Conformable transition rocks cover the primary, and sometimes alternate with them; they are also associated with the lowest beds of the coal formation, so that no well marked division can be traced between them but there is one character, independent of all artificial arrangements, which serves to distinguish transition rocks from the upper secondary strata, in countries where the regular coal formation is found. All rocks under the coal formation, belong either to the transition or primary class; and all the strata above the coal formation belong either to the secondary or the tertiary class. The geological position of the regular coal formation thus serves as a simple and intelligible key to the geology of all countries, wherever it occurs. But where the coal strata are absent, the difficulty of determining the class to which certain rock formations belong, is often very great. Of this we have a striking instance in the perplexed attempts of foreign geologists to classify the vast calcareous formations of the Jura, and the outer range of the Alps; and the perplexity is further increased, by the mistakes which are made in referring to the English mountain limestone, by confounding it with the calcaire alpin, or alpine limestone. The alpine limestone, according to some geologists, is a transition limestone; but according to other geologists it is analogous to the magnesian limestone under the new red sandstone, and also comprises the lias limestones and the oolites. Indeed, I am convinced that in the vicinity of the Alps, rocks analogous to the chalk formation have not unfrequently been classed with transition limestones. These mistakes have arisen from a desire to make observations agree with preconceived theories, and with the artificial arrangements which Werner had invented. Thus it was taken for granted, that the granitic mountains of the Alps being primary, the calcareous mountains must be primary also; and when organic remains were first discovered in them, the geologists in France were greatly surprised, and seemed unwilling to admit the fact: at length, by a painful and reluctant effort, they removed all these mountains from the primary to the transition class. A more Herculean labor remains to be performed, that of removing many of these mountains still higher, to the upper secondary class. In the vicinity of Moutiers, in the Tarentaise, where M. Brochant first observed some organic remains supposed to belong to transition rocks, I discovered the Patella and other fossils, peculiar to the upper secondary strata.

In parts of France at a distance from the Alps and the Jura, the mineral character of the secondary strata might alone serve to identify them with the English lias, oolites, and chalk; but in the range of the Jura and the outer ranges of the Alps, the calcareous formations are of such immense magnitude, and the beds are often so highly indurated and crystalline, that it is only from their relative position and imbedded fossils, that we can trace their analogy to the English strata, or to the secondary strata in the north of France,

CHAPTER VIII.

ON THE LOWER OR GREAT COAL FORMATION.

The Geological Position and Structure of Coal Districts, called Coal-Fields.— Dislocation and Disturbances of Coal Strata by Faults and Dykes.-Mineral Coal, Anthracite, Plumbago, Wood-Coal or Lignite.-Iron-Stone accompanying Coal Strata.-On Carbon as an original Constituent Part of the Globe.On the Origin of Coal Strata, and their Deposition in Fresh-Water Lakes or Marshes. Numerous Repetitions of the same Series of Beds in the same CoalField.-Precautions necessary in the Establishment of Iron Furnaces.-On the Mode of searching for Coal.-Hints to landed Proprietors on the Probability of finding Coal in Districts where it has not yet been discovered.-On the Formation of Coal-Beds in Fresh-Water Lakes.-On the Conversion of Vegetable Matter into Coal.-Imperfect Coal Formations.-Salt Springs in Coal Strata.Coal Mines in France and North America.-Observations on the Consumption of Coal in England, and the Period when the Coal-Beds will be exhausted.

In the transition rocks covering the primary, described in the preceding chapter, we very rarely, indeed, discover any remains of vegetables, either terrestrial or marine. Carbon, which is the principal constituent element of all plants, is seldom found as a mineral substance in these rocks; for, with a very few exceptions, all the vestiges of organic forms which they contain, are of marine animals. Hence we are led to infer, that there were but few islands, or tracts of dry land, rising above the ancient ocean, in which these marine calcareous beds were formed or deposited. The attention of the geological student is now required to contemplate a most important and extensive change in the condition of the globe,-at least, of that part of it which forms the subject of the present chapter. Over the marine rock formations before described, we find a series of strata, two thousand feet or more in aggregate depth, in which remains of marine animals are extremely rare, but which contain, almost exclusively, the remains of terrestrial plants, or such as have grown either on dry land or in marshes. Carbon, in the form of coal, constitutes also numerous beds in the series, varying in thickness from a few inches to thirty feet or more, and alternating with beds of sandstone, indurated clay, and shale or schistose clay. The remains of vegetables are distributed in greater or less abundance throughout the whole series, which, taken together, are called by miners, in the north, coal-measures. The coal strata were, doubtless, deposited in the vicinity of extensive tracts of dry land, containing rivers, marshes, fresh-water lakes, and mountains: the marine beds which are the foundation of the series of coal strata, and also surround them, must, therefore, have been raised from the bottom of the ancient deep, before the vast accumulation of vegetable matter could be formed. To whatever cause we attribute this change in the condition of the globe, it appears to have been attended with another re

markable effect: after this period, metallic veins have been rarely formed, for they seldom rise into the coal strata. The vegetable remains that are in the coal strata, appear principally to belong to plants that abound chiefly in tropical climates, as will be subsequently noticed. In no country have coal-measures been more extensively worked than in England, or the relations of the strata to the rocks above or below them been more fully examined.

Every coal district has its peculiar series of strata, unconnected with any other: there is a general resemblance in the nature of the different beds in each. A district, with its peculiar series of strata, is called a coal-field. The foundation rock on which the coal-fields of Derbyshire, Northumberland, Durham, Shropshire, and North and South Wales immediately rest, is the mountain and transition limestone, described in Chapter VII. In Nottinghamshire, Yorkshire, and Lancashire, the foundation rock has not been sunk to, nor does it rise to the surface; but we have every reason to believe, that it is formed by a continuation of the same limestone, though this is by no means essential to a coal-field. In some parts of France, I have observed the coal strata resting upon granite; being separated from it only by a thick bed of conglomerate. A general view of the arrangement of the Derbyshire coal-field may be taken as affording a type of the whole English coal-fields, with certain exceptions, which will be noticed.

The thick beds of mountain limestone (see Chap. VII.) which form entire mountains, decline in height towards the eastern side of the county, and are covered by the coal-measures. The lowest bed of these measures, or, to speak more correctly, the bed which separates the coal-measures from the limestone, partakes of a mixed character, varying from soft argillaceous shale to hard sandstone; the prevailing color is a dark reddish or blackish brown. This bed has been called limestone-shale: its total thickness varies from five to six hundred feet, but in some situations is much less.

The harder strata of which this great bed is composed, are separated by soft beds that easily disintegrate and fall down; they form the exposed face of Mam Tor, or the shivering mountain, near Castleton. The peculiar circumstance which renders this bed remarkable is, that though it contains chiefly vegetable remains, it contains also occasional patches or limited strata of dark bituminous limestone, with beds and nodules of ironstone, and thin seams of coal, which, however interesting they may be to the geological enquirer, are too inconsiderable to be worked. The next large bed, which is in some situations from three to four hundred feet in thickness, is composed chiefly of strata of hard siliceous sandstone, which is in some places coarse, containing angular fragments of quartz; in other parts it is a fine grained and very durable stone. Some of the strata of this bed were formerly worked for millstones; from which circumstance it received the name of Millstone Grit. It contains, as

far as I have examined, the remains of vegetables exclusively, but no beds of workable coal occur in it. Where the strata crop or basset out, this rock forms abrupt and picturesque cliffs. Above the grit, are laid the regular series of coal-measures or strata, comprising sandstone of various qualities, indurated clay called tlunch, ironstone, softer argillaceous beds called bind, and schistose argillaceous beds, called shale. There are also two argillaceous strata containing numerous shells allied to fresh-water muscles, and hence called Muscle-bind.

A gentleman extensively engaged in the working of coal mines in this district, had an approximate measure taken of the thickness of the different beds, which he sent me, and it was published in the first edition of this work; from which "it appears, that the total depth taken on the level line of the measure of the whole Derbyshire strata, including part of Nottinghamshire, is thirteen hundred and ten yards, in which are thirty different beds of coal, varying in thickness from six inches to eleven feet, making the total thickness of coal twenty six yards of course the above estimate can be regarded only as an approximation to truth, since the thickness of the strata was taken upon a level line, and not perpendicular to the line of their inclination or dip." Making an allowance for excess in the above measurement, the true thickness of the strata may fairly be estimated at about two thousand five hundred feet.

What is particularly deserving of notice in the bed of limestoneshale before mentioned, below the coal-measures, and above the mountain limestone, is, that this bed presents a transition from marine calcareous strata with animal remains, to fresh-water strata with terrestrial vegetables: as both occur in different parts of the bed, it would imply, that the subjacent limestone had been gradually but unequally raised above the sea, and during its elevation some parts remained immersed in the ocean, while other parts were covered with vegetable depositions. In the western side of Durham and Northumberland, the alternations of coal of inferior quality, with beds of mountain limestone, are more distinct, and the transition from marine to fresh-water formations on a larger scale: both prove that the elevation of the beds above the sea was effected by the operation of an elevating force acting slowly, or at distant intervals, a subject which it is proposed to advert to in another part of the volume.

Coal-fields, as before stated, are of limited extent, and the strata frequently dip to a common centre, being often arranged in basinshaped concavities, which appear to have been originally detached lakes, that were gradually filled by repeated depositions of carbonaceous and mineral matter. In some of the larger coal-fields, the original form of the lake cannot be traced, but in the smaller ones it is distinctly preserved.

The different strata under a bed of coal are frequently similar to the strata over it; and the same series is again repeated, in some

mines several times, under different beds of coal, with a perfect similarity both in the succession and thickness of each. In some instances, a single bed of stone of vast thickness separates two beds of coal. In other instances, only a very thin stratum of shale or clay lies between coal beds.

Though numerous beds or seams of coal occur in one coal-field, very rarely more than three of these are worked. The thickness of the coal strata in the same coal-field, often varies from a few inches to several yards; but each stratum generally preserves nearly the same thickness throughout its whole extent. Instances to the contrary sometimes occur, in which the same bed will become narrower or wider, and sometimes be divided by a stratum of incombustible earthy matter, in different parts of its course. Few beds of coal are worked at a great depth, which are less than two feet in thickness. The stratum lying over a bed of coal is called its roof, and the stratum under it the floor. The facility of getting coal depends very much on the compactness of the stone which forms the roof, not only on account of the security from falling, but for keeping out the upper water, and preserving the pit in a dry state. The great expense incurred in supporting the roof when it is loose, frequently prevents a valuable bed of coal from being worked, or absorbs all the profit. In some situations, the roof is indurated clay, impregnated with bitumen and pyrites. When this falls down, and is intermixed with water and small coal at the bottom, it takes fire spontaneously; on which account the miners close up the space with common clay, where the coal has been worked, to prevent the access of air to the combustible matter. This kind of combustible clay is called tow; it is common in the Ashby-de-la-Zouch coal-field, and in Staffordshire. The floor or stratum on which the coal lies, consists of clay in various degrees of induration, and is almost always of that kind which will resist the action of fire, called fire-clay, suited for furnace bricks and crucibles.

It has been before observed that coal strata are frequently bent in concavities, resembling a trough or basin, dipping down on one side of the field and rising on the other. In Plate IV. fig. 2. the section of a coal-field is represented, in which the coal strata c c c, D D are inclined in this manner, but partially dislocated by a fracture or fault at F. The extremities of the lowest stratum c c, are several miles distant in some coal-fields, in others not more than one mile. In the great coal-field in South Wales, which is rather a long trough than a basin, the strata are arranged in this manner over an extent of nearly a hundred miles in length, and a variable breadth of from five to twenty miles. It is partly broken into by Caermarthen Bay, but it forms an extent of surface exceeding twelve hundred square miles. It contains twenty-three beds of workable coal, which are said by Mr. Martin to make together ninety-five feet in thickness of this valuable mineral; this will yield sixty-four million tons of coal