Rev. J. F. Blalce — The Llanheris Unconformity. 215 



fhe junction, which they describe as a surface of original deposit : 

 it must be in the felsitic part, probabl}^ at some supposed junction of 

 felsitic grit with true felsite. Such a fault would have to rotate the 

 conglomerate and bring it out from between the felsitic grit and 

 felsite. Now, the rocks in this area are quite bare, and one can see 

 for certain that there is no fault ; even the mass that discloses the 

 junction with the slate is continuous till it becomes a true felsite. 

 The only possible fault in the neighbourhood is oxi the other side of 

 the mass of purple slate between it and the next exposure of 

 conglomerate, that is, along the face of the cliff above and below 

 the road.^ 



The similar section described by Sir A. Geikie (E, p. 96) is 

 doubtless on the continuation of the line of junction seen below. 

 The purple slate ^ is there also said to be nearly vertically inter- 

 banded with felsitic material which passes towards the west into 

 a true felsite. Sir A. Geikie does not say exactly where this section 

 is, but describes it as separated from " the porphyry of the ridge " 

 by a " zone of conglomerate and grit," so it is most likely where 

 Miss Raisin has inserted felsite on her map. In my map, however, 

 it is included with the " Post-Llanberis," because on the upper 

 surface there are scattered here and there some pebbles of quartz, 

 so that it is covered, as it were, by a skin of conglomerate. Here 

 again, then, if the spot is rightly identified, the plane of junction 

 with the slates is nearly vertical, while that with the conglomerate 

 is horizontal.^ 



This inlet section is really an important one, as it is the only 

 place I know of, except in tunnels, where the felsite and any other 

 rock than the conglomerate can be seen in unbroken sequence. 

 What we see in such a case I regard as one irresistible argument for 

 the unconformity I postulate, and, so long as it holds, the question 

 whether the purple slate here is the workable slate or not is of 

 secondary importance, for if the conglomei'ate is unconformable it 

 may just as well extend over the workable slate as not. The rock 

 is like the workable slate, and like no rock out of that group ; as 

 a fine-grained rock it cannot be a mere local deposit ; and it is in 

 continuation with the worked slate which runs over the hill to the 

 south-west. My critics also recognize in it one of the characteristics 

 of parts of the workable slate series — the " interbanding of fine grit 

 and purple slate," which is quite a distinct thing from the " alter- 

 nation of hard grey slate, transversely cleaved, and coarse grit," 

 characteristic of the Post-Llanberis group. 



^ Professor Bouney and Miss Eaisin also claim a fault on tte other side of the 

 conglomerate, between it and the main mass of the felsite, but the junction may be 

 seen in a block near the water's edge ; one is welded to the other. 



- So gradual is the passage from one rock to the other that Sir A. Geikie considers 

 the parts of the slate nearest to the felsite to be only a cleaved portion of the latter. 



2 A similar argument is applicable to the tramway section on the opposite side of 

 the lake. The felsite there shown next the first conglomerate is followed beyond the 

 conglomerate (and a dyke) by nearly vertical slaty beds, but the summit of the felsite 

 crag shows a covering of conglomerate, indicated by the presence of quartz pebbles. 

 Here, however, the vertical succession is broken. 



