Vol. 50.] OLDER FRAGMENTAL ROCKS IN N.W. CAERNARVONSHIRE. 581 



crags the dip is distinctly as much as 40° to a westerly or south- 

 westerly point (approximately W.S.W.). 



It is then said that " the lower crags are of conglomerate like that 

 of the summit." The rock clearly is a conglomerate, one of those 

 largely made up of material from the quartz-felsite, with pehbles 

 also of quartzite and occasionally of granitoid rock, while the large 

 slate-fragments of the mass at the top of the hill are wanting. Thus 

 it seems to us that lithologically this conglomerate does not resemble 

 that of the summit ; it more probably represents a band at a different 

 level, like that of the tramway-cutting. The argument implied 

 in the words "all is covered by conglomerate and grit" is less 

 strong than it seems, because no small part consists of unbroken 

 sward. So far as any inference is justifiable from the latter fact, 

 it would be that there probably is a softer rock, such as slate, in 

 this part of the hill. 



Thus it does not appear that we can prove the conglomerates on 

 the hill to form a single great mass, or that this is approximately 

 horizontal as implied by Mr. Blake ' : hence we are still not satisfied 

 " that the conglomerate lies unconformably on, and is independent 

 of, the underlying members of the Cambrian Series "(op. cit. p. 463). 



III. "West of Lltn Padarn. 



In the district west of Llyn Padarn we will discuss first that 

 part of the railway-section where the evidence can be best brought 

 to a test examination. This is towards the inlet of the lake at 

 the north of the cutting. It is stated as an argument here that 

 ' Purple Slate ' (op. cit. fig. 6, b ; fig. 7, no. 11) occurs in contact 

 with the felsite with no conglomerate between. 2 The section of 

 the supposed junction is undoubtedly very difficult. We agree with 

 Mr. Blake that both the slate and the felsite-like rock are broken, 

 but after careful microscopic study of both varieties we have come 

 in other respects to rather different conclusions. The purple rock 

 certainly appears to be a variety of slate. The light- coloured one 

 in junction-specimens is almost wholly composed of felsitic material, 

 and might be a crushed condition of the quartz-felsite, but the 

 relation of the two is more like that of sediments, and the darker 

 sometimes contains bands of an intermediate character. Thus the 

 mass is most probably an interbanded dark mud and felsitic grit, 



1 There seems in addition one negative argument which should be considered. 

 If the thick massive conglomerate unconformably overlies Cambrian strata, 

 nearly horizontally or with a low dip to the east, then it would be a curious 

 coincidence that it should have been cleanly removed from over all the purple 

 slates. So far as we know, no representative of it has ever been noticed, 

 although extensive quarries are worked almost continuously along the east of 

 the hill. The conglomerate-and-grit has so successfully resisted denudation at 

 one part that it forms the thick crags of the summit, while less than 100 yards 

 away the same resistant strata have been entirely removed. 



We have to thank the owner, Mr. Menzies, for kindly giving us some inter- 

 esting details relating to the slates worked in the quarries and to the associated 

 grits. 



2 Ibid, p. 451. 



2s2 



