Thus, perhaps, the formation of the bituminous fossils may be satisfactorily explained but by far the greater num ber of vegetable fossils are of a lapide ous nature, and necessarily owe their for.: mation to very:different processes ; which the same author supposes are, in gene ral, preceded by tie process by which bitumen is formed. Many bodies which are evidently of vegetable origin may be now found in a lapideous, either calcareous or siliceous, state ; and many others are found possessing certain marks of the presence of some metallic sub.. stance.
To explain these formations, various opinions have been formed. Some have supposed the injection of the impregnat ing matter, in a state of fluidity, by igni tion ; whilst others have imagined the gradual abstraction of the original parti cles of the body, and the regular depo sition of the impregnating particles in the spaces which have just been left by the original matter. Mr. Parkinson, who does not admit of this substitution, attri butes the formation of this description of fossils to the impregnation of vegetable substances, which have undergone differ ent degrees of bituminization, with wa ter, holding the earths or the metals in solution. Thus with lime is formed the calcareous wood or wood-marble of Ox fordshire and Dorsetshire, of Piedmont and of Bohemia ; with silex is formed the calcedonified, agatified, and jasperified wood (Holzstein) ; and with the addition of alumine, &c. the fossil woods which now partake of the nature of pitch-stone, and waxopal (Holzopal). In other situa tions, metallic impregnations occur ; as in such woods as'are impregnated with the pyrites of iron, so frequently found in our and the beautiful woods of Si beria, containing the hydrate and carbo nate of copper.
Various parts of trees and plants (phy tolithi) are found in a mineralized state. Not only fossil wood (lithoxylon), as has been just noticed, but the leaves (litho pylla or lithobiblia), and fruits (carpo lithi) of different trees or plants are thus found. Of the woods, several, from their form and texture, have been supposed to have been originally oak, willow, and such trees as now exist in a recent state ; whilst others differ, in both these re spects, from any species of wood which is now known.
The impressions of the stalks and leaves of plants are very frequently found in many parts of the world, in lofty mountains, as well as at a consider able depth below the surface ; and not only the impressions, but the sub stance itself of different vegetables are also thus found ; but in no situation more frequent than in the neighbourhood of coal mines.
In general these vegetable remains are found deposited in lamina, in the schistose strata which accompany the coal ; but the most perfect remains are commonly found in roundish nodular masses of ferruginous clay, Which abound in the strata accompanying the coal. These are commonly termed catsheads by the workers of the coal mines, and contain pieces of fern, &c. very few, indeed, of which are found to agree with any known recent plants. One of these plants, preserved in coal slate, is shewn, Plate I. ORYCTOLOGY, fig. 1. The ve getable remains in these fossils appear to confirm the opinion above mention ed, of the bituminization of fossil ve getables ;. since these leaves are com pletely changed into a bituminous sub stance.
The remains of fruits are, perhaps, no where found so abundantly as in the Isle of Sheppey, where they are dug up in great variety ; very few, ,however, being found which agree with any known re cent fruits. Where any resemblance ap pears, it is with fruits which only grow in the warm Asiatic regions. Plate 1. fig. 2, represents a fossil fruit which was found in the cliff of Sheppey.
Fossil roots of plants of trees are very rarely found ; a circumstance not very easily explained ; since they possess (es pecially the roots of trees) that degree of solidity which appears to be favourable to the process of petrifaction. From the want of this necessary property it un doubtedly is, that we possess so few re mains of tender flower leaves, and none of pulpy fruits.
From the same cause, the great prone ness to decomposition, the number of animal fossils is considerably limited : those substances being only preserved in a mineralized state which originally pos sessed a considerable degree of solidity ; such are the bones, teeth, horns, shells, scales, &c. The animal, however, far exceeds the vegetable kingdom in the number and variety of fossils which it yields, as well as in the distinctness of form, and excellency of preservation, in which they are found.