and marshy flats upon which the coal-plants grew were liable to constantly-recurring oscillations of level, the successive land-surfaces represented by the successive coal-beds of any coal-field being thus successively buried beneath accumulations of mud or sand.  We have no need, however, to suppose that these oscillations affected large areas at the same time; and geology teaches us that local elevations and depressions of the land have been matters of constant occurrence throughout the whole of past time.

All the varieties of coal (bituminous coal, anthracite; cannel-coal, &c.) show a more or less distinct “lamination”—­that is to say, they are more or less obviously composed of successive thin layers, differing slightly in colour and texture.  All the varieties of coal, also, consist chemically of carbon, with varying proportions of certain gaseous constituents and a small amount of incombustible mineral or “ash.”  By cutting thin and transparent slices of coal, we are further enabled, by means of the microscope, to ascertain precisely not only that the carbon of the coal is derived from vegetables, but also, in many cases, what kinds of plants, and what parts of these, enter into the formation of coal.  When examined in this way, all coals are found to consist more or less entirely of vegetable matter; but there is considerable difference in different coals as to the exact nature of this.  By Professor Huxley it has been shown that many of the English coals consist largely of accumulations of rounded discoidal sacs or bags, which are unquestionably the seed-vessels or “spore-cases” of certain of the commoner coal-plants (such as the Lepidodendra).  The best bituminous coals seem to be most largely composed of these spore-cases; whilst inferior kinds possess a progressively increasing amount of the dull carbonaceous substance which is known as “mineral charcoal,” and which is undoubtedly composed of “the stems and leaves of plants reduced to little more than their carbon.”  On the other hand, Principal Dawson finds that the American coals only occasionally exhibit spore-cases to any extent, but consist principally of the cells, vessels, and fibres of the bark, integumentary coverings, and woody portions of the Carboniferous plants.

The number of plants already known to have existed during the Carboniferous period is so great, that nothing more can be done here than to notice briefly the typical and characteristic groups of these—­such as the Ferns, the Calamites, the Lepidodendroids, the Sigillarioids, and the Conifers.

[Illustration:  Fig. 108.—­Odontopteris Schlotheimii.  Carboniferous, Europe and North America.]

[Illustration:  Fig. 109.—­Calamites cannoeformis.  Carboniferous Rocks, Europe and North America.]