The nature and manner of formation of the various forms of coal should be dealt with in a later chapter devoted to the subject of plants as rock-builders, but in view of the recent statements as to the algal nature of these bituminous deposits it may not be out of place to state briefly the main conclusions of the French authors.
MM. Renault and Bertrand regard each of the yellow bodies in the European and Australian Boghead as the thallus of an alga. To the form which is most abundant in the Kerosene shale they have given the generic name of Reinschia, while that in the Scotch and French Boghead is named Pila.
Reinschia. Fig. 36, 3.
A section of a piece of Kerosene shale at right angles to the bedding appears to be made up of fairly regular layers of flattened elliptical sacs of an orange or yellow colour. Each sac or thallus is about 300µ in length and 150µ broad (fig. 36, 3). A single row of cells constitutes the wall surrounding the central globular cavity. The cells are more or less pyriform in shape, and the cell-cavities are filled with a dark substance, described by Renault and Bertrand as protoplasm, and the cell-walls are fairly thick. In some of the larger specimens there are often found a few smaller sacs enclosed in the cavity of the partially disorganised mother-thallus. In the larger specimens the wall is usually invaginated in several places, giving the whole thallus a lobed or brain-like appearance. The supposed alga, which makes up ⁹⁄₁₀ths of the contents of a block of Kerosene shale, is named Reinschia Australis; it is regarded by the authors of the species as nearly related to the Hydrodictyaceae or Volvocineae.
Fig. 36, 1. Section of a piece of Scotch Torbanite. Slightly enlarged. 2. Pila bibractensis from the Autun Boghead, × 282 (after Bertrand). 3. Reinschia Australis, from the Kerosene shale of New South Wales, × 592 (after Bertrand).
In the Kerosene shale from certain localities in New South Wales Bertrand recognises a second form of thallus, which he refers to the genus Pila, characteristic of the European Bogheads.
Pila. Fig. 36, 2.
The “thallus” characteristic of the Scotch Boghead has been named Pila scotica, and that of the Autun Boghead, Pila bibractensis.
In the latter form, which has been studied in more detail by MM. Renault and Bertrand, the thallus consists of about 6–700 cells, and is irregularly ellipsoidal in form, from ·189–·225mm. in length, and ·136–·160mm. broad. The surface-cells are radially disposed and pyramidal in shape, the internal cells are polygonal in outline and less regularly arranged (fig. 36, 2). The Pila thalli make up ¾ths of the mass in an average sample of the Autun Boghead. The Autun Boghead often contains siliceous nodules, and sections of these occasionally include cells of a Pila in which the protoplasmic contents and nuclei have been described by the French authors. The evidence for the existence of these supposed nuclei is, however, not entirely satisfactory; sections of silicified thalli which were shown to me by Prof. Bertrand did not satisfy me as to the minute histological details recognised by Bertrand and Renault.