Tol. 50.] OLDEB ERAGMENTAL KOCKS IN U.W. CAERNARVONSHIRE. 585 



Mr. Blake speaks of it there as a ' slight fault,' but it is so only 

 on his own hypothesis ; to argue from that statement is reasoning 

 in a circle. This ' slight fault ' is supposed to account for the 

 grit and conglomerate being carried down to the railway-level ; 

 while at the higher ground they are " all but actually seen to overlie 

 [the Purple Slates] unconformably " (op. cit. p. 454). 



We have discussed the grounds for this view, drawn from the 

 lithological succession and the dip. The remaining argument, as 

 given by Mr. Blake, is : ; ' if I understand rightly the section given 

 by Mr. Maw .... it runs beneath these very grits and con- 

 glomerates." But, apart from the fact that that section repre- 

 sents conglomerate under purple slate, there is a serious misunder- 

 standing, for the adit described begins about 80 yards away from 

 these grits, since it extends from the second to the third Glyn- 

 rhonwy Quarry. 1 



Passing to the north-west, we cannot agree that the conglomerate 

 can be traced ' step by step from 13 by 14, etc., to 19' (op. cit. p. 455). 

 It can be traced from 13 nearly to the streamlet which comes 

 down (past Tan-y-pant) to the corner of the inlet of the lake, but — 

 as we believe — not beyond (3 in our map, p. 594). The conglo- 

 merates south of the streamlet appear to represent the — probably 

 thinner and higher — bands of the railway-section (6, etc., in map, 

 p. 594). Along the boundary next the felsite, we are told that the 

 conglomerate behaves as an unconformable deposit. The variation 

 may be due partly to faulting, partly to the occurrence of inter- 

 changeable deposits of grit and coarser material, much as the local 

 sandy and pebbly beaches of the present day. But the important 

 outcrop is the ' knob of hard Purple Slate ' intervening between 

 felsite and conglomerate. The few such knobs as we saw appear 

 to be part of the purple banded series occurring on the hill, 2 and 

 do not separate the felsite from the conglomerate. At the place 

 (2L) :J marked by Mr. Blake as proving the existence of 'Cam- 

 brian ' slate, we find the outcrops 4 next the felsite to be, firstly a 

 cleaved, rather coarse grit somewhat resembling a ' rain-spot ' 

 breccia, then a purple quartz-felsite grit with pebbles, a purple 

 banded grit and argillite, greenstone, then purple grit, after which 

 the quartz-felsite conglomerate reappears (see map, p. 594). 



Then we turn to the ' associated rocks ' and the evidence of 

 their lamination. Mr. Blake says that " almost wherever seen these 

 laminae, etc., are horizontal .... a circumstance which first excited my 



1 We have to thank W. Roberts, Esq., the manager of the Glynrhonwy Slate- 

 Quarries, for very kindly giving us this and some other information in answer 

 to our enquiries. He mentioned also, what seemed to us a point of some interest, 

 that the goodness of the slate appeared to depend on the occurrence of an 

 adjoining mass of grit ('granite' of the quarrymeu). 



2 Included in Mr. Blake's map in the symbol ' Post-Llanberis conglomerates 

 and grit,' fig. 7 ; also fig. 2. This inclusion of things so different as coarse 

 conglomerate and banded slaty and gritty rocks under one symbol makes 

 Mr. Blake's maps difficult to follow, and likely to mislead an observer unfamiliar 

 with the ground. 



8 Op. cit. p. 452, fig. 7. 



1 At the parts measured by scale on the map. 



