Geological Society of London, 47 



before the Cambrian sediments were deposited upon them is clear 

 from the fact that the pebbles of the Cambrian conglomerates which 

 rest immediately on any portion of the series are almost invariably 

 made up of masses of the rocks below, cemented by gritty materials 

 on an unaltered matrix, and from which the pebbles may be easily 

 removed. The great conglomerates at the base of the Cambrians, 

 everywhere in Wales, indicate that there were beach- and shallow- 

 water conditions over those areas at the time, and that the sea was 

 then encroaching on an uneven land, becoming gradually depressed 

 to receive the subsequent Cambrian sediment. 



3. " On some Precambrian (Dimetian and Pebidian) Eocks in 

 Caernarvonshire." By Henry Hicks, Esq., F.G.S. 



In this paper the author gave an account of the special examina- 

 tion of the great ribs of so-called intrusive felspathic and quartz 

 porphyries which are found associated with the Cambinan rocks in 

 Caernarvonshire, made by him in company with Prof. Hughes, Mr. 

 Hudleston, and Mr. Homfray last summer. He described sections 

 at and near Moel Tryfaen and across the mass from Pen-y-groes to 

 Talysarn, in which he showed that instead of being of an intrusive 

 nature, as hitherto supposed, the whole, with the exception of a few 

 dykes at those parts, is made up of bedded volcanic rocks, lavas, 

 breccias, etc., similar to those found in the Pebidian series at St. 

 David's, and that the Cambrian rocks, instead of being intruded by 

 this mass, rests everywhere upon it unconforaiably, and the pebbles 

 in the conglomerate of the Cambrian at the base are, as at St, 

 David's, identical with, and must have been derived from the rocks 

 below. Similar results were obtained in the examination to the 

 north and south of Llyn Padarn, and the conclusion, therefore, at 

 which the author has arrived with regard to the great mass which 

 extends from Llanellyfine in the south of 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 Eocks of Bangor." By Prof. T. McKenny 

 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- 

 bi'ian 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, he 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 



