mirable chapter on the Eyes of the Trilobite, by Dr. Buckland. B. T. vol. 1, page 396.

In Chapter II. we have given a summary statement of the vegetation of the transition epoch. In the upper series, comprising the great coal formation in very different degrees of latitude, we meet with abundance of fossil remains of large plants, analogous to what now grow in tropical climates. From hence we are led to infer, that our globe possessed generally a very high degree of temperature at the latter part of the transition epoch. With respect to the temperature of the earlier periods, we are less certain; for the vegetable remains in the lower transition rocks are chiefly marine, and the submarine rocks, on which the fuci grew, might derive a local temperature from adjacent igneous rocks.

The rare occurrence of remains of terrestrial plants and animals, in the earlier transition rocks, would indicate, that the surface of our planet was not then covered with extensive islands or tracts of permanently dry land: but that it was progressively advancing to a condition suited to support higher forms of vegetable and animal organization. Of the lower classes of animals, we find abundant remains in the transition series of the middle period. These remains are, chiefly,

1. Radiated: corallines, madre pores, and encrinites.

2. Articulated: the trilobite, and a few small crustaceous animals, confined to particular beds.

3. Molluscous: certain species of bivalves, and the nautilus, with a few species of univalves.

4. Vertebrated: certain species of fish, not hitherto found in more recent strata; but neither remains of reptiles, birds, nor mammalia, have yet been discovered.

It was till recently believed, that the teeth of large reptiles, analogous to crocodiles, had been found in fresh-water limestone, below the coal formation, at Burdie House, near Edinburgh; but M. Agassiz has ascertained, that these teeth belonged to distinct genera of fish, approaching to saurians in the structure of their teeth. To these he has given the name of Sauroid fishes.

In the organic remains of the earliest transition rocks, we appear to trace the first manifestations on our own planet, of creative power displayed in the animal and vegetable kingdoms. This is strongly confirmed, by the similarity of these early remains in the transition rocks of distant regions, and still farther, by observing the progressive advancement to higher forms of organic life, as we ascend through the secondary into the tertiary state, indicating a progressive improvement in the condition of the globe, until it became suited to the residence of man, and the highest orders of terrestrial animals.

Some geologists would discard the use of the term Transition Rocks; a term which has been generally employed ever since geology became a science. Such a change would introduce much unnecessary difficulty and confusion into geological descriptions, and retard the progress of knowledge. The Silurian system, for instance, is named after a tribe of ancient Britons, and the members of which it is composed, are named after obscure localities in England. These names, if unconnected with the Transition series, convey no meaning whatever to the foreign geologist, but if it be stated that the Silurian system comprises a series of Transition beds below the Old Red Sandstone, the true position of these beds will be every where understood.

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.-Ironstone 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 Coal-Field.-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.-Additional Remarks on the same Subject.-Section of the Coal Strata at Ashby Wolds.

In the lower transition rocks covering the primary, described in the preceding chapter, we very rarely 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 few exceptions, 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 lesser 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, and fresh-water lakes: the marine beds which form 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 remarkable 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 analogous to those 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, though there is a general resemblance in the nature of the different beds. 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 or upper transition limestone, described in Chapter VII.* In Nottinghamshire, Yorkshire, and Lancashire, the foundation rock has not been sunk to, 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 only separated from it 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 coalfields, 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 country, and are discovered 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 Main 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,

* At Dudley, in Staffordshire, the coal-field rests on the lower transition limestone, distinguished by its abundant remains of trilobites, hence formerly called the Dudley fossil.

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 above, is in some situations from three to four hundred feet in thickness; it is chiefly composed 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 clunch, ironstone, softer argillaceous beds called bind, and schistose argillaceous beds, called shale. There are also two argillaceous strata containing numerous shells of fresh-water muscles, and hence called Muscle bind.

A gentleman extensively engaged in the working of coal mines in this district, had a measure taken of the thickness of the different beds, which he sent me, and 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 only regarded 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 limestone shale 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 in

The relations of the limestone shale and the millstone grit, to the mountain limestone on the west, and the coal strata on the east, is represented Plate 4, fig. 1.