SILURIAN ROCKS OF NORTH WALES. 155 
On the East of the Vale of Clwyd the flags which occur 
along the hill side S. of Caerwys belong, as we-have seen, to 
this series, passing under the tough grey sandstones of Moel-y- 
parc. Near Conway they are just seen succeeding the grit at 
the south end of the estuary. 
From the Nantglyn Flags, near Nantglyn, I have obtained— 
Monograptus vomerinus Orthoceras primavum 
Actinocrinus pulcher 
Now we arrive at what from the constancy of its occurrence is 
a very important stage in the Silurian Rocks of the Northern 
type. A coarse grit (7%.) generally somewhat calcareous and 
full of fossils immediately underlies the Nantglyn Flags. It is 
seen just above the Penyglog Quarry, near Corwen, and indeed, 
gives the upper limit to the workings. It occurs again on the 
other side of the Dee on Caer Drewyn maintaining its coarse 
character. Further north it is represented by the tough grey 
gritty beds of Garwfynydd, and by the sandstones, partly faulted 
out of sight, in the gorge of the Clwyd, near Meiarth. South 
of Conway it is seen in the cliff below Benarth, and projects in 
ridges out towards the estuary. Here it resumes its coarse 
character and exactly resembles the grit at Caer Drewyn and 
Penyglog. 
These coarse grits show that some very important change in 
the conditions of sedimentation had occurred but, as the bed is 
only a few feet in thickness, we should not infer that it was of 
long duration, though it affected a very wide area. I am not 
able to point out any equivalent in the Southern Sections A & B, 
but in the Northern Section D it seems to be represented by 
the Austwick Grit which separates the flags into an upper and a 
lower division. The flags of Leck Beck and Arco Wood above 
corresponding to the Nantglyn Flags, and the Austwick Flags 
below agreeing with the Penyglog Flags Inthe Lake country 
’ Mr. Avetine has traced some gritty bands about this horizon 
almost across the whole district. 
The next division in descending order(¢.) consists of bluish- 
black flags often finely ribboned with white lines showing the 
original lamination. This is a bed of very constant occurrence 
and uniform character. It is the rock in which the great flag 
and slate quarries of Penyglog occur, and it runs, interrupted by 
some faults, behind the town of Corwen on the East. The flag 
quarry N.E. of the church is in Bala Beds faulted up. The 
Series is seen also in the gorge of the Clwyd between Glynbach 
and Meiarth where it contains a number of thin subordinate 
beds of sandstone (0, 0’ of Section, Fig. 2., Q.J.G.S., Vol. 
xxxv., Nov., 1879, p- 697.) It turns up along the Conway 
estuary forming the cliff for about a mile from opposite the 
Castle to below Benarth. 
c 
