v. GENERAL DEDUCTIONS REGARDING THE TERTIARY BASIC SILLS
If we consider the facts which have now been adduced regarding the position and structure of the sills, we are led, I think, to regard these masses as certainly belonging to the history of the basalt-plateaux, but, on the whole, to a comparatively late part of it. They consist of essentially the same materials as the lavas that form these plateaux, though with the differences of structure which the conditions of their production would lead us naturally to expect. Where they occur in thick masses, which must obviously have cooled much more slowly at some depth beneath the surface than the comparatively thin sheets could do that were poured out above ground, they have assumed a far more largely crystalline texture than that of the superficial lavas. As a rule, we may say that the thicker the sill the coarser is its texture, while the thinnest sheets are the most close-grained. Sills are especially abundant about the base of the basaltic-plateaux. We may examine miles of the central and higher parts of the escarpments without detecting a single example of them, but if the escarpment is cut down to the base we seldom need to search far to find them.
Fig. 329.—View of the same Sill seen from the channel opposite the island of Kolter.
That the efforts of the internal magma to establish an outlet towards the surface were accompanied by powerful disturbances of the terrestrial crust is shown by the abundant dykes which traverse all the volcanic districts from Antrim to Iceland, and some of which ascend even to the very highest remaining lavas of the basalt-plateaux. The parallel fissures filled by these dykes prove that even after the accumulation of more than 3000 feet of basalt-sheets, the movements continued to be so powerful as to disrupt these vast piles of volcanic material. But undoubtedly the highest parts of the plateau-basalts are less cut by dykes than the lower parts. There would no doubt come a time when the dislocations would more seldom reach the surface, when dykes would not be formed so abundantly or up to such a high level, and when the volcanic energies would more and more sparingly result in the opening of new vents or in the discharge of fresh eruptions from old ones.
It appears to me most probable that the injection of the sills was connected with the same terrestrial disturbances that produced the dykes which traverse the plateaux. Besides being dislocated by parallel fissures, the earth's crust in North-Western Europe seems to have been ruptured internally along lines more or less at right angles to the vertical fissures. The deep accumulation of bedded basalts presented an increasing obstacle to the ascent of the magma to the surface. Unable to gain ample enough egress through such vertical fissures as might be formed in the volcanic pile, the molten rock would find its lines of least resistance along the planes of the strata and the lower basalt-beds, either by the aid of terrestrial ruptures there, or in virtue of its own energy. On these horizons, accordingly, the sills occur in extraordinary profusion throughout the volcanic regions. They are no doubt of all ages in the progress of the building up of the volcanic plateaux, but I am disposed to believe that a large number of them may belong to the very latest period of the uprise of basalt within the area of Britain.
One of the most suggestive features of the abundant Tertiary sills lies in the evidence they furnish of the enormous energy concerned in the ascent and intrusion of volcanic material. The infilling of dykes or the outpouring of successive streams of lava at the surface hardly appeals to our imagination so strikingly as the proof that the sills have been impelled into their places with a vigour which, even when guided and aided by gigantic terrestrial ruptures, was capable of overcoming the vertical pressure of hundreds, or even thousands of feet of overlying rock. Had these intrusive sheets been mere thin layers, their horizontal extent and persistence would still have excited our astonishment, but when we find them sometimes several hundred feet thick, and to extend in a continuous series for horizontal distances of 50 miles or more, we are lost in wonder at the prodigious expansive strength of the volcanic forces. Again, the intrusions have not always taken place between the bedding-planes of the stratified or igneous rocks, but, as we have seen, solid sheets of already deeply buried lavas have sometimes been split open and the intrusive material has forced itself between the disrupted portions. Such subterranean proofs of the vigour of volcanic energy teach some of the most impressive lessons in the chronicles of volcanic action in the British Isles.
In closing this history of the accumulation of the great Tertiary volcanic plateaux of North-Western Europe, I would remark that as the result of prolonged eruptions from innumerable vents, the depression that extended from the south of Antrim to the Minch was gradually filled up with successive sheets of basalt to a depth of more than 3000 feet. A succession of lava-fields stretched from the North of Ireland across the West of Scotland, and perhaps even to the Faroe Islands, Iceland and Greenland. That the lava spread round the base of the Highland mountains and ran up the Highland glens, much as the sea now does, is made clear from the position of the outliers of it which have been left perched on the ridges of Morven and Ardnamurchan. So far as can now be surmised, these wide Phlegræan fields were only varied by occasional volcanic cones scattered over their surface, marking some of the last vents from which streams of basalt had flowed. But the volcanic energy was still far from exhaustion. After the accumulation of such a deep and far-extended sheet of lava, those underground movements which produced the fissures that served as channels for the uprise of the earliest dykes continued to show their vigour. The pile of bedded lavas was rent open by innumerable long parallel fissures in the prevalent north-westerly direction, up which basic lavas rose to form dykes, while vast numbers of sills were injected underneath. Whether the outflow of basalt at the surface had wholly ceased when the last of these dykes were injected into the plateaux cannot be told. Nor is there any evidence whether it had ended before the next great episode of the volcanic history—the extravasation of the gabbro bosses. All that we can affirm with certainty is, that the formation of north-west fissures and the uprise of basalt in them were again repeated, for we find north-west dykes traversing even the crests of the later eruptive masses of basic and acid rocks. It is difficult to suppose that none of these latest dykes communicated with the surface, and gave rise to cones with the outpouring of basalt and the ejection of dust and stones. But of such later outflows of basic material over the surface of the plateaux no undoubted trace has yet been recognised.