THE SCOPE OF PALASONTOLOGY. Tt 
rupted by gaps, which not only bear a large proportion to our 
solid information, but which in many cases are of such a nature 
that we can never hope to fill them up. 
Foss1ts.—The remains of animals or vegetables which we 
now find entombed in the solid rock, and which constitute the 
working material of the paleeontologist, are termed “fossils,” * 
or “petrifactions.” In most cases, as can be readily under- 
stood, fossils are the actual hard parts of animals and plants 
which were in existence when the rock in which they are now 
found was being deposited. Most fossils, therefore, are of the 
nature of the shells of shell-fish, the skeletons of coral-zoophytes, 
the bones of vertebrate animals, or the wood, bark, or leaves 
of plants. All such bodies are more or less of a hard consist- 
ence to begin with, and are capable of resisting decay for a 
longer or shorter time—hence the frequency with which they 
occur in the fossil condition. Strictly speaking, however, by 
the term “fossil” must be understood “ any body, or the traces 
of the existence of any body, whether animal or vegetable, which 
has been buried in the earth by natural causes” (Lyell). 
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 bur- 
rows 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. 
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. In some rare instances, as in the case of 
the body of the Mammoth discovered embedded in ice at the 
mouth of the Lena in Siberia, the fossil may be preserved 
* Lat. fossus, dug up. 
