216 
THE TERTIARY VOLCANOES 
BOOK VIII 
situated, The streams of basalt spread around it, not only covering the 
surrounding low tracts of Jurassic rocks, but gradually accumulating against 
the hills, and thus reducing them both in area and in height above the 
plain, 1 Viewed from Canna the western coast of Rum presents a striking- 
picture of the general relations of the volcanic masses of the Inner Hebrides 
and of the enormous denudation which they have undergone (Fig. 267). The 
Torridon Sandstones are there seen to mount into ranges of hills, capped with 
outliers of the basalt-plateau, while behind rise the great eruptive bosses of 
gabbro and granophyre. The edges of the sheets that form the outliers 
would, if prolonged, cover the northern or lower half of the island, where 
pre-Cambrian rocks form the surface. In the southern half, the continuity 
Fig. 267. — View of Rum from the harbour of Canna. 
The ground indicated by single birds' is the area of Torridon Sandstone ; two birds, the plateau basalts ; three 
birds, the gabbro just seen at one point above the granophyre hills ; four birds, the granophyre. 
of the basalt has been partly obscured and partly destroyed by the pro- 
trusion of the great masses of gabbro that form the singularly picturesque 
mountain group to which this island owes its prominence as a landmark far 
and wide along the West Coast of Scotland. 
The most varied and interesting of the fragments of the basaltic plateau 
in the area of the Small Isles is that which forms the island of Canna, with 
its appendage Sanday. Canna measures five miles in length by from half a 
mile to a mile in breadth, and consists entirely of the rocks of the plateau 
and their accompaniments. The basalts are exposed along the north coast 
1 That the lava-iields did not completely bury this nucleus of older rooks has been supposed 
to be shown by the fragments of red sandstone found in the ancient river-bed of Eigg, which was 
scooped out of the basalt- plateau and sealed up under pitchstone. But I am disposed to think 
that these fragments, together with those of Jurassic sandstone, came, not from Rum, but from 
some district more to the north and east, as will be explained in a later page. At Canna, a few 
miles to the west, fragments of red sandstone not improbably derived from Rum are abundant in 
the conglomerates between the basalts. 
