Yol. 6S.'] GEOLOGICAL STRTJCTUKE OF CENTRAL WALES. 333 



be taken to indicate in different districts rocks ranging in age from 

 Menevian to Llandovery inclusive. Different series are indicated 

 by letters where supposed rocks of the series have been recognized 

 or proved. Over the greater part of the area herein considered^ 

 b^ indicates that the rocks were regarded as predominantly of Bala 

 or Caradoc age, while 5*, scattered about mainly on the outcrops of 

 the more gritty beds, suggests that those are of Lower Llandovery 

 age. 



The Ordovician rocks in the neighbourhood of Fishguard have 

 been described by Mr. E. E,. C. ileed,^ while rocks of the same 

 general type have been observed in other parts of Pembrokeshire.. 

 They consist of black graptolitic slates of Arenig and Llandeilo age 

 alternating with felsitic flows, tuffs, and breccias. Their distribu- 

 tion on a large scale may be assumed to coincide generally with the 

 igneous area of North Pembrokeshire ; on a geological map these 

 igneous rocks end off abruptly along a fairly even line (see 

 PI. XXXIY). 



In the Newport district the volcanic series is succeeded by grap- 

 tolitic shales of the age of the black Bicranograj^tus Shales of South 

 Pembrokeshire ; the graptolites recorded by Mr. Reed indicate, as 

 is now known, a low horizon in those shales. 



These are followed north of Newport by a great slaty and gritty 

 series. According to Keeping and Dr. Matley (opp. sujdtci cit.) these 

 consist of black slates containing Diplograptidae, alternating with 

 pale-blue and grey felspathic grits. In the latter, near PwU-y-wrach^ 

 I discovered, some years ago, numerous impressions of shelly fossils ; 

 they deserve further study. The graptolites found in this series 

 by Keeping and Dr, Matley (especially by the former at Cardigan) 

 indicate a horizon about the top of the Dicranogra])tus Shales, which 

 seem, therefore, to be represented in this district by an altogether 

 peculiar facies. 



In the more southerly parts of Pembrokeshire the Dicranograptus- 

 Shales are succeeded by blue mudstones, probably of the same general 

 age as the Eedhill and Slade Beds described by Dr. Marr and the 

 late T. Roberts - in the Haverfordwest district. 



The same general change takes place in North Pembrokeshire, 

 for, in a lane south of the Glogue quarries, between Crymmych 

 Arms and Llanfirnach, I observed typical black Dicranograptus- 

 Shales or slates (for they were highly cleaved) with abundant 

 specimens of Mesograjptus sp. and Dicranogra2:)tiis brevicaulis, re- 

 presenting, therefore, a low horizon in those shales. Similar shales 

 occur at Crymmych Arms and southwards, where they appear to 

 succeed the felsitic rocks of Poel Trigarn. Farther north, between 

 Crymmych and Cardigan, blue-grey cleaved mudstones and slater 

 occupy an enormous area ; as these cannot be compared with any 

 part of any facies of the Dicranograptus Shales or lower beds, it is 

 probable that they occur on a higher horizon. 



1 Q. J. G. S. vol. li (1895) p. 149. 



2 Ibid. vol. xli (1885) p. 476; cf. D. C. Evans, ibid. vol. Ixii (1906) 

 pp. 602 & 640. 



