10 NOTES ON THE GEOLOGY OF 
Silurian rocks of Caer Drewyn and other places in the Dee 
valley near Corwen. 
Thus we get up into the main mass of the Denbigh Grits, 
Sandstones, and Flags, a series many thousand feet thick. These 
are the ‘‘blue rock” which occurs along the Clwydian range, and 
behind the Mountain Limestone from Llandulas to Derwen 
and from Derwen to Llandegla; in fact it occurs at the back 
of the Limestone all round the Vale of Clwyd. The Geological 
Survey has not yet attempted to subdivide these rocks further 
than to indicate, by yellow dots, the more sandy portions; but 
I think that with experience of the Lake District, and of 
Scotland, much may now be done towards subdividing and 
correlating these rocks with those of other areas. The sand- 
stones are not constant either as to horizon or thickness, but 
they are apt to occur at about the same part of the series. 
The paleontology also has not been worked at over a 
sufficiently large area, or in sufficient detail, to allow us yet to 
generalise much upon it. I have given a sketch of the 
sequence and a few short lists of fossils from these beds in 
the paper before cited. 
The Silurian of the Clwydian range does not offer such good 
opportunities of establishing a sequence as that on the west 
of the valley, because it is cut off above and below by faults 
and unconformities, so that there is no well-marked horizon to 
start from; but if the blue mudstones with subordinate striped 
and banded flaggy beds, such as those in the Rhiallt quarry, 
and the grits and sandstones of Moel Arthur, &c., were carefully 
traced and mapped, and the fossil zones, such as that of Tirgwyn, 
followed up, we might hope to do something in the way of 
correlation. 
The following sketches may suggest lines along which the 
sequence might be worked out :— 
In asmall quarry by the road side in Lady Bagot’s drive, about 
100 yards N. of where the g. of Pantglas is engraved on the 
ordnance map, and about 300 yards N. of the farm Yr Hengoed, 
near Rhewl, there are blue flags, with straight or wavy white and 
grey bands and lines, in beds from z to 6 inches thick. There 
are some concretionary bands with stromatopora-like structure, 
In a quarry about 200 yards below the meeting of the streams 
near Gelli there is a similar banded rock with small concretions, 
worm-tracks, and a few fossils, such as Orthoceras primevum, 
Monograptus, too ill-preserved to make out species, but most like 
AL. Colonus. 
About roo yards still further down the stream the bedding is 
less obvious, but the texture of the rock is much the same. 
The sequence, as seen between this and the highest beds seen 
on Careg-y-gath, is shown in the following diagram sketch, 
fig. 4. 
