TUB ORDOYK'IAN SYSTKM 117 



Dr. H. Hicks described the Lower Arenig, near St David's, as 

 consisting of fine black slates and shales, which are best exposed in 

 IL-uiiscy Island, where they have yielded Didymograptus extentu$, 

 Phyllograptus stella, Trigonograptus ensiformis, and T. truncatug. 

 They also contain Ogygia scutatrix and Asaphellus homfrayi. 

 Some of the same beds are seen in Whitesand Bay on the main- 

 land, but are faulted against the Tremadoc slates. He believed 

 this group to be about 1000 feet thick. 



What must be called the Upper Arenig (Middle Arenig of 

 Hicks) are a series of black slates and flagstones with some beds of 

 gritty sandstone, and they have yielded many of the characteristic 

 Arenig trilobites, Ogygia peltata, dEglina grandis, Trinuclew Gibbfri, 

 and Ampyx Salteri, and the graptolites Tetragraptus serra, T. 

 crucialis, and Didymograptus patulus. These beds are about 1500 

 feet thick. 



The equivalents of this series have recently been mapped in 

 Carmarthenshire for the Geological Survey by Messrs. Can trill and 

 H. Thomas, who have termed them the Tetragraptus Beds and 

 have divided them into two stages or zones, (1) that of Didymo- 

 graptus extensus, (2) that of Did. hirundo. The former includes 

 the greater part of the series, and is over 1000 feet thick ; litho- 

 logically it consists of blue-black shales and mudstones with grits 

 and conglomerates at the base which mark it off from the under- 

 lying Tremadoc slates. The upper zone appears to be only about 

 200 feet thick. 



North of Carmarthen the Arenig Series passes under the flexured 

 mass of higher Ordovician rocks which occupy the central area of 

 Wales (Cardigan, Brecknock, and Montgomery), not again coming 

 to the surface till brought up by the great Harlech or Merioneth- 

 shire anticline. Round this it forms a nearly continuous out- 

 crop from Towyn on the coast through the Cader Idris, Aran, 

 and Arenig ranges, and thence through the Migneint district 

 to Manod Mawr, north of Ffestiniog (see Fig. 19), where it seems 

 to be faulted against higher beds. Throughout this tract of 

 country from Cader Idris to the Arenig Mountains, the place 

 of the Arenig Series appears to be entirely below the beds 

 of volcanic ash and the intrusive masses of andesite which wen- 

 formerly included in this series. In the district of Arenig 

 Fawr and Moel Llyfnant the succession of beds has been mapped 

 and described by Mr. Fearnsides, 6 and the position of the 

 Arenig Beds is shown in the section drawn by him and reproduced 

 by his permission in Fig. 31. The series here is only about 600 

 feet thick, but both zones can be recognised, and the sequence 

 is as follows : 



