792 Geological Society. 



districts being so situated that a comparison of their respective 

 developments can be interpreted in the light of the fact that, during 

 Avonian time, the nearest coast lay to the north, with a general 

 east-and-west trend. With the description of the lithological sequence 

 arc included notes on some breccia-like limestones, characteristic 

 of D, and on e lagoon- phases ' and the origin of radiolarian cherts. 

 To the faunal lists are added notes on the D 2 -D phase of the 

 Dibunophylhim Zone, which distinguishes Gower from the rest 

 of the South -Western Province at present known, and on the 

 correlation of that zone with the Upper Bernician of Northumber- 

 land. From the faunal sequence it is concluded that the zones 

 Z, C, S, D t and D, (the K Zone is poorly exposed) are characterized 

 by the same assemblages as in the Bristol area. 



The lithological sequence shows (1) that over the whole area the 

 depth of the Carboniferous sea underwent a complete cycle of 

 intermittent change during Lower Avonian time, the initial deepen- 

 ing being followed by gradual shallowing up to the top of the 

 lower part, C v of the Syringothyris Zone, which was deposited 

 almost at sea-level ; (2) that a similar cycle marked the ensuing 

 period up to the top of the Seminula Zone ; (3) that a similar but 

 smaller cycle took place in the Dibunophyllum Zone, the latter 

 actually reaching the surface ; and (4) that a fourth cycle, com- 

 mencing with a far-reaching physiographic change, characterized 

 the Posidonomya Zone. 



Further, a comparison of the sequences and thicknesses in the 

 three districts shows that, not only were the downward movements 

 of the sea-bottom during the first two cycles greater in the south 

 than in the north, but also that the axis on which the movement 

 during the first cycle hinged was different in direction from the 

 axis during the second cycle. The bearing of these movements on 

 the question of the delimitation of the divisions of the Avonian is 

 then discussed. They suggest that the base of the upper part, C 2 , 

 of the Syringothyris Zone should form the base of the Upper 

 Avonian. On the other hand, the base of C 2 in at least two 

 localities is closely connected, faunally, with the zones below, 

 whereas the fauna of the main mass of C 2 passes into S t without 

 appreciable change other than the introduction of Lithostrotion. 

 It will, therefore, in all probability be decided that the break 

 between the Lower and the Upper Avonian should be taken at a level 

 within C, rather than at the base of the Seminula Zone. For the 

 present, however, this question must be deferred, since it concerns 

 the whole extent of the formation in Belgium, the North of France, 

 and the British Isles. 



The paper concludes with notes on some of the corals and brachio- 

 pods, including one new species of coral and two new species and 

 a new variety of brachiopod. 



