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Vol. 66.] GEOLOGY OF THE DISTEICT AROUND LLANSAWEL. 417 



rocks and predominance of shales, mudstones, and grits ; but 

 striking differences exist, and the Llansawel area seems to hold 

 an intermediate position between the other two. 



The Shon Nicholas Conglomerate and Pen-y-ddinas Grit which 

 form so well marked a base to our Birkhill Series are absent 

 altogether round Pont Erwyd, but are represented by the Cerig 

 Gwynion Grit of Ehayader. The nature of this rock agrees very 

 closely with that of our Pen-y-ddinas Grit, for it is described as 



' a very dense, tough, hard quartzose grit or grauwacke. The colour varies 

 from bluish-grey to greenish -grey.' x 



In this respect, therefore, the district here described more closely 

 resembles Rhayader. But there appears to be around Llansawel, 

 as in Pont Erwyd, a complete passage from Lower to Upper Birkhill 

 rocks, and the great Caban Conglomerate is entirely absent. Also, 

 as far as we can see, there is no overlap at the base of the Gala 

 rocks in Llansawel, but a complete passage up from Birkhill to Gala 

 Shales, as in the Pont Erwyd district. 



With regard to the detailed stratigraphy, there are many very 

 close resemblances in lithology between the Llansawel district, and 

 sometimes one, sometimes the other, of the Central Welsh areas 

 under discussion. The Beili Tew Shales seem to agree very closely 

 with the mudstones of the Drosgol Series, with the 



' curious gnarled and knotted surface on weathering, which is connected with 

 their internal structure, for a fresh fracture shows numerous dark laminee 

 twisted and contorted in a remarkable manner.' 2 



The characteristic grits in our cyphus zone, which cause a rise 

 in ground wherever they occur, may be represented in the Rhayader 

 district by the ' ferruginous sandy and green gritty beds ' 3 ; but in 

 Pont Erwyd they seem to be entirely absent, and there the pro- 

 minent feature is everywhere made by the pale mudstones of the 

 convoluius zone, which are never conspicuous around Llansawel. 



Lastly, the closest resemblance of all is seen between the upper 

 part of the Pengelli Series and the higher part of the Rhayader 

 Pale Shales, which are described as soft pale-grey shales, inter- 

 banded with grits, and weathering to a brilliant orange colour. 4 

 These shales are quite free from cleavage, and the graptolites are 

 well preserved, whereas the Lower Pale Shales were highly cleaved. 

 The resemblance is rendered even more complete by the fact that 

 in the Rhayader district, as around Llansawel, folding and inversion 

 sets in at this Upper Gala horizon. With regard to the faunal 

 characteristics, it is quite impossible to institute a comparison. 

 Only in the Llansawel Group is there a really clear stratigraphical 

 succession, and here the characteristic graptolites occur in the same 

 order as that in which they are found in other districts. For the 



1 H. Lapworth, Quart. Journ. Geol. Soc. vol. lvi (1900) p. 95. 



2 O. T. Jones, ibid. vol. lxv (1909) p. 4(>9. 



3 H. Lapworth, op. supra cit. p. 78. 



4 Ibid. p. 123. 



