Fossil (Lat. fossilis, 'dug out of the earth'), a term formerly applied, in accordance with its derivation, to whatever was dug out of the earth, whether mineral or organic. The term is now restricted to remains and relics of plants and animals which have become embedded by natural causes. These fossils may consist of the harder and more durable parts of animals and plants, or they may be merely the casts or impressions of such remains, or the footmarks or tracks which animals may have left behind them on some soft surface which has been subsequently covered up and consolidated. They occur in nearly all the stratified aqueous rocks, which have on this account been called Fossiliferous strata. It is difficult or impossible to detect them in metamorphic aqueous rocks, for the changes that altered the matrix have also affected the organisms, so as either almost or altogether to obliterate them. In the archæan schists they have escaped notice, if ever they existed; but recently they have been detected in schistose rocks of Silurian age in southern Norway.
The conditions in which fossils occur are very various. In some Pleistocene beds the organic remains are but slightly altered, and are spoken of as sub-fossil. In this state are the shells in some raised sea-beaches, and the remains of the huge struthious birds of New Zealand, which still retain a large portion of the animal basis. In the progress of fossilisation every trace of animal substance disappears; and if we find the body at this stage, without being affected by any other change, it is fragile and friable, like some of the shells in the London clay. Most frequently, however, a petrifying infiltration occupies the cavities left in the fossil by the disappearance of the animal matter, and it then becomes hardened and solidified; hence fossils were formerly, and still often are, called petrifications. Sometimes the whole organism is dissolved and carried off by water percolating the rock, thus leaving a cavity which may be filled up with calcite, pyrite, gypsum, flint, chalcedony, or some other mineral; and we thus obtain the form of the organism, with the markings of the outer surface, but not exhibiting the internal structure. Not infrequently, as in the case of shells of molluscs, &c., after the soft parts of the organism have been removed and replaced by inorganic matter (either before or after burial), the shell itself may be dissolved out so as to leave a cavity which shows the mould of the outer surface of the shell, and a cast of the interior. If the shell-space is not subsequently filled up by introduced mineral matter, the internal cast lies loose in the cavity like the kernel of a nut. Most commonly, however, the shell itself is replaced by hydrated mineral matter. The most advanced and perfect condition of fossilisation is that in which not only the external form, but also the most minute and complicated internal organisation, is retained; in which the organism loses the whole of its constituents, particle by particle, and as each molecule is removed its place is taken by a molecule of another substance, as silica or pyrites. In this way we find calcareous corals perfectly preserved in flint, and trees exhibiting in their silicified or calcified stems all the details of their microscopic structure—the cells, spiral vessels, or disc-bearing tissue, as well as the medullary rays and rings of growth.