150 MR. F. DIXEY AXD PROF. T. F. SI ELY OX THE [a^oI. IxXlii, 



liAS been drawn at a level marked by the appearance of Cijrtlna 

 cdrhonaria} 



The estimates of thickness should be regarded as rough approxi- 

 mations, except in the case of the S., dolomites, the thickness of 

 which can be determined with fair accuracy. Allowance has been 

 made for some probable repetition, by thrusts, of beds Avithin the S., 

 oolites. Both the dolomites of Z-C\ and the oolitic beds of S.^ are 

 much broken and sheared, doubtless ow^ng to the close proximity 

 of the sections to the great dip-fault which runs along the gorge. 

 The dip varies from about 60°, slightly west of noi-th, in the lower 

 beds, to about 20°, north-westwards, in the upper part of S.,. 

 Calcite-veining on both a large and a small scale is greatly developed 

 throughout, and nests of calcite and dolomite are abundant in the 

 dolomites generally. Although the dolomites are mainly contem- 

 poraneous, portions of the sequence, in C., + S^ and in S.,, owe their 

 dolomitic character to vein-dolomitization. The problem of the 

 dolomitization is not discussed here, however. 



Castell-C>)cli Quarry (Glani. 37 SW \ 5), a great quarry whicli extends 

 almost as far north as tlie Walnut-Tree yiaduct, exposes the grey finely- 

 crystalline dolomites which form the lower half of the Main Limestone. 

 These beds cover Z and probably the whole of Ci- As mentioned on p. 131, 

 the section includes the junction of the Main Limestone with the Lower 

 Limestone Shales, and this junction is sharp. The basal beds of the Main 

 Limestone are grey or buft"- coloured argillaceous dolomites, with some very 

 thin- shale-partings. They include some richly-crinoidal seams, and show a 

 small development of crinoidal chert. Their horizon (Zj) is indicated by an 

 abundance of Produchis bniiingtonenais, associated with Sch.eUirie)ieUa cf. 

 crenistrin and Syringotliyris cf. cnspidata, observed on a shaly bedding- 

 plane. 



Beds assignable to C.,-)-S, are exposed (a) in the low-level cutting of the 

 Cardiff Railway, under and immediately north of the Walnut-Tree viaduct, 

 and {b) in the high-level cutting of the Barry Railway-. In the main, these 

 are grey or yellow, fine-grained to medium-grained dolomites, barren of fossils. 

 They include occasional bands of dolomite-mudstone and lenticles of clay, 

 features which foreshadow the strong development of JforfJo?a-phase deposits 

 farther east. They have jielded a few fossils. On the Cardiff Railway, 

 Sijriiigopora cf. i-eticidata (?J and Prodnctus corrugato-liemisphericus have 

 been recorded. On the Barry Railway (Glam. 37 SW A 6), about 110 yards 

 from the viaduct wall, a band of yellow dolomite contains cohmies of 

 LitliOfftwtion ma7-tini Avith the tissue completeh' dolomitized. This band lies 

 about 3.5 feet below the base of S^. Patches of slightly-dolomitized. pale- 

 grey oolite survive in a massive dolomite exposed on the Cardiff' Railway 

 65 yards north of the viaduct. This is the easternmost trace of the normal 

 oolites of C_,-|-Sj discovered in our district. 



The dolomites which form the lowest part of S., are best exposed on the 

 Barry Railway (Glam. 37 SW \ 6). They are grey slightly-crinoidal rocks, 

 fine-grained t(j medium-grained, and tliinly bedded, with occasional clay- 

 partings. At their base, Cyrfiiia cnrhnnaria and Seminida jicoides are 

 represented hy numerous casts in a band of dark-grey dolomite, red-stained 

 in patches. This band, defining the base of S^, lies 140 yards from the wall 

 of the Walnut-Tree viaduct, (m the eastern side of the cutting. Except in 

 this basal band, fossils other than crinoid-ossicles are rare in these dolomites. 

 Prodnctus sp., Seminida fico id en. smd Carcinophglhim sp. have been identified. 



^ Here, as elsewhere east of the Taft', the dolomites with Cijrtina car- 

 ho liar ia which form the lowest part of S^ represent the basal portion of the 

 Seminida Oolite as developed farther west. 



