Vol. 50.] FKAGMENTAL KOCKS IN N.W. CAERNARVONSHIRE. 591 



is still more clearly marked in the banded grit and argillite, which 

 occur above the rounded conglomerate. That the upper con- 

 glomeratic part was deposited continuously on the lower breccia is 

 proved by the finer matrix graduating from the one to the other. 

 This is seen unmistakably both in the cliff and in a microscope 

 section cut from a junction-specimen. 1 



Not only, however, is the unconformity quite disproved, but the 

 only line which might be supposed, and has been supposed (by 

 Prof. Green), to mark it is not shown by Mr. Blake. His drawing 

 improves on the earlier sketch by altering the actual line to a non- 

 existent division. The conglomerate is not mast-headed on the top 

 of the lower part of the cliff as shown by him. The line of 

 separation is not a ' nearly horizontal line.' It more closely 

 approaches that position just at its southern end, but soon curves 

 and slants down to the railway, as is shown roughly in Prof. Green's 

 drawing, and with a little more detail 2 in the sketch on the 

 following page (fig. 3). 3 



We are told, however, not only that there is a ' nearly horizontal 

 line of separation,' but also that in the underlying rock ' different 

 vertical sheets ' have different characters. 4 The ' nearly horizontal 

 line,' as already stated, does not exist, neither do the 'vertical sheets.' 

 Mr. Blake says that the conglomerate (which we prefer to call, for the 

 sake of distinction, the ' rain-spot ' breccia) is ' vertically bedded.' 

 It has, indeed, a roughly vertical cleavage, but of stratification in 

 this direction we could see no evidence. It is stated that it changes 

 " in a horizontal direction to more and more felsitic material till it 

 is almost a felsite or pure felsitic ash." Lithologically we should 

 not object to this description, but the first distinct change, passing 

 from the ' rain-spot ' breccia to the southward, seems marked by a 

 line dipping north-westerly at a moderate angle (about "W.N.W. 

 35°), which hardly can be considered to mark a ' vertical sheet/ 

 Also, in the underlying felsitic rock (which possibly may be a squeezed 

 grit 5 ), a band seems indicated (although indistinctly) showing a 

 similar dip. Thus the dips in the section roughly agree : namely, 



1 In this slide an infiltrated mineral seems to us to be a kind of opaline 

 silica. Other slides prepared from Mr. Blake's ' post-Llanberis unconform- 

 able conglomerate ' A show that the materials are of the same character as 

 those in the underlying breccia, and different from the conglomerates a and i 

 with which he classes it. 



2 From drawings made on three separate occasions by two different 

 observers. 



3 It seems, then, since Mr. Blake himself has not succeeded in correctly 

 representing the cliff, that my words — ' the section is not so clear in nature as 

 in the diagram ' — were not unjustifiable after all. — T. Gr. B. 



4 It must be remembered that the bands originally supposed to denote vertical 

 layers of slate proved on examination to be thin dykes of compact diabase, 

 C. A. Raisin, Quart. Journ. Geol. Soc. vol. xlvii. (1S91) p. 333. 



5 This rock is so much affected by pressure and micro-mineralogical changes 

 that it is very difficult to determine whether it has been originally a felsite or a 

 felsitic grit. In an earlier description by one of us it was regarded as possibly 

 the former faulted up, but we now incline, though with hesitation, to the latter 

 view. [Also, even if a faulted junction occurs, I doubt whether the line of it is 

 visible, as was formerly suggested. — 0. A. R.] 



