Yd. 56.] THE LOWER LUDLOW FORMATION. 433 



I think that these flags, which are about 100 feet thick, should be 

 regarded as the basement-bed of the Ludlow formation. 



Above these again comes a thick series of brownish-grey flaggy 

 shales with thin beds of limestone (C). These are well exposed in 

 the bed of the stream, and at C 1 have yielded Monograptus colonus 

 (Barr.), M. bohemicus, and M. Nilssoni. At the Mill Race (C 2 ) the 

 shales are especially rich in the same graptolites. The right bank 

 of the river at this point, and for some distance up stream, rises almost 

 precipitously to a height of 60 or 70 feet ; and similar beds, con- 

 taining the same graptolites, occur all up the cliff, so that they 

 must be of considerable thickness, probably about 500 or 600 feet. 

 At C 3 the shales yield M. colonus and BetioUtes spinosus, sp. nov. 

 in great abundance, the surfaces of the rock being almost hidden 

 from sight by the graptolites ; the specimens, however, are much 

 squeezed. 



Farther up hard flaggy beds (B 2 ) are again exposed at the curve 

 which the river makes at this spot. Still higher up, the hard con- 

 cretionary flags of A and B are again seen in the bed of the river 

 (A 2 , B 3 ), and the graptolitiferous shales (C 4 ) containing M. colonus, 

 M. Nilssoni, BetioUtes spinosus, etc., forms identical with those seen 

 at C 2 and C 3 , are re-entered. They are succeeded by hard flags (D l ) 

 which are much contorted and folded. The dip of the beds varies 

 considerably both in amount and direction in the higher reaches of 

 the river, while their strike bends gradually round until, in the 

 centre of the syncline of Wenlock Shales west of the Carneddau 

 Hills, it runs in a north-north-easterly and south-south-westerly 

 direction. This graptolite-b earing bed is easily recognized in the 

 field, and forms an important zone almost at the base of the Lower 

 Ludlow Shales. I have also recognized it in the country west of 

 Builth, near Glan Irfon, and at Cilmery Station ; and Miss Elles has 

 traced it farther north near Gaufron-isaf, etc., but graptolites are 

 rare at the last-named locality. 



In the Irfon section two distinct horizons in these Lower Ludlow 

 Beds are, therefore, recognizable : — 



(2) Flaggy shales and thin limestones rich in graptolites: M. colonus, 

 M. bohemicus, M. Nilsso?ii, and BetioUtes spinosus. 500 to 600 feet. 

 (1) Hard calcareous flags with M. vulgaris. 100 feet. 



(6) Builth-Llanddewi'r Cwm Road - Section. — An 

 excellent section, confirmatory in every way of the foregoing, is 

 seen along the road from Builth past the Vicarage to Llanddewi'r 

 Cwm. At the base come the hard flags (b 1 ) which have yielded 

 M. vulgaris and M. dubius; the graptolites, however, are badly 

 preserved, and any identifications must be somewhat doubtful. 

 Owing to their hardness, their outcrop forms a scarp, and thus 

 they are easily mapped : they pass up gradually into the shaly 

 graptolitiferous zone (c). The detailed succession of these beds is 

 particularly well seen for some 40 or 50 yards along the Yicarage 

 road, which here joins the main road (c l ). The beds dip at an 

 angle of 20°, and strike 10° to 20° south of east. 



