part 3] LOWER PALJEOZOIC OF ARTHOG-DOLGELLEY. 317 



the Bifidus Beds, which becomes more pronounced towards the 

 north and east. 



The Cefn Hir Ashes, with the overlying Lower Basic 

 Series, are clearly equivalent to the Lower Platy Ashes and 

 Agglomerates with the overlying Lower Andesitic Series of Arenig. 



(b) Correlation with the Shropshire sequence. — The 

 Arenig strata of the district can only be correlated in a general 

 way with those of Shropshire, 1 and various important differences- 

 are found. On the other hand, a close comparison may be drawn 

 between the Llanvirnian Series in the two districts. The arena- 

 ceous beds — the Stiper Stones and the succeeding Lower Mytton 

 Flags (composed of alternations of arenaceous and argillaceous- 

 material) — probably correspond to the Basement Series at Arthog; 

 but the Shropshire strata are much thicker than their equivalents- 

 in the Cader-Idris and Arenig districts. In the succeeding 

 argillaceous beds of Shropshire, which are likewise of considerable 

 thickness, nothing corresponding to the Lower Acid Volcanic 

 Series occurs. The Shropshire sequence, therefore, resembles that 

 in the Lleyn Peninsula, the two districts being symmetrically 

 placed with reference to the Cader-Idris-Ai'enig line, along which 

 the thinner, more volcanic facies of the Arenig strata is developed. 



Above the Hirundo Zone, however, the successions in the Arthog 

 and Shropshire districts are veiy similar. The junction between 

 the Hirundo and the Bifidus Zones occurs in both areas in a shale- 

 band, the Pont Kings Slates and the Hope Shales respectively. 

 The ' China-stone ' Ashes above the Pont King's Slates are repre- 

 sented by exactly similar ashes among the Hope Shales. The 

 Moelyn and Crogenen Slates together evidently correspond to the 

 main mass of the Hope Shales. The Cefn Hir Ashes and Lower 

 Basic Volcanic Series are correlative with the Stapeley Andesitic 

 Ashes and Lavas. Shale-bands containing Didymograptus bifidus 

 occur intercalated in both volcanic series, while the composition 

 of certain porcellanous ashes is identical in both areas. 



(c) Correlation with the sequence in South Wales. — 

 The sequence in South Wales compares more closely with that in 

 Shropshire than with the sequence at Arthog. Again, only a general 

 correspondence exists between the sequences of North and of South 

 Wales, so far as the Arenig is 'concerned ; whereas, again, the 

 Llanvirnian allows us to institute a close comparison. 



In South Wales, 2 as in Shropshire, the Arenig succession is much 



1 C. Lapworth & W. W. Watts, ' The Geology of South Shropshire ' Proc. 

 Geol. Assoc, vol. xiii (1894) p. 317; and ' Shropshire ' Geol. Assoc. Jubilee 

 Vol. (1910) p. 751 ; also section by C. Lapworth in the ' Summary of Progress 

 for 1915 ' Mem. Geol. Surv. 1916, p. 37. 



2 T. C. Cantriil & H. H. Thomas, ' The Country round Carmarthen r 

 Geology of the South Wales Coalfield, pt. x' Mem. Geol. Surv. 1909, p. 9 ; 

 also D. C. Evans, ' The Ordovician Bocks of Western Carmarthenshire ' 

 Q. J. G. S. vol. lxii (1906) p. 603 ; and A. H. Cox, 'Geology of the District 

 between Abereiddy & Abercastle ' ibid. vol. lxxi (1915) p. 283. 



