70 SECTIONAL ADDRESSES. 



ther«! is evidence of a slight discordance. Except in North Devon, where 

 the Carboniferous rocks succeed marine Devonians, there is a gradual 

 passage from rocks of the Old Red Sandstone facies deposited under conti- 

 nental conditions to rocks of definitely Carboniferous character deposited 

 under marine conditions, and the horizon at which the dividing line 

 between the two formations is drawn is often, as in the Avon section, 

 merely a matter of convenience. Thus in the Avon section the line is 

 drawn above the highest band in which fish remains were found. In the 

 Tenby district Dixon "^ states that the conformable passage is sometimes 

 gradual, sometimes with abrupt change of conditions. In the Forest of 

 Dean^ there is no appreciable development of a transition series. 



In North Devon the lower part of the Pilton beds — a variable series of 

 shaly, gritty and calcareous strata — contains a fauna of predominantly 

 Devonian type, while the upper Pilton, though containing some Devonian 

 species, is more related to the Carboniferous. No full description of the 

 Pilton faunas is yet available, but Vaughan^ correlated the Proditctella 

 ■productoides beds with Km, and stated that in the uppermost Pilton beds 

 the fauna is essentially a p fauna with a few persistent Devonian forms. 

 This would be in accord with Evans' ^'^ correlation of the Baggy and Lower 

 Pilton beds of North Devon with the Upper Famennian. 



The Midland counties of England were mainly land areas in lower 

 Avonian times, and the upper Avonian rocks rest with strong unconformity 

 on Lower Palaeozoic or Precambrian rocks. 



Unconformities and Breaks in the Succession. 



A. In the Culm of Devon and Somerset. 



While the Upper Pilton beds of North Devon apparently represent the 

 K and p beds of the S.W. Province, no representatives of any later horizon 

 have been recognised till one reaches D.^, to which level the lowest zones 

 of the Culm are assigned. 



B. In the Smith-western Province. 



(1) Mid- Avonian breaks and unconformities. 



It is well established that there is evidence of disturbed conditions in 

 mid-Avonian times in the S.W. Province, increasing in intensity as one 

 passes N. and N.W. from the Bristol area. Thus, while in the Mendip 

 region the C beds are to a large extent represented by ' standard ' limestone, 

 in the Avon section and those of Gower and along the S.E. margin of the 

 South Wales coalfield the place of standard limestone is taken by shallow- 

 water oolites, shales and dolomites. Farther west, in Carmarthen, much, 

 if not all, of the C beds and of horizon y is missing, while in most localities 

 the Z beds are also absent, so that at Fan and other places near Kidwelly, 

 and for many miles along the outcrop to the north of the South Wales coal- 

 field, the Sj beds rest directly on the K beds. Along the northern crop 

 of the Pembroke coalfield west of Carmarthen Bay the same state of affairs 



' Pembroke and Tenby Memoir, p. 67. 



8 Sibly, Oeol. Mag., Dec. v., vol. ix. (1912), p. 418. 



• Q.J.O.S., vol. Ixvii. (1911), p. 385. 



w GeoL Mag., Dec vi., vol. vi. (1919), p. 548. 



