6. A review of the geological history of Britain cannot but impress the geologist with a conviction of the essential uniformity of volcanism in its manifestations since the early beginnings of geological time. The composition and structure of the materials erupted from the interior have remained with but little change. The manner in which these materials have been discharged has likewise persisted from the remotest periods. The three modern types of Vesuvian cones, puys and fissure-eruptions can be seen to have played their parts in the past as they do to-day.
Among the earliest igneous masses of which the relative geological date can be fixed are the dykes which form so striking a system among the Archæan rocks of the north-west, and show how far back the modern type of volcanic fissures and dykes can be traced. No relic, indeed, has survived of any lavas that may have flowed out from these ancient fissures, but so far as regards underground structure, the type is essentially the same as that of the Tertiary and modern Icelandic lava-fields.
The early Palæozoic volcanoes formed cones of lava and tuff comparable to those of such vents as Vesuvius and Etna. In the Lake District the pile of material ejected during Lower Silurian time was at least 8000 or 9000 feet thick. In the Old Red Sandstone basins of Central Scotland there were more than one mass of lavas and tuffs thicker than those of Vesuvius.
The puys of the later half of Palæozoic time closely resembled their Tertiary successors in Central France, the Eifel, and the Phlegræan Fields.
Nor, as regards extent and vigour, did the eruptions of the geological past differ in any important respect from those of the present time. There is assuredly no evidence that volcanic energy has gradually waned since the dawn of geological history. The latest eruptions of North-Western Europe, forming the Tertiary basalt-plateaux, far exceeded in area, and possibly also in bulk of material discharged, all the eruptions that had preceded them in the geological record.
7. Nevertheless, while the Tertiary eruptions showed no diminution of vigour, it is undoubtedly true that the volcanic energy has not manifested itself in a uniform way since the beginning of geological time. There have been periods of maximum activity followed by others of lessened force. Thus if we take a broad view of the general features of volcanic action during the Palæozoic ages in Britain, we see clear evidence of a gradual diminution in its vigour. The widespread outpourings of lava and tuff in the Silurian period in England, Wales, Scotland and Ireland were succeeded by the somewhat diminished, though still important, eruptions of the Lower Old Red Sandstone basins. The latter were followed by the still lessened outflows of the Carboniferous plateaux, which in turn were succeeded by the yet feebler and more localized eruptions of the Carboniferous puys, the whole prolonged volcanic succession ending in the small scattered vents of the Permian period. There were of course oscillations of relative energy during this history, some of the maxima and minima being of considerable moment. But though progress towards extinction was not regular and uniform, it was a dominant feature of the phenomena.
8. The Permian volcanoes were the last of the long Palæozoic series, and, so far as we yet know, the whole of the Mesozoic periods within the area of Britain were absolutely unbroken by a single volcanic eruption. The chronological value of this enormous interval of quiescence may, perhaps, never be ascertainable, but the interval must assuredly cover a large part of geological time. It was an era of geological calm, during which the Triassic, Jurassic and Cretaceous formations were slowly accumulated over the larger part of Europe. The stratigraphical quietude was not indeed unbroken. The widespread subsidence of the sea-bottom was interrupted here and there by important upheavals, and considerable geographical changes were in process of time accomplished. But, save in one or two widely separated areas of Europe, there were no active volcanoes over the whole continent.[439] Here again the scarcity or absence of intercalated volcanic rocks is in harmony with the general stratigraphy of the formations.
[439] The Triassic eruptions of Predazzo and Monzoni were important, and traces of others are said to occur in the Cretaceous system in Portugal and Silesia.
9. After the prodigious interval represented by the whole of the Mesozoic and the earlier part of the Tertiary formations, a time of disturbance arose once more, and the great basalt-floods of the north-west were poured forth. Evidence has been adduced in the foregoing chapters that this latest volcanic period was one of vast duration; that it was marked by long intervals of quiescence, and by repeated renewals of volcanic energy. Yet over the area of Britain the whole of its manifestations were probably comprised within the earlier (Oligocene and perhaps early Miocene) part of older Tertiary time. Since its eruptions ceased, another interval of profound quiescence has succeeded, which still continues. But this interval is almost certainly of less duration than that which elapsed between the Palæozoic and Tertiary outbursts. In other words, remote as the date of these Tertiary volcanoes appears to be from our own day, it comes much nearer to us than did the era of the last Permian eruptions to the earliest of the Tertiary series.
10. By the dissection which prolonged denudation has effected among the old volcanic centres of Britain, materials are supplied for studying the sequence of events from the beginning to the end of a volcanic period. These events have generally followed the same tolerably well-defined order.