74 MR. H. LAPWORTH ON THE [Feb. I9OO, 



the limits of the .Rhayader District occur representatives of the 

 Lower Llandovery, Upper Llandovery, and Tarannon 

 formations, capable of being divided into petrological and palaeonto- 

 logical zones. The sequence of these zones is demonstrable by 

 stratigraphical evidence, and they may be correlated, by means of 

 fossils, with their equivalents elsewhere. 



II. Stratigraphical Relations ojf the Rocks of the 

 Rhayader District. 



(A) The G-wastaden Group. 



In describing the rock-sequence of an area it is advisable to 

 establish some typical section to which the reader may turn at any 

 time. In the present case, the rocks flanking the northern side of 

 Gwastaden and forming the floor of the Rhayader Valley are best 

 exposed, curiously enough, along a line that a geologist would 

 naturally select, on first attempting to unravel the sequence. This 

 line may be fixed as running from the summit of Gwastaden to 

 the town of Rhayader itself. Taking its st?rting-point at about 

 \ mile north-east of the cairn and following a small stream at 

 the eastern end of Gigrin Prysg, which may, for the sake of 

 convenience, be termed the Prysg stream, it reaches the Wye at 

 about ^ mile south of the workhouse. From this point the section 

 follows the river upstream for about j mile, and terminates in 

 the centre of the town itself. The rocks occupying the greater 

 part of this section form a continuous ascending series, which 

 I have named after Gwastaden Hill (pron. ' Gwastedden '), along 

 whose slopes their characters may best be studied. By a curious 

 coincidence this Gwastaden Group is not only shown to its 

 greatest advantage along this line of section, but the order of the 

 beds is practically undisturbed by folding or faulting. Moreover, 

 as will be shown subsequently, the Gwastaden Group here attains 

 its maximum development. 



(a) Description of the Sections in the Rhayader Valley. 



(1) Typical Section through Gigrin Prysg and the Wye. 



The ridge of Gwastaden is formed by a series of thickly-bedded 

 grits, which run along the hill from end to end. This ridge is the 

 most prominent feature of the hill itself, and the grit-beds are well 

 exposed in one continuous line for a length exceeding 2 miles along 

 the line of strike. We have thus a convenient datum or base-line 

 for the succeeding three or four sections. At the south-western 

 end of the hill, where these beds sweep down into the Wye, they 

 form a long precipitous crag or cliff over 200 feet high, known 

 as Cerig Gwynion, and it is here perhaps that the finest exposure 

 of these grits is exhibited. The name Cerig Gwynion Grits, 

 then, naturally suggests itself as the most suitable term for this 

 group. 



