DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 121 



when the pent-up volcanic energy would be unable to open new ones. Through the 

 shattered crust the lava would be forced upwards, but the deep overlying cover of 

 bedded basalts would present a formidable obstacle to its further ascent. Unable to find 

 ample enough egress through such fissures as might be formed in the pile of basalts, the 

 molten rock would seek its lines of least resistance along the planes of the strata and the 

 lower basalt-beds; and there, accordingly, we find the sills in extraordinary profusion. 

 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. 



In closing this history of the accumulation of the great Tertiary volcanic plateaux of 

 this country, I would remark that as the result of prolonged eruptions from innumerable 

 vents, the depression that stretched from the south of Antrim to the Minch was gradually 

 in large measure filled up. We know that the pile of basalt-sheets reached in some places 

 a depth of more than 3000 feet, and that not improbably it stretched in one continuous 

 field of black lava along the west of Scotland and across the north of Ireland. 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 a few 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 accumu- 

 lation of such a deep and far extended sea of lava, those underground movements which 

 produced the fissures that served as channels for the uprise of the dykes through all the 

 older rocks continued to show their vigour. The covering of bedded lavas, though 

 several thousand feet thick, was rent open by innumerable long parallel fissures in the 

 prevalent north-westerly direction, up which basic lavas rose to form dykes. Whether 

 the outflow of the bedded basalts had wholly ceased when the last 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 repeated, for we find N.W. basalts 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 

 manifestations of volcanic activity on the surface of the plateaux no undoubted trace can 

 now be recognised. 



