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 as plant or animal remains they are commonly called “fossils”, and it is from their testimony that we must learn all we can know about the life of the past.

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 thin sheets on whose surfaces the flattened fossils show particularly well.

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 a rock it will split so as to show the perfect form of the surface of a stem, while its reverse is left on the stone as is shown in fig. 2. Had we only the reverse we should still have been able to see the form of the leaf bases by taking a wax impression from it; although there is nothing of the actual tissue of the plant in such a fossil. Sometimes casts of leaf bases show the detail preserved with wonderful sharpness, as in fig. 3. This is an illustration of the leaf scars of Lepidodendron, which often form particularly good casts.

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 stem remains as a solid stone. Where this has happened, and the outer part of the plant has decayed so as to leave no trace, the solid plug of stone from the centre may look very much like an actual stem itself, as it is cylindrical and may have surface markings like those on the outsides of stems. Some of the casts of this type were for long a puzzle to the older fossil botanists, particularly that illustrated in fig. 4, where the whole looks like a pile of discs.

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 it was then known that the plant had a hollow pith, with transverse bands of tissue across it at intervals which caused the curious constrictions in the cast.

Fig. 5.—Leaf Impressions of “Fern” Sphenopteris on Shale.