Yol. 65.] SUCCESSION ABOUND PLYNLIMOX AND PONT ERWYD. 493 



The higher strata of the Pont Erwyd Stage: 

 Castell Group (c). 



It has been repeatedly mentioned that the upper beds of the 

 Bheidol Group are normally followed by pale mudstones, differing 

 markedly in their lithological character from the underlying 

 dark-blue shales. The paloeontological features of this new group,, 

 and its relationships to the underlying and overlying strata, are 

 nowhere more clearly exhibited than along the southern flank of 

 the Castell Valley, and for this reason I propose to call it the 

 Castell Group. 



The fault which brings the pale mudstones against the black 

 shales in the Rheidol Gorge has been traced, in a north-easterly 

 direction, into the Castell Valley, the south side of which it skirts 

 for nearly 2 miles, finally leaving it near Cwm Ergyr. Its course 

 is indicated by numerous workings in, and trials for, lead- and zinc- 

 ores. The amount of its throw varies somewhat in different parts ; 

 but, so far as I have traced it, the downthrow is always to the 

 south-east. It usually brings the pale mudstones against various 

 members of the llheidol Group, but in places small areas of the 

 latter beds appear on its downthrow side, especially along the more 

 important anticlinal folds. It is exposed in several artificial 

 sections near the Dyffryn Castell Mine (or West Esgair Lie Mine, 

 as it is named on the 6-inch Ordnance Survey map), Avhich is one 

 of the few mines still at work in this district. 



The most instructive sections through the group are (a) Nant 

 Euches-wen, which enters Nant Euches-gau at the Devil's Bridge 

 road near Troedrhiwgoch ; (6) Kant Meirch, a small stream which 

 joins the Afon Castell from the west, near the .Dyffryn Castell 

 Hotel, and the hillside to the south ; and (c) the steep slope south 

 of Eisteddfa Gurig. 



(a) Nant Euches-wen.— Eor nearly 300 yards to the east of 

 the main road this stream flows in a shallow gorge cut in the 

 higher strata of the Bryn-glas Group, after which there is a space 

 of about 120 yards where no rock is visible. The section (see 

 fig. 9, p. 494) recommences in blue flags with thin shale-bands, 

 which yielded Climacograptus tomquisti and CI. hugliesi. They dip 

 up stream at about 40°, and about 30 feet higher Monograptus 

 rJieidolensis was found with M. atavus (E. 23). A few feet above 

 the latter Moyiograptus cyphus, M. revolutus, and Climacograptus 

 hugliesi were obtained, in association with numerous examples of 

 CI. tomquisti (F. 23). This assemblage suggests that the beds 

 belong to the zones of M. rheidolensis and M. cyphus, an inference 

 confirmed by the finding of a layer of large calcareous nodules 

 about 10 feet above the last graptolitic band. It is obvious that, 

 as these beds occur within so short a distance of the Bryn-glas 

 Mudstones, a fault with a downthrow to the south-east must 

 intervene. 



