666 MISS E. M. R. WOOD ON THE [Nov. I906, 



60 feet is exposed before the section is interrupted. Between this 

 point and Dolgoch the beds are only shown occasionally, and are 

 bent into a series of gentle folds. In some cases the axes of these 

 folds are well exhibited, the most striking perhaps being that on the 

 west side of the railway-line slightly north of Dolgoch, which figures 

 in all the local guide-books as the ' Natural Arch.' It is even more 

 clearly seen along the road on the east side of the railway, and 

 consists of fairly-thick grits with alternating shales. Beyond the 

 * Natural Arch ' there are evidences of other smaller undulations 

 along the roadside, until eventually the beds once more resume their 

 normal north-westerly direction, and the Talerddig Grits pass up into 

 the Dolgau Mudstones. The passage between these is best shown 

 in the bed of the Afon Iaen. This river is confined within an 

 artificial conduit for a distance of 145 yards near the ' Natural 

 Arch ' ; but, from the point where it emerges, a continuous section 

 can be studied for a long distance down its valley-course. The 

 first beds are the characteristic shales and greywackes of the 

 highest shale-band of the Talerddig Group. They were examined 

 carefully for graptolites, but without much success, for these do not 

 appear to be confined to such definite layers as usual. Monograptus 

 priodon, M. subconicus, M. nudus, and M. griestonensis were found 

 at a point about 50 yards from the commencement of the section. 

 The thickness of this shale-band (e), of which only a part is 

 exposed in the railway- cutting, must here be about 250 feet ; while 

 that of the whole Talerddig Group cannot be less than 700 feet, and 

 the true base is not visible. 



Dolgau Beds (Bd) : Lowe.r Green Mudstones (Bd^. — As 

 we descend the stream, the lithological character of the beds is seen 

 to undergo a change. The rocks become lighter in colour, less arena- 

 ceous, and more of the nature of mudstones, and they lack the 

 brilliant weather-colouring of the Talerddig Shales. They resemble 

 the lowest Dolgau Mudstones seen in the Tarannon section, and like 

 those have black shale-bands at their base containing graptolites. 

 At this locality these soft black shales are so slickensided and 

 disturbed (owing probably to a small fault) that the fossils are 

 almost unrecognizable ; but from one of the lowest bands Mono- 

 graptus priodon, M. subconicus, and M. cf. dextrorsus were identified. 

 The highest black band is seen on the right bank, about 17 yards 

 north of the hedge which comes down to the river. It is barely an 

 inch thick, but has yielded a fairly-rich assemblage of species, 

 including : — 



Eetioliies Geinitzianus (f. C). Monograptus nudus (f. C). 



Monograptus subconicus (C). Monograptus Marri (f. C). 



Monograptus griestonensis (f. C). Monograptus discus (K). 



Monograptus priodon. Monograptus crenulatus (?) . 



Comparing this fauna with that of the black bands at the base of 

 the Dolgau Group in the Tarannon section, we find a fairly-close 

 resemblance between the two, the main difference being that the 

 zone-fossil, M. crenulatus, is here doubtfully represented. At other 



