330 MISS C. A. RAISIN ON TUB LOWER LIMIT OP 



rocks which ciui hardly be considered characteristic of different 

 periods. So far as I know, there is no clear distinction between 

 hallefiintas of pre-Cambrian and of Cambrian age ; indeed, 

 Prof. lionney has often insisted on the slight difference between 

 some of the later Pebidian and the early Cambrian rocks, when 

 they are compared lithologically. Also in the newer series at 

 Bangor, we do not find the accumulations of coarse volcanic 

 materials that are so marked a feature of the rocks below the 

 conglomerate. It is true that Mr. Blake emphasizes the fact that 

 these " are arranged in beds after the manner of subaqueous 

 deposits," and states that they " can only be said to be volcanic, as 

 having been derived from the denudation of volcanic products." 

 But the question is whether these were contemporaneous volcanic- 

 products, or were denuded from those of a previous period. Xo 

 evidence is brought forward for the latter view, and I imagine that 

 the volcanic ejectamenta were rolled and partially rounded by the 

 sea of the period to which they belong, as has been described by 

 Prof. Bonnoy *. 



(b) Unconformity heloiv the Cambrian Conglomerate. 



Even if the two sets of strata were strictly conformable, it has 

 already been pointed out by Prof. Hughes that a local coincidence 

 of strike would be quite compatible with a break in the succession, 

 especially when volcanic deposits terminate the underlying series. 

 But have we at Bangor even this local coincidence ? The series, 

 which includes the Tairffynon and Cae-Seri breccias, has been 

 traced by Prof. Bonney with a strike about N.X.W., and Mr. Blake 

 adds that the strike continues still X.X.W. in the grit by Nant 

 Gwtherin, thus leading up, after some hiilleflintoid rock, to the 

 conglomerate of Bryniau. In these strata just below the con- 

 glomerate I find, however, both in the halleflinta and also in the 

 pebbly grit of Nant Gwtherin and Bryniau, a dip to a point S. of 

 E., or about E.S.E., at a fairly high angle (about 50°)t ; and the 

 greenish pebbly grit well exposed south of Br3'jiiau is a continua- 

 tion of that opposite Minffbrdd cottages (as is allowed by Mr. Blake j. 

 Thus it occurs along a N.X.E. strike, and can be traced appa- 

 rently also to the rock in the " Old Quarry," south of the road to the 

 Mount. Hence the conglomerate to the eastward is not " strictly 

 conformable to the underlying beds," and, to prove its position as one 

 of a succession of Cambrian conglomerates, Mr. Blake must rely on 

 the slender argument of lithological resemblance in hiilleflintoid 

 rocks. 



* Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 483. See also Prof. Hughes 

 on the Llyn-Paclarn rocks, whose slateiuent is quoted with approval by Mr. 

 Blake, op. cit. p. 285. [Dr. Geikie, in his Address to the Geological Society in 

 February 1801, expresses the opinion that much of the material came from 

 contemporaneous volcanoes.] 



+ The dip is stated by Prof. Hughes to be about 55° S.E. ; Quart. Journ. 

 Geol. Soc. vol. XXXV. (1870) p. (582. 



