ROCKS IN N.W. CAERNARVONSHIRE. 



287 



From this examination I can- 

 not understand on what Prof. 

 Ramsay's statement is based. I 

 fear it cannot be maintained. 

 Much of the mapping, too, here 

 is acknowledged to be hypothe- 

 tical, and leads, as I conceive, to 

 too great importance being at- 

 tached to these felsites. Thus the 

 porphyry west of Clegyr, which 

 itself consists of two different 

 flows, separated here and there 

 by deposits of slate *, is never 

 seen to cross the road at Llaubabo, 

 and its occurrence beyond, to- 

 gether with the fault, is entirely 

 conjectural, as the whole country 

 is covered deeply in drift, a cir- 

 cumstance in itself suggestive that 

 the underlying rock is not likely 

 to be so hard a rock as felsite. 

 Then the felsite of Moel Gronw 

 is not apparently connected with 

 the former, nor can it be traced 

 beyond the road called Cefn-y- 

 waun. The basal conglomerate 

 does not wrap round it, but forms 

 a band clinging to the east of 

 Clegyr only, and leaving Moel 

 Gronw some distance further east. 

 Q^he Moel-Gronw felsite is thus a 

 later outburst, which is separated 

 from the purple slate above by 

 only a band of grit, which 'p&rhaps 

 is represented in the form of a 

 thin band of conglomerate seen 

 in the railway at a higher level 

 than the great one. The limits 

 of the felsite, as actually seen, are 

 also very much smaller than drawn 

 on the south-west. The line is 

 carried out as far west as Glan- 

 rafon, on the Gorfai Eiver, where 

 there is an exposure of rocks in 

 the deep river-gorge ; but these 

 are quite different from those of 

 Moel Swtan, which is the nearest 

 exposure of the true Llanberis 



* See Bonney, Quart. Journ. Geol. 

 See. vol. XXXV. p. 312. 



