namely, of an indefinite alternation of beds of sandstone, shale, and coal, sometimes with bands of clay-ironstone or beds of limestone, repeated in no constant order, but sometimes attaining the enormous aggregate thickness of 14,000 feet, or little short of 3 miles.  The beds of coal differ in number and thickness in different areas, but they seldom or never exceed one-fiftieth part of the total bulk of the formation in thickness.  The characters of the coal itself, and the way in which the coal-beds were deposited, will be briefly alluded to in speaking of the vegetable life of the period.  In Britain, and in the Old World generally, the Coal-measures are composed partly of genuine terrestrial deposits—­such as the coal—­and partly of sediments accumulated in the fresh or brackish waters of vast lagoons, estuaries, and marshes.  The fossils of the Coal-measures in these regions are therefore necessarily the remains either of terrestrial plants and animals, or of such forms of life as inhabit fresh or brackish waters, the occurrence of strata with marine fossils being quite a local and occasional phenomenon.  In various parts of North America, on the other hand, the Coal-measures, in addition to sandstones, shales, coal-seams, and bands of clay-ironstone, commonly include beds of limestone, charged with marine remains, and indicating marine conditions.  The subjoined section (fig. 107) gives, in a generalised form, the succession of the Carboniferous strata in such a British area as the north of England, where the series is developed in a typical form.

As regards the life of the Carboniferous period, we naturally find, as has been previously noticed, great differences in different parts of the entire series, corresponding to the different mode of origin of the beds.  Speaking generally, the Lower Carboniferous (or the Sub-Carboniferous) is characterised by the remains of marine animals; whilst the Upper Carboniferous (or Coal-measures) is characterised by the remains of plants and terrestrial animals.  In all those cases, however, in which marine beds are found in the series of the Coal-measures, as is common in America, then we find that the fossils agree in their general characters with those of the older marine deposits of the period.

[Illustration:  Fig. 107.  GENERALIZED SECTION OF THE CARBONIFEROUS STRATA OF THE NORTH OF ENGLAND.]

Owing to the fact that coal is simply compressed and otherwise altered vegetable matter, and that it is of the highest economic value to man, the Coal-measures have been more thoroughly explored than any other group of strata of equivalent thickness in the entire geological series.  Hence we have already a very extensive acquaintance with the plants of the Carboniferous period; and our knowledge on this subject is daily undergoing increase.  It is not to be supposed, however, that the remains of plants are found solely in Coal-measures; for though most abundant towards the summit, they are found in less numbers in all parts of the series.  Wherever found, they belong to the same great types of vegetation; but, before reviewing these, a few words must be said as to the origin and mode of formation of coal.