74 SECTIONAL ADDRESSES. 



rocks as thinly and evenly bedded dark argillaceous limestones inter- 

 bedded with shales and including much chert, which is typically black. 

 Fossils are generally abundant, and include crinoids, simple corals 

 {Zaphrentis, CyatJmxonia, Caninia), bryozoa, such brachiopods as frilled 

 athyrids, spinose productids, Syringothyris and Sjnriferina, Leptcena 

 analoga, Rhipidomella michelini, ScMzophoria resupinata. Small trilobites 

 are less abundant. 



Rocks of this type occur in the K and Z beds of various sections in the 

 South-west Province, and are typically represented in the D., {Cyathaxonia 

 beds) of Oystermouth Gower, the Craven lowlands, and Derbyshire. They 

 probably indicate muddy conditions, depth being immaterial. Dixon ^^ 

 points out that the study of the fauna of Zaphrentid phases has upset 

 former ideas as to the zonal importance of certain forms. Thus an upper 

 Avonian Zaphrentid phase such as that of Sj at Bosherton, Pembroke, 

 suggests at first sight a level in Z or y. 



The thin-bedded dark limestones of upper D.^ in Derbyshire and the 

 Cement Stone development of the Northern Province seem to be inter- 

 mediate in character between standard limestones and Zaphrentid-phase 

 beds. 



3. The third phase is styled in Vaughan's table ' Modiola and Posido- 

 nomya phases and other shallow- water deposits.' The deposits included 

 under this head, which are very varied and widespread, are divisible into 

 two main sections : — 



(a) The lainellihnmcli-goniatite or Culm phase. . 



(b) The lagoon-phase deposits (Dixon), including 



(1) Modiola j)^rese=calcareous lagoon-phase. 



(2) Radiolarian phase^cheitj lagoon-phase. 



This large and varied series of deposits seems to me to overweight the 

 third phase, and I think that it would be a distinct convenience to separate 

 the lagoon-phase deposits as a fourth phase. 



(a) The lamellibranch-goniatiie or Culm phase is very widespread 

 and important in the North of England, and is represented also 

 by the Culm of North Devon. 



The rocks consist of thin-bedded black shales, often papery, with thin 

 limestones prevalently of the calcite-mudstone type, and more rarely 

 radiolarian cherts. Sandstone bands may be present, but are not essential. 

 The fauna is somewhat monotonous, and forms with thin shells prevail. 



(&) Lagoon-pihase Deposits. 



The term was introduced by Dixon^^ to connote a group of rocks 

 deposited in wide but shallow coastal areas more or less isolated from the 

 deeper parts of the sea. In siich ' lagoons ' rocks of a peculiar type and 

 containing a peculiar fauna and flora were deposited. Dixon distinguishes 

 two types of lagoon phase : — ■ 



(1) Modiola j:?^ase= calcareous lagoon-phase. 



3° Pembroke and Tenby Memoir, p. 72. 

 " Q.J.Q.S., vol. Ixvii. (1911), p. 511. 



