s of these pebbles are found.

[Illustration: Fig. 34.--View of the Scuir of Eigg from the east. The
lower portion of the mountain is formed of bedded basalt, or dolerite
with numerous dykes and veins of basalt, felstone, and pitchstone; the
upper cliff, or Scuir, is composed of pitchstone of newer age, the
remnant of a lava flow which once filled a river channel in the basaltic
sheets. A dyke, or sheet, of porphyry is seen to be interposed between
the Scuir and the basaltic sheets.--(After Geikie.)]

_Effects of Denudation._--The position of the Scuir of Eigg and its
relations to the basaltic sheets show the enormous amount of denudation
which these latter have undergone since the stream of pitchstone-lava
filled the old river channel. The walls, or banks, of the channel have
been denuded away, thus converting the pitchstone casting into a
projecting wall of rock. That it originally extended outwards into the
ocean to a far greater distance than at present is evident from the
abruptly truncated face of the cliff; and yet this remarkable volcanic
mass seems to have been, perhaps, the most recent exhibition of volcanic
action to be found in the British Isles. It is perhaps, on this account,
the most striking of the numerous examples exhibited throughout the West
of Scotland and the North-east of Ireland of the enormous amount of
denudation to which these districts have been subjected since the
extinction of the volcanic fires; and this at a period to which we
cannot assign a date more ancient than that of the Pliocene. Yet, let us
consider for a moment to what physical vicissitudes these districts have
been subjected since that epoch. Assuming, as we may with confidence,
that the volcanic eruptions were subaerial, and that the tracts covered
by the plateau-basalts were in the condition of dry land when the
eruptions commenced, in this condition they continued in the main
throughout the period of volcanic activity. But the eruptions had
scarcely ceased, and the lava floods and dykes become consolidated,
before the succeeding glacial epoch set in; when the snows and glaciers
of the Scottish Highlands gradually descending from their original
mountain heights, and spreading outwards in all directions, ultimately
enveloped the whole of the region we are now considering until it was
entirely concealed beneath a mantle of ice moving slowly, but
irresistibly, outwards towards the Atlantic, crossing the deep channels,
such as th