670 MISS E. M. E. WOOD ON THE [Nov. 1906,. 



marked black bands, all of which yield graptolites in relief. They 

 include : — 



Monograptus priodon (0). 

 Monograptus crenulatus (f. C). 

 Monograptus subconicus (0). , 



Monograptus nudus (R). 

 Monograptus discus (R). 

 Eetiolites Geinitzianus (f. C). 



This fauna agrees closely with that from the corresponding beds 

 in the Tarannon section and elsewhere. 



Talerddig Group (Be). — The Dolgau Beds pass down with 

 complete conformity into the upper members of the Talerddig Group, 

 which possess .their usual characters. The river, however, for the 

 next mile or so runs through an alluvial flat, and only occasional 

 rock-exposures are met with. Prom these, nevertheless, it is clear 

 that the country for the next half-mile, at any rate, is occupied by 

 the Talerddig Grits bent into gentle folds, similar to those observed 

 in the south-eastern part of this section. 



Gelli Group (Bb). — At a point due south of Llwyn-fTynon 

 there is an exposure of bluish-black mudstones, shales, and small 

 flags which are much disturbed. Their lithological characters 

 suggest that they belong to a lower horizon than that of the 

 Talerddig Group, and some yards lower down the river, just below 

 Pen-y-geulan, the following characteristic Gelli graptolites were 

 obtained: — 



Monograptus crispus (C). 

 Monograptus discus (f. 0). 

 Monograptus Marri. 



Monograptus priodon. 

 Monograptus nudus (?). 



The beds here, and for the remainder of the section, consist of 

 thickly-bedded, deep, bluish-black mudstones weathering into 

 spheroids, separated by very thin sandy shale-partings from which 

 the graptolites were obtained. In this western area, therefore, the 

 Gelli Beds present lithological characters very different from those 

 exhibited in the Tarannon Eiver, but the faunas at the two 

 localities are identical. 



Characteristic Gelli graptolites were also obtained from two 

 bands in the Bhiw-saeson, just below Wynnstay Bridge, 400 yards 

 from the junction of that stream and the Iaen; and the Gelli 

 Beds are probably continued westward as far as Tafolwern, where 

 they are cut off abruptly against another line of disturbance, 

 marked by a large vein of quartz and much crumpling of the strata. 

 jSTo fossils could be detected in the beds lying immediately to the 

 westward and the examination of the section was not continued j, 

 but, from evidence obtained elsewhere, it is fairly certain that 

 they belong to the Brynmair Group. 



Summing up the result of our examination of the Afon-Iaen section,, 

 we find that, in this northern area, it is possible to recognize the same 

 groups of strata, arranged in the same order, as that worked out 



