69 



CHAPTER VI. 



ARENIG, LLANDEILO, AND BALA BEDS. 



The Arenig Beds succeed the Tremadoc slates at 

 St. David's in South Wales, and in North Wales they 

 also overlie the Tremadoc slates between Towyn and the 

 neighbourhoods of Dolgelli, Ffestiniog, Tremadoc, and 

 Criccieth in Caernarvonshire, north of which they also 

 occur in part of the country between Caernarvon and 

 Bangor. They were first distinguished by Professor 

 Sedgwick, and named Arenig slates, and afterwards 

 termed lower Llandeilo beds by Sir Roderick Murchi- 

 son, who had previously included them as part of the 

 Llandeilo flags in his descriptions and sections of the 

 Lower Silurian rocks that lie west of the Stiper Stones 

 near Shelve, in Shropshire. 



In the large district of Merionethshire the Arenig 

 slates appear at the base of the great volcanic series of 

 felspathic lavas and ashes, of which the mountains of 

 Cader Idris, Aran Mowddwy, Arenig, and the Moelwyns 

 form distinguished features in the landscape. They are 

 in these districts never more than about 800 feet in 

 thickness, and the Arenig beds of Merionethshire, at 

 their base invariably consist of beds of grit, sometimes 

 conglomeratic. The higher strata of this sub-forma- 

 tion are generally slaty. For reasons that will after- 

 wards appear, I believe that the Arenig strata, on a 



