Igneous Rocks. 75 



Wales, and the assemblage of fossils in each is very 

 nearly the same. 



I have already mentioned the occurrence of an im- 

 portant episode characterised by volcanic eruptions, 

 during the accumulation of the Lower Silurian strata 

 in Wales. The proof of this is that in Carnarvonshire 

 and Merionethshire extensive interstratified sheets of 

 felspathic lavas and ashes are associated with the 

 Silurian rocks on two horizons, the lower that of the 

 Llandeilo beds, and the higher in the Caradoc series. 

 I do not, however, wish to imply that between them 

 there was a complete cessation of volcanic activity, 

 but simply that in what is now the region of North 

 Wales, there was for a time an interval of comparative 

 repose. 



If any one will examine the Geological Survey maps 

 of North Wales, he will observe that opposite Barmouth, 

 beginning with the hills on the south side of the 

 estuary of the Mawddach, a great series of igneous 

 rocks sweep round the country in a crescent form, in- 

 cluding the mountains of Cader Idris, Aran Mowddwy, 

 the Arenigs, and lastly the Moelwyns, the high southern 

 escarpments of which overlook from the north the 

 beautiful vale of Ffestiniog. These consist of felspathic 

 lavas, and interstratified ashes or tufas, the whole 

 being also associated with bands of Silurian slate, 

 which are sometimes found to be fossiliferous, especially 

 when bedding and cleavage coincide. Among these 

 volcanic rocks, but especially in the Arenig, Tremadoc, 

 and Lingula beds below them, there are numerous lines 

 and bosses of greenstones (diorites, &c.), and also of 

 more purely felspathic traps, which are not interbedded 

 but distinctly intrusive. These I have elsewhere shown 

 give evidence of the underground working of the 



