210 
THE TERTIARY VOLCANOES 
BOOK VIII 
placement is probably more than 1000 feet. Many other minor faults 
in the same district show how much the crust of the earth has been 
fractured here since older Tertiary time. 
A little to the west of Mull, and belonging originally to the same 
plateau, lies the isle of Staffa, the famous columnar basalts of which first 
attracted the attention of travellers, and gave to the Tertiary volcanic rocks 
of Scotland their celebrity (Fig. 266a). 
In spite of the extent to which it has suffered from denudation 
and subterranean disturbance, and indeed in consequence thereof, the 
Mull plateau presents clear sections of many features in the history of the 
basalt-outflows and of the subsequent phases of Tertiary volcanic action 
which cannot be seen in the more regular and continuous tableland of 
Fig. 266h. — V iew of the south side of Statfa, showing the bedded and columnar structure of the basalt. 
The rock in which the cave to the left hand has been eroded is a conglomeratic tuff underlying the 
basalt ; to the right is Fingal’s Cave. These caverns hear witness to the enormous erosive power 
of the Atlantic breakers. 
Antrim. Moreover, it still possesses in its highest mountain, Ben More 
(3169 feet), a greater thickness, and probably a higher series, of lavas than 
can now he seen in any other of the plateaux. 
The difficulties, already referred to in regard to Antrim, of tracing the 
probable form of ground on which the volcanic eruptions began, are even 
greater in the case of the Mull plateau. We can dimly perceive that the 
depression in the crystalline rocks of the Highlands which had, from at 
least the older part of the J urassic period, stretched in a N.N.W. direction 
along what is now the western margin of Argylesliire, lay beneath the sea 
in Jurassic time, and was then more or less filled up with sedimentary 
