part 2] LOWER PALJEOZOIC OF THE LLANGOLLEN DISTEICT. 203 



* bastard slates ' wliicli have yielded the following graptolites 

 indicative of the zone of Cyrtograptus linnarssoni : — 



1 2 



Cyrtograptus linnarssoni Lap- X 



worth. 

 Cyrtograjptus symmetricus Elles. X 

 MoHograptus duhius Suess. 

 M. jiemingi Salter. 

 M. fiemingi var. primus Elles 



& Wood. 



X 

 X 

 X 



Monograptus jiexilis Elles. 

 M. priodon Bronn. 

 M. vomerinus Nicholson. 

 M. vomerinus var. basilicus 

 Lapworth. 



1 



2 



X 



X 



X 



X 



X 



X 



X 





1 — North side of the quarry, north of the upper bridge over the Nant 

 Ty'n-y-twmpath. 



2 = Level in Moel-Fferna Slate-Quarry. 



It was the presence of this fauna at Moel-Fferna that led 

 Mr. P. Lake to institute his ' Moel-Fferna Slates ' ; but the 

 present development of the mine clearly demonstrates that the 

 'bastard slates' (with C. linnarssoni) lie under, and not over, 

 the Pen-y-glog Grit as he thought. 



Pen-y-glog Grit occurs only in the western part of the district. 

 It is a tough, fairiy massive, felspathic grit, often interbedded 

 with hard blue shale. Miss G. L. Elles states that the grit overlies 

 the M,-riccartonensis Beds at Pen-y-glog. If this be so, the base 

 of the grit lies at a lower horizon there than at Moel-Fferna, a 

 fact that may perhaps be related to its greater thickness at the 

 former place, where it is perhaps 120 feet thick. It thins to 

 30 feet at Moel-Fferna, and cannot be traced far to the east of 

 this point. A similar eastward thinning occurs in its northern 

 outcrop near Ffynnon-Tudur (north-west of Bryneglwys), an 

 occurrence of the grit that was overlooked by the makers of the 

 old geological map. 



In the Moel-Fferna district the grit is overlain b}^ hard dark- 

 blue slates, devoid of fossils, which in turn give place upwards to 

 slates and silty bands of the Lower Ludlow type. But in Glyn- 

 Ceiriog there is no trace of the grit, and a considerable thickness of 

 banded, earthy- weathering, ' bastard slates ' succeeds the roofing- 

 slates ; we only know that, near the jSrant-Ty'n-3^-twmpath, the 

 lower part of these represents the linnarssoni zone (see above). 



It appears that the junction with the Lower Ludlow is possibly 

 cut out in places by the Glyn-Ceiriog and Nant Ffrj^dd-isel 

 Faults, but the evidence is meagre and dithcult to interpret. 



In addition to the almost continuous outcrop on the south and 

 west side of the Llangollen Synclinorium, the Wen lock deposits 

 emerge at Caer-Drewyn Hill, north-east of Corwen (see p. 216), 

 north and north-east of Bryneglwys, and on the south side of 

 Cyrn-y-Brain. In the last-named area they have been extensively 

 quarried in the past, on Moel-3^-faen in particular. There is a 

 general similarity of litholog}^ here and in the southern outcrop ; 

 but, on account of severe folding, and because the cleavage often 

 crosses the bedding more or less at right angles, the sequence and 



