130 ilR. F. DIXEY AXD PROF. T. F. SIBLT OX THE [vol. Ixxiii, 



group as thinning out immediately south-west o£ Draethen and 

 reappearing a mile farther up Cwm Draethen. We consider such 

 a thinning-out to be highly improbable, and prefer to regard 

 the outcrop of the limestones as continuous along the steep 

 escarpment slope on the northern side of Cwm Draethen. 



The group is well exposed in some old quarries north of Rhubina, 

 and even better displayed in the railway -cutting south of 

 Castell-Coch Tunnel, Tongwynlais. In both localities, 

 dolomitization has affected the beds in slight degree only. The 

 railwaj^-cutting (Glam. 37 SW \ 4) tm verses the group from 

 base to summit. The beds dip southwestwards at 35° to 40^ 

 throughout most of the section ; but, at the southern end, their dijD 

 uicreases to 60° and over in a sharp roll. At the base, red-stained 

 crinoidal limestones succeed sharply the limestones and shales of 

 the underh'ing division ; at" the top, grey oolite gives place to 

 deeply iron-stained argillaceous limestone and crinoidal limestone, 

 which alternate in thin bands for 8 to 10 feet before the section is 

 terminated by a bank of Glacial gravel. The total thickness of 

 approximately 100 feet comprises about 35 feet of crinoidal non- 

 oolitic limestone below and 65 feet of crinoidal oolite above ; but 

 a grassy gap in the section of the lower beds, equivalent to about 

 12 feet, may indicate some development of shale. The brachiopods 

 in the limestones include Schellwienella cf. creuisfria, Camaro- 

 toecliia mitcheldeanensis, Spirifer clatliratus, Syringotlii/ris cf. 

 cuspidata, and a small ^z^7zy/7S : of these, only the two first-named 

 are abundant. 



The Tongwynlais railway-section shows a development of the 

 limestone group marked by an unusually clear contrast between 

 non-oolitic limestone below and oolite above, although even there a 

 few seams of coarse oolite are intercalated in the crinoidal lime- 

 stones and occasional non-oolitic bands occur within the oolite. In 

 the old (juarries north of Hhubina, crinoidal limestone and oolite 

 alternate repeatedly throughout the group. 



Two disused quarries, lying respectively east and west of Castell 

 Coch, expose the upper beds of the limestone group. Each shows 

 a considerable development of oolite, and each exposes the base 

 of the overlying shale-division, but the sections are much over- 

 grown and difficult of access. In the quarry east of Castell Coch 

 the limestone is much dolomitized.^ 



3. Shales with thin limestones. — K,,. 



General lithology. — Dark-grey to black shales, with sub- 

 ordinate thinly-bedded limestones. The limestones always richly 

 crinoidal and shelly. 



Thickness. — In the Ebbw valley, 50 feet or less 

 to about 100 feet at Tongwynlais in the Taft* valley. 



^ Fig. 4 of the plate in the Ne^vport Memoir, 2nd ed., illustrates a partly- 

 dolomitized oolite from the quarry west of Castell Coch. The selective 

 dolomitization of the ooliths points to vein- dolomitization. 



