156 



:mr. f. dixet and peof. t. t. siblt ox the [vol. Ixxiii, 



These basal beds of S, are grey ciystalline dolomites, fine-grained 

 or medium-grained. Being much more resistant to denuding 

 agents than either the underlying 2Todiola phase or the overlying 

 shales of the Millstone Grit, the}' give rise to a conspicuous 

 feature along much of their outcrop. The}'^ can readily be mapiDed, 

 and at many points the}^ have j^elded the characteristic brachiopod 

 Cyrtina carhonaria and other fossils. 



In Fforest Fawr and the woods adjacent to it on the north, are a few 

 exposures of the S.^ dolomites and many of the overlying oolites. The latter 

 beds are extensively vein-dolomitized in mass, and often rendered highly 

 ferruginous by the development of hasmatite. An old quarry beside the road, 

 immediately south of Pen-y-bryn Cottage, gives a small exposure of ferruginous 

 crinoidal dolomites containing Cyrtina cai-honaria. 



The following tabular list of fossils records most of the fossiliferous 

 exposures of the So beds in their outcrop from Pen-y-bryn to the Rhymney 

 valley. 



Locality. 



Genera and species. 



3. 4. 



Syringopora sp , ... j x 



Lithostrotionmartini^dwa,rds&Jlsiim.e. X X 



Carcinophylhtm sp X X 



Productus corrugato - hemispTiericus 



Vaughan ... x 



Cyrtina carhonaria M.^ Coy x X 



Seminnla fieoides YaAxghsbX). x X 



1. Old Quarry (Glam. 37 SW \ 7), 350 yards north-west of Cefn- 



carnau-fawr. 



2. Old Quarries (Glam. 37 SW A 8), immediately south-east of 



Ty'n-y-ton, north of Thornhill. 



3. Old Quarries (Glam. 37 SW \ 9), immediately west of Cefn- 



carnau-uchaf, north-north-east of Thornhill. 



4. Bed of lane (Glam. 37 NW \ 2), on the northern side of Cefn-On, 



a quarter of a mile west of the Caerphilly railway-tunnel. 



5. Old Quarry (Glam. 37 NE \ 3), in woods north of Cefn-On Farm, 



200 yards east-north-east of Ty'n-y-parc. 



6. Footpath (Glam. 37 NE X 2), 100 to 150 yards north-east of 



Pen-y-waun, north-east of E-udry. 



7. Stream-bed (Mon. 27 SE A 2), about 100 yards west of Ffwrwm, 



eastern side of the Rhymney valley. 



In all cases except one, the fossiliferous beds are grey dolomites. The 

 exception is afforded by the old quarry (Glam. 37 NE X 3) near Ty'n-y-parc, 

 where a band of coarse-grained white oolite yields the fossils. This oolite 

 is probably succeeded immediately by the Millstone Grit. 



Below Ffwrwm in the Rhymney valley, Avhere the S^ beds are last seen, 

 they crop out in the stream-bed and in the very steep northern side of the 

 ravine, and they are seen to overlie the Modiola phase of Sj. About 30 yards 

 upstream from their last outcrop, a band of quartz-grit crosses the stream- 

 bed ; and the quartz-c(mglomerate, of which this grit probably forms the 

 basal bed, undoubtedly cuts out the S_, dolomites in this locality. 



