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 the succeeding chapters constant reference will be made to tissues preserved in “coal balls”, and it may be noticed that not only the relatively hard woody cells are preserved but the very softest and youngest tissues also appear equally unharmed by their long sojourn in the rocks.
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 figured in [fig. 12] shows how the soft tissues of young leaves are preserved, and how their relation to each other and to the axis is indicated.
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.
CHAPTER III
COAL, THE MOST IMPORTANT OF PLANT REMAINS
Some of the many forms which are taken by fossil plants were shortly described in the last chapter, but the most important of all, namely coal, must now be considered. Of the fossils hitherto mentioned many are difficult to recognize without examining them very closely, and one might say that all have but little influence on human life, for they are of little practical or commercial use, and their scientific value is not yet very widely known. Of all fossil plants, the great exception is coal. Its commercial importance all over the world needs no illustration, and its appearance needs no description for it is in use in nearly every household. Quite apart from its economic importance, coal has a unique place among fossils in the eyes of the scientist, and is of special interest to the palæontologist.
In England nearly all the coal lies in rocks of a great age, belonging to a period very remote in the world’s history. The rocks bearing the coal contain other fossils, principally those of marine animals, which are characteristic of them and of the period during which they were formed, which is generally known as the “Coal Measure period”. There is geological proof that at one time the coal seams were much more widely spread over England than they are at present; they have been broken up and destroyed in the course of ages, by the natural movements among the rocks and by the many changes and processes of disintegration and decay which have gone on ever since they were deposited. To-day there are but relatively small coal-bearing areas, which have been preserved in the hollows of the synclines.[2]