PETRIFACTION, in Natural History, denotes the conversion of wood, bones, and some other substances, whether animal or vegetable, into stone. These bodies are more or less altered from their original state, according to the different substances amongst which they have lain buried in the earth; some of them having suffered very little change, and others being so highly impregnated with crystalline, sparry, pyritical, or other extraneous matter, as to appear mere masses of stone or lumps of the matter of the common pyrites. But they are generally of the same external dimensions, and retain more or less of the same internal figure, as the bodies into the pores of which this matter has made its way. The animal substances thus found petrified are chiefly sea-shells, the teeth, bony palates, and bones of fishes, the bones of land-animals, and some others. These are found variously altered, by the insinuation of stony and mineral matter into their pores; and the substance of some of them is found to be wholly gone, leaving only stony, sparry, or other mineral matter remaining in the original shape and form.

Respecting the manner in which petrifaction is accomplished, we know but little. It has been thought by many philosophers, that this is one of the occult processes of nature; and accordingly such places as afford a view of it have been looked upon as great curiosities. However, it is now discovered that petrifaction is exceedingly common, and that every kind of water carries in it some earthy particles, which being precipitated, become stone of a greater or less degree of hardness; and this quality is most remarkable in those jvaters which are much impregnated [7:17:332]with selenitic matter. It has been found by observation, that iron contributes greatly to the process; and this it may do by its precipitation of any aluminous earth which happens to be dissolved in the water by means of an acid, iron having the property of precipitating this earth. Calcareous earth, however, by being soluble in water without any acid, must contribute greatly to the process of petrifaction, as its particles are capable of acquiring a great degree of hardness by means only of being joined with fixed air, on which depends the solidity of our common cement or mortar used in building houses.

The name of petrifaction, as we have seen, belongs only to bodies of vegetable or animal origin; and in order to determine their class and genus, or even species, it is necessary that their texture, their primitive form, and in some measure also their organization, should be still discernible. Thus, we ought not to place the stony kernels, moulded in the cavity of some shell, or other organized body, in the rank of petrifactions properly so called.

Petrifactions of the vegetable kingdom are almost all either gravelly or siliceous; and they are found in gullies, trenches, and other situations. Those which strike fire with steel arc principally found in sandy fissures; those which effervesce in acids are generally of animal origin, and are found in the horizontal beds of calcareous earth, and sometimes in beds of clay or gravel, in which last case the nature of the petrifaction is different. As to the substances which are found in gypsum, they seldom undergo any alteration, either with respect to figure or composition, and they are of very rare occurrence.

Organized bodies, in a state of petrifaction, generally acquire a degree of solidity of which they were not possessed when they were buried in the earth, and some of them are often fully as hard as the stones or matrices in which they are enveloped. When the stones are broken, the fragments of petrifactions are readily found, and easily distinguished. There are some organized bodies, however, so changed by petrifaction, as to render it impossible to discover their origin. That there is a matter more or less agitated,.and adapted for penetrating bodies, which crumbles and separates their parts, and disperses them here and there in the fluid which surrounds them, is a fact of which nobody seems to entertain any doubt. Indeed we see almost every substance, whether solid or liquid, insensibly consume, diminish in bulk, and at last, in the lapse of time, vanish and disappear.

A petrified substance, strictly speaking, is nothing more than the skeleton, or perhaps the form, of a body which has once had life, either animal or vegetable, combined with some mineral. Thus, petrified wood is not, in that state, wood alone. One part of the compound or mass of wood, having been destroyed by local causes, has been compensated by earthy and sandy substances, diluted and extremely minute, which the waters surrounding them had deposited, whilst they themselves evaporated. These earthy substances, being then moulded in the skeleton, will be more or less indurated, and will appear to have its figure, structure, and size, in a word, the same general characters, the same specific attributes, and the same individual differences. Further, in petrified wood, no trace or vestige of ligneous matter appears to exist. We know that common wood is a body in which the volume of solid parts is greatly exceeded by that of the pores. When wood is buried in certain places, lapidific fluids, extremely divided and sometimes coloured, insinuate themselves into its pores and fill them up. These fluids are afterwards moulded and condensed. The solid part of the wood is decomposed and reduced into powder, which is expelled from the mass by aqueous filtrations; and thus the places which were formerly occupied by the wood are now left empty in the form of pores. This operation of nature produces no apparent difference either of the size or of the shape; but it occasions, both at the surface and in the interior, a change of substance, and the ligneous texture is inverted, or, in other words, that which was pore in the natural wood becomes solid in the petrified, and that which was solid or full in the first state becomes porous in the second. In this way, says Musard, petrified wood is much less extended in pores than solid parts, and at the same time forms a body much denser and heavier than the first. As the pores communicate from the circumference to the centre, the petrifaction ought to begin at the centre, and end with the circumference of the organic body subjected to the action of the lapidific fluids. Such is the origin of petrifactions. They are organized bodies which have undergone changes at the bottom of the sea or the surface of the earth, and which have been buried by various accidents at different depths under the ground. See Mineralogy and Geology.