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