252 [Nov. 29, 



tone, &c., coterminous along the Holyhead road and Conway rivers 

 with the last-mentioned principal group ; but bounded tow^ards 

 the north-west by an overlying range of mountain limestone. 



The present paper communicates the results of new researches 

 which, in the company and with the assistance of his friend, Mr. 

 J. W. Salter, the author made, during the summer of 1843, in the 

 eastern portion of his former field of observation : his remarks on 

 the present occasion being directed principally to the geological 

 position and organic remains of the fossiliferous slates which lie 

 to the east of the great Porphyry range of the Arenigs. 



During these excursions, besides correcting the north-western 

 boundary line of the rocks belonging to the second principal group, 

 the author determined their southern boundary. That boundary 

 follows a very sinuous course from the mountain limestone of 

 Llanymynech hill, on the east, to the Dyfi near Mallwydd, on the 

 west ; whence it runs in a south-western direction, down the right 

 or northern bank of that river for several miles. The boundaiy 

 line of the protozoic rocks, both in the north and in the south, was 

 laid down by the author and his companion on the Ordnance Map, 

 from which they have been transferred to the small map annexed 

 to the present Abstract. 



The author has also materially improved the details of the sec- 

 tions which he formerly exhibited to the Society, and has greatly 

 extended his lists of fossils. For the determining of these fossils, 

 for the lists of them appended to this abstract, and for the general 

 observations* which an examination of these lists has given rise 

 to, he expresses himself indebted to Messrs. J. C. Sowerby and 

 J. W. Salter, of whom the latter examined most of the localities 

 where the fossils were obtained. 



§ 2. On the Calcareous Slates and Limestone of Glyn Dyffics on 

 the Holyhead road, West of Corwen and of Rhiwlos, North 

 East of Bala. 



In an endeavour to determine the position of the limestone of 

 Glyn Dyfiws, a series of calcareous and fossiliferous slates was 

 traced from Cader Dinmael, on the north, through Glyn Dyffws 

 and Pen-y-Cerrig, southward, to the hiUs on the left bank of the 

 Merddwr brook, near Llwyn Onn. Here the strike was inter- 

 rupted by enormous dislocations. 



Calcareous slates, passing into limestone, again appear, to the 

 south and west, at Llwyn Jolyn, Craigian-buchan-isaf, Llwyn-y-ci, 

 and again, on the same line of strike, in the high grounds of the 

 Rhiwlas estate N. E. of Bala, and lastly, about a mile above Bala, 

 in the bed of the river Tryweryn. A part of this limestone band 

 has been noticed by Mr. Sharpe, f 



* In this Abstract the observations of Messrs. Sowerby and Salter are an- 

 nexed to Professor Sedgwick's description of the geological position of the 

 fossils; and, for distinction's sake, are printed in smaller type. 



\ Proceedings of the Geological Society, vol. iv. p. 10. 



