482 ME. E. E. L. DIXON AND DR. A. YAUGHAN ON [Nov. 1 9 1 1 , 



Zl = Zaphrentis Zone. 



Z 2 = subzone of Zaplirentis honinchi Edw. & EL, Carr. 

 Z^subzone of Spirifer clathratus M'Coy. 

 Horizon /3. 



Lithological characters. — The lowest part of the Main Lime- 

 stone x ; in descending order : — 

 Z. — Dolomites similar to the Laminosa Dolomites, that is, dark 

 grey, finely crystalline, with small scattered nests of calcite and 

 dolomite ; interbedded with unaltered, thinly-bedded, crinoidal 

 limestones, which are abundant in the lower part but die 

 away to a great extent, upwards, at about 100 feet above the 

 base of the Zone. Through the lowest 50 feet or so occur 

 impersistently-tabular cherts and beekitized fossils. 

 Horizon /3. — More thickly-bedded, crinoidal limestones; several 

 yards exposed, but base not seen. 



Fauna. — See pp. 544-45. 



Thickness.— Of horizon f} + Z 1 :— about 150 feet (Threecliff Bay), 

 on the assumption that the unexposed basal part of horizon /3 is 

 not much more than 10 feet thick. Of Z 2 alone not determinable 

 without difficulty ; of the dolomite-group, Z 2 + Laminosa Dolo- 

 mites, 320 feet (Threecliff Bay). 



Limits. — Horizon /3, at the junction of the Cleistopora and 

 Zaphrentis Zones, is probably also the junction of the Lower 

 Limestone Shales and the Main Limestone. The junction of 

 Z l and Z 2 is not marked by any appreciable lithological change ; 

 the top of Z could not be localized, owing to the prevalent 

 dolomitization, but evidently is also not defined lithologically. 



Typical exposure. — Threecliff Bay, east side: — horizon /3 and 

 the rest of the zone in sequence. 



C 1 = Lower Subzone of the Symingothybis Zone. 



2. Caninia Oolite. 



1. Laminosa Dolomites. 



The presence or absence of horizon y in Eastern Gower could 

 not be determined, owing to the dolomitization prevalent through 

 Upper Z and Lower C . 



Lithological characters and thickness. — In descending 

 order: — 



2. Caninia Oolite. As a rule a remarkably pure limestone, 

 distinguished from neighbouring horizons by its light-grey, often 

 white colour and thick bedding, but especially by the purity 



1 This is merely a convenient name for the main limestone-mass of the 

 lower Carboniferous ; no relationship to the Main Limestone of the North of 

 England is implied. 



