49S 



AIM. W. B. M. KING OS THE 



[vol. lxxix, 



The next section of interest is that at the head of the first 

 tributary of the Marehnant stream coming from the south off 

 Pen-y-bylchau, which joins the main stream near point 1117 on 

 the bridle-path (1-inch Ordnance Survey map). 



The tributary stream rises in some boggy ground, and cuts a 

 small gorge through the upper slopes of the valle3 r -side : here are 

 exposed several hundred feet of strata, the lower beds being exactly 

 similar to those described above and yielding Calymene quadrata 

 in profusion, with the usual assemblage of fossils. In this section, 

 however, these beds are followed by some 45 feet of very tough, 

 blue-grey, blocky mudstones almost free from sandy material, and 

 weathering with orange-coloured rusty joints. The}' have yielded 

 a small, but excellently preserved fauna, the commonest forms 

 being the following (loc. 06, fig. 4, p. 499) : — 



Trinucleus bucklandi Barrande 



(common). 

 Stygina latifrons (Portlock). 

 Cybele loveni Linne. 



Illasnus. 



Bemopleurides (?). 

 Plectambonites cf. qulnquecostata 

 (M'Coy). 



Of these the Trinucleus, Stygina, and Illcenus are represented by 

 complete individuals. 



Ordovician-Silurian Boundary. 



Above the mudstones with Trinucleus just described there is 

 a sudden and complete change of lithology to a massive brown 

 sandstone some 15 feet thick. When fresh, this sandstone is 

 somewhat calcareous and blue-grey in colour ; but, when it is 

 weathered, certain beds are seen to be crowded with brachiopods 

 and crinoid-stems. 



The exact horizon of this sandstone is open to doubt, but it 

 obviously indicates some change in the relationship between land 

 and sea. On cartographical evidence there would appear to be a 

 slight unconformity at the base of the sandstone, for it is signi- 

 ficant that there are 45 feet of blocky mudstones with Trinucleus 

 between the sandstone and the Calymene-quadrata Beds at the 

 c 6 locality (see fig. 4, p. 499) ; and apparently the sandstone rests 

 directly upon the Calymene-quadrata Beds at the Ordnance 

 bench-mark 978*5 in the Blaen-y-cwm valley (32, fig. 3, p. 496), 

 about half a mile distant. Both faunistically as well as litholo- 

 gically, the break is complete. By far the commonest fossil is 

 Jferistina crassa (Sowerby), but associated with it are : — 



Platystropliia biforata A'ar. fissi- 



costata (M'Coy). 

 Myelodactylus. 



OrtMs sagittifera M'Coy. 

 Atrypa marginalis Dalman. 

 Leptsena sp. 

 Strophomena sp. 



The sandstone may be traced almost continuously in a nearly 

 straight line, from above the Vyrnwy near Olan-y-rhyd to beyond 

 the Blaen-y-cwm valley, a distance of 3 miles. It is seen at iihos- 

 fawr, 3 miles west-north-west of Llanfyllin. 



