120 
THE TERTIARY VOLCANOES 
BOOK VIII 
glass, or its more or less completely devitrified representative, often still 
recognizable with the microscope among the individualized microlites and 
crystals throughout the body of a dyke, is also not infrequent as a black 
vitreous varnish-like coating on the outer walls, and occasionally appears in 
strings and veins even in the centre. 
It is the assemblage of dykes presenting these features which I propose 
to describe. Obviously, the age of each particular dyke can only be fixed 
relatively for itself. But when this remarkable community of characters is 
considered, and when the post-Mesozoic age of at least a very large number 
of the dykes can be demonstrated, the inference is reasonable that one great 
system of dykes was extravasated during a time of marked volcanic disturb- 
ance, which could not have been earlier than the beginning of the Tertiary 
period. And this inference may be maintained even when we frankly admit 
that every dyke within the region is by no means claimed as belonging to 
the Tertiary series. 
In spite of their number and the extraordinary volcanic activity to 
which they bear witness, the dykes form a much less prominent feature in 
the landscape than might have been anticipated. In the lowlands of the 
interior, they have for the most part been concealed under a cover of super- 
ficial accumulations, though in the water-courses they not infrequently pro- 
ject as hard rocky barriers across the channels, and occasionally form 
picturesque waterfalls. On the barer uplands, they protrude in lines of 
broken crag and scattered boulders, which by their decay give rise to a 
better soil covered by a greener vegetation than that of the surrounding 
brown moorland. Among the Highland hills, they are often traceable from 
a distance as long black ribs that project from the naked faces of crag 
and corry. Along the sea-coast, their 
peculiarities of scenery are effectively 
displayed. Where they consist of a 
close-grained rock, they often rise from 
the beach as straight walls which, 
with a strangely artificial look, mount 
into the face of the cliffs on the one 
side, and project in long black reefs 
into the sea on the other (Fig. 233). 
Every visitor to the islands of the 
Clyde will remember how conspicuous 
such features are there. But it is 
among the Inner Hebrides that this 
kind of scenery is to be found in 
greatest perfection. The soft dark 
Lias shales of the island of Pabba, for 
example, are ribbed across with scores of dykes which strike boldly out to 
sea. Where, on the other hand, the material of the dykes is coarse in grain, 
or is otherwise more susceptible to the disintegrating influences of the 
weather, it has often rotted away and left yawning clefts behind, the vertical 
1 ,,. '•J, 
JiMui 
Fig. 234. — Fissure left by the weathering out 
of a dyke. 
