part 2] CARBOMFKHUIS 1,1 M KSTuN K UF s. WALKS (OA 1,1 1 Kl.l». 1 li> 



Kast of tlic Taff. 



riu- Main Linu'stoni' is lu-ttcr cxixtst'd than in tlit- aiva west of 

 iiu- Taff. Hut the ovcrstt'}) of tlu' Millstone- (Jrit, wliioh has 

 alivady proi^ivssed lu-low tlu- top of S , i-uts out tlu* whole of that 

 /one lu'fore tlu' Ehhw valK'V is n-ac-liLMl, while doloniitizjition and 

 the develojunent of Modioht phases ohliterate so much of the 

 fauna] succession that onlv two zonal hori/ons (Z and the hast- ot 

 S.) can be traced. 



No trace of the C, fauna has been found, and only vestiges of the C. + S, 

 fauna. It has not V)een j>o.-*sil)k', therefore, to define a b«iundary between 

 C, and C . The line drawn on our niap (PI. XVI) does not, however, 

 represent a conjectured zonal boumlary. From Cefn-eaniau-fawr. about 2 

 miles east of the TafF, to Pen-how. 1 mile west of the lihymney. it is a definite" 

 lithological division-line. It marks the base u{ a band of crystalline dolomites, 

 of O.-f S, a<,'e, which becomes defined, as we i>roceed north-eastwards, by the 

 apjHjarance of luiderlyinjr and overlyinp arjfillaceous Mmliula jduises. This 

 dolomite-band must be assif,nied to C 4-.S, from its po- ition in the se<iuence. 

 which i-an be determine<l accurately at Thornhill and on L'efn-Un (p. 1.">1). 

 But consi.lerations of thickness indicate also that some part of the underlyinj,' 

 Mtnliol<i i)hase belongs to C ,, and lead to the conclusion that this Mudiolo 

 phase is of C, C. age. In "the central portion of the outcrop V)etween the 

 Taff and the Rhyinney. therefore, the line drawn on our map seiiarates an 

 argillaceous Modioln phase of C, C, age from a band of crystalline <lolomit«' 

 belonging to C.jH-S,. West of Cefn-camau-fawr. an<l east of Pen-how, it 

 represents merely the conjectured position of the same level. The argil- 

 laceous ;rroup ceases to be recognizable a little west of Cefn-carnau-fawr, 

 while the dolomite-band appears to die out and give place t(» argillaceous 

 IxmIs east of Pen-how. 



The Main Limt stone on tiic uastcrn side ottlie TalY uor-c. 



Almost the whole thickness of the Main Limestone is expu.sed 

 in tjuarries and railway-cuttiui^s on the eastern side of the Tatf 

 i^oiX'.' The cuttings on the Cardiff Railway, toi^ether with 

 (';jstt'll-('och Quarry and I'ortohelht (Quarry, afford a section which 

 is practically continuous from the hasc of the Main Limestone 

 almost up to the top of the .SV-////y//'AM )olit«- ; while the Harry 

 l»ailway, whicli has crossed the gori^e on the Walnut-Tree viaduct, 

 cutsinto the C,-hS| heds and ))rovides a duj»licate section from this 

 latter horizon upwards (see phototjniph, IM. XIII. liii:. 1 ). 



The succession may he tahvdated as folhiws : 



hWt. 

 J Oolite and <.<.litir linu-stone :l8l» 



^' "^ Dolomite^ i:{" 



C, 



z 



C, [.Doh.mit.- l-'.-.H 



ITr.o 



'I'he Z, fauna is poorly represented, anil tiacsof the C.^-f-S^ fauna 

 are found. The S fauna is well repre.sented. and the ha.se of S^ 



' The sticcessioii ir. briefiy descrilfcd in the Newport .Mi-moir. 2nd»Ml. pp. 20. 

 24 •_».-.. 



