C— GEOLOGY. 



7'J 



of S.^ age. lu the Clee Hill district the Millstone Grit facies (Cornbrook 

 sandstone) comes on at about the base of C,. In all these districts then 

 the ' Millstone Grit ' is wholly or partly {vide infra) Avonian. 



The thickness of the ' Millstone Grit ' in the West of England is 

 insignificant as compared with the thickness of the rocks generally called 

 by the same name farther north, and it was suggested by Vaughan that 

 it may all be of D, age.^^ Kecent work*" shows, however, that the upper 

 part of the ' Millstone Grit ' near Yate contains plants of Yorkian type. 

 This would imply a big unconformity between upper and lower ' Millstone 

 Grit ' of the Bristol area. 



Lithologically, as Kendall*^ points out, the quartzosc grits and con- 

 glomerates which go by this name near Bristol are totally distinct from 

 the arkoses of the Millstone Grit farther north. He consequently suggests 

 that the term ' Millstone Grit' as used in the Bristol area be dropped and 

 replaced by ' Farewell-rock.' There is, however, the objection that the 

 term ' Farewell-rock ' is also used in South Wales, a region where the 

 true post-Avonian Millstone Grit is found; and I am informed by Mr. Dixon 

 and Dr. A. E. Trueman that in parts of South Wales the Farewell-rock is 

 Yorkian in age. Were it not for the fact that the upper beds contain 

 Yorkian plants it would probably be best to use Sibly's term Drybrook 

 sandstone for the ' Millstone Grit ' throughout the whole Bristol area, 

 instead of confining it to the Forest of Dean. The existence of these 

 Yorkian plants necessitates a local name for the Avonian ' Millstone 

 Grit ' of the Bristol district — I suggest the use of the term ' Brandon Hill 

 Grit,' from Brandon Hill, Bristol, where it is well developed. The term 

 * Farewell-rock ' could be used for the Yorkian portion. 



South Wales and the Forest of Dean (see fig. 3). 

 The Millstone Grit of South Wales shows very remarkable and varying 

 relations to the Avonian. ^^ j^ ^j,g south-east part of the South Wales coal- 

 field and in Monmouth it sometimes overlies a big thickness of D beds (Ruthin 

 and Llansanuor), sometimes rests directly on upper S.^ (Miskin), sometimes 



Diagram showing Overstep of the Millstone Grit on the south-eastern margin of the 



South Wales Coalfield. (From figs, and descriptions in the paper by Dixey & Siblv, 



Q.J.G.S., vol. Ixxiii. (1918), pp. 1 1 1-164.) 



" Rep. Brit. Ass., Winnipeg (1909), table 3. 

 *» R. Crookall, Geol. Mag., vol. l.xii. (1925), p. 403. 

 " Handbuch der reg. Geol., iii., 1, s. 156. 

 *-Q.J.G.S., Ixxiii. (1917), p. 119. 



