334 PEOF. 0. T. JONES OX iHE [Sept. 19 1 2, 



Strata of this nature form the coast-line for several miles north 

 of the mouth of the Teifi, and cover an extensive area between 

 Cardigan and Newcastle Emlyn, and around the latter place. They 

 consist generally of greenish-blue or blue-grey mudstones, some- 

 times containing thin bands of fine-grained sandstones or sandy 

 seams. They present often a striped appearance, but in other cases 

 have a very uniform aspect. They are traversed by an imperfect 

 cleavage, vrhich causes them to break up into rough elongated 

 slabs. 



Along the Cardigan road near Newcastle Emlyn the mudstones 

 contain numerous thin bands of dark pyritous shales ; some of these 

 yielded abundantly specimens of a small Climacograptas (probably 

 C.miserabilis Elles & Wood), and near a place called Craig-yr-huad, 

 a mile and a half north of Newcastle Emlyn, a thin dark seam yielded 

 several specimens of Di_plograptus truncatus Lapw. These forms 

 point to an Upper Bala or Upper Hartfell age for these rocks, similar 

 to those of Plynlimon. The most important evidence, however, was 

 obtained in the west, between Aberporth and Penbryn, south-west 

 of Llaugranog, for greenish-blue sandstones of the type described 

 above pass up into dark-blue thick-bedded mudstones with shaly 

 iron-stained bands. These yielded, some years ago, beautifully 

 preserved specimens of I)ij:)Iograptus truncatus, Dicellograptus sp., 

 and Orthoceras, thus establishing beyond question their Hartfell 

 age. It is clear, therefore, that North and South Cardiganshire are 

 both occupied by late Ordovician rocks. 



On account of similarity in lithological characters to the rocks of 

 the Plynlimon district, the mudstones underlying both the Gwasta- 

 den Grits of Ehayader and the conglomerates and sandstones of the 

 Llansawel district are probably of similar age. These rocks appear 

 to form a strip to the west of the Towy valley as far as Ehandir- 

 mwyn, and thence strike north-eastwards towards Rhayader. They 

 were recognized by Sedgwick^ as being below the Bwlch Trebannau 

 conglomerates, which have been shown by Miss Drew & Miss Slater 

 to occur at the base of the Silurian. 



(b) The Silurian Rocks. 



The succession of the Sihirian rocks in Central AVales is sufficiently 

 described in the papers on Rhayader, Tarannon, and Plynlimon 

 quoted above. The peculiar rusty-weathering dark shales and flags 

 forming the lower part of the Silurian have lately been recognized 

 in several localities in North Cardiganshire, as, for example, at the 

 mouth of the Llyfnant valley, near Treddol, east of Talybont, near 

 Craig-y-Pistyll, etc., so that the boundary between the Ordovician 

 and Silurian can be indicated with fair success in that district. 



Similar strata were observed also close to the River Towy, near 

 YstradflBn, about 9 miles north of Llandovery, where they are suc- 

 ceeded by pale striped mudstones of Upper "^Rirkhill type. Some 

 poorly preserved Monograptidae and crowds of Climacograj)tus sp. 



1 Phil. Mag. ser. 4, vol. viii (1854) p. 481. 



