ol-O i'i;oF. W. j. pUGii via the okoLo&i ov the L voi. Ixxix, 



Presumably, the Bhiwlas Limestones and Mudstones correspond to 

 the basal part of the Abercwmeiddaw Group ; but the faunas 

 obtained are quite distinct. 



In the Bala country, ash-bands occur above the main mass of the 

 volcanic rocks. There is no evidence of similar ash-bands in the 

 Corris area, nor of any of the well-marked limestone-bands which 

 are so characteristic and fossiliferous at Bala. 



Girvan area. 1 — There are many points of similarity between 

 the succession at Girvan and that of Corris. The fauna obtained 

 from the Abercwmeiddaw Mudstones recalls at once the fauna 

 obtained by the late Prof. Charles Lapworth from the highest 

 beds of his Whitehouse Group : namely, the Dionide Band. This 

 highly characteristic band at Girvan is underlain immediately by 

 the zone of Dicellograplus complanatiis. 



It appears, therefore, possible that the beds immediately below 

 the Cyclopyge Beds in the Abercwmeiddaw Group correspond 

 approximately to the Pjicellograpitvs-complanatus Zone on the 

 graptolitic time-scale. 



In the lower part of the Whitehouse Group, Prof. Lapworth 

 recorded from black shales Plenrogvctptus linearis, 'Dicellograpttis 

 morrisi, T). forchammeri, etc. The fauna of these graptolitic 

 shales recalls that of the Nod Glas at Corris ; but the presence of 

 Picranograptus clingani in the latter area suggests a somewhat 

 lower horizon : namely, the highest part of the Ardwell Beds at 

 Girvan. Hence at Corris it is possible that the Pleurograptus- 

 linearis Zone may be represented by the very highest black shales 

 of the Nod Glas, and perhaps that part of the Abercwmeiddaw 

 Mudstones which lies beneath the Cyclopyge Beds. 



There is some difficulty in subdividing the Upper Ordovician 

 rocks in the Corris area into the usual Ashgillian and Caradocian 

 Stages, and consecpiently I have not attempted to do so. 



The Conwa}' area. 2 — At Conway, in the Deganwy or PJiacops 

 Mudstones, Ph. ■mucroiiatus has been found about 7 feet above a 

 shale-band yielding Orthograptus truncatus var. abbreviatus. It 

 will be recalled that at Corris the lied Vein yields that graptolite, 

 together with Dicellograptus cuiceps. Outside the Corris area, 

 towards Dinas Mawddwy, Phacops mucronatus occurs in the lower 

 part of the Garnedd-wen Beds immediately overlying the Narrow 

 Vein. The Deganwy Mudstones, therefore, appear to correspond in 

 stratigraphical position with the Abercorris Group. If that be so, 

 then there is a very striking difference in thickness. The Deganwy 

 Mudstones are only SO feet thick; but the Abercorris Group 

 varies from 2-iOO to 900 feet. 



Underlying the Deganwy Mudstones at Conway are the Tri- 

 nucleus or Bodeidda Mudstones, and these appear to possess 



1 C. Lapworth, Q. J. G. S. vol. xxxviii (1882) p. 537. 

 - G. L. Elles, ibid. vol. Ixv (1909) p. 169. 



