Rev. J. F. Blake — The Llanheris Unconformity. 169 



IV. — A Revindication of the Llanberis Unconformity. 



By the Rev. J. F. Blake, M.A., F.G.S.' 



Preliminary Remarks. 



IN a paper published in the Quarterly Journal of the Geological 

 Society for 1893,- I gave an account of the evidence that led 

 me to conclude that certain conglomerates and associated rocks 

 occurring for some distance north-east and south-west of Llanberis, 

 which had hitherto been considered to lie below the workable 

 Cambrian Slates of that area, were in reality unconformable deposits 

 of a later date than those slates. In the year 1894 Professor 

 T. G. Bonney and Miss C. Eaisin published in the same Journal^ 

 a controversial paper criticizing my statements and conclusions. 



It may seem rather late in the day to be replying to a criticism 

 made so long ago, and reasons for the apparent delay must therefore 

 be given. At the reading of their paper before the Society 

 I welcomed it as an attempt to examine the district I had described, 

 and it was only on reading it in full that I realized its true 

 character. I was then just leaving for India, and though a large 

 part of the criticisms might have been replied to at once, there 

 were statements in it which could only be explained and accounted 

 for after another visit to the ground, and this I was unable to 

 make till my return, when I at once presented my reply to the 

 Society — their Journal being, in my opinion, the proper place for it 

 to appear in. As the Council, however, are of a different opinion, 

 such part of my reply as refers especially to the criticisms on my 

 work is here presented in a modified form with additional remarks. 



There was nothing in that paper which in any way suggested 

 to my own mind that I might be wrong, but I can well understand 

 that anyone reading it might believe that my conclusions were so 

 ill-founded that a short visit to the district by another observer might 

 sufSce to upset them. My object, therefore, here is to show ttiat 

 it is not I but my critics who are in error. 



The question whether a certain conglomerate in North-West 

 Carnarvonshire is conformable or not to the underlying rocks may 

 seem at first sight to be of no great consequence, but it involves the 

 larger question as to where the base of the Cambrian system is to be 

 drawn — that is to say, what beds are to be included in or excluded 

 from the Pre-Cambrian series ; and this is a matter in which some 

 of the foremost of present and past geologists have interested 

 themselves. 



This being the case it is probably unnecessary here to point out 

 the bearings of the several factors involved. It will suffice to note 

 what views of the subject have been taken by different observers, 

 in order to show how far my own views or those of my opponents 

 coincide with or differ from those of others. 



1 The substance of a paper read to the Geological Society on December 1st, 1897, 

 with additional remarks. 

 "- Vol xlix, pp. 441-446. 

 3 Vol. L, pp. 578-602. 



