468 MR. 0. T. JONES ON THE HARTFELL-VALENTIAN [Xov. I909, 



YSTWTTH 



Stage. 



Pont 

 Erwyd 

 Stage. 



Pltnlimc 

 Stage. 



1 1 to 

 to 



■{ (5) 



(«) 

 («) 



(6) 

 I («) 



Castell 

 Group 



Rheidol 

 Group. 



Eisteddfa 

 Grou p. 



Rliuddnant (2. Rhuddnant Grits. 



Group. \ 1. Rhuddnant Shales. 

 Myherin C 2. Blaen Myherin Mudstones. 



Group. 1 1. Dolwen Mudstones. 



D G V r i Ju S p Eridge } Mudst0neS With thin g rit - bands> 



f 3. Flags and shales. (Zone of Monograptus sedgwicki.) 

 2. Shales and mudstones. (Zone of Cephalograptus 



cometa.) 



1. Mudstones and shales. (Zone of Monograptus 



convolutus.) 



(~4. Black shales and mud- C S. Leptotheca-ha.nd. 

 stones. J 7. Magnus-hani. 



(Zone of Monograptus | fi. Triangulatus-hani. 

 communis.) \ja. Triangulatus-va.v.-ba.ii&. 



<| 3. Flags and black shales. (Zone of Monograptus 

 I cyplius, sensu stricto.) 



2. Flags with thin shales. (Zone of Monograptus 

 rheidolensis , sp. nov.) 



j 1. Flags and shales. (Zone of Monograptus atavus, 



L sp. nov.) 



f2. Flags, shales, and grits. (Zone of Ceplialograptus 



(?) acuniinatus.) 



1. Flags with thin shales. (Zone of Glyptograptus 



persculptus.) 

 Bryn-glas Group. Mudstones. 

 Drosgol Group. Grits, conglomerates, and mudstones. 



Nant-y-M6ch Flags with thin shales. (Zone of Dicellograptus 



Group. anceps.) 



IV. Detailed Description of the Beds. 



A. The Plynlimon Stage. 



The !N"ant-y-M6ch Beds (Aa) are probably the lowest which 

 are exposed within the limits of the region examined. They crop 

 out with a westerly dip of 36°, in the small stream Maesnant-fach 

 which flows northwards to join a tributary of the Rheidol called 

 Nant-y-M6ch near the farm of the same name. As the strike of 

 the beds coincides with the stream-course, only a thickness of 10 

 to 15 feet is exposed. They consist of blue flags striped with thin 

 greyish-white siliceous seams and dark-blue shaly partings, along 

 some of which fossils were found (F. 1 in the Map, PI. XXIV). On 

 prolonged weathering they become greyish-white to a depth of an 

 inch or two, while the surfaces and joints are coated with light 

 lemon-coloured or orange stains. The abundance in them of crys- 

 tallized pyrite is a striking character ; the mineral occurs as cubes 

 and dodecahedra, either scattered, or aggregated along certain 

 bedding-planes in bands of an eighth to a quarter of an inch thick. 

 With the exception of one small example of Remopleurides and an 



