72 Geological Society: — 



Pre-Cambrian masses, without giving any detailed statement of the 

 evidence in support of so great a change. The author said that he 

 was prepared to bring forward conclusive evidence of the correct- 

 ness of his views from all the areas referred to, and he offered the 

 present paper as a first instalment of the detailed criticism demanded 

 by the Director-General. 



In a former paper he had maintained that there was no evidence 

 to show that the so-called intrusive granite in Anglesey had altered 

 the Cambrian and Silurian rocks in its immediate vicinity, or that 

 they had been entangled in it as described, but that it seemed to be 

 a rock of metamorphic origin, varying much in its general appear- 

 ance at different points. He contended that, instead of being an in- 

 trusive granite, as supposed by the officers of the Survey, it was in 

 all probability the oldest rock in Anglesey. The basal Cambrian 

 conglomerate in contact with it is in an unaltered condition, and 

 at Llanfaelog contains an extraordinary proportion of well-rolled 

 pebbles, identical in mineral composition with the so-called granite 

 immediately below. Fragments of all the varieties of rock found 

 in the granitoid axis are recognizable in the conglomerate, and in 

 precisely the same condition as in the parent rock. Fragments of 

 the various schists of the area were also present ; so that he thought 

 there cannot be the shadow of a doubt that the so-called granite 

 and the metamorphic schists are older than the conglomerate, and 

 therefore Pre-Cambrian. The view maintained by the Survey that 

 the schists are altered Cambrian and Silurian strata, and the granitoid 

 rock an intrusive granite of Lower Silurian age, is consequently 

 quite untenable. 



In Caernarvonshire equally conclusive evidence was obtained from 

 many areas. Fragments of the Dimetian (Twt-Hill type) occurred 

 abundantly in the basal Cambrian Conglomerates at Dinas Dinorwig, 

 Pont Eothel, Moel Tryfane, and Glyn Llifon. Quartz -felsite 

 pebbles in every respect identical with the varieties found in the 

 so-called intrusive ridges between Eangor and Caernarvon, and to 

 the north and south of Llyn Padarn, were found on the shores of 

 the Menai Straits, in the railway-cutting at Bangor, at Llandeiniolen, 

 Dinas Dinorwig, Llyn Padarn, and elsewhere. This evidence, sup- 

 plementary to that previously furnished by Prof. Hughes, Prof. 

 Bonney, and the author, is conclusive as to these areas, since the 

 basal Cambrian conglomerates, which are in contact with these 

 supposed intrusive masses, are composed almost entirely of rocks 

 identical with the latter ; and this could not possibly be the case if 

 the granitoid masses had been intruded among the conglomerates 

 after their deposition. 



2. " On some Pock- specimens collected by Dr. Hicks in Anglesey 

 and N.W. Caernarvonshire." By Prof. T. G. Bonney, D.Sc, F.B.S., 

 Sec.G.S. 



The author stated that pebbles in the blocks of conglomerate 

 collected by Dr. Hicks to the north of Llanfaelog were practically 

 undistinguishable macroscopically and microscopically from the 



