214 
THE TERTIARY VOLCANOES 
BOOK VIII 
A Chioch, and thence northwards into Beinn Ehada. The same lavas are 
likewise found in two outliers, capping Beinn a’ Chraig, a mile further north, 
and I have found fragments of them on some of the loftier ridges to the 
south-east. This highest and youngest group of lavas in the plateaux lias 
been reduced to mere isolated patches, and a little further denudation will 
remove it altogether. Yet it is not less than about 800 feet thick,. and 
consists of bedded andesitic or trachytic lavas, which alternate with and follow 
continuously and conformably upon the top of the ordinary plateau-basalts. 
These dull, finely crystalline or compact, light-grey rocks weather with a 
characteristic platy form, which has been mistaken for the bedding of tuffs. 
The fissility, however, has none of the regularity or parallelism of true 
bedding, and may be observed to run sometimes parallel with the bedding 
of the sheets, sometimes obliquely or even at right angles to it. ' Even 
where this structure is best developed, the truly crystalline nature of the 
rocks can readily be detected. Some of them are porphyritic and 
amygdaloidal, the very topmost bed of the mountain being a coarse 
amygdaloid. Intercalated with these curious rocks there are others in 
which the ordinary characters of the dolerites and basalts of the plateaux 
can be recognised. The amygdaloids are often full of delicate prisms of 
epidote. 
In Mull, as in the other areas of terraced basalts, we everywhere meet 
with gently inclined sheets, which do not thicken out individually or 
collectively in any given direction, except as the result of unequal denuda- 
tions. So far as I have been able to discover, they afford no evidence of any 
great volcanic cone from which they proceeded. Their present inclinations 
are unquestionably due, as in Ireland, to movements subsequent to the for- 
mation of the plateau. In Loch-na-keal they dip gently to the E.IST.E. ; 
in Ulva and the north-west coast to N.N.E. ; near Salen to W.S.W. on 
the one side, and N.W. on the other. Bound the southern and eastern 
margins of the mountainous tract of the island, they dip generally inwards 
to the high grounds. 
The Mull plateau presents a striking contrast to that of Antrim, in the 
extraordinary extent to which it has been disrupted by later protrusions of 
massive basic and acid rocks over a rudely circular area, extending from the 
head of Loch Scridain to the Sound of Mull, and from Loeh-na-keal to Locli 
Buy. The bedded basalts have been invaded by masses of dolerite, gabbro, 
and granophyre, with various allied kinds of rock. They have not only 
been disturbed in their continuity, but have undergone considerable meta- 
morphism. 
Again, further to the north, in the promontory of Ardnamurchan, the 
plateau has been disrupted in a similar way, and only a few recognisable 
fragments of it have been left. These changes will be more appropriately 
discussed in connection with similar phenomena in the other plateaux 
further north. 
