308 UPPER CAMBRIAN OF BERWYN MOUNTAINS AND BALA LIMESTONE, 



The structure of the Berwyn Mountains, which form the upper portion of the great 

 Cambrian System, as well as all the rocks to the north and west of them, will be fully 

 described by Professor Sedgwick • but having traversed these mountains, in company 

 with him, I may be permitted to state, that they mainly consist of coarse slaty rocks, 

 Their overlying masses contain a few impure calcareous courses with casts of fossils, 

 and at intervals thick and massive bands of porphyritic trap as well as thinly laminated, 

 yellowish green bands of compact felspar and other rocks of igneous origin. On their 

 north-western face, these mountains are flanked and underlaid by lower ridges of other 

 slaty rocks, containing many massive beds and elongated concretions of dark grey lime- 

 stone, some crystalline, others earthy and impure, in thickness of 10, 12, and 20 feet. 

 In the hills east of Bala these limestones are pretty extensively quarried, and clearly 

 dip under the chief mass of the Berwyns. Among the fossils, including those of the 

 Bala limestone, the following occur ■ Orthis anomala, Schlot. PI. 21. f. 10. ; 0. Actonice, 

 PI. 20. f. 16. ; O. canalis, PI. 20. f. 8. ; O. compressa, PI. 22. f. 12. ; O. Flabellulum, 

 PI. 21. f. 8. (a) ; 0. lata, PL 22. f. 10. • 0. Pecten, Dalm. ? PI. 21. f. 9. ; 0. protensa, 

 PI. 22. f. 8. 9. ; 0. testudinaria, Dalm. PL 20. f. 9. ; Bellerophon bilobatus, PI. 19. f. 13. ; 

 Lept&na sericea, PI. 19. f. 1,2.; with two or three unpublished species. 



As these -shells abound also in the Lower Silurian Rocks, it would seem that as yet 

 no defined line of zoological division can be drawn between the Lower Silurian and 

 Upper Cambrian groups, and that as our knowledge extends, we may probably fix the 

 lowest limit of the Silurian System beneath the line of demarcation which has for the 

 present been assumed. In the meantime, transitions which, like this, are clearly marked 

 by stratigraphical, lithological, and zoological proofs, are of the highest value in esta- 

 blishing the true classification of sedimentary deposits. Though frequent in the over- 

 lying formations, as developed in the previous chapters, they are necessarily of rarer 

 occurrence among the oldest fossiliferous strata, owing to the greater number of con- 

 vulsions and changes, which they have usually undergone, and hence the geological 

 value of this section. In Caermarthenshire, similar passages will be pointed out; but 

 in that region, the beds in question are neither so largely developed, nor so much 

 charged with fossils as near the Berwyns. To what extent the same species of shells 

 which characterize the Lower Silurian rocks, descend into the Cambrian System, is 

 not yet satisfactorily determined ; nor can it be so, until the oldest fossiliferous rocks 

 of Cumberland, Wales, and Devonshire are brought into close comparison, and their 

 specific contents accurately determined 1 . 



In the accompanying map it will be perceived, that wherever the Lower Silurian 

 rocks pass into the Upper Cambrian, as on the flanks of the Berwyns and in certain 



running in planes mathematically parallel to each other, are not the laminse of deposit. The latter have 

 indeed always more irregular and uneven surfaces, which are frequently to be traced by decomposing edges or 

 furrows on the weathered sides or joints of the rocks, often filled by patches of moss and grass. (See Sedgwick 

 on slaty cleavage, Geol. Trans, vol. 3, p. 460.) 



1 Professor Sedgwick's works on Cumberland and North Wales, as well as a joint memoir by him and myself 

 on the older Rocks of Devonshire, will, it is hoped, in a great measure, clear up this point. 



