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.
[233] Portlock, Report on Geology of Londonderry, etc. (Geological Survey), p. 117.
[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]).
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.
[235] See Explanation of Sheets 7 and 8 of the Geological Survey of Ireland (1888), p. 23.
