C— GEOLOGY. 71 



is found, the S., beds resting directly on the K beds. At Pendine they 

 have a conglomeratic base, but no such rock is known E. of Carmarthen 

 Bay. On the south crop of the Pembroke coalfield, at Wesfc Williams- 

 town, there is only a slight break between C^ and C.^, while at Tenby the 

 sequence is complete, though the C beds are of shallow-water type. In 

 the southernmost part of Pembrokeshire not only is the sequence complete, 

 but the C beds are represented by ' standard ' limestones. 



(2) Post-Avonian Unconformities. 



Throughout South Wales, in the Forest of Dean and in the Glee Hills 

 district, there were upheaval and erosion prior to the deposition of the 

 Upper Carboniferous. At the Titterstone Clee^^ and in the Forest of Dean 

 the Coal Measures rest unconformably on various levels of the Avonian. 

 Along both the south-eastern and northern crops of the South Wales coal- 

 field the Millstone Grit shows remarkable over-stepping relations to the 

 Avonian rocks. These are described in the section dealing with the 

 Millstone Grit. In the Clevedon and Clapton district of North Somerset 

 no representatives of any Avonian horizons higher in the series than the 

 Caninia-doloimte (C.^) are met with. In the Clapton district the Coal 

 Measures rest directly on Zafhrentis beds. In the Clevedon district the 

 relation of the Coal Measures to the Avonian rock is sometimes faulted, 

 sometimes indeterminable. 



It is very rare in the Bristol district and North Somerset to get sections 

 showing the relations of the Millstone Grit to either the overlying or under- 

 lying strata. The best section from the Carboniferous Limestone to Mill- 

 stone Grit is probably that at Wick Rocks, while the best section from the 

 Millstone Grit to the Coal Measures is that described by Dr. H. Bolton ^^ 

 from Ashton Vale. 



In neither of these sections has any break in the sequence been observed, 

 though, as is pointed out more fully in the sequel, the occurrence of 

 Yorkian plants in the upper part of the Millstone Grit shows that if, as is 

 generally believed, the lower part is Avonian, the upper is much later, 

 so that a considerable gap occurs in the sequence. In the Radstock area 

 a second big gap occurs, namely between the Yorkian (upper Millstone 

 Grit) and the Coal Measures, which are represented only by the uppermost 

 group, the Radstockian.^* 



C. In the Midlands and North of England. 

 Mr. J. W. Jackson's work shows the existence of an important un- 

 conformity above the D, beds of North Derbyshire. The Edale shales, 

 which lie between these beds and the Kinderscout grit, were formerly 

 regarded as Avonian, and often grouped with the Yoredalian. Mr. Jackson 

 confirms Hind's i* conclusion that they are of later date, and shows that 

 they contain goniatites of Bisat's zones Rj, H and Upper and Middle E 

 {bisidcafum beds). There is a break below the bisulcatum beds, the 



" Dixon, Hep. Brit. Ass., Sheffield (1910), p. 611, and Geol. Mag., 1910, p. 458. 

 ^■^ Q.J. 0.8. , vol. Ixiii. (1907), pp. 445-69. 



1' The ahove statements are based on information supplied by Dr. R. Crookail 

 and on his papers in the Qeol. Mag., vol. Ixii., 1925. 

 " Oeol. Mag., Dec. iv-, vol. iv. (1897), p. 209. 



