_Vents._--All round the edges of the Limerick basin, where the escarpments of the volcanic groups, rising abruptly above the plain, show that these rocks once extended beyond their present limits, the progress of denudation has revealed a number of bosses which, as above stated, Jukes and his associates looked upon as marking some of the vents from which the lavas and tuffs were erupted. Especially striking is the line of these vents along the southern margin. The rocks now filling them present some unusual and rather anomalous features. They are decidedly more acid than the lavas of the basin, some of them even containing free quartz. Mr. Watts remarks that "though they have a good deal in common with the trachytes, they are crystalline throughout. They are red granite-looking rocks, which are made up chiefly of stumpy idiomorphic prisms of felspar which is mainly orthoclase. Some plagioclase also occurs, and the two felspars are imbedded in interstitial quartz. A trace of hornblende or mica is frequently present, and the rocks contain about 65 per cent of silica." These characters are specially observable in the necks furthest removed from the basin, which may possibly have been connected with the andesitic outflows. Nearer to the basin the necks "contain about 60 per cent of silica, seldom show any interstitial quartz, and stand between trachytes and porphyrites, some perhaps being bostonites."[83] [Footnote 83: _Guide to the Collections of Rocks, etc., Geol. Survey, Ireland_, p. 93, Dublin 1895.] [Illustration: Fig. 195.--View of Derk Hill, a volcanic neck on the south side of the Limerick basin.] A geologist, familiar with the Carboniferous and Permian necks of Scotland, has no hesitation in confirming the surmise of Jukes and his colleagues that the cones and domes around the Limerick basin mark the sites of eruptive vents. On the south side of the basin, at least nine such necks rise into view, partly from among the lavas and tuffs, but chiefly through the limestones that emerge from below these volcanic sheets. One of the most conspicuous of them, Derk Hill (Fig. 195), rises to a height of 781 feet above the sea, and comes through the bedded andesites, as represented in Fig. 196, which gives, in diagrammatic form, the general structure of the Limerick volcanic basin. Around the northern side of the basin a smaller number of necks has been observed, consisting of similar acid rocks. [Illustration: Fig. 196.--Section across the Limerick volcanic basin. 1. Lower limestone; 2. Lower series of lavas and tuffs; 3. Middle and Upper Limestone; 4. Upper series of lavas and tuffs; 5 5. Two volcanic necks; 6. Millstone Grit series.]