T1IK oKliuYIUAN >V>IKM I '-' 1 



Llanvirn Series. This group was separated from tli* 

 In-low and above it by Dr. Hicks in 1881. It takes it> name from 

 tli.it of a village near Llanrain in Pembrokeshire, (see map, Fig. 

 Ifi), where the slates have yielded many of the characteriMi' 

 trilohites and graptolites of the Lower Llanvirnian or /one of 

 JUdymograptus bifidus. This species and ite congener- /'. /?/'. 

 I>. mi nns, and D. Murchisoni are often termed the tuning-fork 

 graptoliteo, as dfatiaguidied from the rxtcn-iform species which 

 prevail in the underlying Arenig Beds. 



The lower stage is probably from 1^00 to 1500 feet thick in 

 Pembrokeshire, and consists of dark-grey flags and slates. The 

 upper stage or zone of D. Murchisoni consists of fine black elates 

 interbedded with felspathic tuft's and has a thickness of about 

 500 feet 



These beds have now been mapped by the Geological Survey 

 from Ha verf oixl west eastward, through Narberth and Carmarthen. 

 Along this tract the lower stage maintains its thickness, but the 

 Murchisoni Beds are much reduced, being only 230 feet near 

 St Clears, while east of Carmarthen they appear to thin out lielow 

 the base of the overlying Ffairfach Grit (Llandilo). 



Passing now to Merioneth there is no doubt that the Llanvirnian 

 can be followed all round the great Harlech geanticline, but they 

 have not yet been separated from the Arenig and Llandilo Serii-. 

 Their position on Arenig Fawr is indicated in Mr. l-Yarn-i'le'- 

 section (Fig. 31, p. 118), the base being marked by a Iwmd of shale 

 with D. bifidus which is overlain by a volcanic series consisting 

 of ash and agglomerate with thick intrusive masses or laccolites 

 of hypersthene-andesite. Above these, however, is an outcrop of 

 shale containing Diplograptus foliaceus and other specie- whu-h 

 suggest a high horizon in the Murchisoni zone. 



The position of the Llanvirnian in the Moelwyn area has not 

 yet been ascertained, but farther west in the promontory of 

 Penrhyn (see Fig. 20), it appears to be faulted down against the 

 basal Arenig grit, for the black slates of Ty Obrey which adjoin 

 that grit have yielded fossils which prove them to VH- of Llanvirn 

 age. Above them are ashy slates and ash beds pacing under a 

 thick volcanic series. 



In Carnarvon the Llanvirn Series has not yet been sedated as 

 a stratigraphical division from the beds above and below, but as 

 mentioned on p. 119, the bifidus zone has been recognised as 

 succeeding the /immrfo-beds on the river Seiont near Carnarvon. 



Its existence in Anglesey has a special interest, because in tin- 

 central part of that island it forms the base of the Ordovician 

 System, and has a basal conglomerate which rests directly on the 





