498 MR. E. E. L. DIXON AND DE. A. VAUGHAN ON [Nov. I91 I, 



in length. Somewhat similar irregularities are known elsewhere 

 in shallow-water deposits, and have been caused by distortion of 

 originally evenly-bedded strata while still plastic, shortly after 

 deposition. It is possible, therefore, that the C&ninia Oolite, also, 

 has been drawn out tangentially while in this condition, though 

 by what agency is not apparent ; in this case, however, it is equally 

 possible that the irregularities are due to some slight subaerial 

 corrosion prior to the deposition of C 2 . 



The basal layers of the Modiola phase of C 2 , which follow the 

 Caninia Oolite sharply and fill the hollows just described, are 

 puckered in a fashion suggestive of parts of landscape-marble. 

 At higher levels in the phase conglomeratic beds are rare in this 

 district ; and dolomite-mudstones appear to be restricted, as in 

 parts of Pembrokeshire, to a buff-weathering, argillaceous group at 

 the top. The fauna of the phase is devoid of standard forms, 

 and a Mitcheldeania-like alga 1 (in minute fragments) is present; 

 otherwise the fossils are those found in the Eastern District. 



The standard limestones of C 2 differ considerably in character, 

 though not in fauna — except in an absence or rarity of Caninia — 

 from the same subzone in the Eastern District. The chief differ- 

 ence lies in an absence of contemporaneous dolomite in North- 

 Western Gower, even the gasteropod-valves, which would be readily 

 affected by ' contemporaneous ' dolomitization, being wholly calcitic; 

 further, the beds are more uniform in character, rather darker 

 and less thickly-bedded than in the Eastern District. They thus 

 approach the facies of the Seminula Zone of the outcrop north 

 of the coalfield. 



Note. — Along the east side of Burry Pill the zone has been extensively 

 vein-dolomitizecl. 



Seminula Zone, 8. — The lower subzone, S , is similar on the 

 whole to the underlying standard limestones of C 2 , but includes, 

 though rarely, thin calcite- or dolomite-mudstones and, possibly, 

 a little dolomite that has ' contemporaneously ' replaced limestone ; 

 chert is absent. The top of the subzone was not seen. 



In the 8 2 -oolites, as in the Caninia Oolite, some ooliths have been 

 replaced by aggregates of microscopic quartz-crystals ; but dolomite 

 is wanting. In the Modiola phase which forms the upper part of 

 the subzone, mudstones, including ' chinastone-limestones ' and 

 nodular beds, appear to be better developed than in the Eastern 

 District, and dolomite is wanting. The change to the standard 

 conditions of Zone D at the close of the phase was rapid. 



Dibunophyllum Zone, D. — The chief divergences in this part 

 of the sequence from the development in Eastern Gower are (1) an 

 absence of dolomite, as in C 2 and most of S ; (2) the presence of 

 a thick oolite at the base of D 1 ; and (3) the presence of D . 



(1) The absence of dolomite is most noticeable where the material 

 is known to be particularly susceptible to dolomitization ; such 



1 By this expression is meant a calcareous alga with a minute, radially- 

 columnar structure; such identification must suffice, until the revision of the 

 genera promised by Prof. E. J. Garwood is published. 



