Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT EEWTD. 481 



points. In some places the fossiliferous beds are underlain by a 

 few feet of dark-blue, flaggy, pyritous mudstones, with bands of 

 grit 3 to 6 inches thick. 



The Eisteddfa Beds next appear in the small stream called ]N T ant 

 Fuches-gau, which flows northwards past the farm of the same name, 

 to join the Castell river about half a mile west of the Hotel. The 

 Bryn-glas Mudstones are exposed in the stream as well as in Kant 

 Fuches-wen, which joins the former just where it passes under the 

 Devil's-Bridge road, and also in a road-cutting near by. They 

 present their usual characters of blue-black, apparently crushed 

 mudstones, which break up into fragments with curved shiuy 

 surfaces ; and they occur at intervals for about 60 yards up Kant 

 Fuches-gau. At this point is a quartzitic band 2 or 3 feet thick, 

 which is followed for about 20 yards by sandy micaceous mud- 

 stones with some pyrite. These mudstones, in the lower part, 

 have the crushed appearance characteristic of the Bryn-glas Group ; 

 but, in ascending the section towards the south, they are seen to 

 become less sandy and micaceous, and gradually to assume the 

 smooth flaggy characters of the rocks of the overlying group. ]S T o 

 fossils were obtained in this part of the section ; but a few yards 

 farther south, a band of highly pyritous shale crowded with grapto- 

 lites was found at a right-angled bend in the stream, below the 

 ford leading to the farmyard (see fig. 5, p. 480). The following 

 species have been identified from this locality (F. 7) : — Meso- 

 graptus modestuS and its variety parvulus, and Glyptogr aphis per- 

 sculptus, the two last-named being abundant and in beautiful 

 preservation. The strike of the beds is nearly parallel to the 

 stream at this point, and the dip is about 30° in a westerly 

 direction. 



Immediately beyond the first fence (F. 8), south of the ford, hard 

 blue flags with darker pyritous bands yielded a large number of 

 the following graptolites, beautifully preserved in full relief in 

 pyrite : — Climacograptiis scalaris vars. normalis & miserabilis, 

 Qlyptographis persculptus, and -Mesograptus modestus (?). 



These beds are probably about 20 feet above those of F. 7. 



About 40 yards up stream similar, though somewhat higher, 

 rocks are exposed in the sides of a small waterfall ; and near the 

 gate which leads to the moorland on the east of the stream 

 another small fall occurs, over still higher beds. In ascending 

 the section the proportion of shales to flags gradually increases, 

 and a pronounced red and yellow weathering of the strata sets 

 in, which is hardly noticeable in the lower beds. About 40 yards 

 south of the gate, the stream is crossed by another fence, beyond 

 which is an exposure of flaggy shales with thin flags and white 

 siliceous stripes. They contain pyrite in the form of minute globules, 

 which on weathering out leave a multitude of small hemispherical 

 cavities on the flat surfaces of the beds. These shales are of a 

 peculiar granular texture, and break rather like cardboard. 



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