154 THE ROCKS OF THE TOURMAKEADY DISTRICT. [May I909, 



the west had nothing in common with the underlying rocks, 

 inasmuch as they had a very different strike, rested upon different 

 horizons in different places, and appeared to be unaffected by the 

 faults which traversed the underlying rocks. 



Mr. J. V. Elsden remarked upon the suggested resemblance 

 between certain of the intrusive andesitic rocks, mentioned in the 

 paper, and rocks from North Pembrokeshire, which he had described 

 as lime-bostonites, on account of their chemical and mineralogical 

 identity with Prof. Brogger's msenite. The Pembrokeshire rock 

 contained irregular chloritic areas and a little quartz, possibly of 

 secondary origin, with occasional large felspars, in addition to the 

 oligoclase-laths of which the mass of the rock was composed. He 

 could not say how far this description agreed with that of the 

 amygdaloidal andesitic rock described by Prof. Reynolds. He 

 recalled the fact that Mr. Cowper Eeed had found a very similar 

 rock in the Waterford area. 



The President (Prof. Sollas) desired to join in congratulating 

 the Authors on their success, in determining the age and succession 

 of the rocks in this difficult and interesting area : the transference 

 of the series from the Silurian to the Ordovician was of great 

 importance in its bearing on general views. Some of the igneous 

 rocks had a remarkably fresh appearance, suggestive of Tertiary age. 



Mr. C. I. Gardiner expressed the thanks of the Authors for the 

 very kind manner in which their paper had been received. In reply 

 to the criticism that there was a very small thickness of Arenig Beds 

 between the Didymograptus-bifidus horizon and the Llandeilo, 

 he pointed out that the latter rocks do not always rest on the 

 same Arenig horizon, and consequently there might very well exist 

 in the district Arenig Beds higher than any now exposed. With 

 regard to the doubt thrown on the possibility of a limestone being 

 deposited, disrupted, and its fragments embedded in a tuff, all in 

 Llandeilo times, he pointed out that the fossil evidence conclusively 

 proved that the limestone and the tuffs were both of Llandeilo 

 age. 



In reply to the suggestion that the coarse conglomerates and grit 

 on the west should be classed as of Llandovery age, he observed 

 that these rocks were merely taken as marking the western limit of 

 the area, and that, as they had previously been taken to be the 

 same bed as the coarse Arenig conglomerate on the east, the Authors 

 had perforce mentioned the reasons which had led them to reject 

 this view. The study of these western deposits was, however, outside 

 the objects of the paper ; and so, as their age seemed a matter of 

 doubt, the rocks were always referred to as of (?) Bala age. 



Prof. Beynolds, in reply to Mr. Elsden, said that the rocks which 

 the Authors had compared with those described as lime-bostonites 

 from the St. David's district, were fine-grained, commonly amygda- 

 loidal rocks, often showing flow-structure. The prevalent mineral 

 was a felspar, apparently oligoclase ; no quartz was present ; and 

 but rarely any ferromagnesian constituent. 



