Measures were formed in an area undergoing slow and gradual subsidence, during which pauses occurred which are marked by the different beds of coal. These seams of coal can sometimes in limited areas be correlated, but looked at in a large way it is scarcely possible to indicate the extent of any one particular seam. That the beds were mostly deposited in a freshwater and fluvio-marine area is proved by the organic remains; at the same time in some localities there are indications of purely marine accumulations. The conditions presented were probably those of an inland sea, bordered by swamps, into which several rivers brought and deposited sand and mud. The coal-beds which indicate a luxuriant vegetation, were formed when either an increase of sediment or a slight elevation produced a land area. Many remains of plants were, however, drifted, although not very far. The cypress swamps of the Mississippi and the Great Dismal Swamp of Virginia appear to furnish the nearest analogues to the conditions prevailing in Coal-measure times.1 There is still a great difficulty in accounting for the pu rity of Coal, and its general freedom from foreign materials. If, however (as Mr. Henry Woodward has pointed out), we picture a vast alluvial plain covered with a Cryptogamic forest of giant Lepidodendra and Sigillariæ growing on a stiff tenacious clay-soil, capable of retaining the rain-fall, then we have the conditions suited for the rapid production of peat ; and that is the purest form we know of any great accumula 1 See Lecture on Coal, by A. H. Green. (Manchester, 1871.) tion of vegetable matter unmixed with foreign material, which is the peculiar feature of the Coal-measures.1 The principal varieties of coal are as follows: Anthracite or Stone-coal, which has a shining conchoidal fracture, does not ignite so readily as other kinds of coal, and has a much larger percentage of carbon. It is sometimes described as non-bituminous, and has probably been altered by pressure. Culm is a variety of Anthracite. 6 Bituminous coal, which is so called because it has a 'more flaming character in burning than anthracite.' The varieties generally recognised are mostly named after their application or chief properties: Free-burning, steam- or smokeless coal, non-caking coal. These, in different grades, approach towards the anthracites, and are chiefly valued for engine and smelting purposes. They often exhibit, in parts of the seams at least, a peculiar fibrous structure, passing into a singular toothed arrangement of the particles, called cone-in-cone, or crystallized coal.' (Smyth.) Caking coals are bituminous' coals, which are generally used for household purposes, having a tendency to cake, emitting jets of gas and giving off much flame and smoke. The Smalls' have the property of fusing together in large masses when duly heated, whence they are turned into coke for iron-smelting and for burning in locomotives. Cannel (candle) coal is a hard, dull, and clean coal, which readily ignites with a yellow flame, and is much used for gasmaking. Splint coal has a slaty structure. There is also a variety called 'Peacock coal,' which exhibits iridescent colours. The term Mother of Coal' is applied to the soft mineral charcoal occurring between brighter laminæ of coal. p. 1 Geol. Mag., vol. viii., p. 500. See also Carruthers, Ibid., vol. vi., 289. Different seams are thus suited for different purposesfor smiths, steam, gas, or the household. They vary in thickness from an inch to thirty feet. Coal has been worked in England since the thirteenth century. In 1259 Henry III. granted a charter to the freemen of Newcastle-on-Tyne for liberty to dig coals. Coal, however, had been known many years before the Christian Era; it was known to the Ancient Britons and to the Romans.2 There are two systems of working the coal, termed respectively the Pillar and Stall,' and the Long Wall' systems. In the former case the coal is worked out in square galleries called stalls, while pillars or posts of coal are left. to support the roof: in the latter case long galleries are driven to the full extent of the mine, and the coal is then worked out as far as possible in the intermediate spaces. The latter plan is usually adopted in working thin coals. The deepest mine in England is at Rose Bridge, near Wigan, 815 yards. Much danger arises in the mines from the escape during the working of the imprisoned gas called 'fire-damp' (carburetted hydrogen), which is highly explosive when mixed with air, and when ignited produces the much dreaded choke damp,' or 'black damp' (carbonic acid gas); so that a good system of ventilation and the use of the Davy lamp are very necessary. In some few districts, as near Radstock in the Somersetshire Coal-field, 'fire-damp' is unknown, and the miners work with naked candles. Amongst the fossils of the Coal-Measures may be mentioned the following:-Plants, Sigillaria, with its root Stigmaria, Lepidodendron, Calamites, Asterophyllites, Alethopteris, Pecopteris, Neuropteris, Sphenopteris; Mol 1 For details on the subject of coal, and the method of working it, see Smyth, Coal and Coal Mining. 2 See Hull's Coal-Fields of Great Britain. luscs, Aviculo-pecten papyraceus, Spirifer, Productus, Goniatites Listeri, Anthracosia, Anthracomya, Lingula squamosa; Annelide, Spirorbis carbonarius; Crustacea, Anthrapalamon, Eurypterus, Prestwichia, Bellinurus, Dithyrocaris; Insects, Myriapods, and Arachnida; Fishes, Megalichthys, Rhizodus, Palæoniscus, and Coelacanthus; and upwards of 30 Labyrinthodont Amphibia. The term coal-field is applied to the several geographical areas over which the coal-measures are exposed, including the area they occupy beneath newer strata, where they are so connected as to form one field; but many of the different coal-fields may be connected at great depths beneath newer strata. The largest coal-field in England and Wales is the Great South Wales Coal-field, comprising strata reaching a thickness of upwards of 10,000 feet, and forming an area of 900 square miles. The total area of the coal-fields in England and Wales is estimated at about 3,000 square miles. [N.B. For a great part of the following details referring to the Coal-fields, I am indebted to the Coal-fields of Great Britain,' by Professor Hull, to which work the reader may refer for more information.] The coal-fields of England and Wales are as follows:- 1. Northumberland and Durham Coal-field. The strata are thus divided: Upper Coal-Measures with thin coals and a band of Middle Series, from the High Main Coal to the Brock- Lower Coal-Measures, with 2 beds of coal, between 2 and 3 feet thick, 150 feet. H |