156 OBSERVATIONS ON THE 
From the Penyglog Flags (c.) the following fossils have 
been recorded :— 
Monograptus priodon, Peny- O. subundulatum, Peny- 
glog, N. of Bryngorlan glog 
Cyrlograptus Murchisoni, Holopella gregaria, Palé, 
Penyglog (Ruddy) 
Retiolites geinitzianus, Peny- Acroculia haliotis, Penyglog 
glog (Marr) 
Orthoceras primevum, Peny- Rhynchonella, Palé (Ruddy) 
glog Lllenus sp., Clegyrmawr. 
These lower flags (Fc.) pass down into red or pale pasty 
rock commonly spoken of as the Tarannon Shale or Pale Slates 
which are rarely fossiliferous ; and these in turn are split up by 
bands of black shale sometimes full of graptolites, which occur 
in zones corresponding to those of Westmorland and Yorkshire. 
In the gorge of the Clwyd for instance, near Clegyrmawr, there 
are black shales at the bottom of the Pale Slates from which the 
following fossils have been determined by Mr. Marr :— 
Monograptus convolutus M, tenuis 
M. triangularis M. sp. tike colonus 
_ _ As is usual, in the case of basement beds, the lowest part of 
the Silurian of North Wales is very variable. At Corwen, it 
consists of irregular beds of sandstone, often coarse; sometimes, 
as in Nant-caweddau, containing pebbles as large as pigeons’ 
eggs. At one place it is made up of somewhat even-bedded 
sandstones with shaley partings; at another, it is concretionary 
and irregular; while close by it consists of a thick mass of grey 
or whitish tough gritty sandstone, with few divisional planes, 
and often no indication of bedding. Many of these characters 
may be seen behind the town of Corwen, from which I have 
named the more sandy development of the series—the Corwen 
Grits. In that neighbourhood they are seen to pass under the 
great mass of flags and sandstones of the Berwyns.—Q.J.G.S., 
Vol. xxxiii., May, 1877, p. 207. Notes—Geol. Vale of Clwyd, 
Proc. Chester Nat. Sci. Soc., p. 8. : 
A few miles to the W. the Clwy4 cuts through the same 
series. Under Dinas we find similar sandstones, but here 
there are bands of pale slate and black mudstone with 
graptolites intercalated in the upper part of the sandy deposits. 
The gritty sandstones of Bod Renail, still further W., are 
probably on the same horizon; and the series is represented by 
grey sandstone, succeeded by pale slaty beds, which is exposed 
on the northern slopes of the great mountain barrier which 
divides the Dee Valley from the Vale of Clwyd. 
The basement series disappears under the great rolling mass 
of “ Denbigh Grits and Flags,” which covers North Wales up 
to the Conway Valley. Here we should look for the basement 
beds about the position of Conway Castle, and the Castle is 
built upon a giitty sandstone very similar in character to the 
