SEDGWICK ON NORTH WALES. 15 



[The Llansaintfraid series, including the slates and two hands of limestone, 

 lies above the porphyries of the Teirw river, and, consequently, far above the 

 fossiliferous beds of Pont Meibion. It is distinct from any other part of the 

 series, with the exception, perhaps, of the beds on the western bank of the 

 Fyrnwy river, above Meifod. (Vide Section VI.) It is loaded with shells of 

 the Wenlock limestone ; among which are Orthis sinuata and O. inflata ; 

 Spirifer crispus, Terebratula crispata, Atrypa affinis, and Euomphalus funatus. 

 It also contains nine or ten Wenlock corals, such as Catenipora, &c. Among 

 the Orthides is a new species, which is found also at Coniston. Several of 

 the corals belong to new species. Besides the above, are several well-known 

 Caradoc sandstone species of shells. 



In addition to the above positive characters, the group is distinguished by the 

 following negative one — that it contains apparently none of the species which 

 are characteristic of the lower parts of the Protozoic series, such as Asaphus 

 Buchii, Agnostus pisiformis, lllcenus Bowmanni, Spirifer crucialis, &c. 



This group, then, seems to form a kind of passage between the lower and 

 upper systems. 



To judge from the fossils only, the Coniston limestone appears to be interme- 

 diate between the Llansaintfraid and the Bala limestones.] 



On the evidence of this Section and of the lists of fossils which 

 belong to it, the author concludes : — 



1. That the highest or Llansaintfraid group cannot be identified 

 with any of the groups in Sections I., II., and III. ; and that if it 

 ever be brought into comparison with any group in those Sections, 

 it must be with the highest group, namely, with that which is 

 found near the crest of the southern Berwyns ; and, therefore, that 

 it lies far above the Bala limestone. 



2. That the rocks from Pont Meibion southwards, and those of 

 Craig -y-Glyn, may be brought into comparison with the lower parts 

 of the Bala series, to the west of the lake, and with the slates east 

 and west of Arenig, which contain Asaphus Buchii. 



The preceding conclusions the author proposes, subject to the 

 modifications which they must necessarily undergo, when his 

 sections and lists of fossils come to be compared with those ob- 

 tained by the gentlemen employed on the Ordnance Geological 

 Survey, from an examination of the mountains of South Wales 

 west of the district surveyed by Mr. Murchison. * 



They exhibit no traces of the lower beds such as occur in North 

 Wales, containing Asaphus Buchii, &c. ; and they disappear when 

 the Porphyries begin. The Coniston limestone appears to be very 

 little lower than the limestones of the Ceiriog, and is therefore 

 probably higher in the series than the Bala limestone. In North 

 Wales, on the contrary, the fossiliferous series has no well-defined 

 base, since fossiliferous beds of vast thickness, extending far below 

 the Bala limestone, there alternate with porphyries. 



* The sections of Cumberland and Westmoreland are not of a nature, in the 

 author's opinion, to throw light on questions having reference to minute points 

 in the classification of the different members of the Protozoic series of rocks. 

 For, in those countries, the Lower Silurian rocks, containing fossils, are of com- 

 paratively small thickness, and have a well-defined base, which the author has 

 formerly described. See "Proceedings of the Geol. Soc." vol. iii. p. 551. 



