Il8 PROCEEDIN'GS OF THE GEOLOGICAL SOCIETY. 



north-eastwards beyond Bala Lake, and there can be but little 

 doubt that they thin out also eastwards under the Upper-Eala 

 rocks. The lavas and tuffs that rise up on a similar horizon among 

 tho Bala rocks of the Berwyn Hills evidently came, not from the 

 Snowdonian vents, but from another minor volcanic centre some 

 miles to the east, while still more remote lay the vents of the 

 Breidden Hills and the sheets of andesitic tuff that probably spread 

 from them over the ground east of Chirbury. 



Restricting myself for the present to the Caernarvonshire volcanic 

 group, I may remark that it extends from north to south for fully 

 thirty miles, with an extreme breadth of about fifteen miles ; while, 

 if we include the rocks of the Lleyn peninsula, the area will be 

 prolonged some twenty miles farther to the south-west. 



The general stratigraphical horizon of this volcanic group has 

 been well determined by the careful mapping of Ramsay, Selwyn, and 

 Jukes on the maps of the Geological Survey. These observers have 

 brought forward ample evidence to show that the lavas and tuffs 

 were erupted during the deposition of the Bala strata of the Lower- 

 Silurian series, that the Bala Limestone is in places full of ashy 

 material, and that this well-marked fossiliferous band passes later- 

 ally into stratified volcanic tuffs containing the same species of 

 fossils *. But the progress of stratigraphical geology, and the in- 

 creasing value found to attach to organic remains as marking even 

 minor stratigraphical horizons, give us reason to believe that a 

 renewed and still more detailed study of the Bala rocks of I^orth 

 Wales would probably furnish data for more precisely defining the 

 platforms of successive eruptions, and would thus fill in the details 

 of the broad sketch which Sir Andrew Bamsay and his associates 

 so admirably traced. There may even be lithological horizons 

 which, like the grit-band and pisolitic iron-ore of the Arenig 

 group, might be capable of being followed among the cwms and 

 crests as well as the opener valleys of Caernarvonshire. Until some 

 such detailed mapping is accomplished, we cannot safely advance 

 much beyond the point where the stratigraphy was left by the 

 Survey. 



Prom the Survey maps and sections it is not difficult to follow 

 the general volcanic succession, and to perceive that the erupted 

 materials jmust altogether be several thousand feet in thickness from 

 the lowest lavas in the north to the highest on the crest of Snowdon. 

 In that mountain the total mass of volcanic material is set down as 



* Mem. Geol. Surv. vol. iii. 2iid edit. pp. 126, 128, 131, 139, &c.. 



