66 DR. T. P. SIBLT OX THE FAULTED IXLIEE OF [Feb. I912, 



(iv) Zonal extent of the Yobster-Quarry section. — 

 The total thickness of Carboniferous Limestone exposed in Tobster 

 Quarry, amounting to some 520 feet, is considerably greater than 

 the thickness of S^ in the typical Mendip section of the Seminula 

 Zone, namely, the Cheddar-Gorge section ^ in the Western Mendips. 

 At Cheddar, where the Semimda Zone as a whole has a thickness 

 of about 700 feet, the thicknesses of S^ and S., are about 300 and 

 400 feet respectively. Having found no evidence of an expansion 

 in the thickness of the Seminida Zone as a whole in the Eastern 

 Mendips, I am led to conclude that the Tobster Quarry-section 

 extends below S., and includes part of S^. I have failed, however, 

 to discover any palseontological evidence establishing the presence of 

 S^ here. If, as I believe to be the case, we have here a portion of S^ 

 from which the distinctive fossils are absent, then the Yobster section 

 affords an interesting parallel to the Burrington-Combe section. 

 At Burrington Combe, certain distinctive elements of the S^ fauna 

 are apparently absent, and consequently a subdivision of the 

 zone into S^ and S., on the typical faunal basis cannot there be 

 established.^ 



(6) The Southern Limestone Mass. 



Exposures in this mass are numerous, occurring in many disused 

 workings and in cuttings on the mineral railway. Those afforded 

 by Yobster Old Quarry are of particular interest, since they show 

 beds of fossiliferous limestone, highly disturbed in places, in 

 contact with the strata of the Grit-&-Shale Mass. 



The workings of Yobster Old Quarry extend east and west of the 

 tramway-tunnel. Earther west, the Carboniferous Limestone is 

 seen in abandoned workings in several cottage gardens. Earther 

 east, it is exposed at various points in the railway-cutting. 



The beds of Carboniferous Limestone exposed in the face of 

 Yobster Old Quarry and also in the yards and gardens to the west 

 are abundantly fossiliferous, and can be assigned definitely to D^, 

 as the following list of fossils shows : — 



Cyaihophyllum rmircliisoni Ed, & H. I 



Troductus ^S.giganteus (Mart.), 'mut. D^ ' Vaughau. I Abundant. 

 Frodtictus hemisphericus Sow. J 



Alveolites septosus (Flem.). "j' 



Carclnophyllum Yaughan. I ^, 

 Lithodrotion martini Ed. & H. \ '-'^^^i^o^^- 

 Daviesiella aff. comoides (Sow.). J 

 BihunopTiyllura <p Yaughan. 

 Athyris cf. expansa (Phill.), Day. 



A section in Yobster Old Quarry, which shows the faulted 

 junction of these D^ beds with the strata of the Grit-&-Shale Mass, 

 is described immediately below. 



1 T. F. Sibly, Q. J. G. S. vol. Ixii (1906) p. 347 ; and Proc. Geol. Assoc, vol. xx 

 (1907-08) p. 68. 



2 T. F. Sibly, Proc. Bristol Nat. Soc. ser. 4, vol. i (1904-07) p. 36 ; and 

 A. Yaughan, Q. J. G. S. vol. Isvii (1911) p. 370. 



