ANNIVEESART ADDRESS OF THE PEESIDENT. 1 47 



laths with magnetite and enstatite, and with porphyritic crystals, 

 often large, of zoned plagioclase, as well as of ilmenite and haematite. 

 The large majority of the sheets, so far as I observed, are basalts. 

 Mr. Watts finds them, both in the lower and upper volcanic group, 

 to contain granular augite and magnetite set in a more or less 

 devitrified glass with microlites of felspar and porphyritic plagio- 

 clase, serpentinized olivine, and some well-marked augite. Among 

 the upper basalts on Nicker Hill certain rocks occur 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. 

 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 far be\ ond their present limits, the pro- 

 gress of denudation has revealed a number of bosses which probably 

 mark 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 have been already referred to 

 as on the whole more acid than the lavas of the basin, including, 

 indeed, felsites and even quartz-porphyries.^ 



IX. PEEMTAN. 



In our survey of the past geological history of this country 

 we have now arrived at a notable interval of quiescence in volcanic 

 action. The portion of geological time represented by the Coal 

 Measures, during which, so far as we know, there were no volcanic 

 eruptions within our borders, must have been of long duration ; 



^ Contemporaneous volcanic rocks have been believed to occur among the 

 Lower Carboniferous rocks of Bear Island, and on the opposite mainland west- 

 ward to White Ball Head at the entrance toBantryBay. There are some 

 interesting agglomerates in that district, but I did not detect any evidence that 

 they are truly bedded among the strata. They seemed to me rather to be in- 

 trusive masses of the nature of necks, for they certainly present in some places 

 clear evidence of having disrupted the strata, though elsewhere lying apparently 

 between them. For the present, therefore, I do not include them among the 

 proofs of Carboniferous volcanic activity. The agglomerate at White Ball 

 Head contains numerous large hornblende-crystals and large flakes of muscovite. 

 Dykes and veins of diabase and felsite are associated with these breccias. 



