Thus the Antrim basalt-plateau, in addition to the high interest of its volcanic history, has the additional claim to our attention that it has preserved, more fully and clearly than any other of the plateaux, the evidence for the latest subterranean movements that followed the long series of volcanic eruptions during Tertiary time. It contains the record of a post-Glacial subsidence that gave birth to the largest lake in Britain.
ii. DISLOCATIONS
Though I have not observed any features among the Tertiary basalt-plateaux of the British Isles that can be compared to the remarkable rifts and subsidences of Iceland, it can be shown that these piles of volcanic material have undoubtedly been fractured, and that portions of them have subsided along the lines of dislocation.
Careful examination of the basalt-escarpments of the Inner Hebrides discloses the existence of numerous faults which, though generally of small displacement, nevertheless completely break the continuity of all the rocks in a precipice of 700 or 1000 feet in height. Not infrequently such dislocations give rise to clefts in the cliffs. Some good illustrations of this feature may be noticed on the north side of the island of Canna, where the precipice has been fissured by a series of dislocations, having a hade towards the west and a throw which may in some cases amount to about 20 or 25 feet. The cumulative effect of this system of faulting, combined with a gentle westerly dip, is to bring down to the sea-level the upper band of conglomerate which further to the east lies at the top of the cliff. Again, the basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch Eynort, is traversed by a series of small faults. On the east side of Skye and in Raasay, a number of faults, some of them having perhaps a throw of several hundred feet, has been mapped by Mr. H. B. Woodward.
The largest dislocation observed by me among the basalt-plateaux of the Inner Hebrides is that already referred to (p. 209), which runs at the back of the Morven outlier, in the west of Argyllshire, from the Sound of Mull by the head of Loch Aline to the mouth of Loch Sunart, along the line of valley that contains the salt-water fjord Loch Teacus and the fresh-water lakes Loch Durinemast and Loch Arienas. While the Cretaceous deposits and the bottom of their overlying basalts rise but little above the sea-level on the south-west side of this line, they are perched as outliers on hill-tops on the north-east side, where they rise to 1300 feet above the sea. The amount of vertical displacement here probably exceeds 1000 feet. The fault runs in a north-westerly direction, and has obviously been the guiding influence in the erosion of the broad and deep valley which marks its course at the surface.
This dislocation is only the largest of a number by which the basalt-plateau has been broken in the district of Morven. Their effects are well shown in the outlier of basalt which caps Ben Iadain, where two parallel faults bring down the lavas against the platform of schists on which they lie (see [Fig. 266]).
Many faults have been traced in the Antrim plateau, and are represented on the Geological Survey Maps. In general they are of comparatively trifling displacement. Occasionally, however, they amount to several hundred feet, as in those already referred to as occurring near Ballycastle and around the southern part of the basin of Lough Neagh.
To what extent the dislocations that traverse the British Tertiary basalts are to be regarded as comparable to those which in Iceland have been referred to subsidence caused by the tapping and outflow of the lower still liquid parts of lava-sheets must be matter for further inquiry. So far as my own observations have yet gone, the faults do not seem explicable by any mere superficial action of the kind supposed. Where they descend through many hundreds of feet of successive sheets of basalt, and dislocate the Secondary formations underneath, they must obviously have been produced by much more general and deep-seated causes.
It is conceivable that, if these dislocations took place during the volcanic period, they broke up the lava-plains into sections, some of which sank down so as to leave a vertical wall at the surface on one side of the rent, or even to form open "gjás," like those of Iceland. But it is noteworthy that the fissures, which have been filled with basalt and now appear as dykes, comparatively seldom show any displacement in the relative levels of their two sides. In Iceland, also, the great lava-emitting fissures seem to be in general free from marked displacements of that kind.
