CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
277 
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the occurrence of true volcanic agglomerate may be accepted as evidence of the 
existence of an eruptive vent communicating with the surface of the earth. 
The agglomerate in the vents associated with the basalt-plateaux, like that ot 
the Palteozoic vents, is generally exceedingly coarse, and without any trace of 
structure. Blocks of all sizes up to masses some ^ 
yards in length, and of the most diversified 
materials, both volcanic and non-volcanic, are 
dispersed confusedly through a granular paste 
of similar miscellaneous composition. 
An instructive example of the general 
characteristics of agglomerate- vents, and of the 
relation of these vents to the surrounding tuffs 
and basalts, is to be found at the island of 
Carrick-a-raide, on the north coast of Antrim, IH-UgblUi! j;lil i 
and on the opposite mainland. The visible 
mass of this neck is about 1000 feet in 
diameter, but the boundaries, except on the IBEfllRilH: 
land side, are concealed by the sea. The 
material filling up the vent is a coarse agglo- 
merate, in which blocks and bombs of basalt, 
with pieces of chalk and flint, are stuck at all 
angles in a dull dirty-green granular tuff. Some 
large and small intrusions of basalt rise through 
it. Owing partly to these intrusions, and partly 
to the grass -covered slope that separates it 
from the line of cliff, the actual contact of this 
neck with the volcanic beds of the escarpment 
cannot be seen. I have no doubt, however, 
that the tuff, which has already been referred 
to as so conspicuous a member of the series 
here, was discharged from this vent. 1 The ■ -I IS 
materials are as usual coarser in the pipe than 
beyond it, but the finer portion or matrix of 
the agglomerate is similar to many bands of 
the tuff. The structure of the locality may T 
be diagrammatically represented as in Fig. 301. 
The bedded tuff is thickest in the neighbour- 
hood of the vent, and gradually dies away on 
either side of it. 
But another important inference may be drawn from this locality. I 
have already pointed out that the lower basalts here reach their minimum 
thickness. Their basement beds thin away towards the vent as markedly 
as the tuff thickens. Obviously they cannot have proceeded from that 
point of eruption. Yet, that they had begun to be poured out before the 
discharge of the tuff is shown by their underlying as well as overlying 
1 See Explanation of Sheets 7 and 8, Geol. Survey of Ireland (1888), p. 81. 
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