Orthis fNicolellaJ actonise var. 



asteroidea Reed. 

 Christiania temticincta (M'Coy). 

 Orthis (Hebertella) crispa (M'Coy). 

 Orthis elegantula, var. dvummucTi- 



part 2] ITS STEUCTUEE AND EOCK-SUCCESSION. 153 



phate nodules in it is characteristic. The fossils from this locality 



include : — 



Phillipsinella parabola Barrande. 



Agnostus agnostiformis (M'Coy), 



Encrinurus sexcostatus Salter. 



Staurocephalus murchisoni Barrande. 



Lichas sp. 



Illsenus davisi Salter. I ensis Reed. 



Cheirurus Mmibcronatus Murchison, Hemicosmites sp. 



Trinucleus sp. i EchinosphseritesarachnoideusSaltev. 



Above this are seen at least 20 feet of pasty Rhiwlas Mudstone, 

 so that the faunal succession compares well with that observed at 

 Gelli-grin. (See figs. 5 & 7, p. 154.) 



As we pass round the syncline to the west it becomes obvious 

 that both limestones are abruptly cut off, and their place taken in 

 the line of strike by the fossiliferous Calcareous Ash, the outcrop 

 of the Grelli-grin Limestone being shifted into the line of the 

 E-hiwlas Mudstones. Thus, the syncline, as seen so far, is affected 

 hy cross-faulting with a downthrow to the north, the result being 

 that orT the south the Gelli-grin Limestone is practically folded on 

 itself, the merest tongue of pasty Khiwlas Mudstone occupying 

 the centre of the fold ; this limestone is not well exposed at the 

 present time, but a double line of workings indicates its outcrop. 

 Oddly enough, the outcrop of the limestone is almost continuous 

 on the south-east side of the fold despite the fault, and both 

 outcrops are trmicated by the Moel-fryn displacement. The small 

 dislocation east of the fold has the effect of diminishing the 

 outcrop of the Calcareous Ash, and brings the Gelli-grin Lime- 

 stone very close upon the Allt-Ddu Mudstones. 



Another small outcrop of limestone seems to belong to another 

 fold, while a highly contorted outcrop \jmg south of this and 

 east of the main outcrop is so affected by the great line of disturb- 

 ance in its immediate neighbourhood that some of the associated 

 mudstones are practically schists. At this localit3^ probably on 

 account of the lenticular nature of the Grelli-grin Limestone, there 

 is a rather greater thickness of rock separating the two limestones, 

 a foot and a half of Calcareous Mudstones, 4 feet of Calcareous 

 Ash, and 4 feet of pasty Rhiwlas Mudstone being clearly visible 

 above the main limestone and below the calcareous development of 

 the Ehiwlas (fig. 7, p. 154). At this locality the lower limestone is 

 richly fossiliferous, and has yielded the following species : — 



Orthis (NicolellaJ actonise (Sowerby). | Plectambonites sericea (Sowerby). 

 Strophomena (Rajinesquina) ex- ■ Calymene sp. 

 pansa (Sowerby). 



Creiglau By chain. 



The group of synclines on Creigiau Bychain shows an excellent 

 series of exposures of the calcareous development of the Bhiwlas 

 Beds, with a maximum thickness of 12 feet ; occasionally the beds 

 are calcareous right down to their junction with the Calcareous 

 Ash, in other places there is a definite band of pasty mudstone 



