Vol. 50.] OLDER ERAGMENTAL ROCKS IN N.W. CAERNARVONSHIRE. 587 



structure equal to that of Cwm-y-glo, and enclosing lumps of a com- 

 pacter and redder felstone, 1 as the principal mass does near Bettws 

 Garmon. Although the massive conglomerate of the western is 

 wanting on the eastern margin of the igneous rock of Moel Goronwy, 

 yet the felsitic grits with occasional seams of quartz-felsite pebbles a 

 indicate a similarity of material (4 in our map, p. 594). But the 

 existence here of a fault seems highly probable from the nearness of 

 the outcrop of purple slates, from the absence of the main mass of 

 the conglomerate, and from the variation found along a continuous 

 line down to the lake. On the railway Mr. Blake states that there 

 is possibly a fault, but makes no allowance for its continuance at 

 Moel Goronwy. 



Tracing next the outcrops along the western boundary of the 

 conglomerates, etc., the most important argument is the sudden 

 change at the north of the quartzite-conglomerate (11, 12), which, 

 if the map given by Mr. Blake were accurate, could be explained only 

 by more than one fault. But instead of the rocks east of the line 

 25 to 11 in his map being rightly represented by the same symbol 

 as the great quartzite-couglomerate, they require a different sign, 

 and the difference between the rocks east and north of 11 — that is, 

 between ' post-Llanberis ' and ' Cambrian ' — does not exist. The 

 same purple and grey-weathered, finely laminated and banded grit 

 and argillite (which we should describe as of Y Bigl type) occurs, 

 followed to the eastward in each case by green banded slaty rocks 

 and grits, like those on the summit of Y Bigl. If the fault which 

 Mr. Blake marks on the railway (op. cit. p. 415, fig. 1, between a and 

 ae) is continued inland, 3 it would account first for the conglomerate, 

 then for the felsite, coming into close association with the banded 

 gritty or slaty rocks (see F 2 in our map, p. 594, south of Tyddyn Du, 

 3, 1, and 7). 



Between the two boundary-lines the rocks of the slate-railway 

 section strike towards the slopes of T Bigl. On this hill several 

 examples are noticed, and two are drawn representing grits overlying 

 Pale Slates (op. cit. pp. 448, 449, figs. 3, 4). We were not satisfied as 

 to this interpretation for the one which we visited, but as the horizon 

 of these beds is part of the point in dispute, the sections would only 

 supply an argument for an unconformity to an observer who already 

 is convinced. We are also told of the difference between all these 

 rocks of Y Bigl and those of the railway syncline. As we shall 

 show, the latter include some banded grits and conglomerate, such 

 as occur over the higher slopes. It is, however, true that here on 

 the hill the pale banded slates of the railway do not seem to extend 

 over a wider area, as should happen in a section of a syncline, 



1 Sir A. Geikie has called attention to both these characteristics, Quart. 

 Journ. Geol. Soc. vol. xlvii. (1891), Pres. Addr. Proc. p. 94. 



2 These apparently are the ' curious breccias derived from the felsite,' to 

 account for which, on his hypothesis, Mr. Blake has to suppose the Moel 

 Goronwy felsite a later eruption, op. passim cit. p. 450. 



3 The fault which lias been suggested by one of us as occurring between the 

 felsite and the conglomerate was not supposed to be great, and might have died 

 out in a short distance from the lake. 



