SILURIAN ROCKS OF NORTH WALES. 153 
stratigraphically and palzontologically, and I look forward to 
its proving of great value in tracing out the subdivisions of the 
Silurian Rocks. 
They form the lower stages of a great sandy series (Z.) 
which extends along the west flank of the Clwydian Range, and 
occupies large and as yet undefined areas between the valleys of 
the Clwyd and Conwy. 
From beneath the Moel-y-Parc Sandstones, the Nantglyn 
Flags (¥. 1.) rise on the east, and rolling over in two large 
anticlinals, disappear under the basement beds of the Carboni- 
ferous Limestone of Caerwys. 
They are the equivalents of the flags of Arco Wood, and of 
Leek Beck (see pp. 142, 149), and have yielded the following 
fossils :— 
Orthoceras primevum Favosites fibrosa 
O. subundulatum Cardiola (?) 
Section No. 1 shows the relative position of the beds above 
described. 
Crossing now to the West side of the Vale, near Denbigh, 
and following the section from Denbigh S.S.W. to Nantglyn, 
we find the tougher and more massive beds giving rise to 
prominent features over a considerable area. These upper 
sandstones and mudstones are here and there fossiliferous and 
are obviously the equivalents of the Bodfari Beds (Z%.) of the 
East side of the Vale. 
Mudstones like those of the Grove overlie, a little off the line 
of section, the alterations of shale and tough sandstone which 
roll over and over between Segrwyd and Brynrobin. I will not 
now discuss the position of the Llansannan Shale, or of the 
Pontyralltgoch fossiliferous Mudstone, or of the Sandstones 
south-west of Abergele. 
The following diagram shows the relation of the various beds 
to one another :— 
Section II, 
In this Section the Sandy Beds which roll over between 
Bryneirin, 24 miles S.W. of Denbigh and Dolhyfryd, are 
obviously on the horizon of the Bodfari Beds (Z%.) I have 
found the following fossils at Bryneirin :— 
Rhynchonella navicula Meristella 
R. nucula Strophomena depressa 
Orthis elegantula var sulcata Cardiola interrupta 
O. sp. (small coarser form with | Ctenodonta 
straight ribs) Orthoceras 
O. sp. (small form with ribs Monticulipora 
bifurcated) | Encrinite stems 
And at Dolhyfryd I have found— 
Rhynchonella navicula Cyclonema 
Orthis elegantula Orthoceras (a smooth form like 
Meristella O primevum.) 
Cardiola interrupta 
