and markings with the removed substance. Any mineral matter being introduced into this mould, acquires, as it hardens, a renewal or redintegration of the external form of the original substance. This cast is sometimes hollow, having been formed by crystallizations which have only invested the inner surface, but have not been sufficient to fill the cavity.

The casts or nuclei of shells, fruits, reeds, &c. require to be examined to ascertain whether the substance of the fossil is similar or not to that of the matrix in which it is found. If it is similar, the cast may be considered as having been simultaneous in its formation with the bed in which it exists: if not, its substance may have been yielded by percolation and subsequent crystallization in the mould; or it may have been formed in some former bed, on the breaking up of which it may have been deposited among the materials of the succeeding rock in which it is now found.

Vegetable or animal substances deprived of life, and exposed to the action of the air, in combination with moisture, are rapidly decomposed; the softer and more volatile of their constituent principles are dispersed under new forms, and their more solid parts, such as wood and bone, are sometimes left cohering and displaying the form of the original skeleton. When in this state, deprived of those principles which conduce to chemical changes, these substances become capable of continuing unaltered in their forms, in different situations, for comparatively very long periods. But either vegetable or animal substances, when placed, under the influence of moisture, in situations excluding the access of atmospheric air, undergo certain peculiar chemical changes.

Vegetable Fossils.

WHEN vegetable matter is accumulated in so large a quantity that the compactness of the mass may in a great degree exclude the atmospheric air from the internal parts of the mass, a considerable and peculiar change is effected: the vegetable matter soon loses its green and acquires a brownish colour; its flavour and odour are changed, and heat is produced, terminating, unless air is freely admitted, in combustion. The vegetable matter, thus changed into Hay, acquires, among its other new properties, that of powerfully resisting any further change upon exposure to the atmosphere.

F

But should vegetable matter be thus accumulated in a situation in which moisture has almost constant access to it, a very different result ensues. Another process takes place, by which the vegetable matter, as the process goes on, loses its original forms, and becomes a soft magma, of a dark colour and peculiar appearance; no traces of its former mode of existence being discoverable, except in the accidental presence of such vegetable matter as shall not have undergone a complete conversion. When dried, it forms a readily combustible substance, of a reddish brown colour, readily absorbing and tenaciously retaining water, and yielding, whilst burning, a strong bituminous odour. This is the substance termed peat, immense accumulations of which are formed in various parts, favourable to the collection of water and the growth of the sphagnum palustre, a plant, by the conversion of which the supply of this substance is chiefly supported. In the peat-bogs or mosses, as the natural magazines of this substance are called, trunks of trees are often found imbedded, and partaking of the nature of the

surrounding bituminous mass. This change is effected in different degrees: the deeper in the mass, and consequently the longer exposed to the process of bituminization, the more perfect is the conversion. Some pieces are found to have nearly lost their ligneous appearance, their respective lines and markings having been molten down in different degrees during their bituminization; whilst others, in which the nature of the substance is also entirely altered, are found still to retain almost all their characteristic markings. This substance has long been known by the designation of bituminous wood.

Wood of a very different character, called Moss Fir, is also frequently found in the peat mosses or bogs. It much resembles, in its colour and general external appearance, ordinary decayed fir-wood; but on examination it appears that the fibre of the wood is strongly imbued with resin, and that all its interstices are filled with resinous matter. It is so highly inflammable, as to be employed, by the poor of the districts in which it is found, not only as fuel, but as torches.

As the real nature of this substance is not perhaps known, it would be very desirable that further enquiries might be made respecting it; it might then be determined whether the opinion which is here offered be correct or not. From its retaining the colour and appearance of decayed wood, it is conjectured to be wood which, by exposure to the atmosphere, had sustained the abstraction of all its constituent parts, except the resin and ligneous fibre impregnated therewith; and from its having been thus rendered almost an entirely resinous mass, it has not been affected by the bituminizating process.

Subterranean collections of bituminized wood and other vegetable matter are found at various depths in different parts of the world. The substance thus

found is generally a compact, light, glossy, combustible substance; of a dark brown colour, and frequently almost black; splitting longitudinally into plates of various thicknesses, breaking transversely with an imperfect conchoidal fracture, with a shining resinous lustre, and sometimes yielding the appearance of the markings of wood. This is the suturbrand of Iceland, the Bovey coal of this country, and the common brown coal of Thomson.

The fossil wood, now described, may be said to pass into jet, which is found, especially in the neighbourhood of Whitby, in Yorkshire, in a state very nearly approximating to that of bovey coal. It exists in plates, generally from half an inch to about an inch in thickness, between which a film of carbonate of lime, with pyrites, is disposed: excepting that it more frequently shows marks of ligneous texture, its characters may be said to be those of jet; its colour, velvet black; internal lustre, shining, resinous; fracture perfect, large, conchoidal; fragments, sharp edged, soft, rather brittle; easily frangible; very light. Jet is found in other situations, in a different form; resembling, in its shape, and the markings of its surface, parts of the branches or trunks of trees, but rarely possessing, internally, any marks of vegetable origin; a circumstance easily accounted for, if its previous softening be admitted.

Cannell Coal, of which some of the finest specimens are found in Lancashire, differs from jet chiefly, perhaps, in its holding a greater portion of earth in intimate mixture with it. It never manifests internally any traces of vegetable structure, but sometimes bears on its surface evident marks of impressions formed on it whilst in a soft state.

Common Coal is composed of a similar bituminous matter, divided by films of calcareous spar mingled

with pyrites, intersecting each other nearly at right angles: its fracture is thus rendered small grained, and uneven, and its fragments mostly cubical or trapezoidal. By this division and enclosure of the inflammable bituminous matter in combustible septa, the accension and combustion of this substance are rendered more slow, and better adapted to the purposes for which it is destined. Traces of vegetable structure are very rarely discoverable in coal, except in the impressions of cactuses and of various dorsiferous and succulent plants.

Impartiality here requires that the opinion of Professor Jameson on this subject should be noticed. The Professor, speaking of the coal found in the neighbourhood of Edinburgh, says, "the coal, which is black coal, occurs in beds, seldom more than a few inches in thickness, and is generally contained in the bituminous shale or slate clay, rarely in the sandstone. By the gradually increasing mixture of clayey matter, it passes into bituminous shale. The accompanying bituminous shale and slate clay contain impressions of ferns, a fact which has been adduced in support of the opinion which maintains the vegetable origin of black coal. We are inclined to call in question the supposed vegetable origin of this kind of coal, and are rather disposed to consider it as an original chemical formation; and that the occurrence of vegetable impressions in the adjacent rocks no more proves its vegetable origin, than the existence of fossil quadrupeds in the gypsum of Paris proves that rock to have been formed from the debris of animals of the class mammalia*."

To these opinions it appears to be sufficient to oppose the following deductions of Dr. Macculloch, from his experiments on certain products obtained from the

* Geognostical Description of the Neighbourhood of Edinburgh.-Edin. burgh Journal, vol. i, p. 354.