144 ME. F. DIXEY A:N'D PROF. T. F. STBLY 0?s" THE [vol. IxXlii,. 



Old Quarry (Glam. 42 NW A 9), north of Ty-isaf, and 500 yards east of the 

 last-named exposure: mainlj^ in the crinoidal limestones of Group 10, but 

 the base of the overlying oolite just appears. The crinoidal limestones 

 contain many dolomitic bands, in one of which casts of Prodndus corrugato- 

 hemisphericus are numerous. The great abundance of Seminula cf. amhigua 

 in one bed recalls a level in Lower C, of Burrington Combe. Mendips. 



Old Quarry (Glam. 42 NW Ml), in Coed Gellihir-uchaf. north of \ 10 : the 

 westernmost exposure of the S^ beds of Group 12. Very variable crinoidal 

 and oolitic limestones in frequent alternation, with lateral change from one 

 type of rock to the other. The uppermost beds are extraordinarily fossili- 

 ferous, and formed largely by masses of Lithostrotion martini -. they yield 

 Carcinophyllvin mendipense, Camaroplxoria isorhyncha, and Atlnjris ingens, 

 an assemblage which recalls the ' Milton-Eoad level ' in S^ of Weston-super- 

 Mare and the Mendij^s.^ 



Quarry (Glam. 42 KW X 2), a quarter of a mile north of Miskin : this quarry 

 illustrates the sequence of the three groups recognized in the 

 standard limestones of C._, and S^ (see photograph, PI. XIV). Extending 

 for more than 200 yards along the strike, it affords an admirable exposui-e of 

 the beds. The section includes the uppermost beds of the crinoidal lime- 

 stones of Group 10. seen to a thickness of 20 to 30 feet : the whole of the 

 grey oolite of Group 11, 40 to 50 feet thick; and about 25 feet of beds 

 belonging to Group 12. The beds assigned to the top of Group 10 are dark- 

 grey crinoidal limestones, with shale-partings, j'ielding Cyathoplxyllum 

 among other fossils. The grey oolite 11. and the thinly-bedded limestones of 

 Group 12. exhibit their typical lithology and fauna (described on pp. 140 - 

 43). More than half-way up in the oolite is a band of thinly-bedded, black, 

 gasteropod-limestone. with abundant BeUeroplion -. this is conspicuous in the 

 photograph (PI. XIY) as a projecting rib. Current-bedding and contemporaneous 

 brecciation are particularly conspiciaous in the uppermost part of the oolite. 

 A thin smut -band.- of carbonaceous shaly matter, up to 8 inches in thick- 

 ness, rests upon an apparently- eroded surface of the oolite, and separates 

 this from the thinly-bedded limestones of Group 12. These latter beds include 

 a band of pink marl with distorted lamination, and a thick bed of marl}' 

 limestone. 



Quarry (Glam. 42 NW A 1), lying immediately west of the last-named : the 

 grey oolite and the base of Group 12. 



Old Quarrj' 100 yards north of the Castell-y-Mynach Arms, near Croftau : 

 a section in the crinoidal limestones of Grouj) 10. 



Small excavation (Glam. 42 NW A 19). 300 yards north-east of the Castell- 

 y-Mynach Arms, near Croftau : crinoidal limestones of Group 10. vein- 

 dolomitized in part, and including a 12-incli band of dolomite-mudstone. 

 Chonetes carinata occurs, in associati(m with Syi-ingofhijrif< cf. cjtspidata. 



Creigiau Quarrj- (Glam. 42 KE A 4) : the great thickness of oolites and 

 oolitic limestones here exposed probably belongs mainly to C^ — Sj, repre- 

 senting Group 12 of the Miskin sequence. Tlie lower beds yield Lithostrotion 

 martini (abundant) and a large Caninid (rare) among many other fossils. 

 Complex faulting and extensiA'e vein-dolomitization have affected the highest 

 beds of the quarry, which probably reju'esent the base of S.,. 



]Main Seininnhi Zone : S^. 



The appearance of Ci/rfinc carbonaria marks the hase, and the 

 occurrence of the DihmwpJn/lhnn fauna dcHnes tlie to]), of S.,. It 



^ T. F. Sibly. Q. J. G. S. vol. Ixi (1905) p. 500. and A. Yaughan. ihid. 

 vol. Ixvii (1911) p. 370. 



'^ Compare the smut-beds in the Lower DihnnoplnjUum Zone of Gower : 

 E. H. Tiddeman. "Tlie Country around Swansea' Mem. Geol. Surv. 1907. 

 pp. 10. 11 ; also E. E. L. Dixon. Gower paper, p. 490. 



