SEDGWICK ON NORTH WALES. 9 



principally to the genus Favosites. (The Chcetetes petropolitana is also very 

 common.) The Ophiura Salteri has been found both in the Bala limestone and 

 at Cader Dinmael. The series, moreover, furnishes two or three species of 

 Cypricardia, a genus not previously found in Lower Silurian rocks.] 



7. Next comes a series of slates (h), of very varied colour and 

 texture, which alternate with bands of greywacke. As these beds 

 dip steadily towards the east at a very high angle, and are more 

 than a mile broad, their thickness must be very great. 



8. The Hirnant limestone (i) follows, and has a remarkable 

 pisolitic structure ; but, as a limestone, it is very impure. This 

 group is of considerable thickness. The beds are highly inclined, 

 and dip to the east, a few degrees south. The group was traced by 

 the author from Aber Hirnant southwards, in the direction of the 

 strike, to Bwlch-y-Groes*, and was laid down on the Ordnance Map. 



[The Hirnant limestone is characterised by its containing only a few species 

 of Orthis ; in which respect it differs in a remarkable degree from the limestone 

 of Bala, Of those which it does contain, two or three (which are new speciesf, 

 and very flat) are found in great abundance. It abounds in a new plaited 

 Terebratula, and in Encrinital stems ; but contains only a few corals]. 



9. With the same easterly dip, and at a high angle of elevation, 

 follows a very thick group of slate rocks (j). Some are dark and 

 earthy, others grey and siliceous, others glassy and chloritic. 

 They alternate with a few bands of cotemporaneous Porphyry. 



[Over the preceding, near the synclinal of the South Berwyns, fossils, resem- 

 bling those of the Bala limestone, appear here and there, but in no great 

 abundance ; and the peculiar species of the Hirnant limestone are lost. These 

 beds seem to possess scarcely any Cor.chifera or Gasteropoda, and not any 

 Orthoceratites. The fossils belong principally to Brachiopoda, and Leptcena 

 sericea is abundant, but so also is Trinucleus Caractaci. Some of the sandy beds 

 contain Encrinital stems, but corals are veiy rare.] 



10. More than a mile to the east of the Hirnant limestone is 

 a synclinal line (a-), beyond which the beds dip towards the west. 

 The lower beds, which were found to the westward, are therefore 

 again brought to the surface, and the Bala limestone (k) reappears 

 in two places near the top of the descent leading to Llangynog. 

 Both these places are on the eastern side of the watershed of the 



* It has been stated by Mr. Sharpe, in a paper read before the Geological 

 Society (see " Proceedings of the Geological Society," vol. iv. p. 13.), that the 

 line of the Bala limestone, as laid down in Mr. Murchison's map of the Silurian 

 formations, is composed of the Bala and the Hirnant limestones. The Bala 

 limestone, along its whole line of strike, and its several quarries, were examined 

 by Professor Sedgwick in the year 1832 ; and were laid down by him in 

 colours on Evans's half-inch map of North Wales. The Hirnant limestone 

 was seen by him in the same year, and recognised as a distinct bed. He 

 supposed it to be continued to the east side of Bwlch-y-Groes, but did not 

 mark its course upon any map. Mr. Murchison, in representing the course of 

 the Bala limestone, merely transferred Professor Sedgwick's coloured representa- 

 tion to his own map ; and in this transfer from a map in which the physical 

 features of a country are very ill represented, to another map in which they are 

 well represented, it is possible that some errors may have been committed. 

 But for these errors Professor Sedgwick states that he is not responsible. 



f Some of these resemble the new species which were found at Cyrn-y- 

 brain, N. of Llangollen. 



