CHAP. XLI 
VEJVTS OF THE BASALT-PLATEAUX 
293 
vents. They may have been mere spiracles, or blow-holes, where the funnels 
drilled by explosive vapours were filled up with the debris of the rocks that 
were blown out. But that lava did rise within them is shown by the basic 
lapilli in the agglomerate, and by the basalt which in both vents lias found 
its way up the chimney. 
In the island of Baasay Mr. Teall, during the summer of 1894, observed 
a group of curious neck -like masses of breccia which pierce the Torridon Sand- 
stone near Brochel (Fig. 309). The blocks in them are large angular unaltered 
pieces of the surrounding sandstones and shales, sometimes ten feet or more 
in length, and the matrix is sometimes pure crystalline calcite like Iceland 
spar. The breccia is generally coarsest towards the outer margin. But 
though the Lewisian gneiss exists immediately below the thin cake of 
Torridonian strata, not a fragment of it could either Mr. Teall or I, when I 
visited the locality with him, find among the components of the breccia. 
N or did we detect any trace of volcanic material. The general ground-plan 
of these masses is elliptical, the most northerly measuring 30 yards in 
diameter. Where the junction of the breccia with the Torridon strata can 
be seen it is a nearly vertical one, the sandstones and shales being much 
jumbled and broken, but not sensibly indurated. This little cluster of 
patches of breccia can hardly be due to local crushing of the rocks. Their 
definite outlines and composition seem rather to indicate spiracles of Tertiary 
time, which never became vents erupting lava or ashes. The absence of 
fragments of the underlying gneiss may be accounted for if we suppose that 
the orifices were completely cleared out by the violence of the explosions 
and were afterwards filled up by the falling in of the walls of the higher 
parts now removed by denudation, which consisted of Torridon Sandstone 
and shale. 1 
Further research may detect at still greater distances from the 
basalt-plateaux ancient volcanic necks that might, with more or less prob- 
ability, be referred to the Tertiary period. As an instance of this kind, 
I refer to the neck at Bunowen, County Galway, recently described by 
Mr. MTIenry and Professor Sollas. Though so remote from the Tertiary 
basalt-plateaux, the rock of this boss is an olivine-basalt presenting a close 
resemblance to some of the rocks of Antrim. 2 
As a final illustration of Tertiary volcanic vents I will now describe the 
Faroe group already alluded to (vol. i. p. 63, vol. ii. p. 256). It was almost by 
a kind of happy accident that these vents were discovered. Noticing at a 
distance of a mile or more from the deck of a steam-yacht that the base of the 
great basalt cliffs on the west side of Stromo were varied by what looked 
like agglomerate, I steamed inshore, and was delighted to find, as the vessel 
drew near to the cliff, that the agglomerate assumed definite boundaries and 
occurred in several distinct patches, until at last it presented the unmistak- 
able outlines of a group of vents underlying and overspread by the bedded 
1 It is on one of these neck -like patches of breccia that Brochel Castle stands, of which Mac- 
culloch gave so sensational a picture in one of the plates of his Western Isles. 
- Trans. Boy. Irish Acad., 1896. 
