I 



Vol. 68.] CAEBONIJFEROUS LIMESTONE AT UPPEll YOBSTEK. 63 



rubbly, Liassic limestone. The Lias here rests upon a perfectly 

 level-planed surface of Carboniferous Limestone ^ (PI. IV, fig. 1). 



Along the northern face of the quarry the dip of the Carboniferous 

 Limestone is found to vary a little in direction and amount, since 

 the beds are slightly curved in places. The average apparent dip 

 has been found to be about 45°, E. 35° S. ; and a calculation based 

 upon this estimate gives 520 feet as the approximate total vertical 

 thickness of Carboniferous Limestone exposed in this quarry. 



This quarry-section represents a great part of the Seminula Zone. 

 As the result of a single visit in 1905, the Avriter concluded, 

 wrongly, as will be shown, that the upper portion of S^ and the 

 lower part of S., were here represented in normal succession from 

 west to east. The fossil evidence merely indicated S, without 

 justifying any subdivision of the zone ; but the lithological charac- 

 ters of certain beds in the eastern half of the quarry were observed 

 to be those of the middle portion of the Seminula Zone, and the 

 eonclusion stated above was based upon this observation. 



When the section was studied more carefully in the summer of 

 1910, it became evident that the succession was abnormal, since 

 the beds in the Avestern part of the quarry were discovered to 

 possess the remarkable lithological characters of the upper portion 

 of §2 : but no further fossil evidence was forthcoming. 



In April 1911, however, it was found that a recent westward 

 extension of the quarry had exposed fresh beds, underlying those 

 hitherto seen ; and these beds yielded fossils which indicate the 

 summit of the Seminula Zone. The inversion of the strata 

 suggested by the lithological succession being thus 

 confirmed by fossil evidence, we must conclude that Vobster 

 Quarry exhibits over 500 feet of Seminula Limestones 

 overfolded and dipping north-westwards at 135°. 



The development of slickensides and calcite-veins in the 

 limestones of this quarry is remarkable. Hand- specimens can be 

 obtained showing both these features, which are also developed on 

 a very large scale. Near the eastern end of the north face of the 

 quarry, the limestone is so largely replaced by masses of coarsely 

 crystalline calcite, and so much cut up by irregular slickensided 

 joints, that the stratification is very obscure (PI. IV, fig. 1). 



Further efi^ects of the powerful forces that acted on this mass 

 of limestones are to be found in the disturbed condition of 

 the beds as the Grit-&-Shale Mass on the south is 

 approached. Mr. WiuAvood's section ^ shows that the limestones 

 at the mouth of the tunnel, closely adjacent to the Grit-&-Shale 

 Mass, are much disturbed ; and signs of distortion can still be seen 

 in the rather poor exposures of limestone situated around the 

 tunnel-mouth. In the mouth of the tunnel itself, about 8 yards 



^ The Lias of this quarry has been described by Mr. L. Eichardsoii, Proc. 

 Geol. Assoc, vol. xxi (1909-10) p. 222. Mr. Kichardson notes that occasional 

 small patches of Rbgetic rock are to bs found adhering to the surface of the 

 Carboniferous Limestone. 



2 Winwood, 1893, folding plate. 



