THE TERTIARY VOLCANOES 
BOOK VII! 
belonged to the volcanic period was proved by the manner in which it had 
been sealed up and preserved under the black glassy lava of the Scuir. Its 
history and the data from which this history is compiled will be narrated 
in a later part of this chapter. 
My examination of the islands of Canna and Sunday, however, brought to 
light other and more abundant evidence of river-action in the same region 
of the Inner Hebrides, but belonging to an earlier part of the volcanic period. 
This evidence leveals that a powerful river, flowing westwards from the 
Highland mountains, swept over the volcanic plain, while the sheets of 
basalt were still being poured forth, arid while volcanic eruptions were 
taking place from cones of slag. 
The basalt-plateau of Canna resembles in all essential particulars those 
of the other Western Isles. 
Its base is everywhere con- 
cealed under the sea, but 
from the fragments of 
Torridon Sandstone in its 
agglomerates we may infer 
that it probably rests on 
that formation, like the 
volcanic outliers in Eum. 
It is formed of successive 
sheets of different basalts 
including the usual banded, 
amygdaloidal and columnar 
forms. Some of them to- 
wards the west are speci- 
ally marked by the great 
abundance and large size 
of their porphyritic fel- 
spars. The magnetic pro- 
perties of the basalts at 
the east end of the island 
have long been known, and 
have given rise to various 
modern myths regarding 
their influence on the com- 
passes of passing vessels. 
But it is in its con- 
glomerates, tuffs and ag- 
glomerates and the light 
Fig. 268. — Section of the cliffs below Compass Hill, Isle of Canna. they east Oil some aspects 
of the volcanic period, 
elsewhere hardly recorded, that the geology of Canna possesses a special 
importance. To these, therefore, we may at once turn. 
The conglomerates are best developed at the eastern end of the island, 
