ANNIVERSARY ADDRESS OF THE PRESIDENT. IO5 



Wexford. It occurs at wholly irregular intervals — sometimes in 

 numerous bands lying close together within a space of a few yards, 

 while elsewhere it may be absent for equal or longer spaces. 



Judging of this material in the field, I regarded it as probably a 

 fine impalpable volcanic dust erupted at irregular intervals by 

 comparatively feeble explosions from vents lying at some consider- 

 able distance from Bray. An examination of it under the micro- 

 scope throws no satisfactory light on its origin, the material 

 appearing so perfectly homogeneous and structureless that, merely 

 from the slides themselves, one might be left in doubt whether the 

 rock is not an exceedingly close-graiued felsite. 



Y. SILURIAN. 

 I. Arenig Period — Merionethshire. 



Placing the upper limit of the Cambrian system at the top of the 

 Tremadoc group, we pass now into the records of another series of 

 volcanic eruptions which marked various epochs during the Silurian 

 period over the area of the British Isles. The earliest of these 

 Tolcanic episodes has left its memorials in some of the most impres- 

 sive scenery of North "Wales. To the picturesque forms sculptured 

 out of the lavas and ashes of that early time we owe the noble 

 range of cliffe and peaks that sweeps in a vast semicircle through 

 the heights of Cader Idris, Aran Mawddwy, Arenig, and Moel Wyn. 

 To the east other volcanic masses, perhaps in part coeval with these, 

 rise from amidst younger formations in the groups of the Berwyn 

 and Breidden Hills, and the long ridges of the Shelve and Conidon 

 country. Far to the south traces of the older Silurian volcanoes are 

 met with near Builth, while still more remote are the sheets of 

 lava and tuff interstratified among the Lower- Silurian rocks of 

 Pembrokeshire which extend even into Skomer Island. 



The most important of these districts is unquestionably that of 

 Merionethshire. In this area there is no evidence of any volcanic 

 period older than that to which I have now to refer. Neither in the 

 slaty Lingula Flags and the Menevian group nor in the vast pile of 

 grits and conglomerates in the Harlech anticline does there appear 

 to be any trace of contemporaneous volcanic action. If any earlier 

 eruptions preceded those of Cader Idris and the other hills, their 

 memorials had been buried under more than 10,000 feet of sediment 

 before the emission of the tuffs and lavas of Merionethshire. 



At the time when the Geological- Survey maps of this region were 



h2 



