Vol. 62.] THE TARANNOX SERIES OP TARANJSTON. 701 



paper. He had had the advantage of being taken over many of 

 the sections by her, and in the field had seen the succession fully 

 demonstrated. He was much impressed by the detailed similarity 

 between the lithological succession of Tarannon with that described 

 by Prof. Lapworth in the central Southern Uplands of Scotland, 

 and suggested that this similarity might be due to the symmetrical 

 position of the two areas on opposite sides of the great Silurian ocean - 

 trough, which, trending north-east and south-west, seems to have been 

 central about some line passing to the south of the Lake District. He 

 noted that, during the Silurian Period, that trough was gradually, but 

 completely, filled with sediment, which was brought in simultaneously 

 from both sides ; and that, as time advanced, the flanking belts of 

 greatest sedimentation continuously approached the central deej)- 

 water trench. He commented upon the extreme thinness of 

 the Llandovery and Tarannon sediments in Anglesey, Conway, and 

 Criccieth, and thinking that these areas cannot have been far from 

 the centre of the then ocean-trough, enquired whether the supposed 

 break, mapped at the base of the Silurian in North Wales generally, 

 was not due rather to the thinning of the beds than to actual 

 unconformity. 



Miss Elles said that she was particularly interested in the upper 

 and lower limits of the Tarannon Series as defined by the 

 Authoress. When working on the Wenlock Shales she had 

 noticed Purple Shales in several places below the lowest Wenlock- 

 Shale zone of Cyrtograptus Murchisoni, and she now considered 

 that these should be referred to the Dolgau Group, although 

 possibly the highest members were overlapped in many instances. 

 With regard to the lower limit, she was glad to see that the 

 Authoress definitely included the zone of Rastrites maximus in the 

 Tarannon Series, for she had noticed in the South of Scotland that 

 the lithological and faunal change was far more conspicuous below 

 the R.-maximus zone than above it, and she had always regarded the 

 inclusion of these beds in the Birkhill Shales as unsatisfactory. 



The Authoress thanked the Fellows for the kind reception that 

 they had given to her paper; and, replying to Dr. Marr, said 

 that she had adopted the original suggestion of Prof. Lapw T orth, that 

 the line between the Llandovery and the Tarannon should be drawn 

 at the top of the zone of Mo nog rapt us Sedgwickii, partly on palaeonto- 

 logical grounds, and partly on account of the constancy and ready 

 detection of that zone in the field. She thought that this classi- 

 fication would prove to have a more than local application, and both 

 at Rhayader and at Conway, at any rate, it seemed advisable on 

 stratigraphical grounds. 



