152 



MK. F. i3IXEi' AXD PKOF. T. ¥. SIEJA' OX THE [vol. Ixxiii, 



In this sequence the divisions (i), (ii), and (iii), and the sub- 

 divisions of (ii),are well-detined Uthologicalh\ The zonal correla- 

 tion of the S., beds is placed beyond question by the occuri-ence of 

 Cijrtina carbouaria and other fossils, but that of the underlying 

 beds is inferred from considerations of thickness, in comparison 

 with the faunal succession established west of the Taff. 



These three divisions of the Cefn-On sequence may be con- 

 veniently adopted for the remaining descri])tic)n of the Main 

 Limestone. 



(i) The crvstalline dolomites of Z-C . 



In the Taf£ valley, these dolomites probably cover the whole of 

 Z and C^ (p. 150). Eastwards, the up])er beds pass laterally into 

 J/or/Zo/r^-phase deposits. As a result of this circumstance, com- 

 bined with actual thinning of the strata, the thickness diminishes 

 from about SCO feet in the Taif valley to 400 feet in the Kbbw 

 valley. 



In general, the rocks are gre}^ finely -crystalline dolomites. 

 Nests of calcite and dolomite are sometimes abundant. Chert is 

 usually developed at or near the base. Oolite, completely dulomi- 

 tized, is developed in considerable thickness, less than 100 feet 

 above the base, in Cefn-garw Quarry and in an adjacent disvised 

 quarry, and has been observed at the same horizon in the scarp- 

 face of Cefn-On. 1 Fossils, other than crinoid-debris, which is 

 generally distributed and sometimes abundant, are usually rare. 

 At two localities to be described, however, they occur abundantly 

 at and near the base ; and at various levels and localities SclidJ- 

 wienella cf. cremsfria and Spirifcr clafhrafus \\^\e been observed. 

 Excellent exposures are afforded by Cefn-i>"arw Quarry, north of 

 Tongwynlais ; (lelli Quarry, north of Hhubina ; quarries at Thorn - 

 hill ; crags and cuttings on Cefn-On ; Machen Quarry : and I)an- 

 y-draig Quarry, Kisca. 



Cefn-garw Quarry (Glana. 37 SW X 2), situated 750 yards east-nortL-east 

 of Castell Coch, gives an admirable section of about 60 feet of beds lying 

 very near the base of the Main Limestone. The loAver beds of the quarry 

 are bhie-black, buff-Aveathering, crinoidal dolomites of very fine grain. The 

 middle beds are ijale to dark-grey richly- crinoidal dolomites. These 

 latter are characterized by a highly-irregular development of black argil- 

 laceous streaks, by extensive silicification resulting in patches of crinoidal 

 chert and occasional nodules of chalcedony, and by an irregular patchj^ iron- 

 staining. Thej^ resemble closely the crinoidal liihestones which form the 

 base of the Main Limestone west of tlie Taft' (p. 135). Their correlation 

 Math Z, (Horizon /3) is determined by the following fossils found in them : — 



Zaphrentis delanouei ('?). 

 Chonetes cf. hardrensis ; common. 

 Leptseiia analoga ; common. 

 Schellivienella cf.crenistria : common. 



Ca ma rotopch in onitch eldea ne7ii>is 



common. 

 Spirifer clathratns. 

 Siji'ingothijns cf. cuspidata. 

 Sjni'lferina octoplkata^ mutation /■}. 



' Specimen E 7423 in the collection of the Geological Survey, obtained 

 from the lower part of the Main Limestone east of the Ebbw river at Risca, 

 is a dolomitized oolite of identical character. See Newport Memoir, 2nd ed. 

 Plate, fig. 5 (microphotograph). 



