a much more limited superficies than that of the beds themselves. The usual form of the carbonate of iron of the coal series is that of oval or kidney shaped balls,―rognons, having a brown or greyish fracture; it is a mixture, more or less rich, of clay, and carbonate of iron. These balls occur stratified in the argillaceous beds. They appear to be assembled and precipitated during periods of repose, when the waters deposited at the same time the argillaceous particles with which they were charged. These spheroidal concretions or rognons, in consequence of their mixture with clay, are often arranged in concentric lamine, and frequently also present in their centre, a nodule of clay or of pyrites; sometimes even a fossil substance,nuclei, which appear to have attracted around them the chemical precipitation.

The position of the nodules of lithoide carbonate of iron is in strata parallel to the coal seams. This is the case at the mine of Treuil, in France, and almost every coal-field in England presents a similar deposition. In those of North America, we find a smaller amount of argillaceous carbonate of iron so interstratified with the coal beds, than in Europe, and the instances where iron works are supplied from these sources in America, at the present day, are but rare.

In the coal basins of England, the carbonate of iron is almost always found in the same beds, extending over very large districts. There are two large beds of ferriferous clay in the Dudley basin,* and sixteen or more in the great anthracite district of South Wales.

According to M. Burat, the numerous coal basins of France are far from containing the carbonate of iron in the same abundance as those of England. Few of the argillaceous beds contain these balls, and still more seldom do they contain those which are concentrically formed. At Saint Etienne, for instance, there exist two which furnish in the concession of Treuil, flattened rognons of fair quality, and contribute to supply the furnaces of Janon; but in the other concessions, the balls are not recognized in the equivalent beds, or they are so small as to be neglected. At some other points, the carbonate of iron appears in great abundance, but with very different characters: at the mine of Cros, it penetrates the entire beds of clay, of one or two yards thickness, and gives to that rock a remarkable solidity and density. But these massive beds are much more impure than the beds with disseminated nodules: besides a large proportion of clay, they also contain pyrites and precipitations of dark silex, to such an extent that the working which ought to have been developed, remains almost unproductive.

The coal basin of Aubin, in France, contains the iron ore in the

*The Dudley coal-field is remarkable as being one of the earliest positions where the argillaceous iron was smelted by means of pit-coal. The experiment was made by the founder of the noble house of Dudley and Ward, who published an account of it in the time of Charles II. He states that in a large stone furnace, twenty-seven feet square, he made seven tons of iron per week, "near which furnace the author discovered many new coal mines, ten yards thick, and iron mines underneath, which coal-works having brought into perfection, the author was by force thrown out of them, and the bellows of his new furnace and invention by riotous persons cut in pieces, to his no small prejudice, and loss of his invention of making iron with pit-coal."

most abundance. It exists, in the first instance, in balls in the beds of clay which accompany the coal, and, as at Saint Etienne, under that form it is the purest mineral. In the other cases, it constitutes a somewhat schistose bed of from three to fifteen feet thickness, which appears to extend under the greater part of the coal area. This bed

is remarkable, inasmuch as it presents at several points a series of contradictions and enlargements, which constitute the arrangement called en chapelet; like a string or chaplet of beads, an arrangement very frequent in all the substances which result from chemical precipitations in sedimentary waters, such, for instance, as is often. presented by the flints in chalk. This structure is, in other respects, independent of the other accidents, faults or disturbances, which equally affect this bed as those of the coal series. The kidney ores and the bed of stony carbonate of iron are worked at several places in the basin, and supply the high furnaces of Decazeville.

There are but very few basins which do not possess beds analogous to the argillaceous carbonate of iron; but they are in such slight amount, that there are no other workings than those of the two basins of Saint Etienne and Aubin, which we have just mentioned.

On the whole, if we compare the beds of lithoide carbonate of iron with the mass of coal formations, we see that their existence, but little developed, although frequent, must only be considered an accidental circumstance. It is equally worthy of remark, that, in every case where there was a formation of coal in the series subsequent to the true coal, viz., the ferruginous elements have anew resumed the composition and the characteristic aspect of this epoch.

Thus in the coals and the shales with vegetable impressions of the epoch of the lias of Yorkshire, we find the carbonate of iron stratified in balls; whilst, in the same formations, when their appearance is in the normal state, the ferruginous infiltrations appear only in the state of oxides.

These variations of composition in the ferriferous minerals, establish no real difference in the origin to which they may be attributed. They tend merely to demonstrate that the iron, collected at certain intermittent epochs and at isolated points, most commonly in the state of oxides, has undergone through the influences of the carboniferous epoch, a mineralogical transformation. The coal period appears generally to have been a period of tranquil deposits. It is, then, natural to find that in it the ferruginous infiltrations are more concentrated than in the periods of the old red sandstone, of the new red sandstone, and even of the trias, where the products of these infiltrations are blended with the general materials of the deposit.

We have already remarked that these infiltrations only become valuable according to their concentration: the formation before mentioned, so highly coloured by the per-oxide of iron, contain, perhaps, altogether much more iron than the coal formation; but, in the latter, it is collected together, and often possesses a concentration of 30 and 40 per cent., constituting serviceable beds. In the red or mot

tled sandstone formations, we find iron everywhere; but the concentrations even amounting to ten per cent. are but rare exceptions. The presence of the iron would then be scarcely remarked, if the glaring colour of these red and variegated formations did not contrast with the gray and dark rocks of the coal deposits which they cover, and with the white and greenish colours of the thick limestones and clays which are about them.*

SECTION III.

FOSSIL BOTANY AND GEOLOGICAL DISTRIBUTION OF VEGETABLE

REMAINS.

IN intimate connection with the matter of the present volume, a knowledge of the forms, the botanical classification, the geological arrangement of the vegetable remains of an ancient world, seems to be almost indispensable. It embraces facts, at least, sufficiently valuable, to ensure for it, as a collateral branch of natural science, a conspicuous section of this book. Independently of its usefulness, there is a never failing interest attached to such an investigation, which enables us to trace the history, as it were, the past condition, the present adaptation of the primeval flora;-that magnificent vegetation which, amidst the mutations of our planet, yet survives for our use; its characters changed, it is true, but only to become more serviceable to man.

A happy provision was it that secured for the ultimate advantage of the human race, ages before its appearance upon the globe, the trees of gigantic size, the densely growing shrubs, the most delicate even of the lesser plants-that flora which covered in such profusion the islands and plateaux, and filled the humid valleys of the early world. A happy provision was it, that amidst the early catastrophes of the earth, those convulsions which modified its entire surface, overwhelmed its primeval forests, and buried them beneath enormous accumulations of earthy debris, of sediment and of rocky debaclestill perpetuated and matured during the lapse of countless ages, that primitive vegetation, which, finally, in the form of mineral combustibles, we are now busy in exploring and mining, and appropriating in a thousand ways, and for a thousand purposes. A happy provision was that—a beneficent one, surely-which at the moment when man is compelled to level the existing forests, to make room for the progress of agriculture, and the cultivation of the present surface, he finds nigh at hand, yet buried beneath that surface, within the shallow basins and woody islands of the antediluvian world, those inex

* Burat.-Géologie appliquée, p. 108.

haustible stores of a combustible now rendered infinitely more precious and effective than that existing vegetable fuel, whose destruction is the inevitable consequence of advancing civilization.

Respecting the wondrous influence which the employment of mineral combustibles has had, even in our own days, upon the whole world, by the acquisition of new forces; by the extension of mechanical powers, of manufacturing capabilities; by the impulse given to the industrial arts, and the creation of new sources of wealth; by rapid and cheap modes of transportation, and enlarged commercial facilities; above all, by the improved condition of the people, we will not here dilate. Abundant evidence of all these will be found in this volume.

FOSSIL BOTANY.

Classification of Plants; their families, classes, and orders.-We shall occasionally have to make mention of the varieties of plants which occur in a fossil state, and which, in common with all other organic remains, are characteristic of, or distinguish with remarkable precision, every geological epoch. It may save the reader some trouble in referring to elementary books, if we briefly explain here the mode observed in the classification of this fossil vegetation; of which the true coal formation alone contains about four hundred known species.

The system generally adopted by botanists is, that of Jussieu, which is termed the "natural system," in contradistinction to that of Linnæus, which is denominated the "artificial system. Mr. Loudon states that the former method has for its object the arrangement of plants according to their greater or lesser degree of resemblance, both externally and internally.

The seed is considered the most important part of the plant; as being destined for its re-production and continuance in the world. The fundamental divisions of this arrangement are, therefore, founded on the characters of seeds.

The first grand division is derived from the presence or absence of seed-lobes; the next on the union or division of the seed-lobes in such as have them. Thus we have the three primitive divisions of Cotyledonex, Monocotyledonex, and Acotyledonex.*

Every one allows, M. Decandolle observes, that plants which resemble each other by their exterior forms, resemble each other also in their internal structure; their mode of vegetation and their properties. The three primitive divisions are divided by this botanist into eleven classes; and, according to the Jussieuan method, all vegetables are furnished with seeds which arrange themselves under one or other of the following heads.

COTYLEDONEÆ.

Exogenous stems.-Furnished with two or more cotyledons, or * Loudon's Encyclopedia of Gardening, p. 113.

seed-lobes; as the bean or the acorn; having a central column or pith, and an external band called the bark, the two being connected by medullary rays; this division being thus subdivided into I. Dicotyledons; II. Monocotyledons; III. Acotyledons.

embracing 1255 genera, and 8612 species.

Plumbagineæ,

Plantagineæ,

Euphorbiacæ,

Amentaceæ, &c.

Laurinæ,

Santalaceæ,

Urticeæ,

Coniferæ,

comprising 172 genera, and 1249 species.

Besides 53 genera and 71 species whose orders are not fully determined.

Fossil dicotyledonous plants of the coal formation-Until recently the fossilized dicotyledones were supposed to occur not lower than the Tilgate or Upper Oolite beds. The coniferæ also were considered as not older than the oolite series. But recent investigations, by distinguished naturalists, have shown that these groups formed the greater portion of the coal vegetation. Thus, for instance, some fossil trees, which were discovered rooted in a coal bed in the Lancashire coal-field, were identified by Mr. Bowman as sigillaria,* while at the same time he showed that medullary rays and coniferous structure existed; a fact which M. A. Brongniart, Lindley and Hutton, Humboldt and others have fully corroborated. Hence, it seems that botanists are inclined to withdraw the Sigillaria altogether from the family of tree ferns, with which they have been heretofore classed, and even from the Endogenous class, or Monocotyledones. We are therefore to understand that the Sigillaria is a dicotyledonous and coniferous plant, and that the arborescent ferns, Caulopteræ, belong to the monocotyledonous group.

Among the dicotyledonous plants of the coal formation are now arranged

Endogenous Stems-furnished with only one cotyledon or seedlobe, [as the lily,] and having neither pith, concentric circles of

* Proceedings Geological Society, London, vol. iii. p. 270; also Mantell's Medals of Creation, p. 132.