On this branch of our subject we do not mean to offer much in the way of argument strictly so called. We shall content ourselves with a simple statement of facts, and leave them to produce their own impression. It will be necessary at the outset to explain some technical matters, that what we have to say hereafter may be the better understood: and if in this we are somewhat dry and tiresome, we will try to make amends by the curious and interesting story of Nature’s long buried works, which we hope in the sequel to unfold.
When the word Fossil was first introduced into the English language, it was employed to designate, as the etymology suggests, whatever is dug out of the earth.[69] But it is now generally used in a much more restricted sense, being applied only to the remains of plants and animals embedded in the Crust of the Earth and there preserved by natural causes. When we speak of remains, we must be understood to include even those seemingly transient impressions, such as foot-prints in the sand, which having been made permanent by accidental circumstances, and thus engraved, as it were, on the archives of Nature, now bear witness to the former existence of organic life.
Now in every part of the world where the Stratified Rocks have been laid open to view, remains of this kind are found scattered on all sides in the most profuse abundance. In Europe, in America, in Australia, in the frozen wastes of Siberia, in the countless islands scattered over the waters of the Pacific, there is scarcely a single formation, from the lowest in the series to the highest, that, when it is fairly explored, does not yield up vast stores of shells, together with bones and teeth, nay, sometimes whole skeletons of animals; also fragments of wood, impressions of leaves, and other organic substances.
Fig. 12.—Fossil Irish Deer (County Fermanagh). In the Museum of Trinity College, Dublin. From Haughton’s Manual of Geology
These Fossil Remains do not always occur in the same state of preservation. Sometimes we have the bone, or plant, or shell, in its natural condition; still retaining not only its own peculiar form and structure, but likewise the very same organic substance of which it was originally composed. Examples innumerable may be seen in the British Museum, or, indeed, in almost any Geological collection: the fine skeletons of ancient Irish Deer, which are exhibited in the Museum of Trinity College, Dublin, and of which all the bones are in excellent preservation, must be familiar to many of our readers.
It happens, however, more frequently that the organic substance itself has disappeared, but has left an impression on the rock, that now bears witness to its former presence. Thus, for instance, when a shell has been dissolved and carried away by water percolating the rock, it has very often left after it, on the hard stone, a mould of its outer surface and a cast of its inner surface, with a cavity between corresponding to the thickness of the shell. In such cases we have the form, the size, and the superficial markings of the organic body, but we have no part of its original substance, and no traces of its internal structure. This form of fossilization, as Sir Charles Lyell has well put it, “may be easily understood if we examine the mud recently thrown out from a pond or canal in which there are shells. If the mud be argillaceous, it acquires consistency in drying, and on breaking open a portion of it, we find that each shell has left impressions of its external form. If we then remove the shell itself, we find within a solid nucleus of clay, having the form of the interior of the shell.”[70] In many cases the space first occupied by the shell is not left empty when the shell has been removed, but is filled up with some mineral substance, such as lime or flint. The mineral thus introduced becomes the exact counterpart of the organic body which has disappeared; and has been justly compared to a bronze statue, which exhibits the exterior form and lineaments, but not the internal organization nor the substance of the object it represents.
There is a third form more wonderful still, in which Fossil Remains are not uncommonly found. The original body has passed away as in the former case, and yet not only does its outward shape remain, but even its internal texture is perfectly preserved in the solid stone which has taken its place. This kind of change is exhibited most remarkably in the vegetable kingdom. Fossil trees of great size have been discovered of which the whole substance has been changed from wood to stone: yet with such exquisite skill has the change been effected that the minute cells and fibres, and the rings of annual growth, may still be clearly traced; nay, even those delicate spiral vessels which, from their extreme minuteness, can be discerned only by the aid of the microscope. Thus the tree remains complete in all its parts; but it is no longer a tree of wood; it is, so to speak, a tree of stone.
The mystery of this extraordinary transformation has not yet been fully cleared up by scientific men; but the general principle, at least, is sufficiently understood. It is thus briefly explained by Sir Charles Lyell: “If an organic substance is exposed in the open air to the action of the sun and rain, it will in time putrefy, or be dissolved into its component elements, consisting usually of oxygen, hydrogen, nitrogen, and carbon. These will readily be absorbed by the atmosphere or be washed away by rain, so that all vestiges of the dead animal or plant disappear. But if the same substances be submerged in water, they decompose more gradually; and if buried in the earth, still more slowly, as in the familiar example of wooden piles or other buried timber. Now, if as fast as each particle is set free by putrefaction in a fluid or gaseous state, a particle equally minute of carbonate of lime, flint, or other mineral is at hand and ready to be precipitated, we may imagine this inorganic matter to take the place just before left unoccupied by the organic molecule. In this manner a cast of the interior of certain vessels may first be taken, and afterward the more solid walls of the same may decay and suffer a like transmutation.”[71] This exposition, so simple and luminous in itself, may, perhaps, be rendered still more intelligible to the general reader by an ingenious illustration of Mr. Jukes. “It is,” he says, “as if a house were gradually rebuilt, brick by brick, or stone by stone, a brick or a stone of a different kind having been substituted for each of the former ones, the shape and size of the house, the forms and arrangements of its rooms, passages, and closets, and even the number and shape of the bricks and stones, remaining unaltered.”[72]