Vol. 56.] GEOLOGY OF NORTHERN ANGLESEY. 243 



The Llanbadrig Series is newer than, and its base may be con- 

 formable with, the Green Series. No fragments derived from the 

 latter have been observed in the lower zones of the Llanbadrig 

 rocks, and there is also a close similarity between the Green Slates 

 and a few of the slaty rocks of the Llanbadrig Series which suggests 

 a close connexion between the two groups. It is a fact that the 

 Llanbadrig Beds are found in juxtaposition with several horizons 

 of the Green Series, but the junction is seen to be a fault in some 

 cases, and in other cases where the junction is obscure it seems 

 more reasonable to assume a fault than an unconformity. 



On the other hand, one of the highest zones, namely the beds 

 (Be) below the highest quartzite, has yielded rock-fragments 

 which, in Prof. Watts's opinion, are ' almost certainly derived from 

 the Green Series.' The best example is a microscopic slide 

 [N. A. 119] l from the beds below the Porth Llanlliana quartzite, 

 where the fragments are chiefly varieties of phyllite and chlorite- 

 schist. The beds containing them cannot be excluded from the 

 Llanbadrig Series (although they have a distinct resemblance to 

 some of the Llandeilo basal rocks) on account of the quartzite and 

 limestone associated with them. The evidence, as a whole, is 

 therefore in favour of the Llanbadrig Series being 

 unconformable to the Green Series. 



As regards the relation of the Llanbadrig Beds to the overlying 

 basal Llandeilo rocks, the occurrence of fragments of limestone 

 and pieces of quartzite in the Llandeilo conglomerates favours the 

 view that the two series are unconformable. But the latter beds 

 cling closely to the quartzite which forms the highest member 

 of the Llanbadrig group, instead of lying indifferently upon the 

 various members of that series ; and where this quartzite and the 

 overlying conglomerates are seen together in the cliffs, as at Porth 

 Llanlliana and to the east of Hell's Mouth, they dip at the same 

 angle in apparent conformity, this fact showing that the quartzite- 

 beds must have been horizontal when the Llandeilo conglomerates 

 were laid down. Moreover, when the Llandeilo and Llanbadrig 

 rocks are faulted together, a quartzite is the rock usually found at 

 the junction, and this rock may be seen (as at Ogof Gynfor and 

 near Porth Padrig) even in the midst of Llandeilo ground. It is 

 therefore a moot-point whether there is an unconformity at all 

 between the Llanbadrig and the Llandeilo Series : it is certainly not 

 a conspicuous one. Yet the variation in thickness of the quartzite 

 as it is followed along its outcrop — a point which will be again 

 referred to shortly — could be very conveniently explained by its 

 erosion at the time when the conglomerates were forming. 



In short, the evidence of the relations of the upper part of the 

 Llanbadrig Series to the Llandeilo conglomerates is conflicting. 

 No fossils have been found in the Llanbadrig rocks to help us to a 

 conclusion, and no more definite assertion of their age 

 is therefore made than that they are pre-Llandeilo. 



1 The numbers in brackets are those of microscope-slides in my cabinet. 



