:4o 
THE TERTIARY VOLCANOES 
BOOK VIII 
no one who knows West-Highland geology can fail to recognize as of 
Torridonian derivation, at once makes it clear that the higher grounds from 
which they were borne probably lay to the north or north-east. The frag- 
ments of white sandstone may also have been derived from the same quarter, 
for the thick Jurassic series of Eigg once extended further in that 
direction. The pieces of quartzite and clay-slate bear similar testimony to 
an eastern or north-eastern source. In short, there seems every probability 
that this old Tertiary river flowed through a forest-clad region, of which 
the red Torridon mountains of Eoss-shire, the white sandstone cliffs of 
Eaasay and Skye, and the quartzite and schist uplands of Western Inverness- 
shire are but fragments, that it passed over a wide and long tract of the 
volcanic plateau, and continued to flow long enough to be able to carve out 
for itself a channel on the surface of the basalt. Its course across what is 
now the island of Eigg took a somewhat north-westerly direction, probably 
guided by inequalities on the surface of the lava-plain. It is there marked 
by the winding ridge of the Scuir, the pitchstone of which flowed into the 
river-bed and sealed it up. Several minor spurs, which project from the 
eastern side of the main ridge, show the positions of small tributary rivulets 
that entered the principal channel from the slopes of the basaltic tableland. 
One of these, on the south-east side of the hill called Corven, must have 
been a gully in the basalt with a rapid or waterfall. The pitchstone has 
flowed into it, and some of the rounded pebbles that lay in the channel of 
this vanished brook may still be gathered where the degradation of the 
pitchstone has once more exposed them to the light. That the Eigg river 
here flowed in a westerly direction may be inferred from the angle at which 
the beds of the small tributaries meet the main stream, and also from the 
fact that the old river-bed at the east end of the Scuir is considerably higher 
than at the west end. 
Several features in the geological structure of this locality serve to 
impress on the mind the great lapse of time represented by the erosion of 
the river-channel of Eigg. Thus at the narrowest point of the pitch- 
stone ridge, near the little Loch a’ Bhealaich, the bottom of the glassy 
lava is about 200 feet above its base on the south side, so that the valley 
cut out of the plateau-basalts must have been more than 200 feet deep. 
Even the little tributaries had cut ravines or canons in the basalts before 
the ground was buried under the floods of pitchstone. In the most northerly 
spur of the ridge, for example, the hill of Beinn Tighe, which represents 
one of these tributaries, shows a considerable difference between the level 
of the bottom of the pitchstone on the east and west sides. 
Again, all along the ridge of the Scuir, the basalt-dykes are abruptly 
cut off at the denuded surface on which the pitchstone rests. This 
feature is conspicuously displayed on the great sea-wall at the west end 
(Fig. 279). The truncation of the dykes demonstrates that a considerable 
mass of material must have been eroded before these lava-filled fissures 
could be laid bare at the surface. And the removal of this material shows 
that the denudation must have been continued for a long period of time. 
