ANNIVERSARY ADDRESS OF THK PRESIDENT. II7 



assume the (l3'ke form, and through the ap:p:loraeratcs and tuffs of 

 llhobell Fawr dykes of olivine-diabasc liavo worked their way. 

 But there has been no widespread fissuring of the ground and 

 uprise of lava in the rents, such as may bo seen in the Archaean 

 gneiss, and iu the later Pah^ozoic, but still more in the Tertiary 

 volcanic regions. This feature becomes all the more notable when 

 it is viewed in connexion with the extraordinary development of 

 sills, and the evidence thereby afforded of widespread and extremely 

 vigorous subterranean volcanic action. 



In the Merionethshire region there certainly was a long period 

 of quiescence between the close of ths Arenig and the beginning of 

 the Bala eruptions. Moreover, no evidence has yet been found that 

 active vents ever again appeared in that district, the subterranean 

 energy at its next outburst having broken out farther north. In 

 Anglesey, however, where, as I shall point out, there is proof of 

 contemporaneous tuffs among the Arenig rocks, it is possible that a 

 continuous record of volcanic action may yet be traced from Arenig 

 well onward into Bala time. I reserve some notice of this subject 

 for the account which I shall give of the Bala volcanic rocks of 

 Anglesey. 



II. Llandetlo and Bala Periods — {a) Caernarvonshire, 



Anglesey. 



Owing to the effects partly of plication and partly of denudation, 

 the rocks of the next volcanic episode in Wales, that of the Bala 

 period, occupy a less compact and defined area than those of the 

 Arenig group in Merionethshire. From the latter they are sepa- 

 rated, as we have seen, by a considerable depth of strata *, whence 

 we may infer, with the Geological Survey, that the eruptions of 

 Arenig, the Arans, and Cader Idris were succeeded by a long period 

 of repose. When the next outbreaks took place the vents, as I 

 have said, are found to have shifted northwards into Caernarvon- 

 shire, where they fixed themselves along a line not much to the 

 east of where the Cambrian porphyries and tuffs now appear at 

 the surface. The lavas and ashes that were thrown out from 

 these vents form the highest' and most picturesque mountains of 

 North Wales, culminating in the noble cone of Snowdon. They 

 stretch northwards to Diganwy, beyond Conway, and southwards, 

 at least as far as the neighbourhood of Criccieth. They die out 



* Estimated at from 6000 to 7000 feet. Mem. Geol. Surv. vol. iii. 2nd edit. 

 p. 131. 



