CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
283 
south, beyond the bottom of the agglomerate declivities, not a vestige 
of these erupted materials can now be found. Westward the escarpment of 
Strathaird remains to assure us that no thick showers of ashes fell at even 
so short a distance as two miles, either before or during the outpouring of 
the successive basalt sheets still remaining there. We may therefore con- 
clude with some confidence that here, as at Ardnamurehan, the \ent is 
younger than at least the older parts of the basalt-plateau. Unfortu- 
nately the uprise of the large bosses of granopliyre that stretch from the Bed 
Hills to Loch Sligachan has entirely destroyed the vent and its connections 
in that direction. There is no certain proof that any molten rock ever 
issued from this orifice, unless we suppose the fragmentary patches of 
amygdaloid on the southern flank of Beinn Dearg Bheag to be portions of 
flows that proceeded from this centre of eruption. The basalt-plateau which 
still remains in Strathaird no doubt formerly extended eastwards over 
Strath and northwards across the site of the Bed Hills and Cuillins, joining 
on to the continuous tableland north of Lochs Brittle and Sligachan. How 
much of the plateau had been built up here before the outburst of the vent 
cannot be ascertained. The agglomerate may possibly, of course, belong to 
the very latest period of the plateau-eruptions, or even to a still younger 
phase of Tertiary volcanic history. The impression, however, made on my 
mind by a study of the evidence from the Western and Faroe Isles is that 
the necks of agglomerate, like those of dolerite and basalt, really belong to 
different epochs of the plateau period itself ; and mark some of the vents 
from which the materials of the plateaux were successively emitted. 
The example of Carrick-a-raide (p. 277) is peculiarly suggestive when we 
regard it in connexion with the great Strath vent. Already the progress of 
denudation has removed at least half of the layer of dust and stones which, 
thrown out from that little orifice, fell over the bare chalk-wolds and black 
basalt-fields of Antrim. The neck that marks the position of the volcanic 
funnel has been largely cut away by the waves, and is almost entirely 
isolated among them. The vents at Ganna, Portree and the Faroe Isles, to 
be afterwards described, unquestionably belong to the eruptions of the 
plateau-period, for their connection with the basalts can be clearly estab- 
' lished. At the Strath vent, however, the march of destruction has been 
greater. The connexion between this vent and the materials ejected from 
it has been entirely removed, and we can only guess from the size of the 
remaining neck what may have been the area covered by the discharges 
from this largest of all the volcanic cones of the Inner Hebrides. 
Other masses of similar agglomerate are observable in the same region 
of Skye, where they not improbably mark the sites ot other vents. Un- 
fortunately their original limits and relations to the rocks through which 
the eruptive orifices were drilled have been much obscured by the uprise 
of the great masses of gabbro and granopliyre of the Guillin Hills. Several 
of these isolated intrusions occur in the midst of the gabbro, as in Harta 
Corry and on the west side of the Blaven ridge. Another mass is interposed 
between the gabbro and granopliyre on Druim an Eidhne and at the base 
