Vol. 56.] SHALES OF THE WELSH BORDERLAND. 373 



My work supplies some further details, and confirms his view 

 that in part the Moel Ferna Slates are approximately on the same 

 horizon as the beds with Cyrtograptus Linnarssoni in the Long 

 Mountain. 



III. The Builth District. 



That part of the Builth district which is occupied by Wenlock 

 Shales, and which I have mapped in detail, includes a roughly 

 triangular area extending from Disserth on the north to Llanafan- 

 fechan on the west and Aberedw Hill on the east. (See Map, fig. 4, 

 p. 385.) 



The conspicuous features in the landscape are due to the igneous 

 rocks associated with the Llandeilos, and the Ludlows also attain 

 a considerable height south-west, south, and south-east of the town, 

 forming Mynydd Aberedw and Mynydd Epynt. 



The Wenlock Shales only give rise to gentle undulations sloping 

 down to the bauks of the Wye and its tributaries, the higher ground 

 being generally occupied by the harder and more flaggy beds of the 

 series. Exposures are less common than in the older beds, and 

 close to the town all are concealed beneath a tract of alluvium. 



The general structure of the rocks in the Builth district seems to 

 have resulted from the superposition of two sets of subsequent earth- 

 movements : these have affected a series of flags and shales, the 

 upper members of which overlap the lower. 



The first set of movements impressed upon the beds formed part 

 of a series of folds whose axes ran north-east and south-west. This 

 set of movements was responsible for the production of the syncline 

 which occupies the northern and north-western parts of the district. 

 The second set of movements belongs to a later period, and the folds 

 have their axes running in a general east-and-west direction. 



It seems probable that one of the anticlinal axes of this later set 

 of movements must have run approximately through Newbridge, 

 judging by the general effect of this folding on the whole area of 

 the Borderland. This would result in the raising of the district on 

 the north and north-west of Builth, to a higher level than the 

 district on the east. Hence this effect, combined with the effect of the 

 previous set of movements and subsequent denudation, has caused 

 beds to be exposed in the north and north-west of the area, which 

 are hidden by overlap to the east. The overlapping is clearly 

 indicated on the west of the town, but is only partial there, while it 

 is complete (as regards the Wenlock Shales) on the east. 



The beds exposed in this area range from Llandeilo to Ludlow, 

 but the Bala and Lower Llandovery rocks do not seem to be present. 

 Hence there is a marked unconformity between the Upper Llandovery 

 and Llandeilo Groups. 



