Fig. 194.—Section of the volcanic escarpment, east of Shehan's Cross-roads, south of Limerick.
1. Limestone; 2 2. Tuffs; 3 3. Lavas.
But besides the andesites there occur also, and, so far as I have observed, in larger number, sheets of true basalt. This rock is typically black, exceedingly close-grained in the central portion of each sheet, but becoming highly slaggy and vesicular along the upper and lower parts. Under the microscope it is found to contain granular augite and magnetite, set in a more or less devitrified glass, with microlites of felspar, porphyritic plagioclase, serpentinized olivine, and some well-marked augite. These rocks form distinct escarpments along the northern rim of the basin as in 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.
The 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.[80] It is possible that the interval is greater in some parts of the district than in others, and if so, the difference may be 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 be 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 gradually dying out.
[80] Explanation of Sheet 154, p. 24.
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 porphyritic 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 38·66 per cent of silica, and thus approaching 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 Pallas Grean.[81] The Survey sections show eight sheets of lava, separated by six bands of tuff and eight intercalations of limestone, the whole passing under the Coal-measures.
[81] See Explanation of Sheet 144, p. 30, where a description with detailed map and sections of this ground will be found.
The upper volcanic group may be 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.[82]
[82] Explanation of Sheet 154, pp. 24, 35.