540 keport— 1884. 



two other areas in Pembrokeshire where he believes that there are out- 

 cropping ridges of Archaean rock. 



(9 ) Carnarvonshire and Anglesey. — This district, since the publication 

 of the first edition of the ' Geology of North Wales ' by Professor Ramsay 

 (vol. iii. of the Memoirs of the Geological Survey), has been the subject 

 of several papers by Dr. Hicks, Dr. Callaway, Professor Hughes, myself, 

 and others. 1 Several of these were published before the issue of the 

 new edition Of the above work (dated 1881), but are not mentioned 

 therein. The facts upon which all geologists agree are that the 

 Cambrian series of Carnarvonshire is a group of quartzose grits and slates, 

 beneath which occur large masses of a compact reddish felsite, and that 

 near the town of Carnarvon is a ridge of granitoid rock, which extends 

 to the north-east for three miles, being flanked by conglomerates and grits, 

 chiefly of quartz, after which the ridge is continued by a felsite just like 

 the last ; this extends nearly to Bangor, and is overlain by grits, breccias, 

 and slates, generally differing much in aspect from the indubitable 

 Cambrian of the Llanbei-is region. The view expressed in the Survey 

 maps and memoir is that the felsite, grits, &c, ai*e metamorphosed beds of 

 Cambrian age, and that near the northern end of Llyn Padarn, the lower 

 part of the Cambrian (here conglomeratic) may be seen to be gradually 

 melted down into the felsite, the granitoid rock being probably a part of 

 an intrusive mass connected with the above metamorphic action. 



The following facts are so patent to every person accustomed to 

 microscopic as well as field work that they may be now regarded as 

 indisputable : — 



(a) That the felstone mentioned above, except for the presence of a 

 devitrified structure and sundry marks of age, is no way chemically or 

 microscopically different from a modern rhyolite, and. is a lava-flow or 

 group of flows. It exhibits fluidal structure, 2 is somewhat porphyritic, 

 and in one place (where perhaps it breaks through the granitoid rock) 

 is spherulitic. 



(b) That the granitoid rock appears in some places to be distinctly 

 gueissose; at Twt Hill, near Carnarvon, however, it very closely re- 

 sembles the granitoid rock of the Wrekin area, and is not unlike the 

 Dimetian of St. David's. 



(c) That the conglomerate at Llyn Padarn shows no signs of melting 

 down into the rhyolite, but is full of fragments (many of them being 

 well-rounded pebbles of considerable size) exactly resembling it. Pebbles 

 also of the Twt Hill rock are occasionally found, and in other localities 

 fragments of hard argillite (usually more angular) abound. 



(d) That at the base of the admitted Cambrian in the region south of 

 the Menai Straits is a conglomerate of well rolled pebbles, chiefly derived 

 from the above rhyolite ; a spherulitic variety has also been found (by Dr. 

 Hicks 3 ). 



(e) That between this conglomerate and the rhyolite is a series of 

 beds — argillites, grits, and breccias — into which indubitable volcanic scoria 

 and fragments of rhyolitic lavas largely enter, probably indicating con- 

 temporaneous volcanic action, or, if not, the destruction of proximate 

 cones connected with the above-named rhyolite flows. As to the exact 



1 These are published in Q. J. G. S., commencing with vol. xxxiv., and shorter 

 papers appear in the Gcol. Mag. for 1878 and following volumes. 

 8 Bonney, Q. J. G. S., vol xxxv. p. 309. 

 * Q. J. G. S., vol. xl. p. 187 ; Bonney, id. 200. 





