Coal in Great Britain, the composition, structure, and resources of the coalfields, visible, and concealed, of Great Britain

Front matter

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Preface To The Second Edition

In this edition such corrections and additions in the text and illustrations as seemed necessary have been inserted. Chapters dealing with the coalfields of North Staffordshire, Yorkshire, and Nottinghamshire have been reset and brought up to date; and the section dealing with the coalfields of Scotland rewritten. A new chapter on the origin and composition of the British coalfields emphasizes a district grouping of the chief coalfields according to their geological history.

WALCOT GIBSON.

Hytun, KENT, 1927.

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Preface To The First Edition

THE present work is intended to supply mining engineers, mine owners, and mining students with a concise account of the more important facts relating to the geology of coal generally, and to the composition, structure, and resources of the coalfields of Great Britain in particular. It is hoped, also, that the subject is treated in a manner which will prove useful to those who, though not directly concerned in getting coal, are dependent on coal for industrial purposes, and also to those who take a scientific interest in | this important industry.

In dealing with so comprehensive a matter within a comparatively small space, many details that are often of great local importance are necessarily omitted; and for the same reason controversial matter has been avoided.

Chapters I. to VIII., which may be regarded as introductory, are reproduced, with many additions, from the author’s work on the ‘Geology of Coal and Coal-Mining,’ which formed the first volume of Arnold’s Geological Series, and which is now out of print. The rest of the book, dealing with the coalfields of Great Britain, is in part a condensation of personal experience and observations in the field during a period of thirty years, and of facts collected from various publications, more especially those of the Geological Survey of the United Kingdom and of the Federated Institute of Mining Engineers, to which the author here wishes to acknowledge his indebtedness.

WALCOT GIBSON.

HAMPSTEAD, 1920.

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Contents

CHAPTER

I, VARIETIES—CHEMICAL AND PHYSICAL PROPERTIES : 1 II. COAL AS A ROCK - - - - - 14 Ill. FORMATION AND ORIGIN - - - =) 2c IV. DISTRIBUTION” - - - - - - 30 Vv. FOSSILS AS ZONAL INDICES ra - : - 39 VI. PROSPECTING AND BORING - - - aie VII. GENERAL STRATIGRAPHY (EXPOSED COALFIELDS) - 66 VIII. GENERAL STRATIGRAPHY (CONCEALED COALFIELDS) - 89 IX. THE BRITISH COALFIELDS, ORIGIN AND COMPOSITION - 108 X. THE COALFIELDS OF SOUTH WALES AND THE FOREST OF DEAN - - - - - - - 122 XI. SOMERSET AND GLOUCESTER COALFIELDS - - 144 XII. THE EAST KENT COALFIELD . - - - 155 XIII. THE COALFIELDS OF NORTH STAFFORDSHIRE - - 166 XIV. YORKS, NOTTS, AND DERBY COALFIELDS - - 181 XV. THE LEICESTERSHIRE COALFIELD - - - 208 XVI. THE WARWICKSHIRE COALFIELD . : - 215 XVII. THE SOUTH STAFFORDSHIRE COALFIELD - - 224 XVIII. THE FOREST OF WYRE, COALBROOKDALE, AND SHREWS- BURY COALFIELDS - - - . - 235 XIX. THE COALFIELDS OF NORTH WALES = - - 240 KX. THE LANCASHIRE AND EAST CHESHIRE COALFIELDS - 248 XXI. THE CUMBERLAND COALFIELD - - - - 261 XXII. THE COALFIELDS OF DURHAM AND NORTHUMBERLAND - 266 XXIII. THE COALFIELDS OF SCOTLAND - . - eerie | XXXIV. THE COALFIELDS OF IRELAND - - : sons Les INDEX - - - - . - - 317

LIST OF ILLUSTRATIONS

Chapter I Varieties—chemical And Physical Characters

THE term ‘coal’ includes those solid combustible substances which have resulted from the decomposition and alteration of once living vegetable organisms, now occurring in a fossil state in geological formations of various periods. Since the stages reached in the decomposition and alteration vary within wide limits, and the changed vegetable matter is more or less mixed with foreign material, it is obvious that what is coal and what is not coal cannot be mineralogically defined. Solid bitumens, soluble in naphtha or benzene, and whether of organic or of inorganic origin, are excluded. Bituminous coals are soluble in pyridine to some extent, leaving a residue that does not coke. Anthracite is insoluble.

Sulphur in combination with iron is usually present in coal, and in the cleanest coal there are always certain impurities, consisting of silica, iron oxides, silicates of alumina, potash, and soda. These foreign substances constitute what is known as the ash.

Plant tissues and coal agree in the essential constituents of both being some combination of carbon with hydrogen, oxygen, and nitrogen, the carbon being largely in excess of the other elements in an ash-free coal. The main bulk of plants, whether of a high or low organization—a forest tree or a moss plant—consists of an organic substance remarkably uniform in composition. In each of these plants, separated so widely in organization, the carbon percentage varies between 48 and 51 per cent., the hydrogen between 5-9 and 6-5 per cent., and the combined oxygen and nitrogen between 41-8 and 44-5 per cent. The inference is drawn that the chemical nature of this organic substance has been the same throughout geological time. 1

While plant tissue throughout the vegetable world possesses a remarkably uniform composition, parts of a plant vary in composition, and one part of a plant differs in the proportion of its chemical components from the corresponding part in another plant. For example, leaves, leatstalks, and bark are richer in carbon, but are poorer in hydrogen and nitrogen, and contain more ash than spores, seeds, and cuticles. Since the different kinds of coal are based on differences in chemical composition, a coal formed out of spores will differ ab initio from one derived from the alteration of bark and leaves.

Ordinary domestic coal usually consists of obviously distinct layers—vitreous (vitrain), clear (clarain), dull and hard (durain); another that is dull-black, silky, and fibrous, and known as mineral ‘charcoal’ or ‘mother of coal’ (fusain, the houille daloide, faserkohle, of Continental writers). Coal, in fact, like any other sedimentary rock, has in most cases a distinctly stratified appearance. Along the dull layers, what look like fragments of charred vegetable tissue— ~ either leaves, leaf-stalks, bark, or wood—are often clearly distinguishable. The appearance of charring is not attributable to the action of heat, but represents wood that was affected by dry rot. In the bright layers organic structure is not evident, but as in the Better Bed Coal of Yorkshire, to cite a well-known example, these bright layers can be seen under the microscope to consist largely of spores and sporangia, especially when the coaly substance has been previously treated with a solution of sodium carbonate, or, better still, with a mixture of nitric acid and potassium chlorate, followed by ammonia. The difference in chemical composition of the dull and bright layers agrees with that of the organic materials composing them; the layers of mineral charcoal, or dull layers, are richer in carbon, but contain more ash than the bright layers. In the case of a coal from South Wales, ‘the dull layers give a composition of: Moisture, 1-68; volatile matter, 14-71; fixed carbonaceous residue, 77-17; ash, 6-44; and the bright layers a composition consisting of: moisture, 1-75; volatile matter, 31-63; fixed carbonaceous residue, 63-96; ash, 2-66. However, it is not safe to assume that coals in which spores are abundant have originated entitely from spores, since these parts are present owing to their greater resistance to destruction by biological and chemical action than plant tissues. |

Besides the organic compounds, all plants, as previously mentioned, contain mineral salts, which are not only different for distinct plants, but also vary greatly both in quantity and nature in different parts of the same plant. Lycopodium clavatum, one of the commonest of the British club mosses, contains 4-7 per cent. of ash, and a closely allied species, Lycopodium chamecyparissum, as much as 6-7 per cent. of ash. The ash contained in forest trees, on the other hand, rarely exceeds 2 per cent., and the spores of the Lycopodiacez also contain an appreciably smaller quantity of ash than the other tissues. The character of the ash is most marked in Lycopodium and in Sphagnum, the common bog moss, both of which possess the power of dissolving alumina out of the soil. From its insolubility in water, alumina remains after the plant has rotted, and after a large proportion of the other soluble mineral constituents has been dissolved out and carried away. It is perhaps significant that the ash present in the mineral charcoal of the Better Bed Coal contains from 28 to 33 per cent. of alumina. Salts of iron and lime, though more soluble than alumina, are less so than salts of potash and soda. The sulphur present in all plants unites to form more or less insoluble compounds. Thus in decaying vegetation certain mineral salts which existed in the living plant will be concentrated, while others will be carried away in solution. Apart, then, from the mineral matter—clay and _ shale (forming the bulk of the ash observed in burning most house coals)—introduced from extraneous sources to the spot where the decaying vegetable matter was accumulating, some, though small, proportion of the inorganic salts found in coal is due to the aggregation of the insoluble salts present in the living matter from which the coal was derived.

Some coals consist of an amorphous mass without any trace of organic structure. The theory that such coals have resulted from eruptions of bitumen, though lately revived, does not receive wide acceptance. But it is far from unlikely that animal matter has not to some extent contributed to the formation of certain kinds of coal.

Changes affecting the decayed vegetation subsequent to its burial beneath sediments, and those resulting from earth pressure and other geological agencies, are dealt with in a succeeding chapter. Each agency has had some effect in producing the varieties of coal recognized by the chemist or in commerce. In describing the several varieties of coal as commonly designated, it is usual to commence with those coals of which the organic origin is the least obscured, and then to pass to those in which the original organic structure is partially, or it may be wholly, indistinguishable even under the microscope. Peats and lignites, though usually excluded from the coal series, will be considered first, since these two kinds of vegetable fuel occur in a fossil state, and also illustrate some of the stages through which vegetable tissue may pass.

Peat.—The purest, thickest, and most extensive accumulations of vegetable débris at the present day occur in peat-bogs and swamps. Peat-bogs are mostly found in the Northern Hemisphere, and extend up to the polar limit of vegetation, the 56th degree north parallel forming the chief zone.

In Ireland peat-bogs cover an area of over 3,000,000 acres, and in some places the peaty accumulations attain a depth of over 50 feet. France contains large tracts of peat, and Russia the largest of all. Except in Madagascar, there are | few peat-bogs of any size in the Southern Hemisphere. The swamps of Virginia and those in the deltas of the Ganges and Brahmaputra afford examples of areas where decaying vegetation is accumulating at the present day. Swamps differ from peat-bogs by the presence of free circulating waters, a factor of some importance in determining the stage to which decomposition of the vegetable matter may reach.

The rate of growth of peat varies considerably. It is stated to be as much, under exceptionally favourable conditions, as 26 inches in a century; but immersion and low temperature tend to retard its accumulation. The upper layers in a peat-bog are light coloured, and the vegetable origin is obvious; but deeper down the layers become darker in colour, until near the bottom they are almost black, and to a considerable degree have lost all visible signs of organic structure. In the ‘Red Bogs’ of the central plains of Ireland the organic matter, partly in a living state, forms a layer from 2 to 6 feet in thickness, termed the ‘ clearing ’; beneath this comes a layer of ‘ white turf,’ passing down into ‘brown turf,’ and thence downwards, until at the bottom of the bog comes the ‘black ’ or ‘stone turf.’ Each layer consists of laminz increasing in the number per square foot from above downwards. The ‘hlack turf’ is the most valuable for fuel purposes, giving out when dried a strong heat and brilliant light. It is interesting to note that the percentage of ash is usually highest in the ‘black turf.’ In some Scottish peat-bogs an interesting alternation of forest beds with peat is observed in a following upward sequence: (1) First Arctic Bed; (2) Lower Forestian; (3) Lower Peat-Bog; (4) Second Arctic Bed; (5) Upper Peat-Bog; (6) Upper Forestian; (7) Recent Peat.

Mosses, monocotyledonous, and, more rarely, dicotyledonous plants enter into the formation of modern peats.

The composition of peat varies within wide limits. The percentage of carbon ranges from 50 to 64; hydrogen, 4-5 to 6-8; oxygen and nitrogen, 28-6 to 44-0. A large quantity of moisture is usually present, air-dried peat containing as much as 15 per cent. Compared with fresh plant tissue, the carbon-hydrogen ratio for peat is 9-8 and 7-2 for fresh tissue; and the carbon-oxygen ratio 1-8 and 0-9 respectively.

A carbonaceous substance, called ‘doplerite,’ was first found in a peat-bog in Austria. It is a black, gelatinous substance, becoming brittle after exposure to air and assuming a coaly appearance. It is insoluble in water, ether, and alcohol, but is entirely dissolved by caustic potash. Dried at 212° F., its composition is: Carbon, 48-06; hydrogen, 4-93; oxygen, 40-07; nitrogen, 1-03; ash, 5-86. At Scranton, Pennsylvania, a similar-looking substance was found at the bottom of a peat-bog, but with an analysis showing carbon, 28-98; hydrogen, 5-17; nitrogen, 2-45; oxygen, 56-98; ash, 6-40.

Peat belongs to the Quaternary or latest period in the geological record. A few thin bands are Pre-Glacial, and some are Inter-Glacial; but the most extensive areas of peat are of recent formation, though at the present day peat is not forming to any appreciable extent. It is almost certain that hill peats and lowland peats are not 1epresented on a large scale in the geological formations older than the Quaternary, though ‘dirt beds’ or ancient soils have been preserved in the secondary formation of this country.

The Great Dismal Swamp of Virginia and North Carolina is the best-known example of a swamp. It extends over a rectangular area forty miles long from north to south, and twenty-five miles from east to west. A small lake, about seven miles in its longest diameter, occupies nearly the centre of the swamp, with a maximum depth of between 6 and 15 feet. The thickness of the vegetable soil varies from 10 to 15 feet, and is stated to increase at a rate of io inch in a year.

Lignite.— In its physical character this variety of fuel forms a connecting link between peat and ordinary coal, but it is not classed with true coals. The term is sometimes restricted to those varieties presenting an appearance to woody tissue; another kind, in which the woody aspect is more or less obscure, being termed ‘brown coal.’ Classified according to external characters, lignites are divided into— (1) woody or fibrous brown coal, with the form and structure of wood; (2) earthy lignite, compact and easily rubbed into a powder ; (3) common brown coal, with the woody structure indistinct, possessing a slaty cleavage, and always black or dark brown in colour; (4) bituminous lignite or ‘pitch coal’ (pechkohle, glanzkohle), having a conchoidal fracture, pitch-black in colour, and frequently resembling coal, and sometimes even anthracite.

Lignites frequently contain fragments of leaves, bark, tissues, spores, macrospores, and pollen grains, all in an | advanced stage of decomposition. They were formed in shallow waters in which infusoria could live, and in which fungi and bacteria were abundant.

Like peats, the chemical composition of lignites varies. The carbon percentage ranges between 60 and 75 per cent., that of hydrogen is about 5-0 per cent., and the oxygen and nitrogen between 20 and 35 per cent., calculated on the ash-free basis. A lignite from Germany shows percentages of: Carbon, 57-28; hydrogen, 6-03; oxygen and nitrogen, 36-16; ash, 0-59; that of a browa coal percentages of: Carbon, 61-20; hydrogen, 5-17; oxygen and nitrogen, 21-28; ash, 12-35.

Lignites are chiefly met with in the Tertiary formation, and form an important source of fuel on the Continent and in North America. In some districts they are the only source of fossil fuel. Many of the deposits on the Continent are mined by open-cast workings, and around Cologne one seam averages 100 feet in thickness and sometimes reaches a thickness of 300 feet. Tests conducted in the United States of America show that ‘gas of a high quality can be obtained from the lignites of that country in North Dakota and Texas, and that one ton of lignite used in a gas-producer plant gives as much power as best Pennsylvanian or West Virginian bituminous coals used under boilers.” In Russia the Blatter Kohle and Papier Kohle have been regarded as lignites of Lower Carboniferous age, but are more frequently referred to the bogheads and Torbanites.

Thin and local layers and pockets of lignite are occasionally found in beds of Eocene age in the south and east of England, but the only occurrence of any account is that of Bovey Tracey, south-west of Exeter, in strata of Tertiary age. Owing to its giving out a disagreeable odour when burnt it is not much used as a fuel, but attempts have been made recently to use it for the extraction of oil and mineral wax. Some of the carbonaceous beds reach a thickness of nearly 50 feet; but the lignite is in beds from 1 to 4 feet in thickness. Analyses give: Carbon, 66°31; hydrogen, 5°63; oxygen, 22-86; nitrogen, 0-57; sulphur, 2-36; ash, 2-27. Analyses so far made do not warrant that the lignite could be profitably extracted as a source of montan wax.

Cannel.—In association with true coal, or else existing in separate beds, dull-black, lustreless, carbonaceous substances, breaking with a conchoidal or splinty fracture, are often met with. The variety breaking with a conchoidal fracture and lighting readily with the emission of a smoky candle-light flame is called ‘cannel.’ Itis frequently capable of taking a fine polish, and, like the jet of the Yorkshire Lias, is made into ornaments. When interstratified with coal it is often named ‘splint coal,’ ‘stone coal,’ ‘ parrot,’ ‘lantern,’ ‘cornish ’; when with shale it is called ‘ hoocannel,’ ‘blend,’ ‘rattler,’ ‘jacks,’ ‘rattler jacks,’ ‘ peel,’ ‘drub.’ From the smell when burning it is known as ‘horn coal.’ Cannel always occurs as small lenticles or in lenticular beds, generally a few inches thick, and only exceptionally met with in beds over 2 feet in thickness. It was evidently laid down in water, which was very little subject to disturbance, and in which fish and other animals must have lived, since their remains are frequently found in an almost perfect state of preservation in the cannel.

_ Best Wigan cannel contains percentages of: Carbon, 80-07; hydrogen, 5-53; oxygen and nitrogen, 10-20; ash, 2-70. Such high-class cannels are rare, and the common kind generally contains a very much higher percentage of ash— sometimes as high as 80 per cent.—rendering it commercially unprofitable. Sulphur is usually present. Cannel and its allies were formerly an important source of gas for increasing the luminosity of gases obtained from bituminous coals; and is still used locally for this purpose. It was also much used for the distillation of oil, and has been lately the subject of experiment. From a ton of ‘bastard cannel,’as the inferior kinds are called, the products obtained in a specially designed apparatus included: Pitch, 100 pounds; semi-solid wax, 50 pounds; soda-soluble oil, 4 gallons; phenolic ethers, 14 gallons; hydrocarbon oil, 61 gallons. The total yield of ammonium sulphate was 35 to 40 pounds per ton, and 6,000 cubic feet the yield of gas.

The origin of cannel is a subject of much dispute. Some cannels are rich in the fructifications of cryptogams, chiefly the spores of Lycopods; others, again, under the microscope appear structureless, but even then it is believed that the substance is largely built up of spores. It is generally accepted that cannel was formed in still, carbonaceous waters. Under such circumstances at the present day a gelatinous mass or slime accumulates at the bottom, consisting of both animal and vegetable matter, and in this material the least destructible organs, such as spores, become embedded. The gelatinous substance is called ‘sapropel.’ When occurring in a fossil state in Paleozoic — strata, the term ‘sapranthrakon’ is used by some Continental writers, and the term ‘saprodil’ when it is found in the Tertiary formations.

Boghead Coal.—Boghead coal, or ‘torbanite,’ as it is sometimes called, from its occurrence at Torbane Hill, Linlithgowshire, is regarded by some authorities as a variety of cannel coal, but others dispute its right to be included in the coal series; indeed, such a dispute resulted in a prolonged lawsuit last century. For several years the Torbane Hill mineral yielded the raw material for the extraction of oil, and some of it was exported for distillation to America and the Continent. Its percentage proximate composition consists of: Volatile hydrocarbons, 61-42; fixed carbon, 8-81; sulphur, 0-27; ash, 29-17; moisture, 0-60. The ultimate composition is: Carbon, 60 to 65 per cent. ; hydrogen, 7-5 to 9-0 per cent.; oxygen, 4-0 to 8-0 per cent.; nitrogen, 0-55 to 1-53 per cent.; ash, 19-75 to 21-32 per cent. The material is of a brown or nearly black colour, with a conchoidal fracture, and a yellow or fawn-coloured streak. It is a mass of structureless carbonaceous matter, mingled with the remains of Szgillaria and Stigmaria that occur throughout the bed. Near igneous intrusions it has been converted into a soft, sticky, brown substance. It was mined to the point of exhaustion, and though somewhat similar carbonaceous materials occur at Methel, Capeldrae, and Rocksoles in Scotland; and in association with the Black Band Ironstones of North Staffordshire, none of these approach the original Torbanite in their yield of oil, which amounted from 90 gallons or less to 130 gallons per ton. In the process of distillation Torbanite left a residue, not of coke, as in the case of a true cannel and bituminous coal, but ash with a small proportion of carbon. The carbon compounds contained in Torbanite are not soluble in benzene, turpentine, or carbon bisulphide, thus resembling those carbon compounds of coal, and not those of petroleum, resin, or true bitumen.

Under the microscope the bogheads of Autun and New South Wales, it has been declared, are seen to consist of aggregations of the oval and circular thalli of Pela and Reinschia (colonial alge) in association with pollen grains of various carboniferous plants. But the identification of thalli has been denied by others, who regard them as the megaspores of vascular cryptogams, in which case bogheads resemble cannels in their organic origin, as they certainly approach them in their behaviour with chemical reagents.

Other combustible substances allied to Torbanite are: Byerite, a bituminous coal from the Middle Parks, Colorado; Tasmanite, a carbonaceous shale of a brown colour found on the River Mersey, in Tasmania, in a bed from 6 to 7 feet thick, and resembling the ‘White Coal’ of Tertiary age in Australia. The ash contents of both is very high—-over 68 per cent. in the case of white coal. Combustible materials which have been mistaken for coal include: Anthrazolite, a coal-like substance of variable composition found in the older formations of Quebec and Ontario, and usually regarded as a form of inspissated petroleum; Albertite, similar in appearance to anthraxolite, is placed among the mineral asphalts. The carbonaceous deposit in the Laxey lead-mine, Isle of Man, is probably some form of one of these minerals.

Coal.—As popularly understood, coal includes the several varieties of fuel intermediate in physical appearance between brown coal and the stone-like or metallic-looking anthracite. Ordinary house coal presents bright surfaces that, however, soil the fingers, and breaks into more or less rectangular lumps. Generally the bright parts of coal are interleaved with dull layers (mother of coal). In many steam coals dull layers form a large proportion of the seam; others, as in many best Welsh steam coals, resemble the more metallic looking anthracite.

Anthracite or Stone Coal is stone-like in appearance, having a semi-metallic lustre, and can be handled without soiling the fingers. True anthracites burn with a feebly luminous but smokeless flame, and are much less easily combustible than any other variety of coal. They are the least widely distributed of all the kinds of coal, the purest coming from South Wales and Pennsylvania. In greatly disturbed and folded regions, as in Pembrokeshire and North Devon, where the seams of anthracite have been subjected to great pressure, the coal has been crushed into a fine powder or natural slack, which is called ‘culm.’ When artificially compacted or kneaded with clay it forms a slowly burning fuel used in Devonshire and West Pembroke. In Devonshire the crushed powder has long been used as the basis of black paint.

Sampling Coal.—The value of an analysis of a coal seam depends in the first instance on the method adopted in the © selection of the samples. This is evidently of the utmost importance, and frequently falls under the care of a geologist or pioneer.

A seam of coal is generally made up of layers possessing different properties and value. Some layers, especially of abnormally thick seams, are worthless as fuel. A small piece selected from one thin layer will therefore give very misleading results in the analysis of a thick seam. Usually in a coal mine a sample of about 20 to 30 pounds in weight is taken from each of at least two of the working faces situated as far apart from each other as possible. To obtain this amount a cut about 3 inches wide is made from roof to floor across the seam, neglecting the bands not sent to market, but including those taken out and sold with the coal. Ifa sample of 20 pounds is too bulky for transport. it is first reduced at the mine by breaking up all the material of the sample into small pieces, thoroughly mixing them, and making four portions of the broken material; and then taking alternate quarters and again mixing and halving. This process, which is called quartering, is continued until a convenient amount is obtained. Whenever possible this breaking, mixing, and quartering should be left to the analyst.

Analyses of Coal.—The chemist generally furnishes two analyses of coal: one called the ‘ proximate ’ and the other the

‘ultimate’ analysis. A proximate analysis furnishes percentages of volatile combustible matter, fixed carbon ash, sulphur, and moisture. An estimate of the yield of coke per ton is usually given, together with its physical character, and the percentages of carbon, sulphur, and ash contained in the coke. The general trade analysis is in this form, but it is not sufficiently reliable as a basis for classificatory putposes. An ultimate analysis is required if the coal is to be carbonized or gasified in producers under ammonia recovery conditions. Ina good ultimate analysis the carbon, hydrogen, oxygen, and nitrogen are expressed in percentages calculated for the ‘pure coal’ (7.e., after deduction for moisture, ash, and combustible matter). The percentage of volatile matter is calculated on the coal exclusive of moisture andash. The ash is given in percentages of the dry coal.

Classification of Coals.—Coals are classified according to their suitability for certain purposes, or according to their chemical composition. In South Wales the coals are classed for general sale purposes into: (1) Bituminous coals, to include house, gas, coking, and manufacturing coals; (2) semi-bituminous or second-class steam coal, applicable for stationary engines, locomotives, and bunkering purposes; (38) ‘first-class steam,’ and seams of a similar character for coal supplied to the Admiralty; (4) °* steam ’ and ‘anthracite,’ used for steam-raising, malting, limeburning, gas-power, hop-drying, and for domestic purposes. The marketable value of a coal varies to a great extent on the amount, chemical composition, and fusibility of the ash, and its lability to attack the material with which it comes in contact. Sulphur, always deleterious, frequently renders a seam worthless; and even in small quantities often makes a coal useless for many metallurgical processes. Phosphorus is always present in coal, though in small and varying quantities. Arsenic, commonly contained in the ‘brasses ’ or iron pyrites of coals, isa dangerous constituent when the coal is used for malting, and the arsenic rises to 7s of a grain per pound.

Humic coal has been suggested as an equivalent term for bituminous coal, since bitumen is never present in any coal. Best coke contains over 90 per cent. of carbon and less

than 9 per cent. of ash. Best gas coal yields on a dry, ashless fuel 30 to 38 per cent. of volatile matter; best coking coal from 20 to 30 per cent., and best steam coal less than 20 per cent.

Various chemical classifications have been suggested, based on (1) percentage of carbon and hydrogen, calculated on pure coal; (2) the relative proportion of carbon to hydrogen; (3) the relative proportion of coke to volatile matter—tfuel ratio—but is unsatisfactory except for anthracite and semi-anthracite coals; (4) on the fixed carbon for the anthracites to the semi-bituminous coals, and on fixed carbon and total carbon for the bituminous and lignitious coals. The fuels previously mentioned are classed according to the fixed carbon basis into: Anthracite, with fixed carbon, over 93; semi-anthracite, fixed carbon, 83 to 93; semi-bituminous, fixed carbon, 73 to 83; high-grade bituminous, fixed carbon, 48 to 73, total carbon, 82 to 88; low-grade bituminous, fixed carbon, 48 to 73, total carbon, 76-2 to 82; cannel, fixed carbon, 35 to 48, total carbon, 76-2 to 88; lignite, fixed carbon, 30 to 55, total carbon, 65 to 73-6.