DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 87 



doubt represents long-continued subserial weathering previous to the outflow of the 

 basalt. Even, therefore, if there were no other evidence, we might infer with some con- 

 fidence from this layer of rubble, that the surface over which the lavas were poured was a 

 terrestrial one. 



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, with no such mountainous masses of eruptive rock as in the other plateaux 

 disturb 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. 



The total thickness of volcanic rocks in the Antrim plateau exceeds 1000 feet; but r 

 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. 



The Lower Basalts have a thickness of from 400 to 500 feet. But, as already 

 mentioned (p. 80), they rapidly die out in about six miles to no more than 40 feet at 

 Ballintoy. They are distinguished by their general cellular and amygdaloidal character, 

 and less frequently columnar structure. The successive flows, each averaging perhaps 

 about 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 sheet of 

 tuff is one which 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, already referred 

 to, is doubtless the vent from which this mass of tuff was discharged (see fig. 29). 

 Owing to the thinning out of the sheets of basalt, 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. * Occasional seams of carbonaceous clays, or even of lignite, 

 appear on different horizons. Beneath the whole mass of basalt, indeed, remains of 

 terrestrial vegetation here and there occur. Thus, near Banbridge, county Down, a 

 patch of lignite, 4 feet 1 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. 



The central zone 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, this zone has been 

 explored more diligently in recent years than any other group of rocks in Antrim, and 

 its outcrop is now known over most of the district in which it occurs. The iron-ore bed 



* See Explanation of Sheets 7 and 8 of the Geological Survey of Ireland (1888), p. 23. 



