126 ME. F. DIXEY AXD PEOF. T. F. SIBLT 02f THE [vol. Ixxiii, 



the dommating escarpment of the Main Limestone by a narrow, 

 level or gently sloping, grassy platform which marks the outcrop 

 of the shales. This characteristic landscape featm-e is well marked 

 on the eastern side of the Ehymney yalley, where the ruins of 

 Castell Meredydd stand on the limestone-scarp, and it is con- 

 spicuously developed almost Avithout interruption from the upper 

 part of Cwm Draethen westwards to the valley between Craig 

 Llanishen and Cefn Carnau (see photographs, PI. XII, and section, 

 tig. 4, p. 120). Xear Rhubina and Tongwynlais it is again well 

 marked, and at the entrance to the gorge of the Talf the limestone- 

 scarp affords a commanding site for Castell Coch. In two localities, 

 south of Cefn-On Farm and north of Craig Llanishen, respectively, 

 a clean-cut displacement of the features, the result of dip-faulting, 

 is admirably shown. 



1. Limestones and shales. — K^. Lower part of the Lower 

 Limestone of the Geological Survey. 



Greneral lithology. — In the Ebbw valley, thinly-bedded lime- 

 stones of varied character, usually crinoidal and shelly, often 

 ostracodal, and sometimes gritty, with subordinate shales. In the 

 Taff valley, the development of shale is little inferior to that of 

 limestone. Ha?matitic limestones of u-type, precisely similar to 

 the Bryozoa-Bed of the Avon section, occur in the lower part of 

 the group. 



Thickness. — About 30 feet in the Ebbw valley, increasing to 

 oO or 60 feet in the Taff valley. 



Fauna: 

 Corals and brachiopods: — 



Cleistopora cf. geornefrica Edwards & ScliellicieneUa cf. crenistria (PhUhjp^). 



Hainie. Orthis michelini L'Eveille. 



Camarofcecliia ynitcheldeanensis 



Prochictns bassiis Vauglian. Yaughan. 



Chonetes cf. hardrensis Phillip?;. Spirifer dathratns M'Coy. 



Chonetes stoddarti Vaughan. Syringotliyris cf. ciispidata (Martin). 



Leptxna analoga (Phillips). Eiimetria carhonaria (Davidson). 



Bryozoa: — Rhabdomeson. Fenestella. Lamellibranchs : — Modiola (?) sp. 

 Gasteropods : — Capulids {Ortlionycliia sp.. Platycems sp.) and small turreted 

 forms. Ostracods. Pishes : — Psephodus sp. 



Good exposures of the beds of this division are found only at the 

 extremities of the outcrop now under consideration. 



On the Avestern side of the Ebbw A'alley, above Pont-y-Mister, 

 the stream which descends from Coed-y-Mochyn exposes, at a Uttle 

 watei-fall and in the cascades and banks below it, the section 

 illustrated by fig. 5 (p. 127). Some 250 yards north of this sti-eam- 

 section, a small disused quarry (Mon. 28 SW AS) affords a second 

 exposure of the same beds, including the calcareous grit at the base, 

 but the section is much grassed over. No shale-bands appear to 

 be developed, and no limestone of a-type has been detected. Some 



