ARENIG SERIES. 6/ 



The beds are not unconformable to the Tremadoc Slate 

 rocks that preceded them, but there is a considerable change 

 in the organic remains, or, in other words, a palaeontological 

 break between the groups. The rocks are developed in Wales 

 and in the Lake District, and comprise slates, flags, sand- 

 stones and limestones, characterized by considerable accumu- 

 lations of volcanic material. 



This group includes three Series (or four if the Llandeilo 

 Flags be placed as distinct from the Bala Series), and the 

 strata may be tabulated as follows : — 



Wales, etc. Lake District, etc. 



' Hirnant Limestone. Ashgill Shales. \ Coniston 



Bala ' (^ J T) ^ ( Coniston Limestone. .Limestone 



Series, i ' ( Dufton Shales. J Series. 



( Llandeilo Flags. jBorrowdale and 



T, • o • ' Skiddaw Series 



Llanvirn Series. ; / ,• , 



i (representmg also 



Arenig Series. j older strata.) 



The thickness of these Upper Cambrian or Ordovician 

 rocks varies much in different districts, in consequence of the 

 unequal distribution of interbedded volcanic rocks, especially 

 in the higher divisions. The estimates for the total thickness 

 thus vary from nearly 12,000 to upwards of 25,000 feet.^ 



WALES, ETC. 



ARENIG SERIES. 



The rocks of this series were described by Sedgwick in 1852, the 

 name being taken from the Arenig mountains of JMerionethshire. 

 They were previously included in Sedgwick's Snowdonian series. 



The rocks consist for the most part of black slates with shales 

 and sandstones, including the quartzose Stiper Stones of Shropshire, 

 The thickness of the Arenig rocks is about 1000 feet and upwards 

 in North Wales, and about 2500 feet in the neighbourhood of St. 

 Davids, South Wales. They rest conformably on the Tremadoc 

 rocks. 



The beds were at one time divided into three stages, but the 

 uppermost being now distinguished as the Llanvirn Series, the 

 following divisions remain : — 



, rr,, I Slates, Flags, and bands of Grit (Garth Grit). About 1500 feet 

 Arenig ) I'P^^' \ (maximum). 



Series. | r , | Fine black slates (Garth Slates), shales and flags (formerly 

 ( grouped as Upper Tremadoc). About 1000 feet. 



' Hicks, Q. J. xxxi 192. 



