TIIK ORDOVICIAN SYSTEM 119 



5. Dark calcareous shales with Didymograptus hirundo and 



Tetragraptus serra . . ... . . . . 200 



4. Erwent or Oyygia limestone, yielding Ogygia Xclwyni, 



Orthoceras sericeum, Monobolina plumbea, etc. . . 20 



3. Heullau ashes hard, grey, comi>act beds with Calymene 



parvifrons ......... 150 



2. Llyfnant flags with Did. extensvs and D. deflexits . . 200 



1. Basal grit, varying in thickness from . . . . to 100 



About 620 



On the north side of the Harlech anticline the series is found 

 in the Moelwyn Mountains, north-west of Ffestiniog, the basal grit 

 being there overlain by 800 feet of hard spotted flagstones in which 

 only a species of Tetragraptus has yet been found. The limestone 

 is absent, and the Did. hirundo shales seem to be represented by 

 bands of slate intercalated with thick beds of volcanic ash and 

 agglomerate. 



Westward the basal grit has been found above the Tremadoc 

 Series, but the overlying beds are so broken by faults that no 

 succession can be made out. They come in again in the southern 

 part of the Lleyn peninsula where grits and flags with extensiform 

 graptolites pass across the edges of the Lingula flags and rest on 

 the Archaean rocks near Aberdaron. On the eastern side theee 

 beds are overlain by a band of grit and breccia 60 to 70 feet thick 

 containing fragments of schist, granulite, etc., derived from the 

 Archaean rocks, and this is succeeded by dark shales which have 

 yielded Didymograptus hirundo. Above them are beds of ironstone 

 and manganese-ore with blue mudstones containing Azygograplut 

 suecicus. 



Arenig slates also form part of the great slate country in 

 Carnarvonshire north-west of the Snowdon range, but the details 

 of this area have not yet been worked out. The complete series 

 has, however, been indicated by Miss G. L. Elles 7 along the banks 

 of the river Seiont at Carnarvon, where the zone of D. extennu 

 consists of calcareous flags and shales containing that species with 

 Azygograptus suecicus and (Eglina binodosa. To the south of these 

 beds are micaceous flags and shales yielding D. hirundo, D. nitidus, 

 and numerous phyllopods, and thee are succeeded by black shales 

 with a Llanvirn fauna. The outcrops extend for a distance of 

 about 2000 yards with a dip of from 40 to 50 to the south-east, 

 and the series seems therefore to be about 1400 feet thick. 



Arenig Beds have also been recognised on both sides of the 

 Menai Strait, but in Central Anglesey they are believed to have 

 thinned out and to be overlapped by the Llanvirn Beds. 



