into their framework, such as the diatoms of which the frustules become remarkably permanent fossils, in the form of diatom-earth or tripoli-powder, which is made up of them (p. 111).

The hard parts of animals may be preserved with little or no chemical change, and remain as durable relics. The hard horny integuments of insects, arachnids, crustacea, and some other animals, are composed essentially of the substance called chitin, which can long resist decomposition, and which may therefore be looked for in the sedimentary deposits of the present time, as well as of former periods. The chitin of some fossil scorpions, admirably preserved among the Carboniferous rocks of Scotland, can hardly be distinguished from that of the living scorpion. Many of the lower forms of animal life secrete silica, and their hard parts are consequently easily preserved, as in the case of radiolaria and sponges. In the great majority of instances, however, the hard parts of invertebrates consist mainly of carbonate of lime, and are readily preserved among sedimentary deposits. The skeletons of corals, the plates of echinoderms, and the shells of molluscs, are examples of the abundance of calcareous organisms, and the frequency of their remains in the fossil state shows how well fitted they are for preservation. Among vertebrates the hard parts consist chiefly of phosphate of lime. In some forms (ganoid fishes and crocodiles, for example) this substance is partly disposed outside the body (exo-skeleton) in the form of scales, scutes, or bony plates. But more usually it is confined to the internal skeleton (endo-skeleton). It is mainly by their bones and teeth that the higher vertebrates can be recognised in the fossil state. Sometimes the excrement has been preserved (coprolites), and may furnish information regarding the food of the animal, portions of undigested scales, teeth, and bones being traceable in it.

FOSSILISATION. - The process by which the remains of a plant or animal are preserved in the fossil state is termed Fossilisation. It varies greatly in details, but all these may be reduced to three leading types.

1. Entire or partial preservation of the original substance. In rare instances, the entire animal or plant has been preserved, of which the most remarkable examples are those where carcases of the extinct mammoth have been sealed up in the frozen mud and peat of Siberia, and have thus been preserved in ice, every portion of the animal substance being retained, and the flesh being fresh enough to be devoured by living carnivores. Insects have been preserved in the resin of trees, and may now be seen, embalmed like mummies, in amber. More usually, however, a variable proportion of the organic matter has passed away, and its more durable parts have been left, as in the carbonisation of plants (peat, lignite, coal) and the disappearance of the organic matter from shells and bones, which then become dry and brittle and adhere to the tongue.

2. Entire removal of the original substance and internal structure, only the external form being preserved. - When a dead animal or plant has been entombed, the mineral matter in which it lies hardens round it and takes a mould of its form. This may be accomplished with great perfection if the mineral is sufficiently fine-grained and solidifies before the object within has time to decay. Carbonate of lime and silica are specially well adapted for taking the moulds of organisms, but fine mud, marl, and sand, are also effective. The organism may then entirely decay, and its substance may be gradually removed by percolating water, leaving a

hollow empty space or mould of its form. Such moulds are of frequent occurrence among fossiliferous rocks, and are specially characteristic of molluscs, the shells of which are so abundant, and occur imbedded in so many different kinds of material. Sometimes it is the external form of the shell that has been taken, the shell itself having entirely disappeared; in other cases a cast of the interior of the shell has been preserved. How different these two representations of the same shell may be is shown in Fig. 113,

FIG. 113.-Common Cockle (Cardium edule); (a) side view of both valves; (b) mould of the external form of one valve taken in plaster of Paris; (c) side view of cast in plaster of Paris of interior of the united valves.

wherein a represents a side view of the common cockle, while c is a cast of the interior of the shell in plaster of Paris. The contrast between a mould of the outside and inside of the same shell is shown by the difference between b and c, which are both impressions taken in plaster.

After the decay and removal of the substance of the enclosed organisms, the moulds may be filled up with mineral matter, which is sometimes different from the surrounding rock. The empty cavities have formed convenient receptacles for any deposit which permeating water might introduce. Hence we find casts of organisms in sand, clay, ironstone, silica, limestone, pyrites, and other mineral substances. Of course, in such cases, though the external form of the original organism is preserved, there is no trace of internal structure. No single particle of the cast may ever have formed part of the plant or animal.

3. Partial or entire petrifaction of organic structure by molecular replacement. - Plants and animals which have undergone this change have had their substance gradually removed and replaced, particle by particle, with mineral matter. This transformation has been effected by percolating water containing mineral solutions, and has proceeded so tranquilly, that sometimes not a delicate tissue in the internal structure of a plant has been displaced, and yet so rapidly, that the plant had not time to rot before the conversion was completed. Accordingly, in true petrifactions, that is, plants or animals of which the structure has been more or less perfectly preserved in stone, the petrifying material is always such as may have been deposited from water. The most common substance employed by nature in the process of petrifaction is carbonate of lime, which, as we have seen, is almost always present in the water of springs and rivers. Organic structures replaced by this substance are said to be calcified. Frequently the carbonate of lime has assumed, more or less completely, a crystalline structure after its deposition, and in so doing has generally injured or destroyed the organic structure which it originally replaced. Where the calcareous matter of an organism has been removed by percolating water, the fossil is said to be decalcified. Another abundant petrifying medium in nature is silica, which, in its soluble form, is generally diffused in terrestrial waters, where humus acids or organic matter are present in solution. The replacement of organic structures by silica, called silicification, furnishes the most perfect form of petrifaction. The interchange of mineral matter has been so complete that even the finest microscopic structures have been faithfully preserved. Silicified wood is an excellent example of this perfect replacement. Sulphides, which are often produced by the reducing action of decaying organic matter upon sulphates, occur also as petrifying media, the most common being the iron sulphide, usually in the less stable form of marcasite, but sometimes as pyrite. Carbonate of iron likewise frequently replaces organic structures; the clay-ironstones of the Carboniferous system abound with the remains of plants, shell-fishes, and other organisms which have been converted into siderite (Fig. 62).

The chief value of fossils in geology is to be found in the light which they cast upon former conditions of geography and climate, and in the clue which they furnish to the relative ages of different geological formations.

1. How FOSSILS INDICATE FORMER CHANGES IN GEOGRAPHY.-Terrestrial plants and animals obviously point to the existence of land. If their remains are found in strata wherein most of the fossils are marine, they usually show that the deposits were laid down upon the sea-floor not far from land. But where they occur in the positions in which they lived and died, they prove that their site was formerly a landsurface. The stumps of trees remaining in their positions of growth, with their roots branching out freely from them in the clay or loam underneath, undoubtedly mark the position of an ancient woodland. If, with these remains, there are associated in the same strata wing-cases of beetles, bones of birds and of land-animals, additional corroborative evidence is thereby obtained as to the existence of the ancient