i. SUBSIDENCES

The mere fact that in many places the lower members of the series of terrestrial lavas have been submerged under the sea may be taken to prove a subsidence since older Tertiary time. Along the west coast of Skye this depression is well shown by the almost entire concealment of the bottom of the plateau under the Atlantic. In the Faroe Isles the subsidence has advanced still further, for not a trace of the underlying platform on which the basalts rest remains above water. In Iceland, too, the complete submergence of the base of the Tertiary volcanic sheets points to a widespread subsidence of that region.

Another strong argument in favour of considerable depression may be derived from a comparison of the submarine topography with that of the tracts above sea-level. It is obvious that the same forms of contour which are conspicuous on the land are prolonged under the Atlantic. If we are correct in regarding the valleys as great lines of subærial erosion, their prolongations as fjords and submarine troughs must be considered as having had a similar origin. We can thus carry down the surface of erosion several hundred feet lower than the line along which it disappears under the waves.

I know no locality where this kind of reasoning is so impressively enforced upon the mind as the west end of the Scuir of Eigg. The old river-bed and its pitchstone terminate abruptly at the top of a great precipice. Assuredly they must once have continued much further westward, as well as the sheets of basalt that form the main part of the cliff. Yet the sea in front of this truncated face of rock rapidly deepens to fully 500 feet in some places. Had any such hollow existed in the volcanic period it would have been filled up by the long-continued outflowings of basalt. Making every allowance for concealed faults and local subsidences, we can only account for this submarine topography by regarding it as having been carved out, together with the topography of the land, at a time when the level of the latter was at least 500 feet higher than it is now.

The subsidence which is thus indicated along the whole of the North-West of Europe probably varied in amount from one region to another. We seem to have traces of such inequalities in the varying inclinations of different segments of the basalt-plateaux. The angles of inclination are almost always gentle, but they differ so much in direction from island to island, and even among the several districts of the same island, as to indicate that certain portions of the volcanic plain have sunk rather more than other portions.

Thus in the Faroe Islands, where the bare cliffs allow the varying angles of inclination to be easily determined, a general gentle dip of the basalts in a south-easterly direction has been noted among the central and northern islands by previous observers. This inclination, however, is replaced among the southern islands by an equally gentle dip towards the north-east. The centre of depression would thus seem to lie somewhere about Sandö and Skuö. The highest angle of inclination which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E. at about 15°.

Among the Western Isles, also, where similar variations in the inclination of the basalt-sheets are observable, it might be possible by careful survey to ascertain the probable position of the areas of maximum depression, and to show to what extent differential movements have affected the originally nearly level volcanic floor. It would doubtless be found that everywhere the dominant movement has been one of subsidence. The vast outpourings of lava would tend to leave the overlying crust unsupported, and to cause it to sink into the cavities thus produced.

Perhaps the most extensive subsidence of this kind, at least that which admits of most satisfactory investigation, because it still remains above sea-level, is displayed by the vast hollow in the Antrim plateau, which embraces the basin of Lough Neagh and the valley of the Lower Bann. This depression measures about 60 miles in length by about 20 in breadth. Its axis follows the N.N.W. trend so characteristic of the volcanic features of Tertiary time. The depression may be said to involve the entire basaltic plateau of Antrim, for with the exception of a few insignificant areas along the borders, especially on the east side between Larne and Cushendall, the whole region slopes inward from its marginal line of escarpments, which reach heights of 1800 feet and upwards, towards the great hollow in its centre (see Map VII.).

Lough Neagh, which occupies the deepest part of this hollow, and covers about one-eighth of the whole area of subsidence, is the largest sheet of fresh water in the British Isles, for it exceeds 150 square miles in extent of surface. Yet, for its size, it is one of the shallowest of our lakes, its average depth being less than 40 feet. Its shallowness, compared with its wide area, marks it out in strong contrast to most of the larger British lakes. Its surface is only 48 feet above the level of the sea.

The origin of Lough Neagh, the theme of various legends, has been seriously discussed by different writers, but most exhaustively by the late E. T. Hardman of the Geological Survey.[434] This author connected the formation of the lake-basin with a series of large faults which are found intersecting the rocks around the basin, and passing under the water in a general north-easterly direction. He showed that these faults have produced serious displacements of the strata, amounting sometimes to as much as 2000 feet, and he believed that it was by the concurrent effect of such dislocations that the depression of Lough Neagh had been caused.

[Footnote 434: "On the Age and Formation of Lough Neagh," _Journ. Roy. Geol. Soc. Ireland_, vol. iv. (1875-76), p. 170; also Explanation of Sheet 35 of the _Geol. Surv. Ireland_ (1877), p. 72.]

It is possible that these displacements may have contributed to at least the earlier stages in the history of the Antrim subsidence. They have undoubtedly taken place after the outpouring of the basalts, for these rocks are involved in their effects. But in the hollow of the Bann valley north of Lough Neagh the faults which have been detected in the basaltic plateau are few and trifling. The bold and bare escarpments, that so clearly display the relations of the rocks, reveal few traces of any important transverse dislocations. Nor has any proof of large longitudinal faults parallel with the axis of depression been obtained within the area of the Bann valley.

The earliest evidence for the existence of a lake on the site of the present Lough Neagh has been supposed to be furnished by certain fine clays, sands, seams of lignite and clay-ironstone, which have been referred to the Pliocene period. These deposits have been regarded as indicating the accumulation of fine sediment with drift vegetation brought down into a quiet lake by streams entering from the south. Their fresh-water origin was believed to be further corroborated by the occurrence of shells belonging to the lacustrine or fluviatile genus, _Unio_.[435]

[Footnote 435: These shells were regarded as forms of _Unio_ by the late W. H. Baily; but Dr. Henry Woodward assigned them to _Mytilus_. See Prof. Hull's _Physical Geology and Geography of Ireland_, 2nd edit. p. 101. The shells have been more recently dug out by Mr. Clement Reid, who has found them to be the common _Mytilus edulis_.]

The thickness of this series of strata, their position above sea-level, and their distribution are important parts of the evidence for the geological history of the locality. At one place the deposits are said to have been bored through to a depth of 294 feet, and Mr. Hardman believed them to be not less than 500 feet deep. The same observer found that they certainly reach a height of 120 feet above the sea, and he was of opinion that in some places their height was not less than 140 feet. The deposition of strata to the depth of 300 feet below a level of 120 feet above the sea would, of course, entirely fill up Lough Neagh, and spread over a large tract of low ground around it. The pottery-clays and lignites, however, appear to be confined to the southern half of the lake, from which they rise gently into the low country around.

The distribution of these deposits and their extraordinary variations in altitude, as described by Mr. Hardman, present great difficulties in the attempt to regard them as the sediments of a Pliocene lake. A more recent examination of the ground by Mr. Clement Reid of the Geological Survey has led that able observer to believe that two totally different groups of strata at Lough Neagh have been confounded. He noticed the _Mytilus_-clay to be a dark blue mass full of derived boulder-clay stones, and yielding _Mytilus edulis_ and seeds of a sedge. This deposit cannot be Pliocene, but must be of Glacial or post-Glacial age, possibly contemporary with the Clyde beds. The junction of this clay with the pipe-clays is not at present seen, but the lithological contrast between the two groups of strata is so strong as to indicate their independence of each other. Mr. Reid found the white, red and mottled pipe-clays with their masses of lignite to present a strong resemblance to the Bagshot group in the Tertiary series. It is possible, as already suggested, that the pipe-clays and lignites may belong to the sedimentary zone that separates the lower and upper basalts of Antrim. At all events they furnish no proof of any Pliocene lake, and may not indicate more than a deeper part of the depression in which the tuffs, lignites and iron-ore were laid down.

The existence of the _Mytilus_-clay shows that in Glacial or post-Glacial times the valley of the Bann was a strait or fjord into which the sea entered. Thick masses of drift have been laid down all round and over the depression now occupied by Lough Neagh, insomuch that had any older lake existed here in Glacial times, it could hardly have escaped being filled up.

The observer, who from one of the basalt-heights looks down upon the expanse of Lough Neagh and the broad peat-covered plain that continues the level platform of the lake-surface down the valley of the Bann, cannot but be impressed with the size of this wide hollow in the heart of the Antrim plateau, and with the evident continuity of the whole depression from the lake to the sea. If he be a geologist, he will be further struck by the fact that while the Chalk and other older rocks appear from under the basalt-escarpments all round the plateau, at heights of many hundred feet above the sea, the floor of this wide hollow is entirely covered with basalt. Had the depression been merely due to denudation, the rocks that underlie the volcanic series would have been exposed to view. The base of the basalts which, on either side of the depression, is often more than 1000 feet above the sea-level, sinks below that level in the hollow of the Bann and Lough Neagh.

This inequality of position may have been partially brought about by faults like those around Lough Neagh, and may thus have been begun long before the Glacial period. But it appears to me to be mainly due to a wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction from the present coast up the valley of the Bann and the basin of Lough Neagh to beyond Portadown.

We may conceive that after the cessation of the outflows of basalt, the territory overlying the lava-reservoir that had been emptied would tend to subside, partly by ruptures of the crust producing faults and partly by a downward movement of a more general kind. In course of time, these disturbances turned the drainage into the hollow now traversed by the Bann. Denudation would necessarily accompany them, and the surface of the country would be continually eroded and lowered.

Lough Neagh has been carefully sounded by the Admiralty, and its chart affords much suggestive material for the consideration of the geologist.[436] From the soundings there given it has long been known that the lake deepens towards its northern end, and attains a maximum depth of 102 feet. But it is not until we trace on the chart a series of contour-lines for successive depths, as shown by the soundings, that we realize the remarkable form of the lake bottom. We then discover that below a depth of 50 feet a well-defined channel extends for rather more than half the length of the lake. This channel begins to be distinctly perceptible between Kiltagh Point and Langford Lodge. It first runs in a northerly course on the west side of the centre of the Lough, but when it comes into a line with Saltera Castle on the western shore, it wheels round so as to conform to the curve of the Antrim coast-line, which it follows northward until, about two miles from the exit of the lake, its outline ceases to be traceable on the gently shelving bottom. Its total length is thus about 12 miles.

[Footnote 436: Lough Neagh surveyed and sounded by Lieut. Thomas Graves, R.N.]

There can hardly be any doubt that this channel is a former bed of the River Bann. It occupies exactly the position which that stream would take if the lake were drained, and its depth and breadth correspond to those of the valley-bottom of the present river. If this conclusion be accepted, some important conclusions may be further deduced from it.

1. The presence of a former course of the Bann on the bottom of Lough Neagh proves the lake to be much younger than the Ice Age. The thick boulder-clays and Glacial gravels which so encumber the country around and descend under the lake, would assuredly have filled up the river-channel had it existed at the time of their deposition. The channel has obviously been cut out of these drifts since the Glacial period. When the erosion took place, the present Lough Neagh could not have existed, but the Bann followed a continuous course across the plain which the lake now covers. The river probably maintained its place for a long period, so as to be able to excavate so wide and deep a bed in the drifts, if, indeed, it did not to some extent slowly carve its bed out of the underlying basalts. It must be remembered that sediment is being continually poured into Lough Neagh, and that some of the silt must have accumulated in the submerged river-course, thus lessening its depth and width. That the channel should still be so marked may be used as an argument for the comparatively late date of the subsidence.

2. The submerged river-course is a clear proof of subsidence. The present Lough Neagh cannot be looked upon as a glacial lake formed by rock-erosion or by irregular deposition of drift. Its floor must have been a land surface when the Bann cut out its bed upon it. The whole area has sunk down, the drainage has been arrested, and some 20 miles of the course of the Bann are now under a sheet of shallow water. This subsidence was not brought about by faults. It seems rather to have resulted from a general sinking of the ground. The movement was probably comparatively rapid, otherwise the river-course would hardly have survived so well.

3. These inferences, based upon purely geological considerations, have an interesting bearing upon the allusions to the origin of Lough Neagh contained in some ancient historical documents. Various legends have from an early period been handed down as to the first appearance of this sheet of water. These myths, though differing in details, agree in describing such a sudden or rapid accumulation of water as destroyed human life, in a district which had previously been inhabited by man. The earliest records indicate that the alleged catastrophe took place in the first century of the Christian era.[437] It appears to me not improbable that the tradition,thus preserved in these legends, may have had its basis in the actual disturbance which, on geological grounds, can be shown to have determined the existence of Lough Neagh. Though the event may go back far beyond the first century, there can be no doubt that, in a geological sense, it was one of the most recent topographical changes which the British Isles have undergone.

[Footnote 437: For versions of the legends, see Dr. Todd's "Irish Version of the Historia Britonum of Nennius," _Roy. Hist, and Archæol. Assoc. Ireland_; Dr. Reeves' "Ecclesiastical Antiquities of Down," etc., p. 370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1 in _Journ. Roy. Hist. and Archæol. Assoc. Ireland_, vol. i. (1870-71), p. 94; _Giraldus Cambrensis_, vol. v. cap. ix. p. 91--"de lacu magno miram originem habente." Moore's well-known lines embody the popular belief that round towers and other buildings were submerged by the inundation.]

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.