272 prof. a. h. eox and me. a. k. wells oisr the [vol. lxxvi, 



J), bifidtis (Hall), and Climacograptiis scharenberqi Lapworth. 

 Didymogruptus nitidus was also obtained from the same slate- 

 band on the crags due south of Llyn Wylfa, and fragments of 

 ■ extensiform graptolites from one or two points elsewhere. 

 D. hifidus and CI. scharenberqi were also found in corresponding 

 slates on the eastern hank of the Gwynant ojmosite the islet at 

 Pont Kings. All the localities mentioned are in the lower of the 

 two slate-bands. 



There is no question that the early forms of Didymogruptus 

 hifidus do occur in actually the same beds as the extensiform 

 graptolites. It is fairly certain, therefore, that we have here the 

 junctions of the zones of Didymograptus hirundo and D. bifid us, 

 or, in other words, the junction of the Arenig and the Llanvirn 

 Series, and that the lower half of the Pont Kings Slates is to be 

 referred to the Arenig and the upper half to the Llanvirn Series. 



Since there is no doubt that the slates form a true intercalation 

 in the Lower Acid Series, it follows that the volcanic series 

 begins in, or just below, the zone of Didymograptus liirundo, and 

 continues up into the lowest part of the Llanvirn Series, closing 

 with the formation of the ' China-stone ' Ashes and associated rocks 

 that immediately overlie the Pont Kings Slates. The represen- 

 tatives of the volcanic series in other districts will be discussed 

 below (see pp. 316 & 318). 



(iii) The 'China-stone? Ashes. — Succeeding the higher of 

 the two Pont- Kings Slate bands comes a well-marked band of ash. 

 The ashes belong predominately to the 'china-stone' t} r pe : that 

 is, they are very fine-grained rocks, and break with a smooth 

 conchoidal fracture into sharp-edged chips. When fresh they are 

 blue-hearted, but they yield the usual white crust on weathering. 

 The colour is sometimes constant over a considerable thickness of 

 rock ; at other times a mottling following the lamination makes its 

 appearance. With the incoming of coarser material they pass over 

 into creamy types of rhyolitic ash, such as those already described 

 (p. 269). On the other hand, all transitions to normal slates may 

 be present within a small thickness of rock. 



These rocks are well exposed along the roadside at Pont Kings, 

 . and thence almost continuously right away to the Arthog Valley. 

 They resemble in all respects, and are probably on the same horizon 

 as, the ashes that occur at or near the base of the Didymograptus- 

 hifidus Zone in Shropshire, 1 and at various localities in South 

 Wales. 3 The petrology of the similar rocks of South Wales has 

 been described by Dr. H. H. Thomas, and also by one of us. 3 



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

 Geol. Assoc, vol. xiii (1894) p. 317. 



2 D. C. Evans, ' On the Ordovician Rocks of Western Carmarthenshire ' 

 Q. J. G. S. vol. lxii (1906) p. 613; T. C. Cantrill & H. H. Thomas, 'The 

 Geology of the Country round Carmarthen' Mem. Geol. Surv. 1909, pp. 24 & 

 25 ; J. Pringle & others, ' Pinal Report on the Geology of Ramsey Island ' 

 Rep. Brit. Assoc. (Australia) 1914, p. Ill ; A. H. Cox, 'The Geology of the 

 District between Abereiddy & Abercastle ' Q. J. G. S. vol. lxxi (1915) p. 298. 



3 Q. J. G. S. vol. lxxi (1915) p. 299. 



