[71] Guide to the Collections of Rocks and Fossils belonging to the Geological Survey, in the Museum of Science and Art, Dublin (1895), pp. 38, 39.
As regards the history of volcanic action in Britain one of the chief points of interest connected with these Irish breccias and lavas relates to their geological age. As no proof has been produced that any portion of them is contemporaneously interstratified in the Carboniferous Limestone which surrounds them, we cannot definitely affirm that the volcanic eruptions which they record took place during the accumulation of that formation. The vents must, of course, be later than that portion of the limestone which they pierce. But the evidence seems to me to be on the whole most favourable to the view that they are of Carboniferous Limestone age, for the following reasons:—
1. The breccias of Croghan Hill do not present a resemblance to any of those belonging to the Tertiary volcanic series in Antrim or the Inner Hebrides. The possibility of their being of Tertiary age may therefore be dismissed from consideration.
2. There are no known Permian volcanic rocks in Ireland. Nor does the Croghan Hill breccia show any resemblance to the ordinary material of the breccias in the Permian necks of Scotland. It is thus not likely to be of Permian age.
3. The peculiar basic pumice of these Croghan Hill vents has many points in common with the palagonite fragments so abundant among the volcanic breccias and tuffs of Carboniferous age in Scotland, Derbyshire, and the Isle of Man, and which occurs also among the Carboniferous tuffs of the Limerick basin. It differs from the general type of the material in its pale colour, in its uniformity of character, in its calcareous cement, and above all in its vast preponderance over all the other materials in the breccia.
4. The saturation of the Croghan Hill breccia with calcite is a singular feature in the composition of the rock. Had the vents been opened long subsequent to the deposition of the Carboniferous Limestone, it is difficult to understand how this calcite could have been introduced. Mere percolation of meteoric water from the adjacent limestone does not seem adequate to account for the scale and thoroughness of the permeation. But if the vents were opened on the floor of the Carboniferous Limestone sea, it is intelligible that much fine calcareous silt should have found its way down among the interstices of the breccia and into the pores of the pumice which, being caked together within the vent, did not all float away when the sea gained access to the volcanic funnel. The effect of subsequent percolation would doubtless be to carry the lime into still unfilled crevices, and to impart to the cement a crystalline structure similar to that which has been developed in the ordinary limestones.
2. THE LIMERICK BASIN
About 70 miles to the south-west of the area just described lies the most compact, and, for its size, one of the most varied and complete, of all the Carboniferous volcanic districts of Britain (Map I.). It takes the form of an oval basin in the Carboniferous Limestone series near the town of Limerick, about twelve miles long from east to west and six miles broad from north to south. Round this basin the volcanic rocks extend as a rim about a mile broad. A portion of a second or inner rim, marking a second and higher volcanic group, partially encloses a patch of Millstone Grit or Coal-measures, which lies in the heart of the limestone basin. (See the section in [Fig. 196].)
But it is evident that, as the denuded edges of the volcanic sheets emerge at the surface all round the basin, the present area over which these rocks extend must be considerably less than that which they originally covered. Some indication of their greater extension is supplied by outliers of the bedded lavas and tuffs, as well as by bosses which doubtless indicate the position of some of the eruptive vents. The distance between the furthest remaining patches is 24 miles. The original tract over which the volcanic materials were spread cannot have been less than 24 miles long by 10 miles broad. If we assume its area to have been between 250 and 300 square miles we shall probably be under the truth.
This volcanic centre made its appearance on the floor of the Carboniferous Sea in the same district which had witnessed the eruptions of Upper Old Red Sandstone time. The two visible vents that crown the Knockfeerina and Ballinleeny anticlines (Chapter xxii.), are only some ten miles distant, and there may be others of the same age even under the Limerick basin. This district thus supplies another instance of that recurrence of volcanic energy in the same area, after a longer or shorter geological interval, which stands out as a conspicuous feature in the history of volcanic action in Britain. That a prolonged interval elapsed between the extinction of the Old Red Sandstone volcanoes and the outbreak of their successors during the accumulation of the Carboniferous limestone series, may be inferred from the thickness of strata which separate their respective tuffs. From the published sections of the Geological Survey there would appear to be about 500 feet of Old Red Sandstone above the volcanic series of that formation. Then comes the Lower Limestone shale, which is computed to be about the same thickness. From the scarcity of observable dip among the Lower Limestones and their variable inclination, it is not easy to form any satisfactory estimate of the depth of this group up to the base of the volcanic series. It may be as much as 800 feet,[72] and if so there would thus intervene a mass of sedimentary material nearly 2000 feet in thickness between the two volcanic platforms. Throughout this thick accumulation of stratified deposits no trace of contemporaneous volcanic activity has been detected. From the descriptions published more than thirty years ago by Jukes and his colleagues in the Geological Survey of Ireland, geologists learnt how full and interesting are the proofs of great volcanic activity contemporaneous with the deposition of the Carboniferous Limestone series in the Limerick district.[73] Nowhere, indeed, is the evidence more complete for the occurrence of a long succession of volcanic eruptions during a definite period of geological time. The officers of the Survey showed that two epochs of activity during the older part of the Carboniferous period were each marked by a group of tuffs and lavas, while the interval of quiescence between them is represented by a thousand feet of limestone. The same observers likewise mapped outside the volcanic ring a number of eruptive bosses, which they regarded as probably marking some of the actual vents of that time.