244 EEV. J. p. BLAKE ON THE KOCKS MAPPED AS 



observation ^ — and after a re-examination of it, I have to confess that 

 this is true. I have been deceived by the slaty aspect of the marginal 

 remnants of the squeezed and altered greenstone — and the section 

 has no bearing on the question in hand — thus illustrating my 

 remark that "it is ill founding a determination on one section." 

 The other two grounds, however, remain intact, and the conclusions 

 on further examination are found to be amply justified. 



It seemed to me, however, that having been thus led into error I 

 could best atone for it by undertaking the work from which I had 

 previously shrunk. It has been a hard and laborious task, but the 

 results will be found, I trust, to amply repay the labour. 



The Cambrian succession as I make it out is, in descending order, 

 as follows : — 



1. Pale Slates. 



2. Upper Purple Slates. 



3. St. Ann's Grit. 



4. Lower Purple Slates. 



5. Rhiw-wen G-rit. 



6. Hard banded Pale Slates and Halleflintas. 



7. Bangor Conglomerate. 



8. Hard banded Pale Slates and Halleflintas. 



9. Bangor Breccfa. 



10. Blue banded Laminated Grits. 



11. Tairffynnon Conglomerate. 



12. Blue banded Laminated Grits. 



13. Brithdir Quartz-felspar Grit. 



As will be seen in the sequel, all these members are not inva- 

 riably present. Thus, when the St. Ann's Grit dies out, the Upper 

 and Lower Purple Slates run into a single mass. The Hhiw-wen 

 Grit is often wanting. The Bangor Conglomerate does not appear 

 to occupy a definite horizon and when it is absent the rocks above and 

 below can scarcely be distinguished, while the same may be said of 

 the conglomerates in the Blue Laminated Grits. Thus the complete 

 series reduces to three main subdivisions : — 1. The Purple Slate 

 series. 2. Pale banded Slates and Halleflintas. 3. Laminated 

 Grits and Conglomerates. 



§ 2. The Upper Limit oe the Seeies considered. 



Before commencing the description of these various members or 

 tracing their range and relations, I must give reasons for excluding 

 from the series the grit which has been hitherto taken to be the top 

 and which is known (amongst other names) as the Bronllwyd Grit. 

 Doubtless, the justification for including it in the Cambrian has 

 been its great resemblance in some places to the St. Ann's Grit, 

 particularly when they both contain small quartz-pebbles. But it 

 cannot be denied that the Cambrian strata below present throughout 

 well-marked peculiarities, and the line of subdivision between these 

 and the Bronllwyd Grit is easily traced. On the other hand, the Silu- 

 rian strata above, as mapped by the officers of the Geological Survey, 



1 Miss C. A. Raisin, Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 329. 



