444 
THE TERTIARY VOLCANOES 
BOOK VIII 
show themselves partly at the margins of the visible bosses, but much more 
profusely in that tract of altered basalt, with intrusive sheets and dykes of 
basalt, dolerite and gabbro, which lies within the great ring of heights 
between Loch na Keal and Locli Spelve. In some areas, the amount of 
injected material appears to equal the mass of more basic rock into which it 
has been thrust. Pale grey and yellowish porphyries and granophyres, 
varying from thick dykes down to the merest threads, ramify in an intricate 
network through the dark rocks of the hills, as shown in the accompanying 
illustration (Fig. 380), which represents a portion of the hillside between 
Beinn Fhada and the Clachaig River. Such a profusion of veins probably 
indicates the existence here of some large mass of granophyre or granite, 
at no great depth beneath the surface. 
In Mull, as in the other islands of the Inner Hebrides, two horizons on 
which protrusions of acid materials have been specially abundant, are the 
base of the bedded basalts of the plateau and the bottom of the thick sheets 
of gabbro. Dykes and veins of granophyre, quartz-porphyry, felsite and 
other allied rocks are sometimes crowded together along these two horizons, 
though they may be infrequent above or below them. 
Illustrations of solitary veins in the midst of unaltered plateau-basalts 
or in older rocks may be gathered from many parts of the Western Isles. 
Some remarkable instances are to be seen among the basalts that form the 
terraced slopes on the north side of Loch Sligachan. Several thick dykes 
of granophyre run up the declivity, cutting across hundreds of feet of the 
nearly level basalt-beds. Some of them can be seen on the shore passing 
under the sea. They trend in a S.S.E. direction towards Glamaig, and they 
are not improbably apophyses from that huge boss, the nearest edge of 
which is three-quarters of a mile distant. Another example may be cited 
from the basalt-outlier of Strathaird, where two veins of felsite, one of them 
a pale flinty rock showing flow-structure parallel to the walls, may be 
seen on the west front of Ben Meabost. In this case, the veins are 
three miles and a half from the granophyre mass of Strath na Creitheach 
to the north, four miles from that of Beinn an Dubhaich to the north- 
east, and nearly three miles from that of Coire Uaigneich at the foot of 
Blath Bheinn. 
A special place must be reserved for the pitchstone-veins. Ever since 
the early explorations of Jameson and Macculloch, the West of Scotland has 
been noted as one of the chief European districts for these vitreous rocks. 
From Skye to Arran, and thence to Antrim, many localities have furnished 
examples of them, but always within the limits of the Tertiary volcanic 
region. That all of the pitchstones are of Tertiary age cannot, of course, be 
proved, for some of them are found traversing only Paleozoic rocks, and of 
these all that can be absolutely affirmed is that they must be younger than 
the Carboniferous or even the Permian system. But, as most of them are 
unquestionably parts of tire Tertiary volcanic series, they are probably all 
referable to that series. Hot only so, but there is, I think, good reason to 
place them among its very youngest members. It is a significant fact that 
