The important mineral compounds, CaCO_3 and MgCO_3, 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. [Illustration: 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. [Illustration: 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