Yol. 62.] ROCKS OP WESTERN CARMARTHENSHIRE. 633 



the dingle south of Waun-rhyddod and Bronysgawen, past Sarnau, 

 to Glanrhyd, south of Eglwys-Fair-a-Churig, where they have 

 yielded fossils of the Melin-nant-yr-eglwys type. 



South of Fron-isaf the outcrop is shifted for some distance north- 

 ward by a fault. From here the black beds follow the lower beds 

 normally, and are typically exposed near Sylvania Factory, where 

 they have yielded Orthis argentea, His. 



A short distance south of Llanglydwen Station a ' lenticle ' of these 

 beds has been faulted in among Didymograptus-bifidus Beds. They 

 can be well seen near the signal-post, where they have yielded a 

 few distorted graptolites and Orthis argentea, His. They are also 

 very pyritous here, as elsewhere. 



(3) Conclusions. — The occurrence of Orthis argentea in these 

 shales tends to show that they cannot be of Llandeiio age. 



The late Thomas Roberts reported the occurrence of 0. argentea 

 at Dyffryn-pwdryn north of Llanboidy and elsewhere, where the 

 succession is practically the same as at Mydrim. Mr. Roberts 

 presumably found his specimens in the black slaty beds, which are 

 the equivalents of the beds at Fron-isaf and Llanglydwen. 



The graptolitic fauna of a considerable portion of these beds is, 

 in the opinion of Miss Elles, more characteristic of Llandeiio than 

 of Bala-Caradoc age ; but further search for fossils, and a closer 

 examination of both beds and fossil-forms, will probably bring forth 

 evidence showing that the upper portion is of Bala-Caradoc age. 

 Until that is done the matter must remain unsettled. 



(/) Bala-Caradoc Rocks. 



Bala-Caradoc rocks of Pembrokeshire type are well developed in 

 the district, and are readily divisible into the four groups enumerated 

 by Dr. J. E. Marr & the late Thomas Roberts in their paper on the 

 Haverfordwest district, 1 namely : — 



(a) Robeston-Watben Limestone. — Limestones and calcareous shales. 



(/3) Sholeshook Beds. — Arenaceous limestones. 



(y) Redkill Beds. — Blue-grey mudstones and shales, with some thin 



sandstone-bands, 

 (o) Slade Beds. — Blue-grey mudstones and shales, with limestone-bands. 



(1) South of the Anticline. 



(a)Robeston-WathenBeds. — These beds are typically exposed 

 immediately north of Llandeilo-aber-cowyn, where the Dicrano- 

 (/rapms-Shales are seen passing up. Some of the limestone-beds are 

 very compact and break sharply under the hammer, but weather 

 into dust. An unusual feature observed here is the occurrence of 

 lines of pebbles in some of the beds. The compact limestone is 

 fairly fossiliferous, but better specimens can be collected from the 

 calcareous shale-partings. (For fossils see the appended list, 

 pp. 634-35.) 



1 Quart. Journ. Geo). Soc. vol. xli (18S5) p. 476. 



2x2 



