Vol. 50.] ERAGMENTAL ROCKS IN N.W. CAERNARVONSHIRE. 595 



conclusion as to the exact position of the purple felsite which has 

 been described by Sir A. Geikie. 1 If we may infer from its high 

 southerly dip that it is an interstratified band on the S.E. side of 

 the fault, then, as Sir A. Geikie believes, it probably would be a small 

 lava-flow higher in the succession than the great quartz-felsite, from 

 which it differs lithologically. If, however, it be on the northern 

 side of the fault, this difference would lead us to suppose it to be an 

 external part of the lava-flow. At parts it has remained uncleaved, 

 and this is especially the case in an outcrop well shown (in a dry 

 season) at the edge of the lake. We have already dealt (p. 585) 

 with the alleged junction of felsite and ' Cambrian slate ' in the 

 crag nearly between these two localities. 



Neither of the faults before mentioned is great, not so large as 

 many of the parallel displacements among higher strata which are 

 shown in the Geological Survey memoir and map. 2 Beyond the 

 stream fault, the interbanded argillite and grit probably belong to 

 the syncline which is well marked, along and above the railway, by 

 bands of purplish conglomerate. The next cutting through bastard 

 slates and greenish grits, which, owing to undulations, extend for 

 some distance, though they have proportionally a rather small 

 thickness, exhibits not a few faults, so that probably the general 

 result has been to let down the whole mass in a kind of trough. 

 Unless this be so, these beds can hardly be equivalent to the green 

 banded halleflinta and grit on the east of Llyn Padarn. The rock 

 adjacent to the conglomerate (south of the bridge in Glyn Padarn 

 grounds) is slickensided and the junction probably is a faulted one. 

 This conglomerate, as we have said, may be the representative of 



[Note xo Map. — A few outcrops in this area have not been visited, 

 and others need re-examination to test the hypothesis here repre- 

 sented. 3 Further, many variations occur (not possible to show on 

 this map), apparently due partly to small faults or rolls, partly to the 

 slope of the synclinal or anticlinal axis. This axis seems to dip with 

 the slope of the bill, sometimes at a less angle. Also the different 

 layers mentioned above are not always sharply distinct, gradations 

 occurring between them, which would be expected, if (as the 

 Authors believe) the rocks form a continuous series. The Authors 

 are fully aware of the defects of the map, but they nevertheless 

 insert it, since, notwithstanding its imperfections, it may be of 

 use to make parts of the accompanying paper more easily under- 

 stood.] 



1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) Pres. Addr. Proc. pp. 96, 97. 



2 The stream fault, apparently the greater, might have a throw of 140 feet; 

 but such an estimate is purely hypothetical. 



3 Thus beyond the road south-west of the Moel G-oronwy felsite (although 

 part is grassed over) conglomerates are exposed, and possibly they include some 

 representative of conglomerate 3, in which case its outcrop might extend con- 

 tinuously to the west of Moel Groronwy. 



At * the ground is largely covered with slate tips, and is partly grass, with 

 some outcrops of grits, etc. 



Q. J. G. S. No. 200. 2 t 



