ON THE MENEVIAN GROUP. 185 



(1) Shallow -water or sJiorc beds for the Harlech group. 



(2) Slow depression for the Menevian group. 



(3) Shallow-iuater or shore accumulations for the Lingtda-Jlag. 



(4) Gentle depression for the Tremadoc. 



(5) And, lastly, deep and decided depression of the sea-bed to receive the 



next great formation — the Ai'cnig Eocks (Lower Llandcilo,Murchison). 



5. Arenig or Skiddaw Itoclc. — A formation at least a thousand feet thick, of 

 vertical beds of black shale, seen in Whitesand Bay, and occupying also more 

 than half of liamsey Island, where they lie in a clistinct trougli of Tremadoc 

 and Lingula-jlags; and everywhere characterized by the following fossils : — 



^glina, 2 species. 



Ogi/gia, 2 large species. 0. peltata and 0, hullata. 



Asaphus, a large species. 



Trinucleus, 2 species. 



Ampyx, n. sp. 



Agnostus, Orthis, Lingula, Theca, Bellerophon. 



And, lastlj', branched and twin Graptolites, viz. Dendrograpsus and Di- 

 dgmograpsus. 



No Graptohtes occur beneath the Skiddaw group in Britain. And though 

 all these species, except the Graptolites, arc distinct from the fossils of the 

 same strata in North Wales and Shropshire, the probable reason is, that the 

 latter were in shallow water, while ours is evidently, like the Skiddaw slate, 

 a deep-water series. We may therefore expect these species to be found in 

 Cumberland. 



Eamsey Island is worth a visit ; for the exhibition of the three sets of rocks 

 (Lingida-JJag, Tremadoc, and Arenig) is very complete on the north side of 

 the island, and fossils are abundant. 



G. To complete the geology of St. David's, one must go to the overlying Llan- 

 dcilo flags of Abereiddy Bay. These line slate-quarries are full of fossils. 

 Trilobites of well-known forms, familiar to us at Builth and LlaudeUo, crowd 

 the slaty bands, and Graptolites in myriads, principally the species called 

 the tuning -forhgraxytolite {Didym. Murehisonce). The chief TrHobite is Ogygia 

 Biichii ; but there are many other Builth species, and some very rare ones, 

 Barrandia Cordai, for instance. A few words on the faults of the district, 

 which are literally innumerable. 



E. and W. faults, sometimes of large amount, but not much indicated on 

 the surface. 



N.N'.W. ones, more conspicuous as lines of valley and marsh; often shift- 

 ing the strata much, and gi^Hing outline to the coast. 



N.E. faults, freqiient, but not of very large amount ; have not been well 

 observed. 



N. and S. faults, believed to be the latest, and they give much impress to 

 the features of the district, forming short valleys, and shifting the strata, but 

 less than the others. 



Summary of the facts stated. 



1. AVe have two axes of elevation in the promontory, viz. the granite of 

 Brawdy and Asheston on the south-east, and the so-called St. David's syenite 

 in the centre. The latter is chiefly altered rock. 



2. Between these two axes, and on either side of them, the purple and 

 grey Cambrian rock forms a steep trough, supporting black shales of the 

 Menevian group, followed by Lingula-flag and Tremadoc rock, and on the 

 north side of the coast Arenig or Skiddaw rocks covered by Llandeilo Aug. 



