540 



UKl'OltT "1884. 



Mi. 



■I ■ t: ■ 



two otlicr avoiis in Pombroko.sliii'o wlioro ho holioves that thoro are out- 

 cropping riilfjfcs of Archican rock. 



(0 ) (Jdriiiirvoiishirti nud Antjlesi'ii. — Tliis district, since the pablication 

 of the first edition of the ' (JeoU)f;y of North Wales ' by Professor Uainsay 

 (vol. iii. of the Memoirs of the (i('olo(,'i('al Survey), has l)(!en the subject 

 of several i)apers by Dr. J licks, Dr. Callawny, Professor llufjhes, myself, 

 and others.' Several of these wen! judjlished before the issue of the 

 new edition of the above woi'k (dated 1*^^!), but are not mentioned 

 therein. The facta upon which all ideologists agree are that the 

 Cambrian series of ('arnarvonsliiro is a group of quartzoso grits and slates, 

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

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

 to the nortli-east for three miles, being (!iiiik('{l hy conglomerates and grits, 

 chietly of (piartz, after which tlu* ridge is continued by a felsito just like 

 the last ; this extends nearly t) IJaugor, atul is overlain by grits, breccias, 

 and slates, generally dilVering nuudi in nspect from the indubitable 

 Cambrian of the Llanberis region. The view expressed in the Survey 

 maps and memoir is that the ielsite, grits, Ac., are metamorphosed beds of 

 Cambrian age, and tliat near the noi-thcrn end of Llyn I'adarn, the lower 

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

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

 an intrusive mass connected with the above metamorphic action. 



The following facts are so ]>atent to every person accustomed to 

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

 indisputable : — 



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

 devitritied structui-e and sundry marks of age, is no way chemically or 

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

 group of flows. It exhibits fiuidid structure,^ 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 

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

 sembles the granitoid rock of the AVrekin area, and is not unlike tlie 

 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) exat^tly I'csembling it. Peblilcs 

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

 fragments of hard argillito (u.sually moi'o 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 



• These arc piiblislied in Q. J. G. S., commencing witli vol. xxxiv., and shorter 

 p.apors apjioar in the Crrol. Mag. for 1878 and following volumes. 



» Bonncy, Q. J. G. S., vol xxxv. p. 30i). 



* Q. J. a. S., vol. xl. p. 187 ; Bonncy, id. 200. 



