CAMBRIAN IN CAERNARVONSHIRE. 251 



give the grit an jiumerited importance. The Allt Ddii slates lie below 

 it again, so there must be some faults in the slate itself, as, indeed, 

 Sir A. Kamsay's careful investigation of the details of the quarries 

 has already shown. The same grit when seen on edge, descending 

 to the Llanberis Lake, seems less important. The last of these 

 green grits in the middle of the slates is seen on either side of 

 Llyn Padarn, that on the southern side soon dying out, as noted by 

 Sir A. Karasay. 



South of this point I have nowhere seen a green grit of any 

 importance, though some bands occur in quarries south of Xantlle. 

 But this is no proof that the St. Ann's Grit does not or did not 

 exist. Within two miles of where it is last seen the whole breadth 

 of the visible outcrop of Cambrian strata is reduced by the uprising 

 of the felsite mass on eitlier side of the Bettws Garmon valley to 

 900 feet, so that there is no room for more than part of the Upper 

 Slates, and though this breadth expands between Moel Tryfaen and 

 Tal-y-Sarn to three-quarters of a mile, yet in the jS'antlle slate- 

 quarries one dip of 45° to the east and another of 70° to the west 

 are observable, so that the slates are undulating and are doubtless 

 cut off on the west (as on the east) by a fault. 



Moreover, it is certain that in this southern portion we do not see 

 the whole of the Cambrian series, but only the uppermost portion. 

 What may lie to the west of the felsite masses is practically 

 unknown, all being covered by Drift, or rarely exposing rocks which 

 cannot be paralleled with the known lower part of the succession as 

 subsequently described. Only in one spot on the west have I seen 

 any purple slate, viz., by the side of the River Rothell, and even 

 here the exposure is so small that the relations are quite obscure 

 and hard to interpret. 



Returning now to the complicated region in the north, we find 

 that no purple slate occurs west of a line drawn from Coch-winllan 

 near Halfway Bridge to the base of Pare Bwlch on Moel-y-Ci. As, 

 however, the general trend of the strata south of this line is to the 

 north-east, its occurrence so far west even as this is due to disloca- 

 tions. The line above indicated is the western boundary of a 

 broken band, the separate pieces of which present no contradiction 

 to the order of succession elsewhere, but the determination of whose 

 position depends upon a knowledge of that succession. The 

 northern patch of Purple Slate included here runs north and south. 

 It belongs to the Upper Slates and is apparently overlain by what 

 seems to be Silurian, but of this I cannot be quite certain. The 

 southern is of Lower Slates and occurs in a faulted wedge. On the 

 eastern side of this broken band the strata are bent into a curve, as 

 indicated in the Geological Survey map, so that the St. Ann's Grit 

 and the slates above and below dip first east, then north, and 

 finally north-west, till they meet the boundary again, and are finally 

 cut off by the Aber fault. 



In a general way, these slates are thus striking towards Baiigor, 

 though the strata there are assuredly neither the actual continua- 

 tion of the arch nor of the broken strip. Now, ai the northern end 



