chap, xlii GENERAL OBSERVATIONS ON THE BASIC SILLS 
325 
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 Phlegrsean 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 
