284} PROCBEDINGS OF THE GEOLOGICAL SOCIETY. 



and is rarely affected by cleavage. The Ludlow rocks of Llan- 

 gollen dip at a low angle, and pass conformably below the moun- 

 tain limestone of Eglyseg Crags. To the west and south they are 

 bounded unconformably by lofty hills belonging to the Wenlock 

 series, which appear to have been elevated before the deposition of 

 the Ludlow rocks ; and the latter, since they were deposited, ap- 

 pear to have been little disturbed. 



Outlier of Mountain Limestone at Guerclas, west of Cor wen; and 

 district of Ludlow Rocks west of Corwen, and of the Nant Mor- 

 wynion fault. — On passing from Corwen along the Holyhead-road 

 to the left bank of the Dee, from Lower Silurian rocks, having 

 an eastern strike, and hereafter to be noticed, you enter on a di- 

 strict of more recent origin, where the beds strike N., N.W., or N.E. 

 The gravel which here covers the valley of the Dee conceals one of 

 the principal faults of the country, the continuation of the Nant Mor- 

 wynion fault, hereafter mentioned. 



On the left bank of the Dee, at the north-east end of Cefn Mawr, 

 is a detached outlier of mountain limestone, about half a mile 

 long and a quarter of a mile wide, which is laid down on Mr. 

 Greenough's map. It is a mile and a half west of Corwen. 



It is well-exposed in open quarries, and consists of thick beds 

 of light grey limestone, alternating with dark and black argillaceous 

 shale, agreeing in character with the shale that, in the neighbour- 

 ing districts of mountain limestone, is found in the lower part of 

 that formation. The author obtained from these quarries many 

 well-known fossils of the mountain limestone. The beds of this 

 mass dip north-east, from a low angle up to 45°. 



To the west of the limestone, and underlying it, is a rotten grey 

 shale, readily decomposing into mud ; which, at Pont Bryn, a mile 

 further west, dips E.N.E. 45°. To the west of the Druid Inn, on 

 the Holyhead-road, is a quarry of the same rocks, dippping E.N.E. 

 30°. These beds the author was able to connect with well-exposed 

 beds of Ludlow shale, which stretch along the west side of the Nant 

 Morwynion fault towards the head of the vale of Clwyd*. There, 

 at the distance of about six miles from Guerclas, they pass con- 

 formably beneath the mountain limestone range of Pwll Naid, near 

 Llanelidan, a range which extends from that point northwards 

 along the western side of the vale of Clwyd. The Ludlow shale 

 therefore is seen to be continuous from the limestone of Guerclas 

 to that of Pwll Naid. 



Wenlock series. — The beds of this series, in descending order, are 

 the following : — 



1. Thin beds of hard blue siliceous schist. 



2. Light blue shales, with grey streaks of a lighter and darker 

 shade. In the lower part the shales alternate with beds of excel- 



* At Tyn-y-Celyn, situate in this district of Ludlow shale, two miles north-west 

 of Bryn Eglws, the author found Orthoceras filosum (Sil. Syst. pi. 9. fig. 3). 

 The Ludlow rocks dip very irregularly, and they are unconformable to the Wen- 

 lock series throughout the whole of this district west of the Nant Morwynion 

 fault. 



