(flowerless) and the phaenogams (flowering), one leading from the lycopodes by the sigillariae, another leading by the cordaites, and the third leading from the equisetums by the calamites. Still further back the characters, afterwards separated in the club-mosses, mare's-tails, and ferns, were united in the rhizocarps, or, as some prefer to call them, the heterosporous filicinae."

In concluding this chapter dealing with the various kinds of plants which have been discovered as contributing to the formation of coal-measures, it would be as well to say a word or two concerning the climate which must have been necessary to permit of the growth of such an abundance of vegetation. It is at once admitted by all botanists that a moist, humid, and warm atmosphere was necessary to account for the existence of such an abundance of ferns. The gorgeous waving tree-ferns which were doubtless an important feature of the landscape, would have required a moist heat such as does not now exist in this country, although not necessarily a tropical heat. The magnificent giant lycopodiums cast into the shade all our living members of that class, the largest of which perhaps are those that flourish in New Zealand. In New Zealand, too, are found many species of ferns, both those which are arborescent and those which are of more humble stature. Add to these the numerous conifers which are there found, and we shall find that a forest in that country may represent to a certain extent the appearance presented by a forest of carboniferous vegetation. The ferns, lycopods, and pines, however, which appear there, it is but fair to add, are mixed with other types allied to more recent forms of vegetation.

There are many reasons for believing that the amount of carbonic acid gas then existing in the atmosphere was larger than the quantity which we now find, and Professor Tyndall has shown that the effect of this would be to prevent radiation of heat from the earth. The resulting forms of vegetation would be such as would be comparable with those which are now reared in the green-house or conservatory in these latitudes. The gas would, in fact, act as a glass roof, extending over the whole world.

CHAPTER II.

A GENERAL VIEW OF THE COAL-BEARING STRATA.

In considering the source whence coal is derived, we must be careful to remember that coal itself is but a minor portion of the whole formation in which it occurs. The presence of coal has indeed given the name to the formation, the word "carboniferous" meaning "coal-bearing," but in taking a comprehensive view of the position which it occupies in the bowels of the earth, it will be necessary to take into consideration the strata in which it is found, and the conditions, so far as are known, under which these were deposited.

Geologically speaking, the Carboniferous formation occurs near the close of that group of systems which have been classed as "palaeozoic," younger in point of age than the well known Devonian and Old Red Sandstone strata, but older by far than the Oolites, the Wealden, or the Cretaceous strata.

In South Wales the coal-bearing strata have been estimated at between 11,000 and 12,000 feet, yet amongst this enormous thickness of strata, the whole of the various coal-seams, if taken together, probably does not amount to more than 120 feet. This great disproportion between the total thickness and the thickness of coal itself shows itself in every coal-field that has been worked, and when a single seam of coal is discovered attaining a thickness of 9 or 10 feet, it is so unusual a thing in Great Britain as to cause it to be known as the "nine" or "ten-foot seam," as the case may be. Although abroad many seams are found which are of greater thicknesses, yet similarly the other portions of the formation are proportionately greater.

It is not possible therefore to realise completely the significance of the coal-beds themselves unless there is also a knowledge of the remaining constituents of the whole formation. The strata found in the various coal-fields differ considerably amongst themselves in character. There are, however, certain well-defined characteristics which find representation in most of the principal coal-fields, whether British or European. Professor Hull classifies these carboniferous beds as follows:—

  UPPER CARBONIFEROUS.
    Upper coal-measures.
      Reddish and purple sandstones, red and grey clays and shales,
        thin bands of coal, ironstone and limestone, with spirorbis
        and fish.

    Middle coal-measures.
      Yellow and gray sandstones, blue and black clays and shales,
        bands of coal and ironstone, fossil plants, bivalves
        and fish, occasional marine bands.

  MIDDLE CARBONIFEROUS.
    Gannister beds or Lower coal-measures.
      Millstone grit. Flagstone series in Ireland.
      Yoredale beds. Upper shale series of Ireland.

  LOWER CARBONIFEROUS.
    Mountain limestone.
      Limestone shale.

Each of the three principal divisions has its representative in Scotland, Belgium, and Ireland, but, unfortunately for the last-named country, the whole of the upper coal-measures are there absent. It is from these measures that almost all our commercial coals are obtained.

This list of beds might be further curtailed for all practical purposes of the geologist, and the three great divisions of the system would thus stand:—

Upper Carboniferous, or Coal-measures proper.

Millstone grit.

Lower Carboniferous, or Mountain limestone.

In short, the formation consists of masses of sandstone, shale, limestone and coal, these also enclosing clays and ironstones, and, in the limestone, marbles and veins of the ores of lead, zinc, and antimony, and occasionally silver.

[Illustration: FIG. 18.—Sigillarian trunks in current-bedded sandstone.
St Etienne.]

As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these were deposited in the waters of the carboniferous ages, and this we can best do by considering the laws under which strata of a similar nature are now being deposited as sedimentary beds.

A great proportion consists of sandstone. Now sandstone is the result of sand which has been deposited in large quantities, having become indurated or hardened by various processes brought to bear upon it. It is necessary, therefore, first to ascertain whence came the sand, and whether there are any peculiarities in its method of deposition which will explain its stratification. It will be noticed at once that it bears a considerable amount of evidence of what is called "current-bedding," that is to say, that the strata, instead of being regularly deposited, exhibit series of wedge-shaped masses, which are constantly thinning out.

Sand and quartz are of the same chemical composition, and in all probability the sand of which every sandstone in existence is composed, appeared on this earth in its first solid form in the shape of quartz. Now quartz is a comparatively heavy mineral, so also, therefore, will sand be. It is also very hard, and in these two respects it differs entirely from another product of sedimentary deposition, namely, mud or clay, with which we shall have presently to deal when coming to the shales. Since quartz is a hard mineral it necessarily follows that it will suffer, without being greatly