172 Rev. J. F. Blake — The Llanheris Unconformity. 



this paper Professor Hughes expressed his agreement that here 

 " the newer series " is " absolutely unconformable to the older," 

 and Dr. Hicks said that " from the present evideuce it is clear that 

 an unconformity does exist." Professor Bonney, however, said that 

 Professor Green's reading of the section (which differed from his 

 own) might be the correct one, but he felt very doubtful. 



It was this genera] consensus of recent opinion, that there were 

 Pre-Cambrian rocks exposed in this district, that led me to examine 

 it in connection with the Pre-Cainbrian rocks of Anglesey, announcing 

 my results in 1888' (E). At the time of writing that paper I was 

 no better acquainted with this part of the district than other writers 

 upon it, but I was led to accept the recent opinions (1) that the 

 felsite was a lava-flow and not intrusive, and (2) that the con- 

 glomerate to the east was derived from it. But no evidence had 

 been given for regarding the conglomerate as the base of the 

 Cambrian, and accordingly I considered it might be high up in that 

 series. Nor could I then see proofs of unconformity at Moel Tryfaen, 

 while the uuconformity shown by Professor Green I could not deny, 

 but I tried to discount it by quoting his opinion that it did not 

 necessarily indicate any great difference in age.^ I thus considered 

 the felsite to be part of the Cambrian succession, in spite of accepting 

 all that previous writers had given reasons for. As to the con- 

 glomerate, quoting previous observers, I considered that the pebbles 

 were of Cambrian age, and yet took the containing rock to be one 

 of the higher conglomerates in that series. 



In 1891 Miss C. Raisin traversed my conclusions as above (F).^ 

 In this paper, amongst many minor criticisms, she made one 

 of great weight (in addition to the correction about the "slate"). 

 Speaking of Moel Tryfaen, she said : " We should have to believe 

 that at some epoch, after the deposition of one of Mr. Blake's 

 successive conglomerates, the slates of which we now speak were 

 deposited, indurated, modified, and worn down to form some of the 

 Moel Tryfaen pebbles — a process of rapid manufacture indeed." 

 This was used as an argument for an unconformity below the 

 conglomerate, which she was then in favour of, even though she 

 could not observe it in Professor Green's section, where she con- 

 sidered it as only " locally absent." 



Five days previous to the reading of this paper Sir A. Geikie 

 treated of the district in his Presidential Address (G).* He denied 

 that the conglomerate forms the base of the Cambrian, or is un- 

 conformable on the rocks below, and, in fact, he agreed exactly with 

 my then published views. But he could not see the unconformity 



» Q.J.G.S., Yol. xliv, pp. 271-290. 



^ In this same paper I endeavoured to support the Cambrian age of the felsite 

 by finding its base. Therein 1 mistook a squeezed relic of a dyke for slate, with 

 the result that the section, which I thought proved my point, proved nothing, either 

 for or against it. In a later paper (F) Miss Raisin did me the service of pointing 

 out this mistake, as I have much pleasure in acknowledging. 



^ Q.J.G.S., vol. xlvii, pp. 3^9-342. 



* Ibid., Proc. 



