136 PROCEEDINGS OF THE GEOLOaiCAL SOCIETY. 



the north coast of Anglesey, which were in eruption probably during 

 Bala time, were the last of the long succession of Welsh volcanoes. 

 If the black shales of Parys Mountain are really referable to the 

 horizon of the Mayhill Sandstone, the two great igneous bands be- 

 tween which they lie would seem to mark an outbreak of volcanic 

 energy during Upper-Silurian time. No other indications, how- 

 ever, of eruptions of that age having been met with in Great Britain, 

 more careful investigation is required before such a position can 

 be safely assigned to any rocks in Anglesey. 



Putting these doubtful rocks aside for the present, we may, in 

 conclusion, contrast the type of eruption in Anglesey with that of 

 the great Snowdonian region. While the Caernarvonshire volcanoes 

 were pouring forth their volumes of felsitic lava, and piling them up 

 for thousands of feet on the sea-floor, the little northern Anglesey 

 vents, not more than some five-and-twenty miles away, threw out 

 only stones and dust, but continued their intermittent explosions 

 until they had strewn the sea-bottom with detritus to a depth of 

 many hundred feet. 



There is yet another feature of interest in this independent group 

 of submarine vents in Anglesey. Their operations appear to have 

 begun before the earliest eruption of the Bala volcanoes in Caer- 

 narvonshire. Their first beginnings may, indeed, have been coeval 

 with the explosions that produced the older Arenig tuffs of Merioneth- 

 shire ; their latest discharges were possibly the last manifestations of 

 volcanic energy in Wales. They seem thus to bridge over the vast 

 interval from Tremadoc to Upper-Bala, possibly even to Upper- 

 Silurian time. But we may, perhaps, connect them with the 

 still earlier period of Cambrian vulcanism. I have referred to the 

 evidence which appears to show that the vents which gave forth the 

 lavas and tuffs of Moel Trefan and Llyn Padarn gradually moved 

 northwards, and continued in eruption until after the beginning of 

 the deposition of the black slates that are generally regarded as 

 Arenig. The Anglesey vents may thus be looked upon as evidence 

 of a still further shifting of the active orifices northward. In this 

 view, while the Aran and Cader-Idris volcanoes broke out in Tremadoc 

 and continued through Arenig time, and the Snowdonian group was 

 confined to Bala time, a line of vents opened to the north-west in 

 the Cambrian period before the epoch of the Llanberis slates, and 

 dying out in the south, continued to manifest a minor degree of 

 energy, frequently discharging fragmental materials, but no lava, 

 over the sea-bottom, until, towards the close of the Bala period, pos- 

 sibly even in Upper- Silurian time, they finally became extinct. 



