There is one striking connection between the sites of the vents and ancient topographical features to which frequent reference has been made in the foregoing chapters. All through the long volcanic history, as far back as such features can be traced, we see that orifices of discharge for the erupted materials have been opened along low grounds and valleys rather than on ridges and hills. The great central hollow of the Scottish midlands was a depression even as long ago as the time of the Lower Old Red Sandstone, and though it has probably been several times since then filled up, and more or less completely effaced, its ancient features have been partially revealed by extensive denudation. This vast depression, 40 miles broad, between the Highland mountains on the one side and the Southern Uplands on the other, was the chief centre of volcanic activity in western Europe during the latter half of Palæozoic time. The vents of the Old Red Sandstone, Carboniferous and Permian series are scattered all over it, but few or none of them are to be found on the high grounds that bound it. Again, in Tertiary time, the great outpouring of lava took place in the hollow that lay between the ridge of the Outer Hebrides and the mainland of Scotland. This wide and long tract of low ground was buried under upwards of 3000 feet of lava and tuff, but these materials were erupted from fissures and vents within its own border and not from the mountains on either side.
But perhaps the most conspicuous example of any in which the vents keep to the valleys is that supplied by the Permian necks of Nithsdale and the neighbouring glens. These depressions are as old as Permian, and even as Carboniferous time, but they appear to be entirely hollows of erosion; at least they have yielded no evidence that their direction has been determined by lines of fault. The chain of vents can be followed from the lowlands of Ayrshire up to the base of the Southern Uplands, down the wide valley cut by the Nith in these hills and up some of the tributary valleys, and though the volcanoes continued for some time in vigorous eruption, not a trace of any contemporary vent has yet been met with on the surrounding hills.
While the position of volcanic vents in lines of valley may be generally due to guiding lines of fissure in the crust underneath, either within or below the rocks visible at the surface, there may sometimes be conditions in which other dominant causes come into play. The curious coincidence between variations in the upper limit of dykes and inequalities in the configuration of the overlying ground, suggest that where the subterranean magma has ascended to within a comparatively short distance from the surface, a difference of a few hundreds or thousands of feet in the depth of overlying rock, such as the difference of height between the bottom of a valley and the tops of the adjacent hills, may determine the path of escape for the magma through the least thickness of overarching roof.
5. Volcanic phenomena cannot be regarded as a mere isolated and incidental feature in the physics of the globe. During the short time within which man has been observing the operations of existing volcanoes, he has hardly yet had sufficient opportunity of watching how far they can be correlated with other terrestrial movements. Nor, when he endeavours to trace some such connection among the records of the geological past, has he yet collected materials enough to furnish a sufficiently broad and firm basis of comparison. One formidable obstacle is presented by the difficulty in determining chronological equivalents in separated groups of rock. Geologists have tried to discover whether the volcanoes of some particular period or region were in any way connected with such geological changes as extensive plication, dislocations of the crust, or elevation of mountain-chains. In regard to the volcanic history of Britain, various possible relations of this kind obviously suggest themselves. Thus the division of geological time comprised within the Lower Silurian period was undoubtedly an interval of considerable terrestrial disturbance in western Europe. The unconformabilities and overlaps in the series of formations belonging to that period, the frequent conglomerates, the great and often rapid changes in the thickness and lithological characters of the strata, all point to instability of land-surface and sea-floor. During these oscillations a prolonged and widespread series of volcanic eruptions took place. The volcanic manifestations began in Cambrian time and continued in intermittent activity till towards the close of the deposition of the Lower Silurian formations. It is certainly a significant fact that the Upper Silurian deposits, in their lithological characters, present a strong contrast to those that preceded them. They point, on the whole, to quiet sedimentation, during an interval of comparative calm in the terrestrial crust. With this evidence of tranquillity there is, over almost the whole of the British Isles, an entire absence of any trace of renewed volcanic activity. With the exception of the Dingle lavas and tuffs, in the extreme west of Ireland, not a single undoubted instance is yet known of an Upper Silurian volcano.
After the deposition of the Upper Silurian rocks an interval of great terrestrial disturbance ensued, and these rocks over a large part of Britain were intensely plicated and crushed. The movements, continued into the period of the Lower Old Red Sandstone, were, in their later stages, accompanied or, at least, followed by the vast outpourings of lava which now cover so much of the tracts of Old Red Sandstone in Scotland and Ireland.[438]
[438] Trans. Geol. Soc. Edin. vol. ii. part iii. (1874).
In proportion as the volcanic energy was vigorous, widespread and long-continued, we may expect it to have been connected with important terrestrial movements affecting extensive regions of the earth. The Tertiary volcanic history seems to afford a remarkable instance of this connection. A wide area of the European continent is dotted over with old centres of volcanic activity which were in eruption at successive epochs throughout the Tertiary period. Of all these centres the most important was that of the north-western basalt-plateaux, where floods of lava were discharged over many thousand square miles from Ireland to Greenland. The geological date of these outpourings probably coincides with the last great orographic movements that gave to the mountain-chains of Europe their latest elevation and dimensions.
But without entering into what must be for the present a field of speculation, we can be assured of one important fact in the connection of ancient volcanoes with movements of the terrestrial crust. A study of the records of volcanic action in Britain proves beyond dispute that the volcanoes of past time have been active on areas of the earth's surface that were sinking and not rising. We usually associate volcanic action with elevation rather than subsidence, and there are certainly abundant proofs of such elevation around active or recently extinct volcanoes. Many of the active vents of the present time, like Vesuvius and Etna, began with submarine eruptions and have been gradually upraised into land. It may be, however, that such uprise is merely a temporary incident, and that if we could survey the whole geological period of which human history chronicles so small a part, we might find that subsidence, and not upheaval, is ultimately the rule over volcanic areas.
Be this as it may, there can be no question that with the one solitary exception of the Tertiary volcanoes, which were terrestrial and not submarine, all the British vents were carried down and eventually buried under aqueous sediments. Even the Tertiary lava-fields have in many places sunk down below sea-level since their eruptions ceased.
That there are any Palæozoic volcanic rocks now visible at the surface is obviously due to subsequent movements not immediately connected with their original conditions of eruption, and to gigantic denudation. The amount of subsidence which followed on a volcanic episode was sometimes enormous, even within the same geological period, as one may see by observing the prodigious piles of sedimentary material heaped over the lavas and tuffs of Arenig time, or over those of the Lower Old Red Sandstone. I do not wish to maintain that the downward movement was necessarily a consequence of volcanic ejections, for we know that it took place over tracts remote from centres of eruption. But I have sometimes asked myself whether it was not possibly increased as a sequel to vigorous volcanic action; whether, for instance, the great depth of the Palæozoic sedimentary rocks in some regions, as compared with their feeble development in others, may not have been due to an acceleration of subsidence consequent upon volcanic action.