316 PEOF. A. H. COX AND ME. A. K. WELLS ON THE [vol. lxxvi r 



On account of the similarity of the sequences, we have so far as 

 possible avoided the introduction of new stratigraphical terms. 



The highly - fossiliferous band in the higher part of the 

 Asaphelhis Flags and Shales of Llyn Wylfa, near Arthog, seems, 

 to correspond exactly in horizon with the famous fossiliferous band 

 of Penmorfa, Tremadoc. The occurrence of a Dictijonema Band 

 above the Asaphellus Beds does not seem previously to have been 

 recorded. 



(2) The Ordovician. 



(<?) Correlation with the sequence at Arenig Moun- 

 tain. — The Ordovician rocks of the Arthog-Dolgelley district 

 naturally find their nearest equivalents in the Arenig area, 1 which 

 lies only 12 miles awaj r to the north-east. There are, however, 

 certain important differences. 



The Basement Series at Arthog evidently corresponds to the- 

 Basal Grits and Extensus Flags of Arenig. The actual basal grit 

 is an inconstant member of the series, both at Arthog and at 

 Arenig. It is noteworthy that the representatives of this series 

 in the Lleyn Peninsula — the Tudwal Sandstones 2 — are con- 

 siderably thicker than at Arthog or Arenig, although the lithology 

 is very similar. 



The lower part of the Lower Acid Volcanic Series is in 

 part represented at Arenig by the Galymene Ash. The marked 

 thickening of the volcanic rocks towards the east in the Arthog 

 area and their greatly inferior thickness at Arenig, together with 

 the absence of lavas in the latter area, indicate that the focus of 

 igneous activity lies between the two localities, probably not far 

 from Dolgelley. Towards the north-west these lowest volcanic 

 rocks appear to die out altogether, as no trace of such a series is 

 found in the Lleyn Peninsula, where the Tudwal Sandstones are 

 succeeded by a thick series of mudstones. 



The lowest part of the Pont Kings Slates is almost certainly 

 equivalent to the Hirundo Beds at Arenig, and in both districts 

 the junction between the Arenig and the Lower Llanvirn Series lies 

 in a shale-band. Again, ash-bands occur in and above the slates 

 in both cases, but are more important at Arthog, where they form 

 a continuation of the lower rhyolitic ashes. 



The Bifid us Beds, although lithologically identical in both 

 areas as regards the slates, yet show certain noteworthy differences 

 in development. At Arthog the Moelyn Slates, together with the 

 Crogenen Slates, attain a maximum thickness of 450 feet, whereas 

 at Arenig the Bifidus Slates are only 20 to 30 feet thick. The 

 peculiar Bryn Brith Beds, which come in between the two series 

 of slates in the Arthog district, are either absent at Arenig, or are 

 represented by a thin ash, there grouped with the Hirundo Beds. 

 These differences are probably accounted for by an overlap within 



1 W. G. Fearnsides, Q. J. CI. S. vol. lxi(1905) p. 618. 

 - T. C. Nicholas, ibid. vol. lxxi (1915) p. 107. 



