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



conclusion that Stauroceplialus diverged from some early Encrinnrid, 

 and while retaining several of its ancestral characteristics underwent 

 a development under somewhat similar conditions as Sphcerocoryphe, 

 leading to similar adaptive changes in certain points of its structure. 



IV. — A Kevindioation of the Llanberis Unconformity. 



By the Eev. J. F. Blake, M.A., F.G.S.i 



[Concluded from the April Number, p. 178.) 



The District South -West of Llyn Padarn. 



THIS is undoubtedly the most difficult district to deal with, and 

 one in which I have had to change my views on certain details. 

 Still, there is one part of it where the evidence is very clear, namely, 

 the ground between the railway and the road at the" Tan-y-pant 

 inlet. This was apparently not examined by Professor Bonney 

 when he visited the locality, and its teaching is evidently not 

 appreciated by his coadjutor. They say that the section at what 

 they call " the supposed junction " is undoubtedly very difficult. 

 One thing, however, is clear, since we all agree about it. The 

 junction of a rock of felsitic character with one like a purple 

 slate is vertical. There are here only two alternatives : either the 

 felsite is intrusive into the purple slate, as the junction at first sight 

 certainly suggests, or the purple rock was deposited on the felsitic 

 one. In spite of appearances, we agree to reject the former 

 alternative ; but my critics seem to think they are correcting me 

 when thej agree with me in accepting the latter. The only possible 

 difference between us is, that what I called felsite, including therein, 

 as I remarked in a note, " felsitic ash," they call " felsitic grit." 

 This difference, which, as far as I am concerned, is merely one of 

 words, is quite irrelevant to the argument that the junction-line 

 between two deposits must have been at first approximately 

 horizontal, and have been subsequently turned on end. But the 

 neighbouring conglomerate lies in a hollow on the present upper 

 surface of the felsite, and must therefore have been deposited after 

 the rocks had been turned on end, and therefore be of later date 

 than the slate. (See Fig. 1.) 



Fig. 1. — Eelations of slate (1), felsite and associates (2), and conglomerate (3) at the 

 inlet by Tan-y-pant, Llanberis. 



It is true that my critics draw a fault somewhere about here, 

 but it is not clear where they think it runs. It cannot be in the 

 slate, as that would throw no light on the question ; it cannot be at 



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

 Tvith additional remarks. 



