THE CAMOKIAN SERIES IN N.W. CAERXARVONSHIRE. 329 



18. On the Lower Limit of the Cambrian Series in N.W. Caernarvon- 

 shire. By Catherine A. Haisin, B.Sc. (Head February 25, 

 189L) 



(Oommunicated by Prof. T. G. Bonnev, D.Sc, F.R.S.) 



Contents. 



Page 



I. The Baxgor District 329 



(a) Lithological Character of the Strata 329 



(/>) Unconformity below the Cambrian Conglomerate 330 



(c) Details of Outcrops 331 



II. The Arenig Uncoxformity at Caernarvox axd the Thick- 

 ness OF THE Cambrian Series 331 



III. The Age of the Southern Felsite 332 



(a) Lithological Comparison of the Sedimentary Strata ... 332 



{b) Value of the Conglomerate as a Base 333 



((') Physical Conditions of the Cambrian Period 334 



IV. The Bryx-Efail Sections 335 



{a) Strata north of the Valley 335 



{h) Quarry at Bryn Efail 336 



(c) Section in the Eoad below the Cottages 339 



• 

 In the course of some investigation of the " schists " of the 

 Lleyn, it became necessary to compare them with the rocks in 

 neighbouring districts. I was therefore led to make some examination 

 of these, especially with reference to the new interpretation which 

 has been recently proposed *. In this Mr. Blake tries to establish a 

 great extension of the Cambrian series, increasing the length of the 

 succession as generally accepted, and including in it the pre- 

 Cambrian beds near Bangor and the Llyn-Padarn felsite. 



I. The Bangor District. 



In the Bangor district, the pre-Arenig rocks include, as Prof. 

 Bonney has shown, a succession of beds from the grits, which 

 overlie the felsite, up to the strata exposed to the south and east of 

 Bangor. This sequence Mr. Blake accepts in the main, but 

 maintains that all these strata are to be placed in the Cambrian 

 series, so that no pre-Cambrian rocks would be exposed here, unless, 

 indeed, the felsite should be so classed. Briefly, the arguments 

 brought forward in favour of this view are the similarity of strike 

 and of lithological character. 



(a) Lithological Character of the Strata. 



As to the latter, it is said that the beds above and below the 

 Bryniau conglomerate are lithologically very similar, though they 

 " ought to be decidedly different/' if the conglomerate is basal 

 Cambrian. The similarity, however, is only in the hiilleflintas, 



* ' On the Cambrian and Associated Eocks in N.W. Caernarvonshire,' Quart. 

 Journ. Geol. Soc. vol. xhv. (1888) p. 271. 

 Q. J. G. S. No. 187. 2 a 



