96 SECTIONAL ADDRESSES. 



In the Whitehaven district^^^ subsequent dolomitisation occurs locally 

 m many of the limestones. 



There is very little dolomitisation in the limestones of Northumberland 

 and Durham, though Dr. S. Smith informs me that the Great or Dryburn 

 limestone of Beadnell (D3) shows subsequent dolomitisation. 



Chert. 



The subject of chert, or even of Carboniferous chert, would alone jaeld 

 material for a lengthy address. I make no attempt to deal with it 

 exhaustively, and there is the less reason to do so since the publication 

 of Mr. H. C. Sargent's^^^ two important papers, which contain full 

 references to the literature, British and foreign. 



Distribution, Stratigraphical and Geographical. 



In the Culm area of North Devon chert is strongly developed in Dg, 

 where the fauna includes forms characteristic of D^x and D^p. 



In the Mendip region cherts are plentiful, particularly at the y level in 

 the Frome area. In the Burrington section chert occurs in Z, and is 

 strongly developed in Cjy and S.^. At Vallis there is a great development 

 of laminated chert in Z.^. There is much chert in ^ and Zj at Portishead. 



In a large proportion of the North Somerset sections the fossils in Zj 

 are much silicified (beekitised), even if massive chert is not present. 



Chert is much less developed in the Gloucestershire sections than in 

 those of Somerset. In the Avon section it is not conspicuous, but a little 

 occurs in Zj and above and below the Seminula oolite in S^. There is 

 no sign of chert in the Sodbury, Tytherington and Wickwar sections. With 

 regard to the Forest of Dean, Dr. Sibly informs me that he does not know 

 any considerable development at any horizon anywhere in the district. 



Chert occurs low down in Z along the S.E. margin of the South Wales 

 coalfield and in the Gower area, where it is also found at three other levels — 

 in Sp in the Black Lias of lower D.^ (Dgx), and in the lagoon-phase rocks of 

 upper D3 (D.,p). 



In the Pembroke and Tenby district Dixon^^* describes chert as 

 occurring at many horizons of the Main Limestone {i.e. ^ to D^), especially 

 in rocks of the Zaphrentid phase, which form most of the sequence in the 

 southern outcrop. Cherts are mentioned as occurring at the base of the 

 Millstone Grit at Lydstep, Gower, and all along the north crop of the 

 South Wales coalfield. 



In the Midland^^^ area there is a great development of nodular and 

 lenticular chert in the thinly-bedded upper D^ limestones. Feeble 

 development may occur in the thickly-bedded Dj limestones. The D., 

 beds also contain abundant chert, while in Flint^^® the development of chert 

 in the upper D, and lower Lancastrian (' Cefn-y-fedw sandstone ') is one 

 of the most remarkable in the British Isles. These cherts are not of the 



"2 Geol. Mag., \ix. (1922), p. 79. 



"3 Ibid., vol. Iviii. (1921), pp. 265-78, and vol. Ix. (1923), pp. 168-83. 

 '" Pembroke and Tenby Memoir, p. 69. 

 "5 Q.J.O.S., vol. Ixiv. (1908), p. 38. 



"^ See Sargent, Oeol. Mag., vol. Ix. (1923), p. 168. Other references will be found 

 here. 



