THE VALE OF CLWYD. 11 
Fic. 4. 
Diagram Sketch of sequence of Silurian from near Rhewl, by 
Cyfylliog to Careg-y-gath. (No scale.) 
a. Pale-grey massive sandy mudstone, slightly cleaved ; bedding gener- 
ally obscure. : ; 
&. Grey tough beds, a few inches thick, in dark grey shale, which 
weathers into a fine ‘‘rab.”” Some concretionary bands concen- 
trically marked in shades of grey. i 
c. Tough grey thin-bedded wavy-banded concretionary mudstone ; 
encrinites, &c., occur in the heart of concretions. Holes as if due 
to weathering out of nebulipora. 
Dark evenly-striped flags. 
Tough massive banded sandstone and flags. 
Pale mudstones, with somewhat wavy lines. 
Alternations of blue flags, with massive rock in which the bedding is 
not clear. 
Even-bedded striped and banded rock. 
Massive grey rock ; texture fine ; bedding obscure. 
Bluish-grey slabby shale, evenly banded with white ; fossils abundant 
in some beds (see lists p. 8.) 
All minor faults and folds omitted. 
SRS BRR 
It is not a fact that grits prevail in the lower part of the 
series, and flags and shales in the upper—rather the contrary. 
As the Coniston Flags are below the Coniston Grit, so also in 
North Wales flaggy and shaley beds predominate in the lower 
part, and tough grey sandstones in the upper, until we get up 
to the horizon of the Bannisdale Slates. 
It will be seen that there is a greater similarity between the 
series in North Wales and the Lake District than there is 
between that of North and that of South Wales. The principal 
character in which they differ is the occurrence of three im- 
portant limestones in the Southern area, which are altogether 
wanting, or represented only by occasional calcareous bands, 
in the North. A considerable part of the Southern series 
consists of rubbly mudstone, while the Northern beds generally 
run into flags or sandstones. A great difficulty meets us at 
once when we try to compare them palzontologically, and that 
is, that the list of Wenlock and Ludlow fossils from the South 
include the fossils of the limestone also, which of course belong 
to different conditions and do not offer fair data for comparison. 
I am glad to learn that Mr. Maw and the Rev. H. G. Day 
are beginning to work out the various horizons in the shale in 
detail, and hope that important results may be arrived at. 
The facies of the fossils of the Denbigh Grits and Flags is 
more like that of the Ludlow than of the Wenlock, as far as any 
difference exists between those two stages. It is more obvious 
when we are searching for fossils in the field than when we are 
examining classified lists of the species that occur in each, as it 
depends upon the abundance of individuals of certain species 
rather than upon the occurrence of species peculiar to one or 
_ other age. 
