35 § 
THE TERTIARY VOLCANOES 
BOOK VIII 
stituent minerals have crystallized out in more definite and conspicuous 
forms, here and there succeed each other so quickly as to impart a bedded or 
foliated look to the body of rock, recalling, as in Skye, the aspect of some 
coarsely crystalline granitoid gneiss. In these respects the Mull gabbro 
closely resembles that of the Cuillin Hills. Occasionally, on the exposed 
faces of crags, portions of such bands or veins are seen to be detached and 
enveloped in a finer surrounding matrix. The thick belts or bands of 
coarser and finer texture alternate, and give an appearance of bedding to the 
mass. Nevertheless they are really intrusive sills, which run generally 
parallel with beds of finer gabbro or with sheets of highly indurated basalt, 
that may be detached portions of the ordinary rocks of the plateau. The 
thick sheet of Ben Buy, like the mass of the Cuillin Hills, is thus the result 
not of one but of many uprises of gabbro. 
Of the thinner sheets of dolerite and gabbro in Mull little need here be 
said. I have referred to their great abundance in the range of eastern 1 fills 
that rise from the Sound of Mull between Loch Spelve and Fishnisli Bay. 
Though obviously intrusive, they lie on the whole parallel to the bedding of 
the basalts. The latter rocks exhibit the usual dull indurated shattery 
character which they assume where large bosses of gabbro have invaded 
them, and which gradually disappears as we follow them down hill away 
from the intrusive sheets to the shores of the Sound. They dip towards 
the centre of the hill group, that is, to south-west in the ridge of Mainnir nam 
Fiadh, Dun da Ghaoithe, and Beinn Meadhon, the angle increasing south- 
wards to 1 5°-20°, and at the south end reaching as much as 35°-40°. Some 
fine crags of gabbro and dolerite form a prominent spur on the east side of 
the ridge of Ben Talaidh, in the upper part of Glen Forsa. These consist 
of successive sheets bedded with the basalts, and dipping south-west. A large 
sheet stands out conspicuously on the north front of Ben More, lying at the 
base of the “ pale lavas,” and -immediately above the ordinary basalts. It 
circles round the fine corry between Ben More and A’Cliioch, some of its 
domes beirrg there beautifully ice-worn. This is the highest platform to 
which I have satisfactorily traced any of the intrusive sheets of Midi. 
Another dyke-like mass emerges from beneath the talus slopes of A’Chiocli, 
on the southern side, and runs eastward across the col between the Clachaig 
Glen and Loch Scridain. 
5. The Gabbros of St. Kilda and North-east Ireland 
Sixty miles to the westward of the Outer Hebrides lies the lonely group 
of islets of which St. Kilda is the chief. As the main feature of geological 
interest in this group is the relation of the acid protrusions to the other 
rocks, the account of the geology will be more appropriately given as a whole 
in Chapter xlvii. I need only remark here that the predominant rocks of 
these islands are dark basic masses, chiefly varieties of gabbro, but including 
also dolerites and basalts. Beasons will be afterwards brought forward for 
regarding these rocks as parts of the Tertiary volcanic series. They present 
