28 DR. STAMP AND MR. WOOLDRIDGE ON THE [vol. lxxix, 



('"Shales. 



| Ashy limestone. 



T , , , ,, , , i Fairly coarse ashes. 



Hardened Mudstones ...-< , „ J , 



| Hornstone. 



| Shales. 



(^Fairly fine ashes. 



This section is of especial interest, since it exposes the shales 

 between the hard bands of ash and ' hornstone.' In this way it 

 fills in the gaps in the first section. It is to be noted that fine 

 ' hornstone ' partly replaces the coarse beds of the Upper Ashes. 

 The jointing of the rocks is well seen in the bed of the stream. 



(d) Quarry west of Nant-Grwyn Farm (east side 

 of the anticline). 



{Band of coarse ash seen for 3 feet. 

 Fine banded ash, well cleaved 12 feet. 

 Bedded ash, with large fragments 2 feet. 

 Coarse ash seen for 6 feet. 



The puckering (in some cases due to the presence of a little 

 volcanic bomb) in the bedding-planes of the banded ash is inter- 

 esting and well shown. 



(5) Structure and Scenery of the District. 



The structure and the scenery of the Llanwrtyd area are so 

 closely connected that it seems advantageous to consider them to- 

 gether. Speaking broadly, there is an important anticline running 

 from north-north-east to south-south-west in Central Wales, known 

 by the name of the Towjr Anticline. 1 Its axis passes from the 

 neighbourhood of Carmarthen eastwards along the Vale of Towy 

 to near Llandovery ; thence it passes in a more northerly direction 

 through the Sugar Loaf to Llanwrtyd and to the east of Rhayader. 

 It is this anticline which brings up the igneous rocks of Llanwrtyd. 



The igneous rocks have proved more resistant to weathering 

 than the overlying slates, and hence occupy an elevated ridge rising 

 to over 1600 feet. The surface of this ridge, especially in the 

 southern part, is roughly the dip-slope of the Upper Ashes. The 

 covering of the Upper Ashes has, however, been pierced by the 

 agents of denudation, and slightly older beds are exposed in places. 

 In some cases, as mentioned above, isolated remnants of the Upper 

 Ashes have been left, and present a veiy curious appearance. 

 Murchison says that 



' the upper parts and summits of these hills exhibit numberless rugged and 

 irregular bosses of trap, sometimes carrying up fragments of altered or 

 indurated schist ' (' The Silurian System' vol. i, 1839, p. 344.) 



The core of the northern part of the range (known as Car Cwm) 

 consists of the spilites. 



1 O. T. Jones, ' The Geological Structure of Central Wales & the Adjoining 

 Begions' Q.J. G.S. vol. lxviii (1912) p. 328. 



