part 8] SUCCESSION or the atonian at cliftox. 229 



traceable up the quarry-face to the top of the gorge ; in this 

 is worn the little cave in the middle of the quarry. The strata 

 at about this level show considerable lateral variability, the massive 

 limestone immediately below the chert in the middle of the quarry 

 passing into china-stones nearer the northern end. 



S^ (5). The Seminula Oolite. (Thickness = about 58 

 feet.) — The thick mass of white rock, mainly oolitic, to which 

 Vaughan gave the above name, immediately succeeds, and, as he 

 pointed out, it contrasts with the other strong white oolitic band 

 (the Cktninia Oolite) in its fossiliferous character. It is somewhat 

 difficult to know what level to take as the top of this oolite, for the 

 oolitic structure becomes very impersistent as the series is ascended. 

 K further difficulty arises from the fact that, wherever it is exposed 

 in the Avon section, the Seminula Oolite tends to be bounded by 

 weathered joint-faces in such wise that its characters are very 

 poorly shown. 



The lower part of S., (/>) is mainly a fine oolite, with the grains 

 frequenth^ formed round foraminifera. Higher up the series, near 

 the remains of the firing platform, the oolitic character is less 

 continuous, and the rocks are mainly foraminiferal limestones 

 partlj^ dolomitized (A. 123). Dolomitization may also affect the 

 oolites, but is not especiall}^ characteristic of them. Calcisplicdra 

 (A 123) is very plentiful in these rocks, and bryozoa are frequent 

 in some bands. Attention may be drawn to three bands showing 

 pieces of oolitic limestone, often rounded and 1 or more inches 

 long, enclosed in the same type of rock, which may be partly 

 dolomitized. The structure is probably due to penecontempo- 

 raneous brecciation, followed b}^ the rounding of the resulting 

 fragments by solution. The highest and best exposed of the three 

 brecciated bands meets the eastern base of the screen in front of 

 the firing platform. In the middle band the matrix is dolomitized, 

 while the fragments are not, and have suffered solution more than 

 the matrix. Except for the fact that the rock showing this 

 structure is oolitic and not a calcite-mudstone, and that the 

 fragments are rounded, it is closely comparable with the ' calcaire 

 grumeleux ' of Dr. H. de Dorlodot. The term grumelous is 

 adopted by Vaughan ^ for this brecciated type of structure. 

 Somewhat similar rocks occur in the Inferior Oolite of the Stroud 

 district. 2 Mr. E. B. Wethered ^ gives an analysis of an oolite 

 from the Grreat Quarry. Miss M. B. Chapman * also gives an 

 analysis, and records the presence of sponge-spicules and quartz- 

 crj^stals with secondary growth. 



S^ (c). Rocks between the Seminula Oolite and the 

 Concretionary Beds. (Thickness = about 240 feet.) — These 



1 Q. J. G. S. vol. Ixxi (1915) p. 26. 



2 Proc. Cotteswold Nat. F. C. vol. ix (1890) pp. 100-101 & pp. 388-92. 

 ^ Q. J. G. S. vol. xlvi (1890) p. 271. 



-* Geol. Mag-. 1912, p. 502. 



