Vol. 56.] FOKMATION AKD ITS GKAPTOLITE-FATJNA. 431 



fairly thick-bedded. The majority of the rocks are of the nature 

 of mudstones or mudstone-flags. The typical mudstones usually 

 weather to a rather light brown, while some of the flaggier and 

 more calcareous beds retain their original dark-grey coloration. 

 These rocks frequently show concretionary structure, some of the con 

 cretions measuring several feet in diameter. Typical flags breaking 

 with a splintery fracture are less common, and are found mainly 

 in the highest beds of the formation. All the rocks contain flakes 

 of mica more or less abundantly. At certain horizons thin bands 

 or nodules of limestone occur in the shales ; these generally 

 weather more deeply than the strata with which they are inter 

 bedded, and thus their position may be readily detected in a section. 



The rocks are for the most part richly fossiliferous. Grapto- 

 lites, however, are practically confined to the shales, limestones, 

 and micaceous flags, while the mudstones yield abundant brachiopods, 

 trilobites, corals, etc. Unfortunately all the beds have been subjected 

 to considerable earth-movement, and in consequence the fossils, 

 especially the graptolites, have been deformed. 



The local development of the Lower Ludlow rocks in this area is 

 of peculiar interest, for not only do the rocks combine the lithological 

 characters of the Ludlow sub-calcareous facies already described 

 with those of the purely mudstone-facies of the areas farther north 

 to be noticed later, but the graptolitic fauna shows a similar 

 intermediate character. The lower limits of the Lower 

 Ludlow formation can be suggested on palaeontolo- 

 gical grounds alone, while, on the other hand, the 

 most natural upper boundary may be best fixed by 

 lithological considerations. The Builth district is ex- 

 ceptionally rich in exposures, and therefore sections can be traced 

 right through the formation from base to summit. 



(3) Area lying South of the Wye. 



(a) River -Irfon Section. (See map, fig. 5, p. 432.) — A 

 complete section is exposed along the banks of the Irfon, from the 

 upper zones of the Wenlock Shales into the Lower Ludlow Shales, 

 and the beds are rich in graptolites throughout. 



Starting from Irfon Bridge, the first beds (A 1 ) which are 

 exposed along the right bank consist of black shales and harder beds 

 with large limestone-nodules, dipping at an angle of about 35°. 

 They have yielded Monograptus Flemingii var. h and M. dubius. 

 These must undoubtedly be grouped with the Wenlock Shales, and 

 form the highest zone of that formation. 



They are succeeded by a band of dark, thickly-bedded, hard cal- 

 careous flags (B 1 ), breaking with irregular fracture and forming a 

 conspicuous feature, which projects into the stream and narrows its 

 bed considerably. Graptolites are rare in the flags, and even when 

 they do occur they are distorted almost beyond recognition. 

 M. vulgaris probably occurs, -and from analogy with other exposures 



2g2 



