2 ELEMENTS OF PALAEONTOLOGY 
as certain portions of the original substance are removed, or are replaced atom 
for atom by foreign matter, the result may be either carbonisation, decom- 
position, total dissipation, or petrifaction. 
Carbonisation is a deoxidising process taking place under water or with 
limited access of air, and especially common among plants. Fossil wood and 
other vegetable matter abound in peat, lignite, and bituminous coal, the 
leaves being transformed into a thin flake of carbon, on which often the finest 
venation is still discernible. In some cases chitinous animal structures also 
become carbonised, as in insects, crustaceans, and graptolites. 
Decomposition as a rule effectually destroys all organic carbon and nitrogen 
compounds. With few exceptions, therefore, animals without hard parts, 
such as worms, infusorians, naked mollusca, most hydrozoa, many anthozoa, 
and the embryoes of vertebrates, leave no traces behind in the rocks. Horn, 
hair, chitin, and similar structures are likewise totally destroyed during the 
fossilisation process, while only under especially favourable conditions, as, for 
instance, in ice or in frozen soil, muscular and epidermal tissues remain 
unchanged ; or else, through the taking up of lime phosphate in argillaceous 
and calcareous deposits, undergo a sort of petrifaction, in which the finer 
structure is but little altered.1_ Even the conservable hard parts of animal 
bodies are deprived of their organic compounds ; bones give up their fats and 
oils, and the shells of mollusks, echinoderms, and crustaceans lose their pig- 
ments and soft substratum. The hard portions, which first become more or 
less porous through loss of their organic constituents, next suffer the gradual 
disintegration of their inorganic compounds, and experience lastly either total 
dissolution, reabsorption, or petrifaction. 
Petrifaction.—In this process foreign substances soluble in water (chiefly 
calcium carbonate and silica, more rarely pyrites, iron oxyhydrate, and other 
salts) impregnate and completely fill all original cavities as well as those 
formed subsequently by decay. Chemical metamorphism takes place 
occasionally, when, owing to the decomposition of certain inorganic con- 
stituents, the original molecules become replaced by those of other substances. 
For instance, we find quartz pseudomorphs after calcareous tests and 
skeletons, and conversely, calcite pseudomorphs after silica, as in certain 
sponges. 
Wherever the space originally occupied by soft parts, as, for example, the 
interior of a shell or other hollow body, becomes filled up with infiltrating 
ooze, while the shell itself or the enclosing wall decays, there is produced a 
cast of the interior, which in most cases (especially where the shell is thin, as in 
ammonites, brachiopods, certain mollusks, and crustaceans) preserves an exact 
copy of the original form, and is susceptible of as accurate determination as 
the real object. Not infrequently fossil organisms leave molds or imprints of 
their shells or skeletons—very rarely of their whole bodies—in the rocks. 
Sometimes, indeed, their presence is indicated merely by tracks or footprints. 
Fossils are often distorted by mechanical agencies, such as faulting, folding, 
crushing, and other deformations of the country rock. Such cases require 
especial attention, and due caution must be observed in their determination. 
Palaeontology and Biology.—Although the fossil remains of ancient 
life-forms yield but a fragmentary record of themselves, are almost never 
1 Reis, Otto, Ueber Petrificirung der Muskulatur, Arch. mikroskop. Anat., Band XLI. 
