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MR. W. G. EEARNSIDES ON THE GEOLOGY OF [Aug. I905, 



They are not abundantly fossiliferous, but yield occasional large 

 and rather well-preserved specimens of Calymene, probably 

 C. parvifrons, Salt., which appear only during the later stages of 

 weathering. With these also occur a few large gasteropods, such 

 as Raphistoma or Maclurea ; and many indeterminable . Ortho- 

 eeratidse, including a possible Endoceras. 



hi order to prevent confusion with the underlying beds, the local 

 name is taken from the western slopes of Craig Henllan. There, 

 unfortunately, the exposures are far from good, and much better 

 collecting-ground is afforded by the higher northern slopes of Moel 

 Llyfnant. The structures and general relations are also well 

 exhibited a short distance south of Ceunant-goch and all along the 

 slopes of Llechwedd Erwent. 



The Erwent or O^^ia-Limestone [10J. 



Towards the top the Calymene-Ashes become mixed with a 

 considerable proportion of shaly material, which, being arranged in 

 thin lenticles not more than an inch or two long, gives the rock a 

 streaky-bacon-like appearance. The rock is still very hard and 

 tough, but is now sufficiently calcareous to become porous and 

 breakable when weathered, and in such conditions usually yields 

 quite good casts of shelly fossils. The matrix of the rock still 

 includes much of the basic ash characteristic of the Calymene-Ashea 

 below, but has in addition a good many chips of that acid plagio- 

 clase which is so characteristic of higher .beds (4644). The most 

 abundant trilobite is now Ogygia Selwynii (Salt.), which must 

 sometimes have attained a length of 8 or 9 inches, although more 

 frequently it is represented by specimens only 2 or 2| inches long, 

 or by numerous tails not more than half an inch across. Tails of 

 this species are much more abundant than heads, and nearly all the 

 moderately-complete specimens are more or less enrolled. Calymene 

 parvifrons, Salt., is still fairly abundant, and specimens up to 4 inches 

 long have been found. An Ampyx (possibly A. Salteri, but very like 

 A. domatus, Linn.) is abundant at certain exposures. Of other fossils 

 an Orthis like Carausii, Salt., or calligramma, Dalm., is characteristic 

 of the lower, and Obolella plumb ea, Salt., of the upper, part of the 

 series. Orihoceras is everywhere, and many-branching dendroid 

 graptolites are occasionally met with in the shaly layers. The 

 entire fauna obtained is as follows : — 



Ogygia Selwynii, Salt. 



Calymene parvifrons, Salt. 



Ampysc Salteri (?) or domains, Linn. 



Orihoceras encrinalc, Salt. 



Orihoceras sericeum, Salt. 



Ort hoc eras sp. 



Raphistoma. 



Maclurea. 



JBellerophon. 



Palcsarca. 



Ctenodonta. 



Orthis calligramma, Dalm. 



Orthis Carausii, Salt. 



Obolella plumbea, Salt. 



Lingida Rouaulti, Salt. 



Callograptus radiatus ? (Hopk.). 



It is noticeable that the whole aspect of this fauna, its general 

 composition and numerical proportion of forms, is exceedingly 



