C— GEOLOGY. 81 



succession on all the Avonian rocks from the Mirk Fell beds, which are 

 high up in the Yoredalian above the Fell Top limestone, to strata probably 

 as low down as D.*^ (see fig. 4). Mr. Hudson informs me that un- 

 published work shows a continuation of this overstep south of the Craven 

 faults in the region of TroUer's Gill, near Burnsall, on to the base of D, 

 and almost on to S.^. Mr. Hudson's work also shows overlap of the successive 

 zones of the Millstone Grit, the pseudobilinque zone of Bisat (lower E) 

 resting on the Avonian at Craven, while at Grassington and Arkendale the 

 bisulcatum zone (upper E) is the lowest level represented. 



While the stratigraphical level at which the so-called ' Millstone Grit ' 

 of the South-west Province comes on is so variable, the base of the true 

 Millstone Grit seems to be at a fairly constant stratigraphical horizon 

 throughout South Wales and the North-western and Northern Provinces. 

 The evidence now available from so many areas shows that the uncon- 

 formity between Millstone Grit and Avonian which was first observed by 

 Prof. 0. T. Jones*^ in the Haverfordwest area is of widespread occurrence, 

 and the establishment of this very important fact has helped and will 

 help to the solution of many problems in Carboniferous stratigraphy. 



There are cases in the North of England where beds with fossils of a 

 characteristically Avonian facies overlie some considerable thickness of 

 grit. This may in some cases be due to early establishment of grit con- 

 ditions, in other cases to a late survival into Lancastrian times of 

 Avonian types. Thus the Snebro Gill beds^'' ol West Cumberland which 

 overlie some 50 ft. of grit are considered by Mr. Dixon to be high in the 

 Yoredalian, but the evidence as to their age is admittedly inconclusive, 

 and Mr. Bisat is inclined to correlate them with the Cayton Gill beds 

 (Lancastrian). In Teesdale, Prof. Garwood's Botany Beds,'^ with a late D 

 fauna, are believed to overlie strata about 200 ft. thick originally mapped 

 as Millstone Grit. 



In Yorkshire, when circumstances were favourable, a marine fauna 

 temporarily invaded regions ^^ where Millstone Grit conditions were 

 thoroughly established. Thus Hind^^ described the Cayton Gill beds of 

 Nidderdale, which lie just below the Kinderscout grit equivalent and have; 

 an Avonian type of fauna. The goniatite fauna of these beds is not 

 sufficient to enable Mr. Bisat to speak confidently as to their age, but he 

 suggests lower R.^* The Colsterdale ^^ marine band of the same area, 

 with a goniatite-lamellibranch fauna, is assigned by Mr. Bisat ^* to his 

 horizon E. Again, the Shunner Fell limestone ^^ of Yorkshire contains a 

 coral and brachiopod fauna such as is usually found in Avonian rocks of 

 Zaphrentid or Cyathaxonid phase, associated with a goniatite {Anthra- 

 coceras glabrum) characteristic of zone E of Bisat's table. 



*^ Well brought out in Chubb and Hudson's map, op. cit., pp. 282-3. 



*^ Haverfordwest Memoir (1914), p. 151. 



*" Eastwood (Mem. Gaol. Surv.), Sum. Progress for 1922, p. (54; and Dixon, Proc. 

 Geol. Ass., vol. xxxvi. (1925), p. 44. 



•^1 Q.J.O.8., vol. Ixviii. (1912), p. 542. 



»= See L. H. Tonks, Proc. Tories Geol. Soc, n.s., vol. xx., pt. 2 (1925), pp. 226-5*), 

 for fuU references on the marine bands. 



S3 Hind, Naturalist {1902), T;i]p. 17-63 and 90-96; also Bisat. I'roc. Yorks Geol. Soc. 

 vol. xix., pt. 1 (1914), p. 20. 



" Ibid., vol. XX., pt. 1 (1924), pp. 40-124. 



•"'S Chubb and Hudson, ibid., pt. ii. (1925), p. 261. Other references are given. 



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