400 
THE TERTIARY VOLCANOES 
BOOK VIII 
even if filled up with fragmentary materials, would no doubt continue to be 
points of weakness. Round the flanks of the Loch Ba’ boss, and here and 
there on its surface, patches of intensely indurated volcanic agglomerate may 
be detected. A little to the south of the tarn called Loch na Dairidh, the 
granophyre is succeeded by the black, flinty felsite or rhyolite already referred 
to. This rock in some places exhibits a beautiful flow-structure, with large 
porphyritic felspars, and encloses a great many fragments of clolerite and 
gabbro, varying from the size of a pea up to blocks several inches in 
diameter. Lying on its surface are detached knolls of much altered 
dolerite, basalt, and coarse breccia or agglomerate. On its southern margin 
one of these patches of agglomerate contains abundant fragments of various 
felsitic rocks, among which are pieces of a compact rock with flow-structure 
like that found in place immediately to the north ; also rounded pieces of 
quartzite, and of compact and amygdaloidal basalt wrapped up in a very 
hard matrix which seems to consist largely of basalt-dust. Flo bedding can 
be made out in this rock, and the mass looks like part of a true neck. 
Further down the slope the bedded basalts appear. The actual junctions 
of the different rocks cannot be satisfactorily traced, but the structure of 
a, basalts ; &, dolerite ; c, volcanic agglomerate ; d, black felsite ; e, granopliyre. 
the ground appears to me to be as shown in Fig. 356. A patch of similar 
agglomerate appears a little to the south-west of the last section in front of 
a cliff of the felsite, and seems to be enclosed in the latter rock, and other 
exposures of agglomerate, underlain and intensely indurated by the felsite, 
may be noticed on the ground that slopes towards Loch Ba’. 
That these agglomerates do not belong to the period of the eruption of 
the granophyre and felsite, but to that of the bedded basalts, may be inferred 
from their intense induration next the acid rocks, and also from the fact that 
similar breccias are actually found here interposed between the bedded 
basalts. This is well shown on the hill above the Coille na Srbine, where 
the accompanying section can be seen (Fig. 357). The broad dyke-like 
mass of black flinty felsite already referred to runs as a prominent rib 
over the southern end of Beinn a’ Chraig into the head of the Scarrisdale 
glen (see Fig. 352). It cuts across the bedded basalts, and immediately to 
the south of where these appear, a thin intercalated bed of breccia crops out, 
of the usual dull-green colour, with abundant fragments of basalt and many 
of yellow and grey felsite. 
From these various facts we may, I think, conclude that along the strip 
of ground now occupied by the Loch Ba’ boss of granophyre and felsite, 
there once stood a line or group of vents, from which, besides the usual 
