1846.] SEDGWICK ON THE FOSSIL SLATES OF N.WALES, ETC. J 53 



the highly metalliferous rocks of Coginan, Cwm Ystwyth, &c. — the 

 great alternating masses of grits and slates, so largely developed in 

 the ridges of Plynlimmon (both the coarser beds and the finer often 

 exhibiting cleavage-planes), and all the more soft and earthy slates, 

 grits and flags which extend from the east side of the Plynlimmon 

 ridge, and of Cwm Ystwyth, as far as Llangurig*. 



The structure of this great zone is much varied. In the vicinity 

 of the mineral veins we generally find the rock indurated, and the 

 slates sometimes passing into a structure resembling that of the 

 slates of North Wales and of Cumberland, and resembling them 

 also in colour ; but generally they are of more earthy structure, and 

 alternate with bands of flagstone and indurated shale, pyritous and 

 decomposing. All the masses exhibit, here and there, a rude con- 

 cretionary structure ; and in a few places we find brown decom- 

 posing bands of rotten-stone with numerous impressions of fossils, 

 e.g, near Devil's Bridge, DyfFryn Castell, on the south flank of 

 Plynlimmon, and farther east on the road from Dyffryn Castell to 

 Llangurigf. 



3. Rhayader Slates. — Under this name is included a very remark- 

 able zone of slates, marked, in the Map of the " Silurian System," 

 on the west side of its base-line. They are generally of a pale 

 colour, leaden-grey passing into greenish- grey. They are inter- 

 sected by beautiful cleavage-planes, generally inclined at a high 

 angle towards the north-west ; while the beds are continually thrown 

 into a series of low undulations, the axes of which are ill-defined. 

 Many of these slates exhibit a glossy crystalline surface like the 

 older slates of Cumberland and Wales ; but they are not associated 

 with contemporaneous trap, or elevated by the protrusion of any 

 igneous rock which is shown at the surface. Their low angles of 

 inclination and the highly inclined undulations of the masses on both 

 sides of them, make it diflflcult to ascertain their exact place in the 

 transverse section. I once endeavoured to class them with the 

 Upper Silurian groups, which might, I supposed, be brought in by 

 the undulations, and mineralized into the form of Rhayader slate ; 



* The coarser grits pass, though rarely, into the form of a conglomerate, 

 t List of fossils from Devil's Bridge and Dyffryn Castell (Salter MS.). 



Spirifer octoplicatus. 



Leptsena sericea. 



— transversalis. 



— small convex species. 

 Orthis elegantula. 



— applanata. 



— grandis (?). 



— sp. a simple-plaited small shell. 



— calligramma (?). 

 Terebratula (?) with forked ribs. 



— marginalis or lacunosa. 

 Atrypa crassa, young. 



— convex species. 



Ceraurus Brightii (Dyffryn Castell). 

 Calymene Blumenbachii. 

 Encrinites (-f +), &c. 

 All the species are small. 



