^^^' 55-] GEOLOGY OF NOETHEEN" ANGLESEY. 647 



There still remains the point brought forward by Sir A. Geikie, 

 that a limestone with Bala fossils exists on the shore in the north- 

 wester!! part of the island, and that it forms part of the Green 

 Series. This limestone occurs in the Ordovician tract near Mynachdy, 

 at the base of the Black Slates in a cove called Porth Padrig.^ It 

 is not found as a bed, but as abundant limestone-blocks, more or 

 less rounded, lying in a thick greyish breccia, below which lies a 

 patch of black shale, and streaks of this shale run up in irregular 

 strings into the limestone-breccia, denoting considerable dis- 

 turbance. Above this breccia are black shales of the ordinary 

 Ordovician type, with which are interbedded several breccia-bands. 

 These thinner breccias also contain limestone-blocks, some almost 

 as thick as the bands in which they lie. The coarse fragmental 

 deposits in which the limestone-masses occur appear to be the local 

 equivalents of the basal conglomerates of Ogof Gynfor and Porth 

 Llanlliana, and therefore may be considered of Upper Llandeilo 

 age, while the limestone-fossils, according to Mr. Peach, are also 

 Upper Llandeilo. Although, therefore, the limestone-blocks look 

 like pebbles and boulders of ordinary clastic origin, the palasonto- 

 logical evidence is against this view, and we seem to be confronted 

 with the same phenomenon as in beds of about the same horizon in 

 Southern Scotland, where limes tone-nodules of contemporaneous 

 age lie in the midst of conglomerates. This confirms Pamsay's 

 conclusions"^ respecting this limestone, as he speaks of these masses as 

 ' irregular bands,' ' thin irregular bands,' and ' concretionary half- 

 crystalline limestone.' In their lower part they may also have under- 

 gone some breaking-up by movement. Mr. Peach adds the following 

 to his notes respecting the fossils from this locality : — ' From what 

 I have seen of the limestones in the Southern Uplands of Scotland, 

 I should think that this limestone is of approximately the same date 

 as the conglomerate in which it occurs : that is, that the limestone- 

 masses are not derived pebbles, but that they are in some way 

 formed in place. The manner in which the quartz-pebbles 

 are enclosed in them points to this conclusion. The Stinchar Lime- 

 stone, where it thins out, is represented as a series of knots or 

 masses in the heart of the Benan Conglomerate with which it is 

 associated. The same is the case '^ ith the limestones of AYrae and 

 Glencotho, and also with the fossiliferous conglomerates of Kilbucho 

 and Duntercleuch in the Lead Hills, where the fossils are found in 

 calcareous concretions and masses similar to these limestone-knots.' 

 These inconstant nodules are therefore Llandeilo limestones lying 

 in Ordovician breccias. They do not form part of the Green Series, 

 nor can we correlate them with the Cemaes limestones. The latter 

 are unfossiliferous, differ in texture, weather in a different way, and 

 retain their characteristic appearance in an exposure at Carmel 

 Head not very far from the outcrop of this Llandeilo rock. The 

 presence of these fossils, therefore, throws no light on 

 the age of the Green Series. 



^ Not to be confounded with the larger Porth of the same name near 

 Cemaes. 



2 Mem. Geol. Surv. ' Geol. N. Wales,' 2nd ed. (1881) pp. 229, 242, & 246. 



