Yol. 50.] OLDER FRAGMENTAL ROCKS IN N.W. CAERNARVONSHIRE. 601 



Tryfaen slide (' post-Llanberis '). The other classes of fragments 

 make up the bulk of the rocks enumerated, bcth the ' Cambrian ' 

 so-called and the ' post-Llanberis.' Yery much of the ground- 

 mass apparently has been derived from the felstone, for in some 

 cases it is by no means easily distinguished from a crushed variety 

 of that rock. 



To render the comparison more easy, we have separated the 

 fragments of the acid from those of a basic type ; but they may 

 both occur in the same mass, as for instance, in the thinner bands 

 of conglomerate or of grit at the synclinal by the railway on the 

 western side of Llvn Padarn. 



The results obtained from microscopic examination are thus clear 

 and definite. Whether we compare rocks below the supposed 

 unconformity with those above, on one side of the lake, or on the 

 other, or on both — whether we compare the rocks of more basic or 

 of more acid material, the coarser or (so far as they afford any 

 evidence) the finer — the same kind of fragments can be recognized 

 in both sets. Eocks of very different composition were being denuded 

 before the supposed epoch of disturbance, and the same set of rocks 

 was being worn down after that epoch. It would be a difficulty, 

 as we have pointed out, if we had to suppose that the felsite was 

 twice uncovered, but when we have in addition to believe that the 

 same varieties of more basic rocks were in each case associated, so 

 that identical conditions were reproduced before and after the great 

 interval and interruption, this would be a coincidence suggesting 

 the necessity for the strongest positive evidence of the uncon- 

 formity. But at the crucial section, as we have shown, there is 

 perfect gradation and continuity in the materials. Microscopically, 

 as macroscopically, the unconformity does not exist. 



Discussion. 



Prof. Blake was glad that the Authors had attempted seriously 

 to examine the question dealt with. Criticisms in such a spirit 

 were in any case of value. The points dealt with in the abstract 1 

 read to the Society were covered by the remarks in his original 

 paper, some of which he repeated. 



Dr. Hicks said that he could not understand the position now taken 

 up by Prof. Blake, as the evidence given in the paper clearly showed 

 that the great felsite-ridge was at the base, and evidently older 

 thau any of the recognized Cambrian rocks. The Conglomerate 

 resting on the ridge was sometimes diminished in thickness by 

 faults and in other places represented by grits ; but it contained 

 everywhere material derived by denudation from the ridge, along 

 with pebbles of other pre-Cambrian rocks. On the table were 

 pieces of this Conglomerate which contained cleaved felstones, and 

 large pebbles of a granitoid rock exactly like that forming the 



1 [This expression, unfortunately, is not quite accurate. I did not read an 

 abstract, but gave au abridgment of the paper, sometimes reading, but mostly 

 speaking from notes. — T. G. B., August, 1894.] 



