70 UPPER CAMBRIAN (ORDOVICIAN). 



BALA SERIES. 



LLANDEILO FLAGS. 



The name is taken from the town of Llandeilo in Caermarthen- 

 shire, where the rocks were described by IMurchison in 1834. The 

 beds were first clearly defined by Sir A. C. Ramsay in 1842. 



They consist of bluish-grey and black micaceous and calcareous 

 flags, with black shales or slates at the base. Associated with 

 them are beds of limestone and many igneous rocks. The total 

 thickness of the series is estimated at from 3300 to 4000 feet, 

 including sometimes 2500 feet of lavas. 



Two divisions have been made as follows, a third and lowermost 

 division determined by Dr. Hicks in Pembrokeshire being now 

 placed by him with the Llanvirn Series :^ — 



!Yj f Black slates, flags, and flaggy sandstones, with interbedded 



^" ' t igneous rocks. 2000 feet. 

 , ( Calcareous shales, flags, and black slates, with limestone. 



j^owc-r. ^ ^^^^^^ jQ^Q ^^^^_ 



The Llandeilo Flags contain the earliest limestone of import- 

 ance, a marine band occurring in the lower division. Trilobites 

 were recorded from Llandeilo as early as 1698 by Lhwyd, and the 

 name Trilobite Schists was originally applied to these rocks by 

 Murchison. Actinozoa are found in these rocks ; also Sponges 

 of the group ReccptaaditidiV. represented by the genus hchadites. 

 Phyllopora (formerly Reiepora) is also found. The Upper Beds, 

 which are developed at Builth and St. Davids, yield Ogygia Btichii, 

 Calj viene diiplicata, Cheirurus Sedgwickii, Trinucleus fimbriatus, 

 Ampyx midtis, Barrandea Cordai, LiJiguIa Ramsayi, Bellerophon 

 pertiirbaius, etc. 



The Lower Beds, seen at Llandeilo, Builth, Lampeter Velfrey, 

 Musclewick Bay, and Aber-eiddy, contain Asaphus tyranmis, Trinu- 

 cleus Lloydii, T. favus, Calymene Cambtensis, Lingula attenuala, 

 Orthis stn'alula, Halysites catcnidarms, etc. 



In North Wales the Llandeilo Flags are represented by black 

 shales near Tremadoc, and in Anglesey by the mudstones of 

 Llanfaelrhys, beds which in both cases yield Graptolites. Near 

 Builth, the Builth Flags of Murchison rise in a boss in the midst 

 of the Silurian rocks, which lie unconformably upon them. Cader 

 Idris is formed of Llandeilo Flags with their included felspathic 

 ashes, etc.^ In Shropshire the beds are represented by flagstones 

 overlying the Stiper Stones. 



In South Wales the beds are well shown on the north shore 

 of Aber-eiddy Bay, near St. Davids. The lower beds, comprising 



Q J. xxxi. 172, 192 ; G. Mag. 1876, p. 156 ; Pop. Science Review, 1881. 

 An early description of Cader Idris was given by A. Aikin, T.G.S. (2) ii. 273. 



