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Petrifaction. This preservation might be affected in two ways. By the first, the decomposition of the body is simply prevented, more or less completely, so that the body remains, more or less entirely, in the same state that it was in when it was first buried. If the enclosing substance be a very fine pure clay which does not itself contain any active chemical substance, and does not allow of the percolation of water, and if the body, when buried, consisted of only one substance, that would remain inactive so long as it was left to itself, the conditions for such a preservation would be most complete. Shells, with all their nacreous lustre and iridescent colours, are found in clays of very ancient date, so ancient that, if stated in years and centuries, an indefinite number of millions of either would have to be assigned to them. Wood, so little altered as to be still in the state of wood, easily scratched or indented with the nail, is found in similar clays. In some very ancient shales fragments of leaves may be found that, when first laid open, retain a greenish tint, and are semitransparent, and have the dampness of vegetation upon them, although a few minutes' exposure to the air suffices to wither them into decay.
The second method of preservation of bodies by petrifaction, is that by which the organic substance loses more or less of its organic structure and composition, and acquires those belonging to a mineral. Some shells buried in clays, have lost the carbonate of lime of which they were originally composed, and now consist of iron pyrites, or some other mineral. This replacement of one mineral substance by another, however, has taken place so gradually, particle by particle, that the external form, and even the most delicate markings and minutest structure possessed by the original shell, is retained in the new substance. Shells or other calcareous bodies buried in calcareous mud, are commonly petrified by the addition of some of the same substance as that of which they were originally formed (Calcite or car
bonate of lime) filling up their interstices, and taking a crystalline structure instead of the cellular one which was that derived from the animal. Sometimes, however, an organic body buried in limestone is changed into flint, especially if it contained some silica in its original composition.
In sandstones, which commonly allow of the free percolation of water, it is more usual to find only casts or impressions of the bodies enclosed in them, the substance of the body itself having been dissolved and carried away by water. These are often merely external casts, the outside of the body having left its mark in the enclosing rock; and this impression is sometimes so faithful as to show the most delicate striæ or wrinkle in the original form. Sometimes the inside of the body itself was filled either by fine mud or by some crystalline mineral, before it decomposed, so that a cast of its interior is left with all the muscular impressions, or other internal marks, which the recent body could have displayed.
Traces even of the skin, or soft parts, of animals, and of the most delicate venation of the leaves and stems of plants, are sometimes preserved in these impressions. The scaly coats of fish are often retained, almost as perfect as if taken freshly from the animals ; while teeth and bones are obviously as indestructible as shells, and being composed, like them, chiefly of salts of lime, will obey the same laws of petrifaction.
Even the tracks or footprints of animals crawling or walking over sand in shallow water, or on the sea-shore at low tide, are preserved on the surfaces of some flag-stones in the most wonderful way, as also the holes and burrows of annelids or sea worms. · A footprint, especially if it be covered by a little film of clay before the next layer of sand is tranquilly deposited on it, will be as likely to be retained unaltered as any other form, and will give us as sure an indication of the existence and character of the animal as did the footprint on the shore of
the desert island to Robinson Crusoe, which assured him that a strange man had passed that way.
In a similar manner plants may be petrified : either by being altered into coal, through the abstraction of their gases, so that the residue becomes more and more nearly pure carbon; or by the replacement of their particles, some other mineral matter, such as carbonate of lime or silica, taking the place of the carbon. The woody stem will then become limestone or flint (wood opal), in which, perhaps, all the fibrous and cellular structure of the original wood may be retained in consequence of the minuteness of the particles which are gradually abstracted and replaced.
The mere amount of change, then, which the fossil has undergone, is not by any means a proof of the length of time that has elapsed since it was buried in the earth ; as that amount depends so largely on the nature of the material in which it was entombed, and on the circumstances that have since surrounded it. Some of the most ancient fossils may be very little petrified, some of the most modern may be completely so. Nevertheless, since the longer a body is buried the more chance it has of being acted on by those conditions which conduce to petrifaction, it follows that the majority of the most recent fossils are less altered than the majority of the more ancient ones.
But it may be asked here, If the antiquity of a fossil is not to be judged of by anything in the state of the fossil itself, how is it to be determined ? The answer to this is that the age of a fossil can only be known, in the first instance, by that of the beds in which it is found ; and the relative antiquity of the beds can only be determined by their relative position.
Distribution of fossils in stratified rocks.—We know that in the seas of the present day different animals frequent different places; some love clear open water, some muddy or sandy shores, some live in the deep sea, some only in shallow water. We should expect, then, to find a difference in the kinds of fossils in different rocks; some kinds occurring chiefly in old indurated muds, some in sandstones, others in limestones. If, however, the animals were free swimmers, like fish, or if the body floated after death, or the fragments of it were carried far by currents, the nature of the bottom where it came to rest and was buried would have no connection with the habits of the animal. So of plants and animals carried from the land into the sea : they might float far and be entombed in any kind of rock; still, as sands and muds are generally formed more near to the land than limestones are, we should expect more often to find land plants or animals in the former than the latter. We have here one set of circumstances bearing on the distribution of fossils.
Another arises from the geographical distribution of animals and plants. The mud and sand forming round the shores of the British Islands now, cannot contain precisely the same shells as those which are forming in the Mediterranean, or the West Indies, or the Cape of Good Hope, or Tasmania, or Terra del Fuego, or the Japanese Islands. The shells and other animals now living in the seas of these places are almost entirely different, no two of them having the same assemblage of species, and some of them not a single species in common. So in past ages of the globe, the fossils which were deposited at the same period in widely different parts of the Earth would be different from each other. If they were not it would be a matter for surprise on our part.
These facts show us that fossils will probably observe what may be called a lateral distribution, and that beds formed at the very same time will not always contain identically the same fossils, and that even the very same continuous group of beds may vary in this respect. For when we come to trace groups of beds across wide countries, we find that the same group often varies, so that, consisting almost entirely of limestone at one place, it will in another perhaps contain
little or no limestone, and be made up almost, or altogether, of clay, or sandstone, or both. The very same sea will have the waste of the land swept into it from one side, while at a greater distance from the land limestone only may be formed.
Vertical distribution of fossils.—The fossil corals, shells, scales, bones, teeth, and other hard parts of animals, have now for many years been the subject of careful examination and comparison by many of those men, in all countries, who were best acquainted with the corresponding living animals. The fossil leaves, roots, and stems of plants, have in like manner been examined by some of the best botanists of all countries. The conclusion they have all come to is that the remains found fossil in the stratified rocks, belonged chiefly to species which were different from any now living on the globe. Some of them are merely extinct species of genera which have other species now living: extinct species of oysters or cockles among shells, extinct species of crocodiles or turtles among reptiles, or elephants or rhinoceroses among mammals. Others, however, are so different from any living animals, that new generic names have to be invented for them, and there are even groups of such extinct genera making families, or even in a few cases orders, which are distinct from any of the living orders of animals.
If, however, we were simply to take fossil animals and plants on the one hand, and living ones on the other, classing all the fossil ones together indiscriminately, we should lose sight of the most interesting and important facts which are to be learnt from them. Fossils do not occur indiscriminately; not only do they differ, as previously mentioned, according to the kinds of rock, and according to the different parts of the world in which they are found, but in the very same country, and in rocks of precisely the same mineral character, a law of distribution is observable in fossils according to the order of superposition of the groups of stratified rock in which they occur.