LLANVIRN SERIES. 69 



LLANVIRN SERIES. 



This series was defined by Dr. Hicks in 1881, the name being 

 taken from Llanvirn near St. Davids. The beds were formerly 

 grouped with the Arenig series and Llandeilo Flags. 



The Llanvirn series consists of dark slates and shales, with con- 

 temporaneous beds of volcanic tuff. It has an average thickness 

 of about 2000 feet. The fauna is rich in Trilobites, amongst which 

 the genus Placoparia is unknown elsewhere in Britain. Several 

 genera appear for the first time, such as Ilh^niis, Barrandca, Phacops, 

 and Acidaspis. The Graptolites Diplograptus and Didymograptus 

 Miirchiso7ii are found, as well as large Cephalopods, Gasteropods, 

 Brachiopods, and Lamellibranchs. Dr. Hicks divides the beds 

 as follows : ' — 



Ti • ( TT,. (Fine black slates interstratified with beds of ash and tuff, 

 Llanvirn ) Upper. \ . ^, nr i ■ ■ 



c • \ \ containing Didymograptus Miirchisoni. 



{ Loiuer. — Dark grey flags and slates with Placoparia, etc. 



The lower portion of the so-called Llandeilo Flags at Abereiddy 

 Bay belongs to the Llanvirn Series. Graptolites were discovered 

 in these beds by Sir A. C. Ramsay in 1841. The Didymograptus- 

 Shales found near Narberth, Haverfordwest, by Messrs. Marr and 

 Roberts, may probably be grouped with this series." The beds are 

 also represented near Shelve, in Shropshire, and near Caernarvon. 



At Tai-hirion near Arenig, and Ty C3bry near Portmadoc, there are 

 beds formerly grouped as Upper Arenig, and which should now be 

 placed with the Llanvirn Series, according to Dr. Hicks's classifica- 

 tion. The fossils, however, according to Mr. J. E. Marr, are more 

 closely related to those of the Arenig Series. The fine-grained 

 slates of Ty Obry (Ty Obry beds) have yielded /Eglina caliginosa, 

 Calymene paivifrons, Dionide atra, Deiidrograpltis, etc. At Tai-hirion 

 Calymene pannfrons, Ogygia Selwymi and other fossils have been 

 obtained. 



Near Caernarvon, Mr. Marr has collected Graptolites and other 

 fossils from rocks which probably belong to the Llanvirn Series ; 

 and near Llangwyllog church in Anglesey, Prof. Hughes has 

 obtained evidence of similar fossiliferous strata.^ Other fossils from 

 this series are Phacops Llanvirnensis, lUcemis Hughesii, Calyjiune 

 Hopkinsoni, Trinucleus Etheridgii, T. Ramsayi, Pleurotomaria, Bclle- 

 rophoji pertiirbatus, Orihoceras, Lingula attemiata, etc, 



1 Hicks, Pop. Science Review, 1881, p. 289. 



2 Q.J. xli.477. 



^ Hicks, Q. J. xxxi. 175, 192 ; J. E. Marr, Classification of the Cambrian and 

 Silurian Rocks, p. 35, and Q. J. xxxii. 134; Ramsay, Geol. N. Wales, edit. 

 2, p. 357, etc. 



