CHAP. XXXVII 
THE ANTRIM P LA TEA U 
203 
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 . 1 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. 
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 
Fig. 264. — Diagram-Section of the Antrim Plateau. 
1. Triassic series ; 2, 3. Rliaetic strata and Lias ; 4. Greensand ; 5. Chalk ; 0. Gravel and soil ; 7. Lower group 
of basalts ; 8. Group of tuffs, clays and iron-ore ; 9. Upper group of basalts. 
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 dis- 
integration of the surface of the basalt on which it rests, before tbe 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 inter- 
1 See Explanation of Sheets 7 and 8 of the Geological Survey of Ireland (1888), p. 23. 
