696 PROF. T. MSKENNY HUGHES ON THE SILURIAN 
sandstone a few inches thick in dark grey shale. Some concretionary 
masses are marked by bands of colour due to infiltration from joint- 
surfaces like the Whetstones of Moughton in Yorkshire, but they are 
not red in the Ffriddfawr rock. The shale weathers into a fine rab. 
Below these are tough, grey, thin-bedded, wavy, banded, concretionary 
mudstones, full of holes such as are sometimes left by the decom- 
posing out of fossils like Mebulipora. Encrinites occur in the concre- 
tions. There is nothing very distinctive about these beds; but they 
agree very well in all their characters with the base of the Coniston 
Grit. We find now a clearer section; and after crossing near 
Cyffylliog alternations of paler and darker banded sandy mudstone 
and grey sandstones, agreeing very well in character with the passage- 
beds from Coniston Grit to Coniston Flags, we come, near Pontuchaf, 
to fossiliferous flags with subordinate thicker and more sandy beds. 
These can be seen in descending section along the sides of the 
valley, and in the bed of the stream to near Berth, where they are 
cut off by a fault which brings the basement beds of the Carboniferous 
against them on the E. 
Measuring from the base of the grits near Cyffylliog, there must 
be about 4000 feet of beds to be referred to this group. — Fossils - 
occur at the various horizons indicated on the section. 
In the cutting N. of Hengoed I observed the following :— 
Encrinite stems. Rhynchonella nucula, Sow. 
Monograptus, sp. | , Sp. 
Atrypa reticularis, Linn. | Cardiola interrupta, Brod. 
_ Rbynchonella navicula, Sow. | Orthoceras primzvum, Forbes. 
At Pontuchaf the following occur :— 
Monticulipora,probably the “Cushion- | Orthis, sp. (var. of O. elegantula). 
like coral” of Salter. Rhynchonella navicula, Sow. 
Encrinite stems. —— nucula, Sow. 
Atrypa reticularis, Linn. , Sp. 
Chonetes lata, Von Buch. Strophomena depressa, Dali. 
Orthis elegantula, Dalm. Orthoceras primxvum, Lorbes. 
From the valley of the Clywedog (see fig. 1) the flags can be traced 
S.W. to the upper waters of the Clwyd in a generally descending 
section as they turn up on the S., till on Garwfynydd we come to 
tough grey gritty sandstones cropping out from below the flags, and 
forming a marked feature across the country. Below these grits, 
which are probably on the horizon of the Penyglog Grits of Corwen, 
and the Austwick Grits of Yorkshire, there are more flags, passing 
down into pale slates, which rest on grey gritty sandstones with 
wavy bands in parts. These last appear to be the equivalents of the 
Corwen Grit*. They are well seen about 4 mile S. of Bod Renail, 
where they are the cause of a series of small waterfalls. 
After some curves, as if in a struggle to run into the Dee by 
Bettws y Gwerfilgoch, the Clwyd is turned back and, making an 
angle of about 70° with its former course, crosses the same series of 
beds again, and running over the Corwen Grits under the old camp 
* Quart. Journ. Geol. Soc. vol. xxxiil. p. 207. 
