2 ELEMENTS OF PALEONTOLOGY 



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 i^lants. Fossil wood and 

 other vegetable matter abound in peat, lignite and bituminous coal, the 

 leaves being transformed into a thin flake of carbon, on Avhich 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. AYith few exceptions, therefore, animals without hard parts, 

 such as worms, infusorians, naked mollusca, most hydrozoa, many anthozoa, 

 and the embryos 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 

 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.^ Even the conservable hard parts of animal bodies are deprived 

 of their organic compounds ; bones give tip their fats and oils, and the shells 

 of mollusks, echinoderms and crustaceans lose their pigments and soft sub- 

 stratum. 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 finally undergo 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 skeletal 

 parts, 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. 



Paleontology and Biology. — Although the fossil remains of ancient 

 life-forms yield but a fragmentary record of themselves, are almost never 

 perfectly preserved, and are usually more or less altered in appearance, yet 

 on the whole, they readily fit into place in the great framework of zoological 



^ Reis, Olio, Cber ]>etrilicirte Muskulatur. Arch. Miki'os. Anat., vols. xli. xliv. Hi., — ])ccm, 

 B., Preservation of nuiscle-fibres in sharks. Amer. Geol., 1902, vol. xxx. 



