154 ORSERVATIONS ON THE 
In the lower part of this series I have identified a very 
important horizon. In the bed of the stream at Pont Lawnt, 
there is a fossiliferous mudstone. in which I found the very same 
Acidaspis which occurs in the lower part of the Coniston Grit 
on Casterton Low Fells (see pp. 142, 147), and which I have 
detected at the same horizon in various other localities on the 
south-east borders of Westmorland. With it were associated a 
very similar group of fossils to those found in the corresponding 
beds in the North of England. 
At Pont Lawnt I found— 
Rhynchonella navicula P. subfaicata 
R. nucula Acidaspis Hughestt 
Cardiola interrupta Orthoceras primevum 
Pterinea tenuistriata 
Under these sandstones and mudstones (£%.), with the 
Acidaspis zone near their base, coarser beds corresponding to 
the Moel Fammau Sandstones (£c.), are thrown into view here 
and there in the adjoining area by the irregular undulation of 
the strata, and finally rise into prominence near Brynrobin, 
capping the hill at Garth. They consist of ary sandstones 
(Zc.) with a concretionary structure owing to which they show 
wavy white lines or small lenticular whitish beds, or sometimes 
even spheroidal masses with a rough concentric arrangement 
of white and grey. The sandstones of Moel Ganol are a good 
example, as also those of Moel Gasvdd, 3 miles S.W. of 
Denbigh. This is the sandstone which runs along the crest of 
the Clwydian ridge from Moelyparc, near Bodfari, in the N., 
to nearly opposite Ruthin, in the S. 
The Nantglyn Flags (¥a.) form a fairly homogeneous series, 
splitting into flags where the cleavage is weak or nearly 
coincident with the bedding; but, where this is not the case, 
breaking up into striped rab or shiver, or irregular blocks and 
slabs, or sometimes splitting into slates. It is only here and 
there that quarries are opened in it for flags or slates, and so 
the great thickness and extent of the series has been somewhat 
lost sight of. I take as a typical section that exposed along 
the hill sides and in the valleys near Nantglyn where the flags 
have been extensively worked for a very long time. 
In the gorge of the Clwyd the flaggy beds, which succeed 
the sandstones below Meiarth and extend by Derwen, pass 
under the rough sandstones of Moel-y-Gasedd on the one 
hand and of Garth and Brynrobin on the other. I think we 
must refer to this series the dark grey sandy flags which extend 
up the Clywedog valley to beyond Cyfylliog where they pass 
under the grey concretionary sandstones of Moel Ganol (£c.) * 
* See Q.J.G.S., Vol. xxxv., Nov., 1879, p. 695. 
