CHAP. XXX 
THE LIMERICK BASIN 
47 
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 re- 
vealed 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 ortho- 
clase. 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 porphyntes, some 
perhaps being bostonites. 1 . 
A geologist, familiar with the Carboniferous and Permian necks of 
Fig. 195.— View of Derk Hill, a volcanic neck on tie south side of the Limerick basin. 
Scotland, has no hesitation in confirming the surmise of Jukes and his 
colleagues that the cones and domes around the Limerick basin mar 
the sites of eruptive vents. On the south side of the basin, at l ea ^ nin ® 
such necks rise into view, partly from among the lavas and tufts, n 
1 Guide to the Collections of Rocks, etc., Geol. Survey, Ireland, p. 93, Dublin 1895. 
