Vol. S50 



GEOLOGY OF NOETHEEIS' ANGLESEY. 



673 



fall into place as Ordovician, and we have no difficulty with the 

 Orthis Bailz/ana-heds which are part of this series, though perhaps 

 faulted against the conglomerate. 



The remaining Ordovician outcrops are not extensive. To the 

 east, between Perth Wen and Bull Bay, is a narrow strip, about 

 I mile long, of conglomerate forming the ridge of Mynydd Pant-y- 

 gaseg ; near it, in Perth Pridd, are the black shales noticed by 

 Prof. Blake. Finally, in Porth Padrig are the shattered black shales 

 which have yielded Glenkiln graptolites ; their horizon appears to 

 be near that of the Penterfyn beds.^ They strike inland a short 

 distance, but are soon faulted out. It will be perhaps needful to 

 add various small exposures of broken-up rocks to this summary of 

 the Ordovician strata. 



6e/ia 



Eig. 13. — Purple band in cleaved grits and 

 conglomerates, on the coast about 400 yards 

 west of Porth Llanlliana. 



[The cleavage is not shown.] 



This Ordovician sequence has a general correspondence with that 

 established in Central Anglesey, where the series commences with 

 conglomerates and passes up through grits and sandstones into 

 black slates, in which ironstone is occasionally present. 



[d) Relation of the Northern Complex to the Ordovician. 



"What, then, is the age of such strata in the Northern Complex 

 as are not included in the Ordovician? Some of them, though 

 shattered to ' crush-conglomerates,' are clearly portions of the Green 

 Series. The more typical members, like the limestones and the 

 quartzites, come in between the Green Series and the Ordovician, 

 and they may be in upward conformable sequence with the green 

 strata. The Ogof Gynfor section shows that the limestone has 



^ I brought away from the seaweed- covered rocks of Porth Padrig, near the 

 black shales, a specimen of oolitic ironstone. It seems to be the equivalent of 

 the Penterfyn ironstoxie, though my notes do not make it clear whether the 

 specimen came from the black beds or from the neighbouring rocks. 



Q. J. G. S. No. 219. 2 x 



