ARCH^AN. 41 



less contemporaneous outpouring of lavas and ashes, partly sub- 

 aerial, but, as regards the Pebidian especially, in a great measure 

 remodelled and deposited by water.^ 



In a section in one of the cuttings of the railway which runs 

 from the Dinorwig quarries along the north-eastern shore of Llyn 

 Padarn, the Cambrian basement conglomerate is shown resting in 

 marked unconformity on volcanic tuffs, like the Pebidian rocks of 

 St. Davids. Prof. A. H. Green, who has drawn attention to the 

 section, remarks that the unconformity, strong as it is, does not 

 necessarily indicate any great difference in age between the con- 

 glomerate and the breccia on which it lies. These breccias are of 

 volcanic origin, and the irregular and restricted upheavals and dis- 

 turbances which are always liable to occur where volcanic activity 

 is going on, are quite competent to bring about marked uncon- 

 formities, which may be purely local.'' 



Dr. Hicks maintains that fragments of the Dimetian rock are to 

 be detected in the basement conglomerate of the Cambrian, a view 

 strongly supported by Mr. T. Davies, and also by Prof. Hughes 

 and Prof. Bonney. Whether Dr. Hicks is right in separating 

 Arvonian from Pebidian is not generally admitted : but that the 

 latter division forms a Volcanic series, on the whole distinct 

 from the overlying true Cambrian rocks, is admitted by most 

 geologists who have studied the complicated regions in which 

 these rocks are displayed.' And the desirability of separating the 

 Pebidian from the Cambrian rests rather on the general distinction 

 in character between the rocks, than on any great break in the 

 sequence of events. Dr. Hicks, however, asserts that fragments of 

 Pebidian rocks, metamorphosed and cleaved, have been found in 

 the Cambrian conglomerates. Whether the Dimetian of St. Davids 

 should be regarded as older than the other members of the Pre- 

 Cambrian, as Dr. Hicks maintains, or whether it be regarded as 

 intrusive among these old rocks, is one of the questions which 

 further independent evidence may answer to the satisfaction of 

 all. At present the Doctors disagree. 



Pre-Cambrian rocks were recognized in North Wales by Sedg- 

 wick. With them Dr. Hicks places the so-called intrusive 

 masses at Caernarvon and Llyn Padarn, and the altered Cambrian 

 of Moel Tryfaen and Tal-y-sarn. He identifies Pre-Cambrian 

 areas at Glynllifon, Craig-y-Dinas, and in the Lleyn promontory, 

 also at Ffestiniog, Dolgelly, and in the Harlech mountains. 

 In the neighbourhood of Bangor and Caernarvon Prof. T. 

 McK. Huffhes would thus divide the Pre-Cambrian rocks:* — 



1 Q. J. xxxiv. 168. See also E. B. Tawney, G. Mag. 1882, p. 552 ; and Proc. 

 Bristol Nat. Soc. (2) ii. 113. 



- Q. J. xli. 74. 



^ See T. McK. Hughes, Q. J. xxxiv. 169 ; xxxix. 328. 



* Q. J. G. S. xxxiv. 137 ; xxxv. 682 ; Proc. Geol. Assoc, viii. 197. See also 

 Hicks, Q. J. xxxiv. 137 ; xxxv. 295 ; xl, 1S7 ; G. Mag. 1880, 519; and T. G. 

 Bonney, Q. J. xxxix. 478. 



