282 
THE TERTIARY VOLCANOES 
BOOK VITI 
weathers into large tabular forms, the parallel surfaces of which clip to south- 
west ; but this is probably due only to jointing. Here and there, dykes of 
basalt cut the rock in a general north-westerly direction, but their number 
is remarkably small when compared with the prodigious quantity of them in 
the limestone at the bottom and opposite side of the valley, some of which 
may possibly mark the fissure on which the vent was placed. More abund- 
ant and extensive are the masses of granophyre that rise particularly along 
the outer margin of the agglomerate near Loch Kilchrist. These may be 
connected with the great boss that forms the Bed Hills, of which further 
details will be given in Chapter xlvi. 1 
The important question of the relation of this agglomerate to the plateau- 
basalts does not admit of satisfactory treatment, owing to destruction of the 
evidence by the intrusion of the granophyre, and likewise to enormous 
denudation. Nevertheless, some traces still remain to indicate that the 
basalts once stretched over the site of the vent, which probably rose through 
them. Looking westward from the flanks of Beinn Dearg Bheag to the other 
side of Loch Slapiu, the geologist sees the bold basalt-escarpment of Strathaird 
presenting its truncated beds to him at a distance of only two miles. That 
these lavas were once prolonged eastwards beyond their present limits is 
obvious, and that they stretched at least over these two intervening miles 
can hardly be doubted. But we can still detect relics of them on the flanks 
of Beinn Dearg. As we follow the agglomerate round the margin of the 
granophyre that mounts steeply from it, we lose it here and there under 
beds of amygdaloidal basalt. The rocks next the great eruptive mass of the 
mountain are so indurated and shattered that it is difficult to separate them 
from each other and determine their relative positions. But, so far as I 
could ascertain, these basalts are fragments of beds that overlie the agglo- 
merate (Fig. 303). 
This is not the only place along the flanks of the Bed 
Hills where portions of the bedded basalts have 
survived. Other localities will be subsequently 
alluded to. 
The Strath vent has been drilled through 
the Cambrian limestone, and as the result of 
protracted denudation it now towers steeply 500 
or 600 feet above that formation on the floor 
of the valley. Of the material discharged from 
Fig. 303.— Diagram to show the it over the surrounding country no certain 
probable relations of the rocks t race now remains. We may infer from the 
nature of the rock which fills it that towards 
the end, if not from the beginning of its activity, 
its discharges consisted mainly of dust and 
stones. A cone, of which the remains are two miles in diameter, 
must surely have sent its fragmentary materials far and wide over 
the surrounding region. But on the bare platform of older rocks to the 
1 The granophyre intrusions in this agglomerate have been found by Mr. Harker to have taken 
up and dissolved a considerable proportion of fragments of gabbro, Chapter xlvi. p. 392. 
on the southern flank of Beinn 
Dearg Bheag. 
a , agglomerate; &, amygdaloidal and com 
pact basalt-rocks ; c, granophyre. 
