46 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
the foregoing section east from Shehan’s Cross-roads (Fig. 194). From the 
summit of this ridge, which is about 600 feet above the sea, the eye looks 
northward over the plain, across which low outliers of the volcanic series 
are scattered, and southwards across the basin to the corresponding line of 
volcanic heights forming the southern rim. 
I he upper volcanic group has been estimated by the officers of the 
Geological Survey to lie about 1000 feet higher in the Carboniferous 
system than the lower, the intervening strata consisting of the Upper 
Limestone. 1 It is possible that the interval is greater in some parts of the 
district than in others, and if so, the difference may he due either to greater 
local accumulation of volcanic materials, or to local prolongation of the 
eruptions into higher stratigraphical horizons. The outcrop of the upper 
volcanic band forms about half of a ring round the little cup of Millstone 
Grit or Coal-measures which lies within the volcanic basin. On the north- 
west side of the cup the volcanic rocks disappear. Hence the upper band 
has a much more restricted area than the lower. But if the tuffs 
immediately around Limerick are assigned to the upper group, its extent 
will he proportionately increased. There can be little doubt, however that 
neither in thickness nor in superficial area did the lavas and tuffs of the 
second group equal those of the first. The volcanic energy was graduallv 
dying out. J 
The lavas of the second period are characteristic dull, black, compact 
basalts, like those of » the first period, becoming here and there strongly 
amygdaloidal, and being occasionally separated by slaggy or conglomeratic 
partings. But they include also certain rocks wherein the felspar diminishes 
in quantity, while augite and olivine become conspicuous, together with a 
little enstatite. The augite occurs in large porphyritie forms, as well as of 
medium size and in small prisms. The olivine, as usual, is now in the 
condition of serpentine. These rocks are more basic than the ordinary 
basalts, containing only 3 8 ‘66 per cent of silica, and thus approach]' no- the 
limburgites. With these basic lavas are associated dull green tuffs and 
conglomerates, made up largely of basalt-debris, together °with abundant 
pieces of finely vesicular basic pumice and lapilli of a palagonitic material 
The manner in which the lavas and tuffs have alternated with each 
other, and also with the limestones, is well seen on Nicker Hill above Balias 
Grean. 2 The Survey sections show eight sheets of lava, separated by six 
bands of tuff and eight intercalations of limestone, the whole passirm under 
the Coal-measures. 
The upper volcanic group may he as much as 600 or 800 feet thick 
It appears to have been left, at the close of the eruptions, with a very 
uneven surface, some portions being so low as to be overspread with the 
Upper Limestones, other parts so high as not to be covered until the Coal- 
measure shales and flagstones came to be deposited. 3 
1 Explanation of Sheet 154, p. 24. 
2 See Explanation of Sheet 144, p. 30, where a description with detailed map and 
this ground will he found. a Explanation of Sheet 154, pp. 24, 35. 
sections of 
