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