Monograptiis diibius Suess. 

 M. varians Wood. 



part 2] PALEOZOIC rocks of the llangollen district. 207 



The uppermost beds of the group form the south side of the 

 Pengwern Valley. Here, about 200 yards south by east of Ty'n-y- 

 celyn the following assemblage, possibly indicating the lower part 

 of the Monograptits-scanicios Zone, was found : — 



Monograptus chimsera Barrande. 

 M. chiinaera, var. saliveyi Lapworth. 

 M. colomis Barrande. 



These beds are some 300 to 400 feet below the lowest horizon 

 at which Monograptus leintwardinensis has been found. 



On the north side of the Llangollen Fault the Nant-y-Bache 

 Group is involved in the severe folding (PI. Y). 



(iii) Vivod G-roup (^M. -leintwardinensis Beds). — The 

 rocks composing this group are largely thinly-bedded flaggy shales 

 with silty, often laminated bands at frequent intervals. They are 

 less well cleaved than any of the rocks so far dealt with, the result 

 (it is believed) of their sandy nature. Fossils are rare in them, but 

 M. leintivardinensis has been found at several places. The group 

 is perhaps 1200 to 1300 feet thick. 



South of the Llangollen Fault the characteristic fossil was 

 discovered at Craig-y-ddualt (south of Pengwern) and near 

 Pant-Dafj^dd-goch, and probably all the high land between the 

 Castle-Mills Fault and the Carboniferous of Chirk Castle is 

 occupied by this group. 



North of the Llangollen Fault, the rocks underlying the Vivod 

 Valley, Berwyn,i and part of Llangollen itself appear to belong 

 to this subdivision. 



The lateral shift effected by the branches of the Llangollen 

 Fault is well shown by the outcrops of the beds ; but the exact 

 distribution of the rocks east of Llangollen is very obscure. 



{h) Lower Ludlow Rocks outside the Llangollen 

 Synclinorium. 



The stratigraphy of the Lower Ludlow inside the Llangollen 

 Synclinorium has been dealt with at some length, because the 

 mutual relationship of the beds is clearer here than elsewhere in 

 the area under description. There seems, however, little doubt 

 that the rocks maintain the same general characters outside the 

 synclinorium, and there is pala3ontological evidence that the usual 

 zones are present. Thus : — The M.-leintivardinensis Zone, 

 indicating synclinal cores, is found south of Moel-Truan, near 

 Bryn-Tangor, 2J miles west-south-west of Bryneglwys ; near 

 Nant-Clywd, on the west of the Vale of Clwyd ; and possibly on 

 the eastern flank of the Clwydian range near Moel-yr-acre. Tlie 

 scanicus and tumescens zones occur west of the Vale of Clwyd. 

 The nilssoni zone covers a large area between the Bala and 

 Llanelidan Faults. The Dinas-Bran Beds have not been identified 

 with certainty. 



1 See E. M. E. Wood (Shakespear), Q. J. G. S. vol. Ivi (1900) p. 446. 



