208 
THE TERTIARY VOLCANOES 
BOOK VIII 
ferruginous clay though, as I have said, less frequent perhaps than in the 
lower group, continue to show that the intervals between successive erup- 
tions were of sufficient duration to admit of some subserial decay of 
the surface of a lava before the outflow of the next bed. Occasional thin 
layers of tuff also, and even of pisolitic iron-ore, have been observed 
among these higher basalts. But the most interesting and important inter- 
calations are inconstant seams of lignite. One of the most conspicuous of 
these lies immediately above the basalt of the “ Causeway,” where it was 
long worked for fuel, and was found to be more than six feet thick. But 
it is quite local, as may he seen at the " Organ ” over which it lies, having a 
thickness of only 12 inches and rapidly dying out so as to allow the basalts 
above and below it to come together. The removal of the upper portion 
of the basalts by denudation has destroyed the records of the latest part of 
the volcanic history of the Irish plateaux. 
It is obvious that nowhere in Antrim does any trace exist of a central 
vent or cone from which the volcanic materials were discharged. There is 
no perceptible thickening of the individual basalt-sheets, nor of the whole, 
series in one general direction, in such a manner as to point to the site of 
some chief focus of eruption. Nor can we place reliance on the inclination 
of the several parts of the plateau. I have pointed out that the varying 
dip of the beds must he attributed mainly to post- volcanic, movements, or at 
least to movements which, if not later than all the phases of volcanic action, 
must have succeeded the outpouring of the plateau-basalts. There has been 
a general subsidence towards the central and southern tracts now occupied 
by the valley of the Bann and Lough Neagh. But nowhere in the depres- 
sion is there any trace of the ruins of a central cone or focus of discharge. 
The Antrim plateau, in these respects, resembles the others. But as has 
already been remarked, it differs from them in one important particular. 
It has nowhere been disrupted by huge bosses of younger rocks, such as 
have broken up the continuity of the old lava-fields further north. Yet 
it also is not without its memorials of younger protrusions. It 
contains not a few excellent examples of true volcanic vents, and, as above 
stated, it includes some small acid bosses that may represent the great 
protrusions of the Inner Hebrides, and may have been connected with 
superficial outflows of rhyolitic lava and showers of rhyolitic tuff. 
ii. MULL, MOUVEN AND ARDNAMURCHAN 
This plateau covers nearly the whole of the island of Mull, embraces a 
portion of Morven on the Argyleshire mainland, and, stretching across Loch 
Sunart, includes the western part of the peninsula of Ardnamurchan (Map VI.). 
That these now disconnected areas were once united into a continuous lava- 
field which extended far beyond its present limits is impressively indicated 
by their margin of cliffs and fringe of scattered islands and outliers. The 
plateau went west, at least, as far as the Treshnish Isles, which are composed of 
basalt. On its eastern border, a capping of basalt on the top of Beinn Iadain 
