THE KOCKS MAPPED AS CAMBEIAX EN^ CAEKNAPvYONSHIPvE. 243 



18. On the EocKs mapped as Camekian in Caeenaevonshiee. By 

 the Eev. J". Y. Blake, M.A., E.G.S. (Eead December 9th, 1891.) 



[Plate YI.] 



Part I, — Contexts. 



Page 



§ 1. Introduction 243 



§ 2. The Upper Limit of the Series considered 244 



I a The Pale Slates 249 



§ 4. The Purple SI ates and the St. Ann's Grit 249 



§ 5. The Rhiw-weu Grit 252 



§ 6. The Pale Banded Slates and Halleflintas 254 



§ 7. The Bangor Conglomerate 256 



§ 8. The Bangor Breccias 257 



§ 9. The Blue Laminated Grits and Tairfiynnon Conglomerate. 258 



§10. The Brithdir Grit 259 



§ 11. Conclusions -. 261 



Part IL (Abstract) 261 



§ 1. iNTEODrCTIOJf. 



In a previous communication^ devoted to a consideration of the 

 question whether certain rocks in Korth-west Caernarvonshire, 

 which had been considered pre-Cambrian, were really of that age, I 

 arrived at the conclusion that they were not, mainly on three 

 grounds : (1) that the pre-Arenig rocks of Bangor form one indi- 

 visible series whose general features attach them to the Cambrian ; 

 (2) that the felsite at Bryn Efail appeared to have altered the sedi- 

 mentary rocks with which it is in contact ; and (3) that the 

 series on the west of Bryn Efail is essentially different from the 

 series on the east. At the time of writing the paper now alluded to 

 I was perfectly aware that the proper elucidation of the question 

 required more than this. The only thoroughly satisfactory way of 

 proving that a set of rocks are not pre-Cambrian is to show what 

 they are, that is, to trace out their relations with the other rocks of 

 the district and to show, if possible, that they are part and parcel 

 of the Cambrian succession. But the Cambrian succession had 

 never been made out. Sir Andrew Eamsay in his Memoir describes, 

 of course, the Purple Slates, and makes mention of various grits and 

 conglomerates ; but he arranges the members of the series in no 

 definite order, and supplies no detailed description of their charac- 

 teristic features. Hence, to determine this succession requires the 

 detailed survey of all the rocks which are mapped as Cambrian in 

 the district. This was a task from which I shrank, having at the 

 time no further interest in the rocks if they were not pre- 

 Cambrian. 



Eecently, however, my conclusions have been attacked, and the 

 grounds for them called in question. One of these grounds — the 

 section at Bryn Efail — has been stated to be founded on erroneous 



' Quart. jQurn, Geol. Soc. vol. xliv. (1888) p. 271. 



