210 MINERALS AND GEOLOGY 



Certain fossil forms belonging to the most recent geological 

 deposits are identical with existing species ; others are akin to these 

 generically, although specifically distinct ; and many are wholly 

 without representatives in existing nature. In some cases the fossils 

 found in rocks were evidently entombed as living forms ; whilst, in 

 other cases, they have been drifted after death to a greater or less 

 distance with the sediments of which they now form part. The 

 process of fossilization is a gradual replacement (as in the case of 

 many mineral pseudomorphs) of the original substance of the body 

 by mineral matter. The fossilizing material is either the general 

 substance of the enclosing sediments, or some special substance 

 mostly silica, calcic carbonate, or ferrous carbonate. The latter is 

 frequently converted by after changes into hydrated ferric oxide, 

 or occasionally into iron pyrites. 



PLANT REMAINS. 



The relations of fossil plants to existing vegetable types except 

 (and that not always) in the case of sea-weeds, ferns, and the plant 

 remains of the less ancient rock-formations are generally more or 

 less obscure. This arises from the comparatively imperfect state of 

 preservation of these bodies, their more characteristic structural 

 parts being commonly destroyed ; whilst, at the same time, the 

 fragmentary remains of distinct species and genera are often mixed 

 up together, and much uncertainty in their determination is thus 

 occasioned. Plant remains in the fossil state are much less common 

 than the fossilized remains of animals. . The reason is obvious : 

 aquatic types, literal and lacustrine forms especially, are those most 

 favourably situated for fossil preservation, and these, in the plant 

 world, are for the greater part composed of easily-perishable cellular 

 tissue, whereas most aquatic animals possess shells, bones, or other 

 hard secretions, composed largely of mineral and comparatively 

 indestructible matter. The shells of most mollusca, for instance, 

 as well as most coral structures, contain more than 90 per cent, of 

 calcic carbonate ; in tee bh, the inorganic matter varies from 60 to 

 70 per cent. ; and in ordinary bones, from 50 to about 60 per cent.* 

 Land plants, when preserved as fossils, are generally found in 



* The inorganic matter in ordinary fish-scales exceeds 50 or 55 per cent. ; whilst in the scales 

 of reptiles (as in the nails, horns, and hair of mammals) it is under 5 or 6 per cent. This 

 explains the rare occurrence of reptilian scales, even as impressions, in strata, whilst those 

 of fishes are comparatively abundant. 



