Fig. 3.—Microscopic section of the wood of the common Larch (Abies larix), cut in the long direction of the fibres. In both the fresh and the fossil wood (fig. 2) are seen the discs characteristic of coniferous wood. (Original.) instead of being composed of the original carbonaceous matter of the wood, is now converted into flint. The only explanation that can be given of this by no means rare phenomenon, is that the wood must have undergone a slow process of decay in water charged with silica or flint in solution. As each successive particle of wood was removed by decay, its place was taken by a particle of flint deposited from the surrounding water, till ultimately the entire wood was silicified. The process, therefore, resembles what would take place if we were to pull down a house built of brick by successive bricks, replacing each brick as removed by a piece of stone of precisely the same size and form. The result of this would be that the house would retain its primitive size, shape, and outline, but it would finally have been converted from a house of brick into a house of stone. Many other fossils besides wood—such as shells, corals, sponges, &c.—are often found silicified; and this may be regarded as the commonest form of fossilisation by replacement. In other cases, however, though the principle of the process is the same, the replacing substance may be iron pyrites, oxide of iron, sulphur, malachite, magnesite, talc, &c.; but it is rarely that the replacement with these minerals is so perfect as to preserve the more delicate details of internal structure.
CHAPTER II.
THE FOSSILIFEROUS ROCKS.
Fossils are found in rocks, though not universally or promiscuously; and it is therefore necessary that the palæontologist should possess some acquaintance with, at any rate, those rocks which yield organic remains, and which are therefore said to be "fossiliferous." In geological language, all the materials which enter into the composition of the solid crust of the earth, be their texture what it may—from the most impalpable mud to the hardest granite—are termed "rocks;" and for our present purpose we may divide these into two great groups. In the first division are the Igneous Rocks—such as the lavas and ashes of volcanoes—which are formed within the body of the earth itself, and which owe their structure and origin to the action of heat. The Igneous Rocks are formed primarily below the surface of the earth, which they only reach as the result of volcanic action; they are generally destitute of distinct "stratification," or arrangement in successive layers; and they do not contain fossils, except in the comparatively rare instances where volcanic ashes have enveloped animals or plants which were living in the sea or on the land in the immediate vicinity of the volcanic focus. The second great division of rocks is that of the Fossiliferous, Aqueous, or Sedimentary Rocks. These are formed at the surface of the earth, and, as implied by one of their names, are invariably deposited in water. They are produced by vital or chemical action, or are formed from the "sediment" produced by the disintegration and reconstruction of previously existing rocks, without previous solution; they mostly contain fossils; and they are arranged in distinct layers or "strata." The so-called "aerial" rocks which, like beds of blown sand, have been formed by the action of the atmosphere, may also contain fossils; but they are not of such importance as to require special notice here.
For all practical purposes, we may consider that the Aqueous Rocks are the natural cemetery of the animals and plants of bygone ages; and it is therefore essential that the palæontological student should be acquainted with some of the principal facts as to their physical characters, their minute structure and mode of origin, their chief varieties, and their historical succession.
The Sedimentary or Fossiliferous Rocks form the greater portion of that part of the earth's crust which is open to our examination, and are distinguished by the fact that they are regularly "stratified" or arranged in distinct and definite layers or "strata." These layers may consist of a single material, as in a block of sandstone, or they may consist of different materials. When examined on a large scale, they are always found to consist of alternations of layers of different mineral composition. We may examine any given area, and find in it nothing but one kind of rock—sandstone, perhaps, or limestone. In all cases, however, if we extend our examination sufficiently far, we shall ultimately come upon different rocks; and, as a general rule, the thickness of any particular set of beds is comparatively small, so that different kinds of rock alternate with one another in comparatively small spaces.
As regards the origin of the Sedimentary Rocks, they are for the most part "derivative" rocks, being derived from the wear and tear of pre-existent rocks. Sometimes, however, they owe their origin to chemical or vital action, when they would more properly be spoken of simply as Aqueous Rocks. As to their mode of deposition, we are enabled to infer that the materials which compose them have formerly been spread out by the action of water, from what we see going on every day at the mouths of our great rivers, and on a smaller scale wherever there is running water. Every stream, where it runs into a lake or into the sea, carries
Fig. 4.—Sketch of Carboniferous strata at Kinghorn, in Fife, showing stratified beds (limestone and shales) surmounted by an unstratified mass of trap. (Original.) with it a burden of mud, sand, and rounded pebbles, derived from the waste of the rocks which form its bed and banks. When these materials cease to be impelled by the force of the moving water, they sink to the bottom, the heaviest pebbles, of course, sinking first, the smaller pebbles and sand next, and the finest mud last. Ultimately, therefore, as might have been inferred upon theoretical grounds, and as is proved by practical experience, every lake becomes a receptacle for a series of stratified rocks produced by the streams flowing into it. These deposits may vary in different parts of the lake, according as one stream brought down one kind of material and another stream contributed another material; but in all cases the materials will bear ample evidence that they were produced, sorted, and deposited by running water. The finer beds of clay or sand will all be arranged in thicker or thinner layers or laminæ; and if there are any beds of pebbles these will all be rounded or smooth, just like the water-worn pebbles of any brook-course. In all probability, also, we should find in some of the beds the remains of fresh-water shells or plants or other organisms which inhabited the lake at the time these beds were being deposited.