C— GEOLOGY. 67 



The symbol y was originally applied to rocks at the level of 

 co-occurrence of Zaphrentis and Caninia. In the table of succession of 

 the Bristol paper these arc grouped with Z, but in that of the Gower paper 

 with C. In the Burrington paper Vaughan re-defined horizon y as beds 

 with abundant Caninia patula and without C. cyUmlrica (Scouler) Salee, 

 and extended it so as to include the major portion of C, (styled C'ly in 

 the illustrations). As Dixon* remarks, the value of y as a means of 

 comparing distant developments has been much enhanced, C. patula 

 having a wide distribution but apparently limited range. 



3. The most important change since the publication of Vaughans 

 original paper has been in the dividing line between Tournaisian and 

 Visean. Vaughan originally drew the line at the top of the Syringothyria 

 zone. Dixon,'^ however, showed by the study of the Gower section and 

 others in South Wales that a slight discordance, becoming more pronounced 

 along the outcrop north of the coalfield, occurs in places between C, aud 

 C.J, and that it was at the top of C, that emergence in the south-western 

 province temporarily interrupted subsidence, to give way in turn to 

 renewed subsidence. He further showed that over a large area, particu- 

 larly in Pembrokeshire, C, aud S, are not sharply separable, either as 

 regards lithology or fauna. Consequently he drew the line between 

 Tournaisian and Visean at the top of C„ and in this he has generally beer, 

 followed by workers in the South-western Province. Vaughan, however, on 

 fossil evidence preferred to draw the line somewhat above the break, t.e. 

 in the middle of C.„ and this is also the level adopted by Delepine m 

 Belgium. 



4. In the Bristol paper horizon S, that of overlap between the C and S 

 zones, is shown in the table of succession, but its limits and faunal characters 

 are not defined, as was done in the case of horizons [3 and y. 



The term horizon S is practically dropped in all the earlier papers on the 

 British Carboniferous Limestone. In his later paper (Correlation of 

 Avonian and Dinantiau) Vaughan revives the term in an emended and 

 extended sense so as to include upper C.^ and S, of the original classifica- 

 tion, i.e. from the maximum of Cyathophyllum 4> to the incoming of Cyrtina 

 carbonaria. Vaughan •* gives full details as to the faunal characters of this 

 level, and indicates the corresponding level in a number of sections through- 

 out the British Isles and in Belgium. 



Vaughan introduced the term Avonian, which is nearly equivalent to 

 the Belgian Dinantian, to replace the somewhat cumbersome designation 

 Carboniferous Limestone Series. He also habitually used the terms 

 Tournaisian and Visean for the lower and npper Avonian respectively, 

 though the use of the terms was not quite identical with their use in 

 Belgium. He suggested the terms Clevedonian aud Kidwellian as alter- 

 natives, and if it be essential that terms be used strictly in the sense in 

 which they were originally employed, these should replace Tournaisian 

 and Visean. The terms Tournaisian and Visean are now so thoroughly 

 established with us that it would probably be impossible, even if desirable, 

 to displace them. British geologists will doubtless continue to use them 



* Pembroke and Tenby Memoir, p. 65. 

 ■ Q.J.G.S., vol. Ixvii. (1911), p. 542. 

 « Ibid., vol. Ixxi. (1915), p. 17. 



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