Vol. 56.] FORMATION AND ITS GRAPTOLITE-FAUN A . 429 



These zones are not all equally rich in graptolites, for nearly the 

 whole of the abundant graptolitic fauna of the Lower Ludlow 

 occurs in the zones of Monograptus Nilssoni and M . scanicus, while 

 the two highest zones — those of M. tumescens and M. leintwardinensis 

 — each contain practically only one species. Again, the two lower 

 zones yield numerous graptolites throughout their whole extent, 

 while in the two upper zones graptolites are for the most part 

 rare, occurring abundantly only in certain bands. 



The zones of M. Nilssoni and M . scanicus are perhaps less clearly 

 denned than the higher zones, and it is a matter of some difficulty to 

 decide which are the best graptolites to select for the zonal forms. 

 The three most characteristic graptolites of these beds are un- 

 doubtedly M . bohemicus, M. Nilssoni, and M . scanicus. M. bohemicus 

 is very abundant in the lowest beds ; owing, however, to the long 

 range of this graptolite, it is not advisable to select it for the zonal 

 form. M. Nilssoni and M . scanicus occur in association ; but, as was 

 well seen in the Elton-Lane section, M. Nilssoni is more abundant 

 in the lower beds, and M. scanicus more characteristic of the upper. 

 It is convenient, therefore, to group the lower 350 feet of Lower 

 Ludlow Shales, so rich in graptolites, into two zones — a lower zone, 

 that of M. Nilssoni, containing M. bohemicus, M. colonus var. com- 

 f actus, M. uncinatus var. micropoma, M. varians, M. dubius, etc. ; 

 and an upper zone, that of M. scanicus, with M. Rcemeri and 

 M. chimwva as the characteristic graptolites. 



The zones of M. tumescens and M. leintwardinensis are well 

 marked, both lithologically and palseontologically. M. leintwardin- 

 ensis clearly ranges up into the Aymestry Limestone, above which 

 no graptolites are known, and therefore the Aymestry Limestone 

 should be included in the zone of M. leintwardinensis. 



(B) The Builth District. 

 (1) Physical Features and Structure. 



The Builth district, as examined by me, includes the range of 

 the Aberedw Hills from Ehiw Ehwstyn on the north to Aberedw 

 on the south, and also the ;area lying immediately south and 

 south-west of the town of Builth. The Carneddau Hills, which 

 form part of a buried mountain-range composed of Ordovician 

 rocks, rise up as an anticline, the axis of which runs north-east 

 and south-west, and occupies the central part of the area. Resting 

 unconformably upon them is a thin band of Llandovery Grit, and 

 above this come the various zones of the Wenlock Shales. These 

 are folded into a broad syncline on the west of the Carneddau Hills, 

 in the centre of which occur the lowest members of the Ludlow 

 Shales. The Wenlock Shales pass up conformably into the Lower 

 Ludlow without any marked lithological change. 



The Lower Ludlow Beds occupy the rising ground between the 

 valley of the Wye and the high hills of Mynydd Eppynt on the 

 south and Aberedw on the east, which attain an elevation of 



Q.J.G.S. No. 222. 2> 



