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., Baud XLI. 



