ANNIVERSARY ADDRESS OF THE TRESIDENT. lOI 



been welcome, for it would at once have accounted for the super- 

 position of Lower-Silurian strata directly upon the Cambrian 

 volcanic series and for the disappearance of the Llanberis slates 

 and grits which form so conspicuous a feature above the tuffs and 

 oongiomeratos at Llyn Padarn. In the absence of such a struc- 

 ture we must accept the order of succession as apparently unbroken 

 and rely on some such explanation as was proposed by Sir Andrew 

 liamsay to account for the overlap of the Arenig rocks on every- 

 thing older than themselves as they are traced northwards *. liut 

 this explanation will not entirely remove the difficulties of the 

 case. The inosculation of the volcanic group of Bangor with the 

 base of the Lower-Silurian series cannot be accounted for by any 

 such overlap ; it seems only explicable on the supposition that the 

 volcanic activity which ceased in the Llyn-Padarn district about 

 the time that the Llanberis Slates were deposited was continued in 

 the Bangor area until Arenig time, or was then renewed. The 

 thick volcanic group of Bangor would thus be the stratigraphical 

 equivalent not only of the thin volcanic group of Llyn Padarn, 

 but of the overlying mass of strata up to the Arenig rocks. In 

 confirmation of this view I shall subsequently attempt to show 

 that volcanic action was prolonged in Anglesey to a still later 

 geological period, that it appeared during the deposition of the 

 Arenig strata, and that it attained a great development throughout 

 the time of the Bala group. 



During the last ten years, thanks mainly to the labours of Dr. 

 Callaway and Professor Lapworth, much new light has been cast 

 upon the occurrence of Cambrian rocks in the west and centre of 

 England. AVe now know that portions of the Cambrian system pro- 

 trude in the very heart of the country, and that below these strata 

 a distinct volcanic group may be recognized. The tract where the 

 relations of this group to the overlying Cambrian strata are best 

 seen is along the north-east slopes of the long ridge which comes 

 out from under the Warwickshire coal-field between Atherstone 

 and Xuneaton. Grey, black, purple, and green shales containing a 

 characteristic Upper-Cambrian fauna rest upon a mass of bedded 

 quartzite which may be about 1200 feet thick. Beneath this 

 quartzite, and presenting the same general strike and amount of 

 metamorphism, comes a marked volcanic group consisting of well- 

 stratified ashes, with fclsite which may possibly be intrusive in them, 

 and a diabase-porphyrite which traverses the felsite. The base of 

 * Mem. Geol. Surv. vol. iii. 2nd edit. p. 252. 



