478 MR. 0. T. J(XN T ES OX THE HARTFELL-VALENTIAN [Nov. I909, 



the effects of the weather. The centre of the syncline is, therefore, 

 occupied by relatively low, ill-drained ground with a smooth 

 undulating peat-covered surface, whereas the underlying beds 

 stand out as a prominent rocky ridge, forming a rim to the trough 

 of softer rocks. It may be remarked here that this relation of the 

 form of the ground to lithological character has been observed 

 along this junction throughout the district, the passage from the 

 underlying barren mudstones to the graptolitiferous red-stained 

 beds being almost everywhere attended by a sharp change in the 

 appearance of the ground. AVhen this fact is appreciated, the 

 details of the geological structure can be picked out with ease from 

 some point of vantage. It is a little unfortunate that this line does 

 not quite coincide with the base of the Pont Erwyd Stage, which 

 lies about 100 feet down in the underlying mudstones. 



If now we take a traverse towards the south-east from the axis 

 of the anticline in Afon Tarenig (fig. 3, p. 476) we pass over the beds 

 with Diplograptidse once more ; they are well-exposed in the road- 

 cutting near the watershed, where they yielded Climacoyraptus 

 scalaris var. normalis in abundance. The immediately succeeding 

 strata are not exposed, but on ascending the steep hillside south of 

 Eisteddfa Gurig the rocks are seen to crop out in numerous places, 

 though they are highly-cleaved and fossils are difficult to extract 

 from them. The lowest beds on the slope are flags and shales, 

 with several fine-grained grits up to 8 inches thick; they are 

 followed by a group of blue-hearted shales, with occasional thin 

 flags of a decidedly paler colour. The shales weather deeply to 

 an ashen grey, and their surfaces are stained to various shades of 

 rusty brown ; no fossils were obtained in the lower part, but near 

 the top they yielded a few specimens of Monograptus atavus, sp. nov., 

 and M. rheidolemis, sp. nov. The strata are then concealed for a 

 short space, a slight depression in the hillside suggesting a belt of 

 soft rocks. On resuming the section beyond the depression, one is 

 immediately struck by a great change in the lithological character 

 of the strata. The soft dark-blue shales have disappeared, and 

 their place is taken by very pale greenish mudstones presenting 

 no characters in common with the underlying rocks. They are 

 extremely tough and splintery, and give rise to a precipitous slope 

 with bold crags. It is evident that one is dealing with a new 

 group of rocks wholly distinct in their lithology from those which 

 they succeed, and I propose, therefore, to leave them until the lower 

 group has been more fully described. 



"We learn from this section that the flaggy beds with Diplo- 

 graptidse are followed by more shaly strata, in which occur, for the 

 first time in the ascending sequence, representatives of the genus 

 Monograptus, and that these are succeeded by hard pale mudstones. 

 The group at the base of the Pont Erwyd Stage, characterized by 

 Diplograptidse only, may be named the Eisteddfa Group — a 

 name first given to it by Prof. Charles Lapworth in the appendix to 

 Walter Keeping's paper. 



