278 



J. E. MARK ON THE CAMBEIAN AND 



Epoch. 



Formations. 



Lake district. 



Dee valley. 





f Upper Ludlow. 



Kirkby-Moor Flags. 



Unrepresented. 





Lower Ludlow and 



Bannisdale Slates. 



do. 





Upper Wenlock 









{ 



1 1 



Coniston Grits. 



do. 





1 1 



iii. Upper Coldwell 



Dinas-Bran Beds. 







Beds. 







8. Wenlock, con- ■ 



'Middle Coldwell 

 .. J Beds. 



Grits above Penyglog 



I— 1 



tinued. 



quarry &c. 



p 



'^' Lower Coldwell 

 [ Beds. 









• 





'^ 



i. Brathay Flags. 



Flags of Penyglog 

 quarry, Maeshir, &c. 





1 7. Tarannon Shales. 



1 



Pale Shales. 



Tarannon Shales. 





1 

 6. Graptolitic Mud- 



Graptolitic Mud- 



Graptolitic Mudstones. 





stones. 



stones. 







{5. May-Hill Group. 



Basement-beds of 



Corwen Grits, Cerrig-y- 







Silurian. 



drudion Grits, &c. 





(4:. Upper Bala. 



AshgiU Shales. 



Hirnant Limestone ? 



^" 







O.-alternata beds, Cer- 



^ 







rig-y-drudion ? 



s 



3. Middle Bala. 



Coniston Limestone. 



Bala Limestone and 



pq 







overlying shales. 



Q 



1 2. Middle Bala in 



Borrowdale Series. 



Shales and Andesitic 



1 part, LowerBala. 





ashes of Berwyns. 





1^1. Arenig. 



Skiddaw Slates. 



Beds of Taihirion and 

 Arenig. 



1. Tlie Arenig Beds. 



The lowest beds of the Dee-valley area have yielded fossils at 

 Taihirion, near Arenig (see Cat. Cambr. & Sil. Poss. of Woodw. 

 Mus. p. 22 et seq.). It is needless to treat of them at length, as 

 their contemporaneity with the Skiddaw Slates is generally allowed, 

 and also their lithological resemblance to that formation, and the 

 fact that they show signs of great volcanic activity in Wales, but 

 not in the Lake district. 



2. The Lower Bala Beds. 



At the close of the Arenig period vulcanicity appears to have 

 shifted its action from North Wales to the Lake area ; and the Lower 

 Bala of the Lake district consists of a series of ashes and lava-flows, 

 well known as the Borrowdale series of Professors Harkness' and 

 Nicholson. 



The Dee-valley beds of this age are made up of imperfectly 

 cleaved shales, with a few ash beds, and some doleritic masses, 

 which occur along lines parallel to the strike of the beds, as mapped 

 by the Geological Survey. The ashes and doleritic rocks are seen on 



