FOSSIL (Lat. 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, but now restricted to the remains of plants and animals imbedded in the eatth's crust. They were formerly, and are sometimes still, called petrifactions. They occur in nearly all the stratified rocks, which have, on this account, been called fossiliferous strata. 1t is difficult or impossible to detect them in the metamorphic rocks, for the changes that altered the matrix have also affected the organisms, so as either almost or altogether to obliterate them. In the fundamental mica-schist and gneiss they have escaped notice, if ever they existed; anchit is only within the last few years that their presence has been detected in the gneiss and other rocks, which are the greatly metamorphosed represent atives of the lower Silurian measures in the n. of Scotland.
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 fossilization, 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. tIost frequently, however, a petrifying
infiltration occupies the cavitiesleft in the fossil by the disappearance of the animal mat ter, and it then becomes hardened and solidified. Sometimes the whole organism is dissolved and carried off by water percolating the rock, and its former presence is indi cated by the mold of its outer surface, and the cast of its inner in the rocky matrix, leaving, a cavity between the cast and the mold agreeing with the size of the fossil. This cavity is occasionally filled up with calcareous spar, flint, or some other mineral; and we thus obtain the form of the organism, with the markings of the outer and inner sur faces, but not exhibiting the internal structure. The most advanced and perfect condi tion of fossilization is that in which not only the external form, but also the most minute and complicated internal organization is retained; in which the organism loses the whole of its constituents, particle by particle, and as each little molecule is removed, its place is taken by a little molecule of another substance, as silica or iron 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 disk-bearing tissue, as well as the medullary rays and rings of growth.