'Vol. 52.] GEOLOGY OF THE NEIGHBOURHOOD OF CARMARTHEN. 533 



Lower Arenig. 



Phyllograptus angustifolius, Hall, is found in Britain, both in the 

 Lower and Upper Zones of the Arenig, and has a wide range in 

 Scandinavia, so that its occurrence at Glan Pibwr is not sufficient 

 to determine the horizon of the beds. The absence of Calymene 

 parvifrons, var. Murchisoni, Salt., at Glan Pibwr and Nant Pwntan, 

 and the very great abundance of Ogygia marginata, which is 

 also common in the Tremadoc Beds of Cwm Ffrwd, suggest that 

 possibly these beds are of Lower Arenig age. The evidence which 

 we obtain from their position in the field supports this opinion. The 

 Glan Pibwr Beds are only separated from the soft Tremadoc shales of 

 Cwm Ffrwd by a continuation of low-lying ground, presumably of 

 the same age ; and the Nant Pwntan exposure lies between the 

 Tremadoc Slates of Cwrt Hir and the graptolite-shales of Hafod- 

 wen, in the latter of which are fossils from the Tetragraptas-zone. 

 It is impossible to compare these beds with those of other areas 

 until more fossils are obtained. 



Upper Arenig. 



In the shales of Hafod-wen occur Didymograptus nitidus, Hall, 

 D. constrictus, Hall, Dictyograptus, sp., and JEglina binodosa, which 

 determine the Upper Arenig age of these beds. Although we have 

 not found the characteristic Tetragraptus, yet the similarity of our 

 fossils to those obtained by the late Mr. T. Roberts, at Talfan etc., 

 suggests that at Hafod-wen we have the same beds reappearing. 

 The shales may be correlated also with the Upper (formerly Middle) 

 Arenig of St. David's, and with part of the Tetragraptus-zone of the 

 Skiddaw Slates described by Mr. J. E. Marr. 1 



We have not been able to determine whether the mudstones of 

 the Roman Eoad and Swansea Road stream underlie the Hafod- 

 wen shales, or whether they are on the same horizon but of a 

 totally different facies. The association of Calymene parvifrons, 

 Salt, (a typical Arenig fossil of North Wales, where it is found along 

 with Ogygia Selwynii), with Ogygia marginata, shows that we cannot 

 class these beds with the Llanvirn Group. We find also Calymene 

 parvifrons, var. Murchisoni, Salt. ; as this fossil is recorded by 

 Dr. Hicks from Porth Hayog, Ramsey Island, together with Didy- 

 mograptus nanus, D. bifidus, D. indentus, and D. patulus, its occur- 

 rence in the Carmarthen beds points to the probability that they 

 are near the upper limit of the series. 



3. Didymograptus bifidus-shales (Llanvirn). 



These beds contain PJiacops llanvirnensis, Hicks, Didymograptus 

 bifidus, Hall, and other fossils common in the Llanvirn ( = Lower 

 Llanvirn of Dr. Hicks) of St. David's, with which beds they are 

 exactly comparable. The occurrence of Acidaspis Buchii, Bnrr., 

 connects the Carmarthen beds directly with Stage D d 2 of Bohemia, 



1 Marr, ' Notes on the Skiddaw Slates,' Geol. Mag. 1894, p. 122. 



