202 
THE TERTIARY VOLCANOES 
BOOK VIII 
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. 
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 moun- 
tainous 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). 
The total thickness of volcanic rocks in the Antrim plateau exceeds 
1. Crystalline schists ; 2. Cretaceous strata ; 3. Lower Basalts ; 4. Group of tuffs, clays and iron-ore 
5. Upper basalts ; /. Fault. 
1000 feet; but, as the upper part of the series has been removed by de- 
nudation, 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, 
neai 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 
eoaise 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 
