accumulation of basaltic lava-flows, piled layer upon layer, with
intervening beds of bole and tuff, up to a thickness, according to
Geikie, of about 3,500 feet. At the grand headland of Gribon, on the
west coast, the basaltic sheets are seen to rise in one sheer sweep to a
height of 1,600 feet, and then to stretch away with a slight easterly
dip under Ben More at a distance of some eight miles. This mountain, the
upper part of which is formed of beds of ashes, reaches an elevation of
3,169 feet, so that the accumulated thickness of the beds of basalt
under the higher part of the mountain must be at least equal to the
amount stated above--that is, twice as great as the representative
masses of Antrim. The base of the volcanic series is seen at Carsaig and
Gribon to rest on Cretaceous and Jurassic rocks, like those of Antrim;
hence the Tertiary age is fully established by the evidence of
superposition. This was further confirmed by the discovery by the Duke
of Argyll,[2] some years ago (1850), of bands of flint-gravel and tuff,
with dicotyledonous leaves amongst the basalts of Ardtun Head. The
basement beds of tuff and gravel contain, besides pebbles of flint and
chalk, others of sanidine trachyte, showing that highly acid lavas had
been extruded and consolidated before the first eruption of the
plateau-basalts; another point of analogy between the volcanic
phenomenon of Antrim and the Inner Hebrides. These great sheets of
augitic lava extend over the whole of the northern tract of Mull, the
Isles of Ulva and Staffa, and for a distance of several miles inwards
from the northern shore of the Sound of Mull, covering the wild
moorlands of Morvern and Ardnamurchan, where they terminate in
escarpments and outlying masses, indicating an originally much more
extended range than at the present day. The summits of Ben More and its
neighbouring height, Ben Buy, are formed of beds of ash and tuff. The
volcanic plateau is, according to Judd, abruptly terminated along the
southern side by a large vault, bringing the basalt in contact with
Palaeozoic rocks.[3]

(_b._) _Granophyres._--The greater part of the tract lying to the south
of Loch na Keal, which almost divides Mull into two islands, and
extending southwards and eastwards to the shores of the Firth of Lorn
and the Sound of Mull, is formed of a peculiar group of acid (or highly
silicated) rocks, classed under the general term of "Granophyres." These
rocks approach towards true