92 
THE TERTIARY VOLCANOES 
BOOK VIII 
rhyolites and felsites, while the porphyritic felspars are arranged with their 
long axes in the direction of flow. A good example of these characters 
may be seen on the summit of the Dun Can — the remarkable truncated cone 
which forms the highest point on the Island of Baasay. The rock is a 
black olivine-basalt, partly amygdaloidal, with zeolites tilling up the cavities, 
and its flow-lines are prominent on the weathered faces where they lie 
parallel to the general bedding of the lavas. Another illustration may be 
observed in the basalt already cited from Loch-na-Mna, in the island of 
Eigg, where the rock presents in places a remarkable streaky structure 
which, though hardly visible on a fresh fracture, reveals itself on a weathered 
face in thin nearly parallel ribs coincident in direction with the upper and 
under surfaces of the mass. 
Great variety is to be found in the thickness of different sheets of lava 
in the plateaux. Some of them are not more than 6 or 8 feet ; others reach 
to 80 or 100 feet, and sometimes, though rarely, to even greater dimensions. 
In Antrim, the average thickness of the flows is probably from 15 to 20 
feet. 1 In the fine coast-sections at the Giant’s Causeway, however, some 
bands may be seen far in excess of that measurement. The bed that forms 
the Causeway, for instance, is about 60 or 70 feet thick, and seems to 
become even thicker further east. Along the great escarpment, 700 feet 
high, which rises from the shores of Gribon, on the west coast of Mull, there 
are twenty separate beds, which give an average of 35 feet for the thickness 
of each flow. On the great range of sea-precipices along the west coast of 
Skye, which present the most stupendous section of the basalts anywhere to 
be seen within the limits of the British Islands, the average thickness of the 
beds can be conveniently measured. At the Talisker cliffs some of the flows 
are not more than 6 or 8 feet ; others are 30 or 40 feet. The chief precipice, 
957 feet high (Fig. 286), contains at least 18 or 20 separate lava-sheets, 
which thus average of from 47 to 53 feet in thickness. In the cliffs that form 
the seaward margin of the tableland of Macleod’s Tables (Fig. 283) fourteen 
successive beds of basalt can be counted in a vertical section of 400 feet, 
which is equal to an average thickness of about 28 feet. But some of the 
basalts are only about 6 feet thick, while others are 50 or 60. The Hoe of 
Duirinish, 759 feet high, is composed of about sixteen distinct beds, which 
thus have a mean thickness of 46 feet. The average thickness of the 
successive flows on Dunvegan Head, which is 1000 feet high and contains 
at least twenty-five separate sheets, is about 40 feet. Still further north, 
the cliffs, 800 feet high, comprise sixteen successive flows, which have thus 
an average of 50 feet each. Among the Faroe Islands the average thickness 
of the basalt-sheets seems to be nearly the same as in Britain. Thus in the 
magnificent ranges of precipices of Kalso, Kuno and Boro, forty or more 
sheets may be counted in the vast walls of rock some 2000 feet high, giving 
a mean of about 50 feet. 
Each bed appears, on a cursory inspection, to retain its average thick- 
ness, and to be continuous for a long distance. But I believe that this 
1 See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11. 
