ON THE PRE-CAMBRIAN ROCKS OF BANGOR. 137 



12. On the P re-Cambrian Bocks of Bangor. By Prof. T. M c Kennt 

 Hughes*, M.A., F.G.S. With a Note on the Microscopic 

 Structure of some Welsh Bocks, by Prof. T. G. Bonney, H.A., 

 F.G.S. (Read December 5, 1877.) 



In a paper entitled " Outline of the Geological Structure of North 

 Wales," read before the Geological Society, June 21, 1843, and pub- 

 lished in abstract in the Proc. Geol. Soc vol. iv. pp. 212-224, Prof. 

 Sedgwick gives a general sketch of the Cambrian rocks from the 

 Bangor and Caernarvon series to the Bala beds. The general character 

 he describes as follows : — " The rocks occupying the region are chiefly 

 composed oifelstone (compact felspar) and felstone porphyry, trappean 

 conglomerates, plutonic silt (exactly like the chloritic varieties of 

 German Schaalstein), and other erupted or recomposed igneous pro- 

 ducts ; and the above-named rocks alternate indefinitely with fine 

 masses of roofing- slate, and with great masses of greywacke — and 

 with greywacke slate, often calcareous, but rarely containing beds 

 and masses of limestone " (p. 215). 



Further on he states that " the group of chlorite, slate, &c. contains 

 no organic remains, and forms no passage into the rocks of the other 

 division ; it therefore offers no sure means of classification ; but it 

 seems to be inferior to the other slate-rocks in the southern pro- 

 montory of Carnarvonshire " (p. 219). Passing over these, and 

 " commencing with the line of the Nant Franeon and Llanberis 

 slate-quarries, the author (Prof. Sedgwick) describes a series of 

 regular ascending sections, continued through a horizontal distance 

 of three miles," i. e. up into the Bala beds. 



In a subsequent paper, read before the Geological Society, Dec. 16, 

 1846, and published in the Quart. Journ. Geol. Soc. vol. iii. p. 133, 

 though it is clear he has not yet worked out his subdivisions, nor 

 made up his mind as to the value of some tentative correlation of 

 beds not seen in continuous sections, he shows that he has got hold 

 of the fact that the Bangor beds are not an altered state of the 

 Lower Cambrian, but a distinct underlying series. He says, 

 p. 136, " In short, the slates near Bangor and Carnarvon are among 

 the very oldest rocks of North Wales ; of course," he adds, " ex- 

 cepting the crystalline and hypozoic groups of Anglesea and of the 

 S.W. shore of Carnarvonshire, which I do not at present wish to 

 notice." " On the shore near Bangor they alternate with Trappean 

 conglomerates and Trappean shales (Schaalstein)." He points out 

 that the series is " traversed by the railway- tunnel, that it is inter- 

 rupted by two great ribs of porphyry, which he considered intru- 

 sive ; but he says that these Bangor and Caernarvon beds are repeated 

 in symmetrical undulations towards the base of the higher moun- 

 tains of the Caernarvon chain, and thinks that the second great rib 

 of porphyry very nearly resembles in position and structure the more 



* For the discussion on this paper see p. 165. 





