Geological Society of London. 141 



F.G.S. With an Appendix on their Microscopic Structure by the 

 Eev. Prof. T. G. Bonney, M.A., F.B.S., F.G.S. 



In this paper the author gave the results of some further re- 

 searches made in Caernarvonshire and Anglesey since his previous 

 communication to the Society on Dec. 5, 1877. A brief statement 

 of some of the results was read at the last meeting of the British 

 Association in Dublin ; but much additional evidence was now 

 brought forward, besides many important facts obtained since by 

 microscopical examination of the rocks. Concerning the areas de- 

 scribed in his former paper much additional information was given, 

 and the boundary in one case greatly extended. This new area lies 

 to the west of Moel Tryfaen, and includes now, in addition to the 

 central or quartz-felsite ridge, the whole of the rocks marked in the 

 Survey maps as altered Cambrian, extending as far west as Glynllifen. 

 Many of the large masses in south-west Caernarvonshire and the 

 Llyn promontory, hitherto supposed to be intrusive rocks of Silurian 

 or Post-Silurian age, were discovered, during these researches, to be 

 of Pre-Cambrian age, and conclusive evidence obtained that the so- 

 called altered Cambrian rocks there, and in Anglesey, were also of 

 that age. In these various areas the three Pre-Cambrian formations 

 found in Pembrokeshire were recognized by having similar litholo- 

 gical characters, and in holding almost identical stratigraphical 

 positions in their relations to one another. Dimetian rocks were 

 recognized at Twt Hill, Ehos Hirwani, near Ffestiniog, and in 

 the so-called granitic ridge in Anglesey. Arvonian rocks between 

 Caernarvon and Menai Bridge, in the Eifl Bange, Nevin Mountain, 

 and near Ty Croes, in Anglesey, etc., etc. Pebidian rocks to the 

 east of Glynllifen, Bangor, at the lower part of the Lynn promon- 

 tory, and in many places in Anglesey. Some notes on the section 

 near Ty Croes by Prof. Bonney accompanied the paper, in addition 

 to an appendix by him on the microscopic examination of rock- 

 specimens from each of the areas examined. 



4. " On the Quartz-felsite and Associated Bocks at the Base of 

 the Cambrian Series in North-western Caernarvonshire." By the 

 Eev. Prof. T. G. Bonney, M.A., F.E.S., F.G.S. 



The great masses of quartz-felsite (or quartz-porphyry) which 

 occur in the vicinity of Bangor, Caernarvon, and Llyn Padarn, are 

 coloured in the Survey map as intrusive, and in the memoir regarded 

 as most probably the result of an extreme metamorphosis of the 

 lower beds of the Cambrian series. 



The author showed that these quartz-felsites exhibited, in places, 

 all the characteristics of true igneous rocks — flow- structure, fissile 

 structure, and the more ordinary structure of rhyolitic rocks ; that 

 they were, in one place, at least, associated with masses of agglo- 

 merate, and in another parted by a band of comparatively unaltered 

 slate. He also showed that in several places there succeeded a grit 

 formed of fragments of it, that larger fragments of perfectly 

 characteristic structure, associated with others of a more slaggy and 

 scoriaceous type, occurred repeatedly in the overlying beds up to the 

 base of the Cambrian, described by Prof. Hughes and Dr. Hicks, the 



