i g6 
THE TERTIARY VOLCANOES 
BOOK VIII 
basalts in the Antrim plateau there occur bands of a pale fawn-coloured 
conglomerate largely made up of more or less rounded fragments of rhyolite, 
like some ol the varieties of the rock which occur in place on the plateau. 
The rhyolitic debris is often mixed with pebbles of basalt. Sometimes it 
becomes so fine as to pass into pale clays. 
(S) Breccias of non-volcanic materials. — These, the most exceptional of 
all the fragmentary intercalations in the plateaux, consist almost wholly of 
angular blocks of rocks which are known to underlie the basalts, but with 
a variable admixture of basalt fragments. They are due to volcanic 
explosions which shattered the subjacent older crust of rocks, and dis- 
charged fragments of these from the vents or allowed them to be borne 
upwards on an ascending column of lava. Pieces of the non-volcanic 
platform are of common occurrence among the fragmentary accumulations, 
especially in the lower parts of the plateaux basalts. But I have never 
seen so remarkable an example of a breccia of this kind as that which occurs 
near the summit of Sgurr Dearg, in the south-east of Mull. The bedded 
basalt encloses a lenticular band of exceedingly coarse breccia, consisting 
mainly of angular pieces of quartzite, with fragments of amygdaloidal 
basalt. In the midst of the breccia lies a huge mass or cake of erupted 
mica-schist, at least 100 yards long by 150 yards wide, as measured across 
the strike up the slope of the hill. To the west, owing to the thinning out 
of the breccia, this piece of schist comes to lie between two beds of basalt. 
A little higher up, other smaller but still large blocks of similar schist are 
involved in the basalt, as shown in Pig. 262. As the huge cake of mica- 
schist plunges into the hill, its whole dimensions cannot be seen ; but there 
are visible, at lea.st, 15,000 cubic yards, which must weigh more than 
30,000 tons. Blocks of quartzite of less dimensions occur in the basalts 
on Loch Spelve, in the same district. There can be no doubt, I think, that 
these enormous fragments were torn off from the underlying crystalline 
schists which form the framework of the Western Highlands, and were 
floated upward in an ascending How of molten basalt. Had the largest 
mass occurred at or near the base of the volcanic series, its size and position 
would have been less remarkable. But it lies more than 2000 feet up in 
the basalts, and hence must have been borne upward for more than that 
height. A similar but less striking breccia occurs on the south coast of 
the same island, near Carsaig, made up chiefly of pieces of quartzite 
and quartz. 1 
Some remarkable agglomerates, near Forkhill, Armagh, probably belong- 
ing to the Tertiary volcanic series, will be described in the account of the 
Irish acid rocks (Chapter xlvii.). I hey consist entirely of non-volcanic 
stones and dust and are traceable for some miles along the line of a fissure. 
Where they have been discharged through granite they consist entirely of 
the detritus of that rock, but where they have been erupted in the Silurian 
area they consist of fragments of grits and shales. They seem to have been 
produced by teriform discharges, without the uprise of any volcanic magma, 
1 This is noticed by Mr. Starkie Gardner, Quart. Journ. Geol. Soc. xliii. (1887), p. 283, note. 
