  Of the rocks which form the solid earth of to-day, a very large proportion have been built up from the deposits at the bottom of ancient oceans and lakes. The earth is very old, and in the course of its history dry land and sea, mountains and valleys have been formed and again destroyed on the same spot, and it is from the silt at the bottom of an ocean that the hills of the future are built. The chief key we have to the processes that were in operation in the past is the course of events passing under our eyes to-day. Hence, if we would understand the formation of the rocks in the ancient seas, we must go to the shores of the modern ones and see what is taking place there. One of the most noticeable characters of a shore is the line of flotsam that is left by the edge of the waves; here you may find all kinds of land plants mixed with the sea shells and general rubbish, plants that may have drifted far. Much of the dÉbris (outside towns) is brought down by the rivers, and may be carried some distance out to sea; then part becomes waterlogged and sinks, and part floats in to shore, perhaps to be carried out again, or to be buried under the coarse sand of the beach. When we examine sandstone rock, or the finer grained stones which are hardened mud, we find in them the remains of shells, sometimes of bones, and also of plant leaves and stems, which in their time had formed the flotsam of a shore. Indeed, one may say that nearly every rock which has not been formed in ancient volcanoes, or been altered by their heat, carries in it some trace of plant or animal. These remains are often very fragmentary and difficult to recognize, but sometimes they are wellnigh as perfect as dried specimens of living things. When they are recognizable Fig. 1.—The Face of a Quarry, showing layers or “beds” of different rock, a, b, and c. The top gravel and soil s has been disintegrated by the growing plants and atmosphere. If we would find such stones for ourselves, the quarries offer the best hunting ground, for there several layers of rock are exposed, and we can reach fresh surfaces which have not been decayed by rain and storm. Fig. 1 shows a diagram of a quarry, and illustrates the almost universal fact that the beds of rock when undisturbed lie parallel to each other. Rock a in the figure is fine-grained limestone, b black friable shale mixed with sand, and c purer shale. In such a series of rocks the best fossils will be found in the limestone; its harder and finer structure acting as a better preservative of organisms than the others. In limestone one finds both plant and animal fossils, very often mixed together as the flotsam on the shore is mixed. Many limestones split along parallel planes, and may break into quite It is, however, with the plant fossils that we must concern ourselves, and among them we find great variety of form. Some are more or less complete, and give an immediate idea of the size and appearance of the plant to which they had belonged; but such are rare. One of the best-known examples of this type is the base of a great tree trunk illustrated in the frontispiece. With such a fossil there is no shadow of doubt that it is part of a giant tree, and its spreading roots running so far horizontally along the ground suggest the picture of a large crown of branches. Most fossils, however, are much less illuminating, and it is usually only by the careful piecing together of fragments that we can obtain a mental picture of a fossil plant. A fossil such as that illustrated in the frontispiece—and on a smaller scale this type of preservation is one of the commonest—does not actually consist of the plant body itself. Although from the outside it looks as though it were a stem base covered with bark, the whole of the inner portion is composed of fine hard rock with no trace of woody tissue. In such specimens we have the shape, size, and form of the plant preserved, but none of its actual structure or cells. It is, in fact, a Cast. Fossil casts appear to have been formed by fine sand or mud silting round a submerged stump and enclosing it as completely as if it had been set in plaster of Paris; then the wood and soft tissue decayed and the hollow was filled up with more fine silt; gradually all the bark also decayed and the mud hardened into stone. Thus the stone mould round the outside of the plant enclosed a stone casting. When, after lying for ages undisturbed, these fossils are unearthed, they are so hard and “set” that the surrounding stone peels away from the inner part, just as a plaster cast comes away from an object and retains its shape. There are many varieties of casts among fossil plants. Sometimes on breaking Fig. 2.—A, Cast of the Surface showing the Shape of Leaf Bases of Sigillaria; B, the reverse of the impression left on the adjacent layer of rock. (Photo.) In other instances the cast may simply represent the internal hollows of the plant. This happens most commonly in the case of stems which contained soft pith cells which quickly decayed, or with naturally hollow stems like the Horse-tails (Equisetum) of to-day. Fine mud or sand silted into such hollows completely filling them up, and then, whether the rest of the plant were preserved or not, the shape of the inside of the Fig. 3.—Cast of the Leaf Bases of Lepidodendron, showing finely marked detail. (Photo.) Fig. 4.—“Sternbergia.” Internal cast of the stem of Cordaites. The true nature of this fossil was recognized when casts of the plan were found with some of the wood preserved outside the castings; and Fig. 5.—Leaf Impressions of “Fern” Sphenopteris on Shale. (Photo.) Another form of cast which is common in some rocks is that of seeds. As a rule these casts are not connected with any actually preserved tissue, but they show the external form, or the form of the stony part of the seed. Well-known seeds of this type are those of Trigonocarpon, which has three characteristic ridges down the stone. Sometimes in the fine sandstone in which they occur embedded, the internal cast lies embedded in the external cast, and between them there is a slight space, now empty, but which once contained the actual shell of the seed, now decayed. Casts have been of great service to the fossil botanists, for they often give clear indications of the external appearance of the parts they represent; particularly of stems, leaf scars, and large seeds. But all such fossils are very imperfect records of the past plants, for none of the actual plant tissues, no minute anatomy or cell structure, is preserved in that way. A type of fossil which often shows more detail, and which usually retains something of the actual tissues of the plant, is that known technically as the Impression. These fossils are the most attractive of all the many kinds we have scattered through the rocks, for they often show with marvellous perfection the most delicate and beautiful fern leaves, such as in fig. 5. Here the plant shows up as a black silhouette against the grey stone, and the very veins of the midrib and leaves are quite visible. Fig. 6 shows another fernlike leaf in an impression, not quite flat like that shown in fig. 5, but with a slight natural curvature of the leaves similar to what would have been their form in life. Though an impression, this specimen is not of the “pressed plant” type, it almost might be described as a bas-relief. Sometimes impressions of fern foliage are very large, and show highly branched and complex leaves like those of tree ferns, and they may cover large sheets of stone. They are particularly common in the fine shales above coal seams, and are best seen in the mines, for they are often too big to bring to the surface complete. In most impressions the black colour is due to a film of carbon which represents the partly decomposed tissues of the plant. Sometimes this film is cohesive enough to be detached from the stone without damage. Fig. 6.—Impression of Neuropteris Leaf, showing details of veins, the leaves in partial relief. (Photo.) Fig. 7.—Leaf Impression of Ginkgo, of which the film was strong enough to peel off complete In fig. 7, the impression is that of a Ginkgo leaf, and after treatment the cells of the epidermis were perfectly recognizable under the microscope, with the stomates (breathing pores) also well preserved. This is shown in fig. 8, where the outline of the cells was drawn from the microscope. In such specimens, however, it is only the outer skin which is preserved, the inner soft tissue, the vital anatomy of the plant, is crushed and carbonized. Leaves, stems, roots, even flowers (in the more recent rocks) and seeds may all be preserved as impressions; and very often those from the more recently formed rocks are so sharply defined and perfect that they seem to be actual dried leaves laid on the stone. Fig. 8.—Outline of the Cells from Specimen of Leaf shown in fig. 7 c, Ordinary cells; s, stomates; v, elongated cells above the vein. Much evidence has been accumulated that goes to show that the rocks which contain the best impressions were originally deposited under tranquil conditions in water. It might have been in a pool or quiet lake with overshadowing trees, or a landlocked inlet of the sea where silt quietly accumulated, and as the plant fragments fell or drifted into the spot they were covered by fine-grained mud without disturbance. In Impressions in the thin sheets of fine rock may be compared to dried specimens pressed between sheets of blotting paper; they are flattened, preserved from decay, and their detailed outline is retained. Fossils of this kind are most valuable, for they give a clear picture of the form of the foliage, and when, as sometimes happens, large masses of leaves, or branches with several leaves attached to them, are preserved together, it is possible to reconstruct the plant from them. It is chiefly from such impressions that the inspiration is drawn for those semi-imaginary pictures of the forests of long ago. From them also are drawn many facts of prime importance to scientists about the nature and appearance of plants, of which the internal anatomy is known from other specimens, and also about the connection of various parts with each other. Sometimes isolated impressions are found in clay balls or nodules. When the latter are split open they may show as a centre or nucleus a leaf or cone, round which the nodule has collected. In such cases the plant is often preserved without compression, and may show something of the minute details of organization. The preservation, however, is generally far from perfect when viewed from a microscopical standpoint. Fig. 9 shows one of these smooth, clayey nodules split open, and within it the cone which formed its centre, also split into two, and standing in high relief, with its scales showing clearly. Similar nodules or balls of clay are found to-day, forming in slowly running water, and it may be generally observed that they collect round some rubbish, shell, or plant fragment. These nodules are particularly well seen nowadays in the mouth of the Clyde, where they are formed with great rapidity. Fig. 9.—Clay Nodule split open, showing the two halves of the cone which was its centre. (Photo.) Another kind of preservation is that which coats over the whole plant surface with mineral matter, which hardens, and thus preserves the form of the plant. This process can be observed going on to-day in the neighbourhood of hot volcanic streams where the water is heavily charged with minerals. In most cases such fossils have proved of little importance to science, though there are some interesting specimens in the French museums which have not yet been fully examined. A noteworthy fossil of this type is the Chara, which, growing in masses together, has sometimes been preserved in this way in large quantities, indicating the existence of an ancient pond in the locality. There is quite a variety of other types of preservation Though such remains tell us of the existence of plants at the place they were found at a known period in the past, yet they tell very little about the actual structure of the plants themselves, and therefore very little that is of real use to the botanist. Fortunately, however, there are fossils which preserve every cell of the plant tissues, each one perfect, distended as in life, and yet replaced by stone so as to be hard and to allow of the preparation of thin sections which can be studied with the microscope. These are the vegetable fossils which are of prime importance to the botanist and the scientific enquirer into the evolution of plants. Such specimens are commonly known as Petrifactions. Sometimes small isolated stumps of wood or branches have been completely permeated by silica, which replaces the cell walls and completely preserves and hardens the tissues. This silicified wood is found in a number of different beds of rock, and may be seen washed out on the shore in Yorkshire, Sutherland, and other places where such rocks occur. When such a block is cut and polished the annual rings and all the fine structure or “grain” of the wood become as apparent as in recent wood. From these fossils, too, microscopic sections can be cut, and then the individual wood cells can be studied almost as well as those of living trees. A particularly notable example of fossil tree trunks is the Tertiary forest of the Yellowstone Park. Here the petrified Such specimens, however, are usually only isolated blocks of wood, often fragments from large stumps which show nothing but the rings of late-formed wood. It is impossible to connect them with the impressions of leaves or fruits in most cases, so that of the plants they represent we know only the anatomical structure of the secondary wood and nothing of the foliage or general appearance of the plant as a whole. Hence these specimens also give a very partial representation of the plants to which they belonged. Fortunately, however, there is still another type of preservation of fossils, a type more perfect than any of the others and sometimes combining the advantages of all of them. This is the special type of petrifaction which includes, not a single piece of wood, but a whole mass of vegetation consisting of fragments of stems, roots, leaves, and even seeds, sometimes all together. These petrifactions are those of masses of forest dÉbris which were lying as they dropped from the trees, or had drifted together as such fragments do. The plant tissues in such masses are preserved so that the most delicate soft tissue cells are perfect, and in many cases the sections are so distinct that one might well be deluded into the belief that it is a living plant at which one looks. Very important and well-known specimens have been found in France and described by the French palÆobotanists. As a rule these specimens are preserved in silica, and are found now in irregular masses of the nature of chert. Of still greater importance, however, owing partly to their greater abundance and partly to the quantity of scientific work that has been done on them, are the masses of stone found in the English coal seams and commonly called “coal balls”. The “coal balls” are best known from Lancashire Fig. 10.—Mass of Coal with many “coal balls” embedded in it a a, In surface view; b b, cut across. All washed with acid to make the coal balls show up against the black coal. (Photo by Lomax.) In external appearance the “coal balls” are slightly irregular roundish masses, most generally about the size of potatoes, and black on the outside from films of adhering coal. Their size varies greatly, and they have been found from that of peas up to masses with a diameter of a foot and a half. They lie embedded in the coal and are not very easily recognizable in it at first, because they are black also, but when washed with acid they turn greyish-white and then can be recognized clearly. Fig. 10 shows a block of coal with an exceptionally large number of the “coal balls” embedded in it. This figure illustrates their slightly irregular rounded form in a typical manner. By chemical analysis they are found to consist of a nearly pure mixture of the carbonates of lime and magnesia; The important mineral compounds, CaCO3 and MgCO3, are mixed in very different quantities, and even in coal balls lying quite close to each other there is often much dissimilarity in this respect. In whatever proportion these minerals are combined, it seems to make but little difference to their preservative power, and in good “coal balls” they may completely replace and petrify each individual cell of the plants in them. Fig. 11.—Photograph of Section across Stem of Sphenophyllum from a Lancashire “coal ball”, showing perfect preservation of woody tissue W, wood; c, cortex. Fig. 11 shows a section across the wood of a stem preserved in a “coal ball”, and illustrates a degree of perfection which is not uncommon. In the course of Fig. 12.—Photograph of Section through a Bud of Lepidodendron, showing many small leaves tightly packed round the axis. From a “coal ball” The particular value of the coal balls as records of past vegetation lies in the fact that they are petrifactions, not of individual plants alone, but of masses of plant dÉbris. Hence in one of these stony concretions may lie twigs with leaves attached, bits of stems with their fruits, and fine rootlets growing through the mass. A careful study and comparison of these fragments has led to the connection, piece by piece, of the various parts of many plants. Such a specimen as that Hitherto the only concretions of the nature of “coal balls” containing well preserved plant dÉbris, have been found in the coal or immediately above it, and are of PalÆozoic age (see p. 34). Recent exploration, however, has resulted in the discovery of similar concretions of Mesozoic age, from which much may be hoped in the future. Still, at present, it is to the palÆozoic specimens we must turn for nearly all valuable knowledge about ancient plants, and primarily to that form of preservation of the specimens known as structural petrifactions, of which the “coal balls” are both the commonest and the most perfect examples. |
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