444 MISS E. M. R. WOOD ON THE LOWER LUDLOW [May 1 900 y 



in this area, three at any rate of those recognizable near Builth 

 are well developed : that is, the zones of Monograjptus vulgaris, 

 M. Mlssoni, and M. leintwardinensis. It may be found convenient 

 in any future mapping of the area to divide the second of these 

 into two sub-zones : (a) sub-zone of M. Nilssoni and (6) sub-zone 

 of M. Rosmeri ; but the evidence for this division is at present not 

 quite conclusive. The succession is, broadly speaking, as follows : — 



(3) Light-coloured flaggy inudstones. Zone of Monograjptus leintwardi- 

 nensis var. incijpiens nov. 900 feet. 



(2) Light-coloured thinly-bedded shales. 1 



(b) Zone of Monograjptus Boemeri. I 350 to 400 feet. 

 («) Zone of Monograjptus Nilssoni. J 



(1) Massive flaggy shales. Zone of Monograjptus vulgaris. 600 to 700 feet. 



In the typical Long-Mountain section from Middletown to Upper 

 Trefnant, however, beds containing M. varians, M. Nilssoni, and 

 Retiolites sjoinosns occur above the M.-Rcemeri beds and a short 

 distance below the M.-incipiens zone. The presence of these species 

 at so high an horizon is peculiar, and if further research should 

 verify it, then the sub-zone of M. Rosmeri can no longer be distin- 

 guished. It is possible that the beds are here folded or faulted (as 

 they certainly are locally in other parts of the district), and that the 

 visible succession is consequently not the true one, for I have failed 

 to find these species elsewhere in the district, except at what seems 

 to be a much lower horizon. 



The palaeontological boundary between the Wenlock and Ludlow 

 formations is marked here, as at Builth, by a change in the litho- 

 logical characters of the beds, and therefore its exact position in the 

 field can be mapped with considerable accuracy. The upper limit 

 of the Lower Ludlow Beds, on the other hand, cannot be determined 

 on palaeontological evidence alone, as the graptolites appear to die 

 out very gradually. But there is a lithological change near the 

 horizon where the graptolites disappear, which coincides fairly well 

 with the boundary drawn by the officers of the Geological Survey 

 between their local Wenlock and Ludlow formations previous to the 

 working out of the graptolites ; and this line may be looked upon very 

 naturally as the boundary between the Lower and Upper Ludlow. 



(3) Southern Area of the Long Mountain . 



The evidence obtained from an examination of the Lower Ludlow 

 graptolites of the southern side of the Long Mountain is by no 

 means so satisfactory as that derived from the study of those of 

 the northern side. The rocks in the southern area on the whole 

 are coarser and more arenaceous, and the graptolites are rarer and 

 more indifferently preserved. So far, however, as I have examined 

 the sections in this southern area, the graptolitic succession agrees 

 well with that in the northern part of the district. 



(a) Northern Sections. — In this sub-area the lowest horizons 

 of the Ludlow, namely those of the M. -vulgaris zone, are 

 well developed in the neighbourhood of Aston Rogers and Aston 



