Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/63

ENGLISH COAL-FIELDS.] commencement of the carboniferous period is marked by a mass of limestones known as the Carboniferous or moun tain limestone, which contains a large assemblage of marine fossils, and has a maximum thickness in S.W. England and Wales of about 2000 feet. The upper portion of this group consists of shales and sandstones known as the Yoredale Rocks, which are highly developed in the moor land region between Lancashire and the north side of Yorkshire. These are also called the upper limestone shale, a similar group being found in places below the limestone, and called the lower limestone shale, or, in the North of England, the Tuedian group. Going north ward the beds of limestone diminish in thickness, with a proportional increase in the intercalated sandstones and shales, until in Scotland they are entirely subordinate to a mass of coal-bearing strata, which forms the most pro ductive members of the Scotch coal fields. The next member of the series is a mass of coarse sandstones, with some slates and a few thin coals, known as the Mill stone Grit, which is about equally developed in England and in Scotland. In the southern coal-fields it is usually known by the miners name of Farewell Rock, from its marking the lower limit of possible coal working. The Coal Measures, forming the third great member of the car boniferous series, consist of alternations of shales and sand stones, with beds of coal and nodular ironstones, which together make up a thickness of many thousands of feet from 12,000 to 14,000 feet when at the maximum of deve lopment. They are divisible into three parts, the lower coal measures, the middle or Pennant, a mass of sandstone con taining some coals, and the upper coal measures, also con taining workable coal. The latter member is marked by a thin limestone band near the top, containing Spirorlis carboiiarius, a small marine univalve. The uppermost portion of ihe coal measures consists of red sandstone so closely resembling that of the Permian group, which are next in geological sequence, that it is often difficult to decide upon the true line of demarcation between the two formations. These are not, however, always found together, the coal measures being often covered by strata belonging to the Trias or upper New Hod Sandstone series. The areas containing productive coal measures arc usually known as coal fields or basins, within which coal occurs in more or less regular beds, also called seams or veins, which can often be followed over a considerable length of country without change of character, although, like all stratified rocks, their continuity may be interrupted by faults or dislocations, also known as slips, hitches, heaves, or troubles (fig. 2). Fia. 2, representing a seam of coal k, worked towards m, interrupted by faults or hitches. The fault at AC is called an upthrow, that at BD a downthrow. The thickness of coal seams varies in this country from a mere film to 05 or 40 feet; but in the south of France and in India masses of coal are known up to 200 feet in thickness. These very thick seams are, however, rarely constant in character for any great distance, being found commonly to degenerate into carbonaceous shales, or to split up into thinner beds by the intercalation of shale bands or partings. One of the most striking examples of this is afforded by the thick or ten-yard seam of South Staffordshire, which is from 30 to 45 feet thick in one connected mass in the neighbourhood of Dudley, but splits up into eight seams, which, with the intermediate shales and sandstones, are of a total thickness of 400 feet in the northern part of the coal-field in Cannock Chase. Seams of a medium thickness of 3 to 7 feet are usually the most regular and continuous in character. Cannel coals are generally variable in quality, being liable to change into shales or black-band ironstones within very short horizontal limits. In some instances the coal seams may be changed as a whole, as for instance in South Wales, where the coking coals of the eastern side of the basin pass through the state of dry steam coal in the centre, and become anthracite in the western side. British Coal-fidds. There are about twenty principal coal-fields of Great Britain, besides several smaller ones, whose position is shown in Plate I., which may be classed under three heads: 1. Those forming complete basins, entirely cir cumscribed by the lower members of the carboniferous series ; 2. Those in which one limb of the basin only is visible, the opposite one being obscured by Permian or other strata of newer date; and 3. Those in which the boun daries are formed by faults, which bringdown the upper overlying strata into contact with the coal measures. The South Wales and Dean Forest basins are examples of the first of the above classes, the North of England and Yorkshire and Derbyshire fields of the second, and the South Staffordshire of the third. The last two classes are of the greatest geological interest, as giving rise to the important problem of their probable extension within workable limits beneath the overlying strata. Examples of the three different cases are given in Plate II., the first being represented by the section across the Forest of Dean, fig. 1 ; the second by that of the Lancashire coal-fields, fig. 2 ; and the third by the North Staffordshire section, fig. 3. The largest and most important of the British coal-fields South is that of South Wales, which extends from Pontypool in Wales Monmouthshire on the east, to Kidwelly in Pembrokeshire, a length of about 50 miles, and from Tredegar on the north to Llautrissant on the south, a breadth of about 18 miles, in addition to which a further narrow slip of about 20 miles long, E. and W., extends across Pembrokeshire. Excluding the latter portion, it forms a complete basin of an approximately elliptical shape, surrounded by older rocks, the Carboniferous limestone and Devonian shale dipping generally towards the centre. The basin-shaped structure is, however, modified by a central anticlinal axis, which brings the lower bed within reach of the surface. The total thickness of the coal measures is estimated at about 11,000 feet on the south, and 7000 feet on the north side in the western district. In the central portion between Britton Ferry and the River Taff, it diminishes to 4800 feet on the north side, and is still further reduced in Mon mouthshire and on the eastern side generally to about 2500 feet. The coal-bearing portions are divisible into three groups, known as- - 1. Upper Pt imant series. 2. Lower Pennant series. 3. White Ash series. The Upper Pennant series attains the maximum develop-

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