168 STKATIGKAPHICAL GEOLOGY 



and are well exposed in the cutting near Ledbury station, west of 



Malvern, where the sequence is as follows : 



Feet. 

 Old Red marls and sandstones ..... 



f Ledbury shales ....... 400 



Downtonian -I Downton sandstone ...... 90 



{ Upper Ludlow shales ... . 140 



Aymestry limestone (seen in the tunnel) 



From the Ledbury shales Lintfula cornea with remains of 

 Cephalaspis and Pteryyotus have been obtained. 



The most southerly occurrences of Silurian rocks of Salopian 

 type are at Fortworth, in Gloucestershire, and in the Mendip Hills, 

 where a small inlier has recently been discovered and described. 

 It is noteworthy that where the base of the Devonian comes to the 

 surface in Cornwall it does not rest on Silurian but on Ordovician 

 and older rocks, having probably overlapped the Silurian. 



2. Wales 



The Valentian attains its greatest thickness in Central Wales 

 where it occupies large parts of Cardigan and Eadnor, and has a 

 maximum thickness of 5000 feet. The most complete development 

 of the Llandovery Beds is found within the western part of the 

 area near Rhyader in Radnor, and has recently been worked out 

 by Mr. H. Lapworth, from whose account the following is taken. 

 He describes the Lower Llandovery under the name of the Gwastaden 

 Group, the Upper Llandovery as the Caban Group, and the Tarannon 

 shale as the Rhyader pale slates. The Gwastaden Beds seem to 

 succeed the Bala Beds without any break, but there is a strong 

 unconformity between the Gwastaden and Caban Groups, and the 

 latter is overlapped by the Rhyader slates, so that the structure of 

 the district is somewhat complicated. Fig. 54 is a diagrammatic 

 view constructed by Mr. Lapworth of the succession and of the 

 relations of the rock-groups when faults and folds are eliminated. 



He estimates the thickness of the Gwastaden Series at over 1000 

 feet, that of the Caban Group as 1500 (maximum); that of the 

 Rhyader (Tarannon) slates is not given, but they extend over a 

 large area of ground between Rhyader and Llanidloes, where they 

 are probably 1500 feet thick, so that in this district the Llandovery 

 or Valentian Series has a total thickness of nearly 5000 feet. 



When traced eastward or south-eastward the Gwastaden Beds 

 are found to thin out entirely and the thickness of the Caban Beds 

 and Tarannon shales is greatly reduced. North-east of Builth 

 they form quite a narrow band and rest unconformably on Llandilo 



