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