^86 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



boundary runs south-east through Llansaintfraid glyn Ceiriog to 

 the mountain limestone north of Selattyn. Along this line the 

 Wenlock beds rest unconformably upon the dark roofing-slate which 

 is the uppermost of the Lower Silurian beds*. North of the Dee 

 the Wenlock beds dip exactly south from 40° to 80°, and the clea- 

 vage planes usually dip north 45°. South of the Dee the prevailing 

 dip of these beds is north-east from 5° to 10°; and the prevailing 

 dip of the cleavage is north-east 50°. There are also several other 

 faults, the principal of which are noticed by Mr. Bowman. 



Recapitulation of the Authors Bala Section. 



Since the author, in the sequel of his paper, makes frequent re- 

 ference to the succession of Lower Silurian and Cambrian rocks 

 near Bala, of which he has given an account in vol. iv. p. 10 of the 

 Proceedings of the Society, it may be as well, for the clear under- 

 standing of the present abstract, to recapitulate the members of his 

 Bala section in descending order. Of these he now ranks the first 

 seven amongst the Lower Silurian beds; and the eighth and ninth 

 amongst the Cambrian. 



1. Good dark blue roofing-slate and flags, ending downwards in soft worthless 

 argillaceous slate. 



2. Dark blue fossiliferous limestone, with calcareous slates and soft brown shales 

 — the Upper Bala limestone of Mr. Sharpe, and the Hirnant limestone of Pro- 

 fessor Sedgwick. 



3. Light grey, rather argillaceous schist and indurated shale, with few fossils. 



4. Dark blue limestone, with calcareous shales and grits full of organic remains 

 — the Bala limestone of Professor Sedgwick. 



5. Very hard, grey, slaty grits, streaked occasionally or passing into brown — 

 fossiliferous. The " Bala grits " of the author. 



6. Grey impure limestone — fossiliferous. The Rhiwlas limestone of the author 

 and Professor Sedgwick. 



7. Slaty grits of Rhiwlas — fossiliferous. 



8. Grey rotten clay-slate, weathering brown. 



9. Dark blue slate of poor quality. 



Mr. Sharpe now considers that he underrated the thickness of these 

 beds in the paper referred to : in other respects he adheres to the 

 views there given. 



Lower Silurian Formation. 



Lower Silurian Rocks north of the Dee. — The dark grey slate of 

 Moel-y-faen, north of Llangollen, is the uppermost bed of the Lower 

 Silurian formation : it is of good quality and is largely quarried. 

 The beds dip S.W. by S. 70°; and the cleavage dips N.E. by N. 

 50°. It is continued through Bwlch-uchaf, on the southern flank 

 of Cyrn-y-brain, when it strikes east. This slate is considered 



* The main chain of lofty hills which separates the vale of Clwyd from that of 

 the Alyn consists of Wenlock rocks, which are bounded on each side by a range 

 of mountain limestone. At Moel Acre, near the south end of the chain, Creseis 

 occurs in a hard liver-coloured slaty rock. East of Ruthin is a blue flagstone, 

 alternating with shale, belonging to the middle of the Wenlock series. This 

 chain in Mr. Greenough's map is coloured as belonging to the Ludlow series. 



