138 



MR 



r. DIXEY AND PROF. T. Y. SIELY OX THE [vol. Ixxiii, 



While the fauna admits of no doubt in assigning the beds to 

 yC^, a separate recognition of Horizon y is not possible. Specially 

 characteristic featui'es are the occurrence of Can i nice a:ff. cornv- 

 copice and the abundance of Zaphrentis densa, Ortliis micheliniy 

 and Spiriferina cf. laminosa. 



Large vesicular Caninids {^Ccniinia paiulu, C. cijlindrica) area 

 conspicuous feature of the fauna at this level in the Avonian 

 generally. Xo specificalty determinable corals of this t^'pe have 

 been found in our district ; but the upper beds of the crinoidal 

 limestones contain man}' patches of coarsety-crystalline calcite, 

 the form and size of which suggest large Caninids, and in some of 

 these patches remnants of thickened septa have been observed. 



Eastwards from (xroes-faen, the crinoidal limestones of 

 this group pass rapidly into dolomites, and dolomitization has 

 usually obliterated the fossils, with the exception of the highly- 

 resistant crinoid-ossicles. Immediately east of Creigiau, however, 

 a small quarry (Glam. 42NEX3), situated 170 yards north-north- 

 west of Craig Channel, exposes crinoidal dolomites, together mth 

 some beds of black crinoidal limestone that ha^T suifered only 

 a patchy dolomitization. Corals and brachiopods are numerous 

 throughout the beds of this section : the}^ include Zaphrentis 

 omalhisi, Zuplirentis sp. nov., Orthisresupinafa, and Spiriferina 

 cf. laminosa. a fauna which seems to warrant the reference of 

 the beds to yC\. 



In the dolomite-series of the Taff valley, the crinoidal 

 dolomites that form the upper beds of Ty-nant Quarry (Grlam. 

 87SW/\1) include a richly-fossiliferous band which represents 

 either the base of our yC^ group, or a slightly-lower horizon, in Z.,. 

 This band consists of red completely-crystalline dolomite, in which 

 all the brachiopods and corals, and many of the crinoid-ossicles, 

 are repi'esented by casts or moulds. The following corals and 

 brachiopods occur : — 



Zaphrentis lonincTci; common, 

 SchelUi-ienella cf. crenistHa ; abim- 



dant. 

 Orthis michelini. 

 Ortliis resupinata ; very abundant. 



Camarotoechia mitcheldeanensis; rare. 

 Spirifer clathratus ; common. 

 Syringothyris cf. cuspidata. 

 Spiriferina cf . laminosa ; abundant. 



7 & 8. Laminosa Dolomite and Caninia Oolite: C, 

 (upper part). — These two divisions have been traced from the 

 western side of the Dawen valle3^ north of Penlline, as far east 

 as the neighbourhood of Groes-faen, but it is only between the 

 latter place and Miskin that they are well displayed. 



Between Misl^in and Groes-faen, the collective thickness of the 

 Laminosa Dolomite and the Caninia Oolite probably remains 

 fairly constant at about 340 feet. Each division has been proved 

 to a thickness of over 100 feet, but it has been impossible to 

 determine even approximately a boundary between the two. 



