[Illustration:  FIG. 27.—­Fenestella retipora.  Mountain limestone.]

[Illustration:  FIG. 28.—­Goniatites.  Mountain limestone.]

We have evidence of the existence in the forests of a variety of centipede, specimens having been found in the erect stump of a hollow tree, although the fossil is an extremely rare one.  The same may be said of the only two species of land-snail which have been found connected with the coal forests, viz., pupa vetusta and zonites priscus, both discovered in the cliffs of Nova Scotia.  These are sufficient to demonstrate that the fauna of the period had already reached a high stage of development.  In the estuaries of the day, masses of a species of freshwater mussel (anthracosia) were in existence, and these have left their remains in the shape of extensive beds of shells.  They are familiar to the miner as mussel-binds, and are as noticeable a feature of this long ago period, as are the aggregations of mussels on every coast at the present day.

[Illustration:  FIG. 29.—­Aviculopecten papyraceus.  Coal-shale.]

CHAPTER III.

VARIOUS FORMS OF COAL AND CARBON.

In considering the various forms and combinations into which coal enters, it is necessary that we should obtain a clear conception of what the substance called “carbon” is, and its nature and properties generally, since this it is which forms such a large percentage of all kinds of coal, and which indeed forms the actual basis of it.  In the shape of coke, of course, we have a fairly pure form of carbon, and this being produced, as we shall see presently, by the driving off of the volatile or vaporous constituents of coal, we are able to perceive by the residue how great a proportion of coal consists of carbon.  In fact, the two have almost an identical meaning in the popular mind, and the fact that the great masses of strata, in which are contained our principal and most valuable seams of coal, are termed “carboniferous,” from the Latin carbo, coal, and fero, I bear, tends to perpetuate the existence of the idea.

There is always a certain, though slight, quantity of carbon in the air, and this remains fairly constant in the open country.  Small though it may be in proportion to the quantity of pure air in which it is found, it is yet sufficient to provide the carbon which is necessary to the growth of vegetable life.  Just as some of the animals known popularly as the zoophytes, which are attached during life to rocks beneath the sea, are fed by means of currents of water which bring their food to them, so the leaves, which inhale carbon-food during the day through their under-surfaces, are provided with it by means of the currents of air which are always circulating around them; and while the fuel is being taken in beneath, the heat and light are being received from above, and the sun supplies the motive power to digestion.