part 4] DISTRICT AROUND OOERlS AND AHEULDEFENXI. 541 



lithological characters very similar to those of the Abercwmeiddaw 

 Mudstones which underlie the Abereorris Group. At Corris they 

 yield most commonly species of Tri nucleus ; but there are many 

 differences in fauna between the two groups. If the Bodeidda 

 Mudstones are correlated tentatively with the Abercwmeiddaw 

 Group, there is again a marked difference in thickness from 

 350 feet at Conway to 1500 feet at Corris. 



The highest beds of the Cadnant Shales may, perhaps, be corre- 

 lated with part of the Nod Glas, but there are again many 

 differences in fauna. The Dicellograpti which are so abundant at 

 Corris appear to be absent at Conway, and there is no indication in 

 the latter area of those graptolites which at Corris suggest the 

 soinewdiat higher zone of Pleuroffrcqitus linearis. 



In conclusion, I wish to express my thanks to Prof. O. T. Jones, 

 who has given me much valuable advice and assistance in the 

 identification of the fossils, and much encouragement during the 

 progress of the investigation ; also to Mr. G. J. Williams, I.S.O., 

 who, in the early stages of the work, informed me of several 

 localities where he had collected fossils. 



EXPLANATION OF PLATE XXVII. 



Geological map of the district around Corris and Aberllefenni, 

 on the scale of 2 inches to the mile, or 1 : 31,680. 



Discussion on the two foregoing Papers. 



Miss G. L. Elles said that she was more particularly interested 

 in the questions connected with the Black-Shale Group in the two 

 areas, and in the occurrence of the Dionide fauna. Mr. King's 

 paper dealt with an area which was close to the Bala country, where 

 no Black Shales occur at the horizon that he had noted; nevertheless, 

 this apparent discrepancy probably seems greater than it actually 

 is, for at the top of the Allt-ddw Mudstones these graptolitic shale- 

 bands do. occur intercalated in the mudstones, showing that the 

 conditions for the accumulation of graptolite-shales were not very 

 far away even there. What these conditions were was another 

 question ; Mr. King considered that his Black Shales had accumu- 

 lated under ' lagoon ' conditions. In the opinion of the speaker 

 that depended upon the meaning of the term 'lagoon' : if it meant 

 merely shallow, quiet water, she would agree absolutely ; but, if it 

 implied restriction in communication with the open sea, she would 

 disagree just as absolutely. She did not think that any worker on 

 graptolite-shales at the present day considered them deep-sea 

 deposits : the factor that controlled their deposition was not depth 

 as depth, but the other factors that go with depth, especially 

 quietness of the waters, as indicated by the perfect condition in 

 which these delicate little organisms are found. This had been 

 pointed out as long ago as 1897 by the late Prof. Charles Lapworth, 



