So abundant and varied are the sedimentary formations of Carboniferous time, and so fully have they preserved remains of the contemporary plants and animals, that it is not difficult to realise in some measure the general aspect of the scenery of the time, and the succession of changes which it underwent from the beginning to the end of the period. The land was green with a luxuriant if somewhat monotonous vegetation. Large pine trees flourished on the drier uplands. The lower grounds nourished dense groves of cycads or plants allied to them, which rose as slim trees twenty or thirty feet high, with long hard green leaves and catkins that grew into berries. The swamps and wetter lands bore a rank growth of various gigantic kinds of club-moss, equisetaceous reeds and ferns.
Nor was the hum of insect-life absent from these forests. Ancestral types of cockroaches, mayflies and beetles lived there. Scorpions swarmed along the margins of the shallow waters, for their remains, washed away with the decayed vegetation among which they harboured, are now found in abundance throughout many of the dark shales.
The waters were haunted by numerous kinds of fish quite distinct from those of the Old Red Sandstone. In the lagoons, shoals of small ganoids lived on the cyprids that peopled the bottom, and they were in turn preyed on by larger ganoids with massive armature of bone. Now and then a shark from the opener sea would find its way into these more inland waters. The highest types of animal life yet known to have existed at this time were various amphibians of the extinct order of Labyrinthodonts.
The open sea, too, teemed with life. Wide tracts of its floor supported a thick growth of crinoids whose jointed stems, piled over each other generation after generation, grew into masses of limestone many hundreds of feet in thickness. Corals of various kinds lived singly and in colonies, here and there even growing into reefs. Foraminifera, sponges, sea-urchins, brachiopods, gasteropods, lamellibranchs and cephalopods, in many genera and species, mingled their remains with the dead crinoids and corals to furnish materials for the wide and thick accumulation of Carboniferous Limestone.
Looking broadly at the history of the Carboniferous period, and bearing in mind the evidence of prolonged depression already referred to, we can recognize in it three great eras. During the first, the wide clear sea of the Carboniferous Limestone spread over the centre and south of Britain, interrupted here and there by islands that rose from long ridges whereby the sea-floor was divided into separate basins. Next came a time of lessened depression, when the sea-bottom was overspread with sand, mud and gravel, and was even in part silted up, as has been chronicled in the Millstone Grit. The third stage brings before us the jungles of the Coal-measures, when the former sea-floor became a series of shallow lagoons where, as in the mangrove-swamps of our own time, a terrestrial vegetation sprang up and mingled its remains with those of marine shells and fishes.
Such a state of balance among the geological forces as is indicated by the stratigraphy of the Carboniferous system would not prepare us for the discovery of the relics of any serious display of contemporary volcanic activity. And, indeed, throughout the Carboniferous rocks of Western Europe there is for the most part little trace of contemporaneous volcanic eruptions. Yet striking evidence exists that, along the western borders of the continental area, in France as well as over much of Britain, which had for so many previous geological ages been the theatre of subterranean activity, the older half of Carboniferous time witnessed an abundant, though less stupendous and prolonged, renewal of volcanic energy.
From the very commencement of the Carboniferous period to the epoch when the Coal-measures began to be accumulated, the area of the British Isles continued to be a scene of active volcanism. In the course of that prolonged interval of geological time the vents shifted their positions, and gradually grew less energetic, but there does not appear to have been any protracted section of the interval when the subterranean activity became everywhere entirely quiescent.
The geologist who traces, from older to younger formations, the progress of some persistent operation of nature, observes the evidence gradually to increase in amount and clearness as it is furnished by successively later parts of the record. He finds that the older rocks have generally been so dislocated and folded, and are often so widely covered by younger formations, that the evidence which they no doubt actually contain may be difficult to decipher, or may be altogether concealed from view. In following, for instance, the progress of volcanic action, he is impressed, as he passes from the older to the younger Palæozoic chronicles, by the striking contrast between the fulness and legibility of the Carboniferous records and the comparative meagreness and obscurity of those of the earlier periods. The Carboniferous rocks have undergone far less disturbance than the Cambrian and Silurian formations; while over wide tracts, where their volcanic chapters are fullest and most interesting, they lie at the surface, and can thus be subjected to the closest scrutiny. Hence the remains of the volcanic phenomena of the later Palæozoic periods present a curiously modern aspect, when contrasted with the fragmentary and antique look of those of older date.
The history of volcanic action during the Carboniferous period in Britain is almost wholly comprised in the records of the earlier half of that period, that is, during the long interval represented by the Carboniferous Limestone series and the Millstone Grit. It was chiefly in the northern part of the region that volcanic activity manifested itself. In Scotland there is the chronicle of a long succession of eruptions across the district of the central and southern counties, from the very beginning of Carboniferous time down to the epoch when the Coal-measures began to be accumulated. In England, on the other hand, the traces of Carboniferous volcanoes are confined within a limited range in the Carboniferous Limestone, while in Ireland they appear to be likewise restricted to the same lower division of the system. During the whole of the vast interval represented by the Coal-measures volcanic energy, so far as at present known, was entirely dormant over the region of the British Isles.
These general statements will be more clearly grasped from the accompanying table, which shows the various sections into which the Carboniferous system of Britain has been divided, and also, by black vertical lines, the range of volcanic intercalations in each of the three kingdoms.