Vol. 56.] SILTJKIAN" SEQUENCE OE EHATADEE. 83 



beds. Between the Gigrin Mudstones (Ad) at the summit of the 

 Gwastaden Group and the overlying Pale Shales (C) there is an 

 almost complete break in the life-sequence, indicating a great lapse 

 of time between the periods of deposition of the Rhayader Pale 

 Shales above and the strata of the Gwastaden Group below. 

 The foregoing conclusions may be tabulated as follows : — 



(C) Rhayader Pale Shales. 



(Palreontological break.) 



(A) Gwastaden Group. 



(Ad) Gigrin Mudstones. Feet. 



(Ad 3 ) Pale Grey Mudstones 2501 



(Ad 2 ) Zone of Monograptus convolutus j 9 gQ V 510 



(AdJ Calcareous- Nodule Beds J " J 



(Ac) Ddol Shales. 



(Ac 3 ) Zone of Monograptus fimbriatus 150 ] 



(Ac 2 ) Zone of M. cyphus 200 \ 600 



(Ac-J Zone of M . tenuis 250J 



(Ab) Dyffryn Flags. 



(Ab 3 ) Diplograptus-modestus Flags 300 "j 



(Ab 2 ) Eottenstone Beds 50 V 530 



{Ab l ) Micaceous Flags and Grits , 180 J 



(Act) Cerig Gwynion Grits 170 



1810 

 Cleaved Blue -black Shales. 



(2) Confirmatory Sections. 



(a) Dyffryn Wood and Woodlands. 



About \ mile south-west of our first traverse a very fair con- 

 firmatory section of the rocks is displayed in the sides of a 

 small stream running from off Gwastaden. This stream rises some 

 distance west of the Cairn, and crosses the Builth road to the west 

 of Dyffryn Cottage. It joins the Wye below the road ; and, if the 

 section be continued through Woodlands House on the opposite 

 side of the river, it affords a check on the complete succession as 

 described in the foregoing pages. (See fig. 2, p. 85.) 



Commencing our examination west of the Cairn, where the 

 Cerig Gwynion Grits (Aa) rise to their greatest height above 

 the valley, we find, as before, cleaved Blue-black Shales below 

 them. The basement-grits are here about 200 feet thick. From 

 the topmost bed of the grits to the northward there is a con- 

 siderable break in the exposures along the stream itself ; but here 

 and there to the right or left isolated patches of the Micaceous 

 Flags and Grits (AbJ maybe seen peeping out from the bracken 

 and gorse on the hillside. About 60 paces above the top of the 

 wood, rocks once more become visible in force, and are fairly 

 continuous down to the road. At the commencement of this 



g2 



