356 ME. T. P. SIBLY ON THE CAEBONIFEROUS [May I906, 



In the Bristol area, shallow-water conditions prevailed during 

 the greater part of Syringothyris-time, as evidenced by the extensive 

 development of pure oolite, shales, and dolomites in the Syringothyris- 

 Zone. The consequent interruption of the faunal progression in this 

 part of the Avonian sequence forms a prominent feature in the 

 Bristol development. Additional evidence of physiographical dis- 

 turbance is afforded by the occurrence of contemporaneous igneous 

 rocks at the base of the zone (Horizon y) at several localities in the 

 Bristol area. 



The Mendip area, on the other hand, remained comparatively 

 free from disturbance, 1 and shows little or no interruption of the 

 faunal sequence. The Syringothyris-Zone, which is considerably 

 thicker than the equivalent part of the Bristol sequence, consists 

 entirely 2 of fossilif erous limestone, and possesses a well- characterized 

 faunal assemblage, in which the decline of the Clevedonian fauna, 

 and the genesis of the Kidwellian, is well displayed. The lowest 

 portion of the zone is slightly dolomitized. 



The ' BelUrophon-'Bed&f in the Syringothyris-Zone of Failand 3 

 and Tickenham, 4 in the Bristol area, exhibit the palaeontological 

 characters of the Upper Syringothyris-Zone of the Mendip area, 

 and indicate a temporary establishment of the Mendip facies in that 

 part of the Bristol area. 



The Weston-super-Mare district, 5 which lies between the Bristol 

 and Mendip areas, exhibits an intermediate type of development in 

 the Syringothyris-Zone. The lowest part of the zone is appreciably 

 dolomitized, and is succeeded by a thick band of pure oolite, but 

 the upper and main part of the zone exhibits a continuously- 

 fossiliferous sequence, similar to that of the Mendip area. 



Since this zone, in the Mendip area, contains an abundant coral- 

 and brachiopod-fauna which is but feebly represented in the 

 equivalent part of the Bristol sequence, a detailed comparison of 

 palaeontological features would be superfluous. The following 

 points, however, are worthy of special notice : — 



(1) Syringothyris aff. laminosa is an important fossil in the laminosa-subzone 



of the Bristol area (= Lower Syringothyris-Zone of the Mendips), but 

 is never common in this zone in the Mendip area, where it attains its 

 maximum in the Upper Zaphrentis-Zox\e. 



(2) Syringothyris aff. cuspidata, mut. C attains its maximum in this zone in 



the Mendip area, but in the uppermost part of the Zaphrentis-Zone 

 in the Bristol area. 



1 Igneous rocks associated with the Carboniferous Limestone have been found 

 at one locality only in the Mendip area, namely at Uphill, in the western- 

 most part of the area. The igneous rock is there associated with Upper 

 Zaphrentis-be&s, probably very near Horizon y, but there is no direct evidence 

 to establish its contemporaneity. See C. Lloyd Morgan & S. H. Reynolds, 

 Quart. Journ. Geol. Soc. vol. lx (1904) pp. 146, 150. 



2 An occasional, very local development of unfossiliferous oolites is found, 

 as, for example, at Hutton. 



3 A. Vaughan, Quart. Journ. Geol. Soc. vol. lxi (1905) p. 217. 



4 A. Vaughan, ibid. p. 231. 



5 See T. R Sibly, ibid. pp. 548-61. 



