We shall find, in fact, that many of the objects which we have to study as "fossils" have never themselves actually formed parts of any animal or vegetable, though they are due to the former existence of such organisms, and indicate what was the nature of these.
Thus the footprints left by birds, or reptiles, or quadrupeds upon sand or mud, are just as much proofs of the former existence of these animals as would be bones, feathers, or scales, though in themselves they are inorganic.

Under the head of fossils, therefore, come the footprints of air-breathing vertebrate animals; the tracks, trails, and burrows of sea-worms, crustaceans, or molluscs; the impressions left on the sand by stranded jelly-fishes; the burrows in stone or wood of certain shell-fish; the "moulds" or "casts" of shells, corals, and other organic remains; and various other bodies of a more or less similar nature.
[Footnote 3: Lat.

_fossus_, dug up.] Fossilisation.-- The term "fossilisation" is applied to all those processes through which the remains of organised beings may pass in being converted into fossils.

These processes are numerous and varied; but there are three principal modes of fossilisation which alone need be considered here.

In the first instance, the fossil is to all intents and purposes an actual portion of the original organised being--such as a bone, a shell, or a piece of wood.