A somewhat different crystallisation phenomenon is illustrated by the extremely fine section of a Lepidodendroid plant shown in fig. 15. The tissues of the primary and secondary wood (x1 and x2) are well preserved throughout in silica, but scattered through the siliceous matrix there occur numerous circular patches, as seen in the figure. One of these is more clearly shown in fig. 14 B drawn from a longitudinal section through the secondary wood, x2; it will be noticed that where the concentric lines of the circular patch occur, the scalariform thickenings of the tracheids are sharply defined, but immediately a tracheid is free of the patch these details are lost. It would appear that in this case silicification was first completed round definite isolated centres, and the secondary crystallisation in the matrix partially obliterated some of the more delicate structural features. The same phenomenon has been observed in oolitic rocks[117], in which the oolitic grains have resisted secondary crystallisation and so retained their original structure.
Among the most important examples of silicified plants are those from a few localities in Central France. In the neighbourhood of Autun there used to be found in abundance loose nodules of siliceous rock containing numerous fragments of seeds, twigs, and leaves of different plants. The rock of which the broken portions are found on the surface of the ground was formed about the close of the Carboniferous period.
At the hands of French investigators the microscopic examination of these fragments of a Palaeozoic vegetation have thrown a flood of light on the anatomical structure of many extinct types. Sometimes the silica has penetrated the cavities of the cells and vessels, and the walls have decayed without their substance being replaced by mineral material. Sections of tissues preserved in this manner, if soaked in a coloured solution assume an appearance almost identical with that of stained sections of recent plants. The spaces left by the decayed walls act as fine capillaries and suck up the coloured solution[118].
Fig. 16. Internal cast of a sclerenchymatous cell from the root of a Cretaceous fern (Rhizodendron oppoliense Göpp.). After Stenzel (86) Pl. III. fig. 29. × 240 and reduced to one-half.
In the Coal-Measure sandstones of England large pieces of woody stems are occasionally met with in which the mineralisation has been incomplete. A brown piece of fossil stem lying in a bed of sandstone shows on the surface a distinct woody texture, and the lines of wood elements are clearly visible. The whole is, however, very friable and falls to pieces if an attempt is made to cut thin sections of it; the tracheids of the wood easily fall apart owing to the walls being imperfectly preserved, and the absence of a connecting framework such as would have been formed had the membranes been thoroughly silicified. It is occasionally possible to obtain from petrified plant stems perfect casts in silica or other substances of the cavity of a sclerenchymatous fibre, in which the mineral has been deposited not only in the cavity but in the fine pit-canals traversing the lignified walls. Such a cast is represented in fig. 16, the fine lateral projections are the delicate casts of the pit canals. Numerous instances of minute and delicate tissues preserved in silica are recorded in later chapters. A somewhat unusual type of silicification is met with in some of the Gondwana rocks of India, in which cycadean fronds occur as white porcellaneous specimens showing a certain amount of internal structure in a siliceous matrix. Specimens of such leaves may be seen in the British Museum.
COAL-BALLS.
In the Coal-Measures of England, especially in the neighbourhood of Halifax in Yorkshire, and in South Lancashire, the seams of coal occasionally contain calcareous nodules varying in size from a nut to a man’s head, and consisting of about 70% of carbonate of calcium and magnesium, and 30% of oxide of iron, sulphide of iron, &c.[119] The nodules, often spoken of by English writers as ‘coal-balls,’ contain numerous fragments of plants in which the minute cellular structure is preserved with remarkable perfection. It should be noted that the term coal-ball is also applied to rounded or subangular pieces of coal which are occasionally met with in coal seams, and especially in certain French coal fields. To avoid confusion it is better to speak of the plant-containing nodules as calcareous nodules, restricting the term coal-ball to true coal pebbles. A section of a calcareous nodule, when seen under the microscope, presents the appearance of a matrix of a crystalline calcareous substance containing a heterogeneous mixture of all kinds of plant tissues, usually in the form of broken pieces and in a confused mass.
Fig. 17. A thin section of a calcareous nodule from the Coal-Measures. Binney collection, Woodwardian Museum, Cambridge. Very slightly reduced.