218 PROF. S. II. ]{EY>X)LDS OX THE LITHOLOOICAL ~Vol. Ixxvii, 



Ju the Buri'ington and Failaiid sections there is a stronglv- 

 marked band of coarsely oolitic limestone at this level, and at 

 Abbot's Leigh, less than a mile and a half away, the oolitic character 

 is conspicuous. Xo oolitic band has, however, been observed at 



this level in the Avon Section. 



(2) The ' Bryozoa - Bed." horizon a. (Thickness = about 

 2-5 feet.) — This is well exposed in both the upper and the lower 

 Avonmouth lines. In the former it shows up very prominently 

 below the arches at the mouth of the tunnel through Durdham 

 Down. The Bryozoa-Bed is also well seen in the riverside ex- 

 posure and by the road. 



W . W. Stoddart,! from an examination of the insoluble residues, 

 gave the first description of the Bryozoa-Bed (' Microzoal Bed'), 

 figuring gastropods, ostracods. bryozoa. and crinoids from it. 

 Mr. E. B. AVethered- recognized the presence of corals, in addition 

 to the organisms described by Stoddart. ]Mr. E. E. L. Dixon 

 has also described the characters of that bed.-^ The Bryozoa- 

 Bed consists of about six main bands of limestone, separated by 

 shaly partings. It is a red, mainly crinoidal, but partly bryozoal 

 limestone, sometimes very fine-grained, sometimes more coarsely 

 crystalline. A special feature, as has been noticed by previous 

 observers, is the rounding of the small fossils. Bryozoa ^ are very 

 plentiful at certain levels, but absent at others. Quartz-grains 

 are also plentiful in some bands. The haematite which gives the 

 rock its characteristic colour is sometimes uniformly distributed 

 as a general stain : but its most characteristic method of occurrence 

 is when the cavities of the bryozoa and small gastropods are filled 

 by it, and the crinoidal ossicles have theii* structure picked out, 

 while the main part of the rock is only slightly stained. 



Some 4 feet from the base of the Bryozoa-Bed is a band showing 

 the peculiar structure first denominated by E. H. Tiddeman ^ 

 ■ pseudobrecciation." As this structure is far more characteristic 

 of the D beds, it will be described more fully when I deal with 

 them fp. 234i. In this case the change has led to much sepam- 

 tion of iron and some of argillaceous material. A point of interest 

 is that the pseudobrecciation here affects a crinoidal, not (^as is 

 usually the case) a foraminiferal, rock. 



Immediately above the pseudobrecciated bed is a baud of 

 dolomite-mudstone 6 inches thick. This is the most important 



^ Ann. & 3Iag. Xat. Hist. ser. 3. vol. viii (1861) pp. 486-90. 



2 Q. J. G. S. Tol. xHv (1888) p, 190. 3 j^jVZ. vol. IxtII (1911) p. -515. 



^ The bryozoa from the K-beds of the Avon section, and especiallr from 

 the Bryozoa-Bed, have been generally assigned (as by Yanghan, Bristol paper, 

 p. 183) to the genns Rliahdomeson. Prof. J. W. Gregory, however, who kindly 

 examined some sections, "writes that one of the organisms ' consists of long 

 angular tubes, very few diaphragms, some acanthopores on the snrface. and a 

 cnp-like expansion at the distal end. It must be a genus closely related to 

 BatastomeUa, but the genus is hardly determinable without more specimens.' 



' ' Geology of the South "Wales Coalfield : pt. viii — The Country around 

 Swansea' Mem. Geol. Surv. 1907, p. 10. 



