In the case of fissure-eruptions, rents formed in the crust of the earth and communicating with the surface have allowed lava to rise and flow out above ground, either from the lips of the fissures or from vents opened along the lines of chasm. The thousands of parallel dykes in Britain remain as evidence of this mode of the ascent of the molten magma. Lines of large cones of the Vesuvian type may be presumed to have risen along guiding fissures in the terrestrial crust.
But it is evident from a study of the British examples that the existence of a fissure in the visible part of the crust is not always necessary for the production of a volcanic vent. In hundreds of instances, communication from the internal magma to the surface was effected by successive explosions, which finally blew out an orifice at the surface with no visible relation to any fissures or dykes. Of course, beneath the formations that now form the surface, and through which the necks rise, there may be lines of fault or weakness in older rocks which we cannot see. But, in what can be actually examined, vents have commonly been drilled through rocks independently of faults.
The discharge of explosive vapours was sometimes the first and only effort of volcanic energy. Generally, however, fragmentary volcanic materials were ejected, or, if the eruption was more vigorous, lava was poured out. In a vast number of cases, especially in the later ages of Palæozoic time, only ashes were projected, and cones of tuff were formed. In the earlier ages, on the other hand, there was a much larger proportion of lava expelled. Towards the close of a volcanic period, the vents were gradually choked up with the fragmentary materials that were ejected from and fell back into them. Occasionally, during the process of extinction, an explosion might still occur and clear the chimney, so as to allow of the uprise of a column of molten rock which solidified there; or the sides of the crater, as well as of the cavernous funnel underneath, fell in and filled up the passage. Heated vapours sometimes continued to ascend through the debris in the vent, and to produce on it a marked metamorphism.
There seems to have been commonly a contraction and subsidence of the materials in the vents, with a consequent dragging down or sagging of the rocks immediately outside, which are thus made to plunge steeply towards the necks.
When the vents were plugged up by the consolidation of fragmentary matter or the uprise of lava in them, the final efforts of the volcanoes led to the intrusion of sills and dykes, not only into the rocks beneath the volcanic sheets, but also, in many instances, into at least the older parts of the sheets themselves. These subterranean manifestations of volcanic action may be recognized in almost every district. They vary greatly in the degree to which they are developed. Sometimes, as in the Cader Idris, Arenig and Snowdon regions, they attain considerable importance, alike as regards the number and thickness of the sheets. In other cases, they are exhibited on so small a scale that they might be overlooked, as in the tract of Carboniferous puy-eruptions in the north of Ayrshire. But they are so generally present as to form a remarkably characteristic feature of the volcanic activity of each geological period from the earliest time to the latest. The basic sheets in the Dalradian series of Scotland display early and colossal examples. All through the successive eruptive periods of Palæozoic time, sills are found as accompaniments of superficial ejections.
The Tertiary basalt-plateaux supply numerous and gigantic examples of intruded sheets. Tertiary cones of Vesuvian type are not found in Britain, but where on the continent they have been sufficiently laid open by denudation, they present sometimes an astonishing series of sills. As a striking illustration of this structure reference may be made to the sheets of trachyte that have been injected between and have marmorized the Cretaceous strata on which Monte Venda stands, among the Euganean Hills.[440]
[440] G. vom Rath, Zeitsch. Deutsch. Geol. Gesellsch., xvi. (1864), p. 461. E. Suess, Sitzungsber. k. Akad. Wien., lxxi. (1875), p. 7; Antlitz der Erde, vol. i. p. 193. E. Reyer, Die Euganeen, 1877. This volcano is further referred to, postea, p. 477.
It is obvious that the time of intrusion of the sills cannot be precisely determined. They were not likely to be injected at an epoch when the volcanic magma could find ready egress to the surface. That they did not arise before such egress was obtained may be inferred from their petrographical characters, which are usually those of the later and not of the earlier outflows of the magma; and from the fact that they not only lie among the rocks below the volcanic series, but intersect the lower parts of that series, sometimes even the higher parts. We may therefore, with every probability, regard the sills as among the closing phases of a volcanic period.
As the lavas and tuffs of each volcanic period are intercalated among the successive geological formations, a definite beginning and end to the period are stratigraphically fixed. We see exactly where in the sedimentary series the first showers of ashes fell, and where the last mingled with the ordinary sand and mud of the sea-door. The same record shows that the volcanic accumulations were finally washed down, that they subsided with the rest of the ground around them, and that usually they were buried under overlying conformable sedimentary deposits. Thus cones of ashes and lava which may have been several thousand feet high completely disappeared.
10. A consideration of the distribution of the volcanic rocks in time shows not only how singularly uniform the course of volcanic activity has been, but that there is no evidence of the cessation of any of the broader petrographical types during geological history. Quite as much variety may be observed among the erupted materials of Tertiary time in Britain as among those of the early ages, when the earth was younger and its volcanic vigour might be supposed to have been greater and more varied than it is now. The table on the following page will make these features at once apparent. From this table it will be seen that while some of the acid rocks have not always been extruded, the basic masses have played their part in every volcanic period.