232 Geological Society: — 



extends from Llanellyfine in the south to St. Ann's chapel in the 

 north is that it is entirely Precambrian, and that it belongs to the 

 series described by him under the name Pebidian at St. David's. 



The other mass, extending from Caernarvon to Bangor, he con- 

 sidered also entirely Precambrian ; and from the mineral characters 

 exhibited by a portion of this mass directly behind Caernarvon, he 

 thought it would prove to be, at least at this part, of Dimetian age. 

 The altered beds near Bangor and their associated quartz felsites he 

 considered entirely of Pebidian age, as there is no evidence that the 

 Dimetian rocks are exposed there. 



4. " On the Precambrian Hocks of Bangor." By Prof. T. M c Kenny 

 Hughes, M.A., F.G.S. 



The author described a series of slates, agglomerates, and por- 

 phyritic rocks which, near Bangor, are seen to pass under the Cam- 

 brian and seem to rest conformably upon the quartz felsites and 

 granitoid rocks of Caernarvon. He thought that the Bangor beds 

 were the equivalents of the felsitic and porphyritic series of Llyn 

 Padarn ; and, in order to bring his interpretation into harmony with 

 the observations of Prof. Eamsay, be explained away the apparent 

 melting of the ends of the Cambrian beds in that section by twists, 

 faults, and dykes. He referred the apparent unconformity recorded 

 by Mr. Maw entirely to rock-structure, produced by cleavage on beds 

 of different texture. 



He considered that in the main the Bangor beds were the equi- 

 valents of the Pebidian of Dr. Hicks, while the Caernarvon beds 

 nearly represented his Dimetian. But he thought there was as yet 

 no proof of an unconformity between these formations. He would 

 explain the apparent unconformity at St. David's by a continuation 

 of bends and faults and joints mistaken for bedding, and would refer 

 the brecciated rock of Low Moor, near St. David's, to the Pebidian, 

 thus taking it on the wrong side of the supposed unconformity. He 

 thought that the green beds in the Dimetian were, in all the cases 

 where he had been able to examine them, originally dykes. 



He saw, therefore, no reason from an examination of other areas 

 to suspect any different explanation from that suggested by the ex- 

 amination of the Bangor and Caernarvon district, viz. that we have 

 in the Bangor and Caernarvon beds one great volcanic series, on 

 which the Cambrian conglomerates and grits rest with a probable 

 unconformability. 



An appendix by Prof. Bonney on the microscopical examination of 

 the rocks referred to accompanied this paper. 



December 19.— Prof. P. Martin Duncan, M.B., F.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. " On Argillornis longipennis, Owen, a large bird of flight, from 

 the Eocene Clay of Sheppey." By Prof. Owen, C.B., P.E.S., 

 F.G.S., &c. 



