Chapter xlix

., in connection with the subsidences and dislocations which have affected the region since the close of the volcanic period.]

It is evident, therefore, that the present position of the Chalk platform is far from agreeing with that which it presented to the outflow of the sheets of basalt. But, on the other hand, there can be no doubt that its surface at the beginning of the volcanic outbursts was not a level plain. It was probably a rolling country of low bare chalk-downs, like parts of the South-east of England. The Irish Chalk attains its maximum thickness of perhaps 250 feet at Ballintoy. But it is liable to rapid diminution. On the shore at Ballycastle about 150 feet of it can be seen, its base being concealed; but only two and a half miles to the south, on the outlier of Knocklayd, the thickness is not quite half so much. On the west side of the plateau also, there are rapid changes in the thickness of the Chalk. Such variations appear to be mainly attributable to unequal erosion before the overflow of the basalts. So great indeed had been the denudation of the Cretaceous and underlying Secondary formations previous to the beginning of the volcanic eruptions, that in some places the whole of these strata had been stripped off the country, so that the older platform of Palæozoic or still more ancient masses was laid bare. Thus, on the west side of the escarpment, the basalt steals across the Chalk and comes to rest directly upon Lower Carboniferous rocks.

The authors who have described the junction of the Chalk and basalts in Antrim have generally referred to the uneven surface of the former rock as exposed in any given section. The floor on which the basalt lies is remarkably irregular, rising into ridges and sinking into hollows or trenches, but almost everywhere presenting a layer of earthy rubbish made of brown ferruginous clays, mixed with pieces of flint, chalk, and even basalt.[233] The flints are generally reddened and shattery. The chalk itself has been described as indurated, and its flints as

## partially burned by the influence of the overlying basalt. But I have

not noticed, at any locality, evidence of alteration of the solid chalk, except where dykes or intrusive sheets have penetrated it.[234] There can be no doubt that the hardness of the rock is an original peculiarity, due to the circumstances of its formation. The irregular earthy rubble, that almost always intervenes between the chalk and the base of the basalt, like the "clay with flints" so general over the Chalk of Southern England, no doubt represents long-continued subærial weathering previous to the outflow of the basalt. Even, therefore, if there were no other evidence, we might infer with some confidence from this layer of rubble, that the surface over which the lavas were poured was a terrestrial one. Here and there, too, we may detect traces of the subsidence of the basalt into swallow-holes dissolved in the chalk subsequent to the outflow of the basalt-sheets.

[Footnote 233: Portlock, _Report on Geology of Londonderry_, etc. (Geological Survey), p. 117.]

[Footnote 234: See Portlock, _op. cit._ p. 116.]

The Antrim plateau is not only the largest in the British Islands, it is also the most continuous and regular. It may be regarded, indeed, as one unbroken sheet of volcanic material, not disrupted by any such mountainous masses of intrusive rock as in the other plateaux interrupt the continuity of the horizontal or gently inclined sheets of basalt. Around its margin, indeed, a few outliers tower above the plains, and serve as impressive memorials of its losses by denudation. Of these, by much the most picturesque and imposing, though not the loftiest, is Knocklayd already referred to, which forms so striking a feature in the north-east of Antrim (Fig. 263).

[Illustration: Fig. 263.--Section of Knocklayd, an outlier of the Antrim basalt-plateau lying on Chalk.

1. Crystalline schists; 2. Cretaceous strata; 3. Lower basalts; 4. Group of tuffs, clays and iron-ore; 5. Upper basalts; _f_. Fault.]

The total thickness of volcanic rocks in the Antrim plateau exceeds 1000 feet; but, as the upper part of the series has been removed by denudation, the whole depth of lava originally poured out cannot now be told. A well-marked group of tuffs and clays, traceable throughout a large part of Antrim, forms a good horizon in the midst of the basalts, which are thus divisible into a lower and upper group (Fig 264).

The Lower Basalts have a thickness of from 400 to 500 feet. But, as already mentioned (p. 194), they die out in about six miles to no more than 40 feet at Ballintoy. They are distinguished by their generally cellular and amygdaloidal character, and less frequently columnar structure. The successive flows, each averaging perhaps above 15 feet in thickness, are often separated by thin red ferruginous clayey partings, sometimes by bands of green or brown fine gravelly tuff. The most extensive of these tuff-bands occurs in the lower part of the group at Ballintoy, and can be traced along the coast for about five miles. In the middle of its course, near the picturesque Carrick-a-raide, it reaches a maximum thickness of about 100 feet and gradually dies out to east and west. The neck of coarse agglomerate at Carrick-a-raide, is doubtless the vent from which this mass of tuff was discharged (see Fig. 301). Owing to the thinning out of the sheets of basalts, as they approach the vent, the tuff comes to rest directly on the Chalk, and for some distance westwards forms the actual base of the volcanic series.[235] Occasional seams of carbonaceous clays, or of lignite, appear in different horizons among the basalts. Beneath the whole mass of basalt, indeed, remains of terrestrial vegetation here and there occur. Thus, near Banbridge, County Down, a patch of lignite, four feet ten inches thick, underlies the basalt, and rests directly on Silurian rocks. Such fragmentary records are an interesting memorial of the wooded land-surface over which the earliest outflows of basalt spread.

[Footnote 235: See Explanation of Sheets 7 and 8 of the Geological Survey of Ireland (1888), p. 23.]

[Illustration: Fig. 264.--Diagram-Section of the Antrim Plateau.

1. Triassic series; 2, 3. Rhaetic strata and Lias; 4. Greensand; 5. Chalk; 6. Gravel and soil; 7. Lower group of basalts; 8. Group of tuffs, clays and iron-ore; 9. Upper group of basalts.]

In looking at the great basalt-escarpments of Antrim, the Inner Hebrides or the Faroe Islands, and in following with the eye the successive sheets of lava in orderly sequence of level bands from the breaking waves at the base to the beetling crest above, we are apt to take note only of the proofs of regularity and repetition in the outflows of molten rock and to miss the evidence that these outflows did not always rapidly follow each other, but were separated by intervals of varying, sometimes even of long duration. One of the most frequent and conspicuous proofs of such intervals is to be found in the red layers or partings above referred to which, throughout all the basalt-plateaux, so commonly intervene between successive sheets of basalt. These red streaks cannot fail to arrest the eye on the coast-precipices where by their brilliant contrast of colour, they help to emphasize the bedded character of the whole volcanic series.

Examined more closely, they are found to consist of clay or bole which shades into the decomposed top of the bed whereon it lies, and is usually somewhat sharply marked off from that which covers it. This layer has long, and I think correctly, been regarded as due to the atmospheric disintegration of the surface of the basalt on which it rests, before the eruption of the overlying flow. It varies in thickness from a mere line up to a foot or more, and it passes into the tuffs and clays which are sometimes interposed between the sheets of basalts. It may be looked upon as probably furnishing evidence of the lapse of an interval sufficiently extended to permit a considerable subserial decay of the surface of a lava-sheet before the outflow of the next lava. But an attentive study of the plateaux discloses other and even more remarkable indications that the pauses between the consecutive basalt-beds were frequently so prolonged as to allow extensive topographical changes to be made in a district. Nowhere is the long duration of some of these intervals more impressively taught than in the central zone of sedimentary strata in Antrim.

This persistent group of tuffs, clays, and iron-ore is generally from 30 to 40 and sometimes as much as 70 feet thick. From the occurrence of the ore in it, it has been explored more diligently in recent years than any other group of rocks in the district, and its outcrop is now known over most of the plateau. The iron-ore bed varies from less than an inch up to 18 inches in thickness, and consists of pisolitic concretions of hæmatite, from the size of a pea to that of a hazel nut, wrapped up in a soft ochreous clayey matrix.[236] Where it is absent, its place is sometimes taken by an aluminous clay, worked as "bauxite," which has yielded stumps of trees and numerous leaves and cones. Beneath the iron-ore or its representative, lies what is called the "pavement,"--a ferruginous tuff, 8 to 10 feet thick, resting on "lithomarge,"--a lilac or violet mottled aluminous earth sometimes full of rounded blocks or bombs of basalt. The well-known horizon for fossil plants at Ballypallidy is a red tuff in this zone. The section of strata between the two basalt-groups at this locality may serve as an illustration of the nature and arrangement of the deposits.[237]

[Footnote 236: Consult a good essay on the Iron-ore and Basalts of North-east Ireland by Messrs. Tate and Holden, _Quart. Journ. Geol. Soc._ xxvi. (1870), p. 151. In this paper the nature, composition and modes of origin of the iron-ore and its associated strata are fully discussed.]

[Footnote 237: A. M'Henry, _Geol. Mag._ (1895), p. 263.]

Upper Basalt, compact and often columnar sheets.

Brown laminated tuff and volcanic clays.

Laminated brown impure earthy lignite, 2 feet 3 inches.

Brown and red variegated clays, tuffs and sandy layers, with irregular seams of coarse conglomerate composed of rounded and subangular fragments of rhyolite and basalt, 3 feet 4 inches.

Brown, red and yellowish laminated tuffs, mudstones, and bole, with occasional layers of fine conglomerate (rhyolitic and basaltic), pisolitic iron-ore band and plant-beds, 8 feet 10 inches.

Lower basalt, amygdaloidal.

In some of the Ballypallidy tuffs the most frequent lapilli are pieces of green and brown glass, which Mr. Watts compares with the pitchstone of Sandy Braes, though rarely containing phenocrysts as that rock does. He has found also in these strata a smaller proportion of lithoidal rhyolites and occasionally fragments of basic rock.

The pale and coloured clays that occur in this marked sedimentary intercalation have doubtless been produced by the decomposition of the volcanic rocks and the washing of their fine detritus by water. Possibly this decay may have been in part the result of solfataric

## action. From true bauxite or aluminium-hydrate, the sediments vary in

composition and specific gravity and pass into aluminous silicates and iron-ores. They seem to indicate a prolonged interval of volcanic quiescence when the lavas and tuffs already erupted were denuded and decomposed.[238]

[Footnote 238: See a note on Bauxite by Professor G. A. Cole, _Scientif. Trans. Royal Dublin Soc._ vol. vi. series ii. (1896), p. 105.]

The area over which this interesting series of stratified deposits now extends is obviously much less than it was originally. It has indeed been so reduced by denudation into mere scattered patches that it probably does not exceed 170 square miles. But the group can be traced from Divis Hill, near Belfast, to Rathlin Island, a distance of 50 miles, and from the valley of the Bann to the coast above Glenarm, more than 20 miles. There can be little doubt that it was once continuous over all that area, and that it probably extended some way further on each side. If the so-called Pliocene clays of Lough Neagh be regarded as parts of this group of strata, its extent will be still further increased. Hence the original area over which the iron-ore and its accompanying tuffs and clays were laid down can hardly have been less than 1000 square miles. This extensive tract was evidently the site of a lake during the volcanic period, formed by a subsidence of the floor of the lower basalts. The salts of iron contained in solution in the water, whether derived from the decay of the surrounding lavas or from the discharges of chalybeate springs, were precipitated as peroxide in pisolitic form, as similar ores are now being formed on lake-bottoms in Sweden. For a long interval, quiet sedimentation went on in this lake, the only sign of volcanic energy during that time being the dust and stones that were thrown out and fell over the water-basin, or were washed into it by rains from the cones of the lava-slopes around.

It may here be remarked that the tendency to subsidence in the Antrim plateau seems to have characterized this region since an early part of the volcanic period. The lake in which the deposits now described accumulated was entirely effaced and overspread by the thick group of upper basalts. But long after the eruptions had ceased, a renewed sinking of the ground gave rise to the sheet of water which now forms Lough Neagh.[239]

[Footnote 239: This subject will be discussed in