440 MISS E. M. R. WOOD ON THE LOWER LUDLOW [May I9OO, 



lying south and south-west of Builth shows that this phenomenon 

 is well marked in the succeeding beds. Each formation in turn 

 overlaps on to the one beneath it, so that at Llangadock the Upper 

 Wenlock (? Lower Ludlow) Beds rest on the Llandovery, the former 

 are overlapped by the Ludlow Beds, which in their turn are almost 

 concealed by the Old Red Sandstone. Still farther south the 

 Wenlock and Ludlow Beds are completely overlapped, and the Old 

 Red Sandstone rests upon the Ordovician rocks. Similar con- 

 ditions, too, prevailed in the Llandovery period, as Mr. H. Lapworth 

 has clearly shown in the Rhayader district. It would seem, then, not 

 unlikely that what happens in the case of the formation as a whole 

 will be found to occur in the various minor subdivisions of that 

 formation ; and I think that this has probably taken place here, 

 thus accounting for the absence of the Monograptus-scanicus zone 

 in the southern and south-western portions of the Builth district. 



(6) General Summary. 



Summing up, we see that in the Builth district there are four main 

 graptolite-zones, the succession being as follows, in descending 

 order : — 



(4) Zone of Monograptus leintwardinensis, including the probable repre- 

 sentatives of the Aymestry Limestone. 

 (3) Zone of Monograptus scanicus. 

 (2) Zone of Monograptus Nilssoni. 

 (1) Zone of Monograptus vulgaris. 



(C) The Long Mountain District. (Map, PI. XXYI.) 



(1) Structure and General Features. 



The Long Mountain lies directly east of Welshpool, and is formed 

 of a broad syncline of Silurian rocks, ranging from the middle of the 

 Wenlock Shales, through the Lower and Upper Ludlow, to the base 

 of the Old Red Sandstone, a few of the lower beds of which form a 

 shallow outlier on its summit. The area is well defined on all sides : 

 on the north by the Breidden Hills ; on the west by the valley of the 

 Severn ; and on the south by the Ordovician ground of the Shelve 

 district. On the east the Lower Palaeozoic rocks are unconformably 

 overlain by the Carboniferous and Permian formations. 



On the Geological Survey maps, a considerable thickness of 

 Wenlock Beds is represented occupying the lower slopes all round 

 the district, while the Ludlows are confined to the high ground 

 near the centre, the small patch of Old Red Sandstone occurring 

 towards the north-eastern extremity. It has long been known, 

 however, since the researches of Prof. Watts, that much of the 

 area mapped as Wenlock Shales belongs rather to the Lower Ludlow 

 formation, but the exact boundary between the two has not hitherto 

 been definitely fixed. My own work in this district has confirmed 

 and extended his research, and his large collection of graptolites, so 

 kindly placed at my disposal, and supplemented by my own, together 



