420 Dr. T. F. Sihly—Tlie Forest of Bean Coalfield. 



correlation of the "Whitehead Limestone with the Upper Seminula- 

 zone.^ Indeed, I have as yet found no determinable corals or 

 brachiopods in this subdivision ; and at present I would only point 

 out that, on the evidence of the underlying beds, the Whitehead 

 Limestone represents some horizon not far removed from the summit 

 of the Si/rinffothi/ris-zone. The study of the south-western margin 

 of the coalfield will probably throw light upon this question ; for 

 in proceeding south-westward from Mitcheldean we approach the 

 Chepstow area, in which the Semmida-zone is finely developed. - 



Brybrooh. Sandstone. 



This formation has yet to be studied in detail. It is composed 

 very largely of pure quartz-sandstones, often coarse-grained and 

 friable. Conspicuous bands of quartz-conglomerate occur; some 

 clays and shales are intercalated with the sandstones ; and some 

 haematite deposits are developed. The thickness of the beds at their 

 outcrop in the neighbourhood of Ruspidge, south of Cinderford, is at 

 least 600 feet. 



Mr. Wethered has recorded' aZepidodendron ^^ allied to Z.ffri^tht'i, 

 Brongn." from the lower beds of this formation at Drybrook. No 

 other determinable fossils have yet been recorded. 



The Lower Carboniferous age of the 'Millstone Grit' in the 

 Mitcheldean district was demonstrated by Dr. A^aughan * in 1905. 

 The term Drybrook Sandstone — from Drybrook, near Mitcheldean, 

 Gloucestershire — is now proposed instead of ' Millstone Grit '. "We 

 have seen that the base of the Drybrook Sandstone lies conformably 

 upon limestones whose horizon cannot be far removed from the top 

 of the Syringothyris-zone, and we may conclude that the whole of 

 this formation is older than the Millstone Grit proper. 



C. The Intra-Cakbonifeeous Unconformity. 



Mr. E. A. Newell Arber has proved conclusively that the Coal 

 Measures of the Forest of Dean belong mainly, if not wholly, to the 

 Upper Coal Measures.^ I find that the relation of the Coal Measures 

 to both the Carbojiiferous Limestone and the DryhrooTc Sandstone is one 

 of pronounced unconformity. 



On the eastern margin of the coalfield, the strata of the 

 Carboniferous Limestone and Drybrook Sandstone dip steeply 

 westward, and consequently their outcrop forms only a narrow 

 band. Between Mitcheldean on the north and Buspidge on the 

 south, that is, tliroughout a distance of about 4^ miles, the super- 

 incumbent Coal Measures, wliich dip westward at a small angle, rest 

 upon the Drybrook Sandstone. South of Buspidge, however, the 

 Coal Measures overstep eastward into the Carboniferous Limestone 

 and conceal the Drybrook Sandstone. The transgression is gradual 

 at first, but near Howbeach Colliery, about 2 miles south of 



1 Q.J.G.S., vol. Ixi, p. 252, 1905. ^ ^_ Vaughan, ibid., p. 251. 



=* Q.J.G.S., vol. xxxix, p. 215, 1883. ^ Q.J.G.S., vol. Ixi, p. 252, 1905. 



^ E. A. Newell Arber, " On the Fossil Flora of the Forest of Dean Coalfield 

 (Gloucestershire), and the Relationships of the Coalfields of the West of 

 England and South Wales " : Phil. Trans. Eoy. Soc, vol. ccii, B, pp. 233-81, 

 1912. 



