Lr. Wheelton Hind — Loiver Culm of North Devon. 395 



If palaeontology is of any value, and the distribution of fossils 

 does indicate homotaxis, then we are particularly fortunate in the 

 Culm, which, though it is not richly fossiliferous, yet contains 

 peculiar and well-marked species, which do indicate well-marked 

 horizons in the Carboniferous series further north. 



There are certain stratigraphical facts which are well known^ 

 such as the general succession of the Devonian series and its 

 relation to the Culm. It is agreed that the geological structure 

 of Devonshire is a synclinal, and that in the north the rocks are 

 all very highly tilted and dip steadily south at high angles ; that 

 the dip is not simple, but that in each member of the series there 

 are many secondary folds, owing to which it is impossible to 

 estimate the thickness of each division. There appears to be na 

 unconformity between the Upper Devonian or Pilton Beds and the 

 Lower Culm. 



The Lower Culm consists of two very distinct series of rocks : 

 the Coddon Hill Beds, which are composed of thin-bedded, hard, 

 siliceous, or cherty, light-grey or fawn-coloured rocks, full of 

 radiolarian remains, and containing a small but distinctive fauna, 

 and the Venn or Black limestones, of hard, splintery, black, semi- 

 crystalline limestone and calcareous shales with Posidonomya Becheri 

 and Glyphioceras crenistria. 



The first and important point to settle is the true succession in 

 the Lower Culm, that is to say, what is the relation between the 

 peculiar thin siliceous beds of Coddon Hill and the Black limestones 

 yielding Fosidonomya Becheri of Venn, Barapton, and West Leigh ? 



Sedgwick (Trans. Geol. Soc, ser. vi, vol. v, p. 670) thought that 

 the Coddon Hill Beds were below the Posidonomya beds. Phillips and 

 many others thought the reverse ; but Mr. Ussher (" Culm-measure 

 Types of Great Britain," Trans. Inst. Min. Engineers, vol. xx, 1901, 

 p. 362), in his table of the classification of strata, favours the view 

 that the Coddon Hill cherts are below the Fosidonomya beds. This 

 Mr. Hamling tells me has been his opinion, and together we examined 

 the ground carefully with a view to ascertain the evidence for this 

 succession. 



Mr. Hamling writes me as follows : — " I see at the reading of 

 Hinde & Fox's paper on ' A well-marked horizon of Eadiolarian 

 Eocks in the Lower Culm of Devon, etc.,' I expressed agreement 

 with their conclusions, which suggest that Venn beds ai'e below 

 Coddon Hill (Q.J.G.S., vol. li, 1895, p. 609). I have worked these 

 beds very closely since then, and now believe the Coddon beds are 

 below Venn. Perhaps you can explain this change of opinion 

 since 1895." 



Standing on Coddon Hill and looking north and east, it is to be 

 noted that the Coddon Hill range runs east to beyond the village of 

 Swimbridge, and that it meets east of that village a ridge which 

 runs into it from the west, forming a Y ; i.e., there is a synclinal of 

 Coddon Hill Beds which west of Swimbridge has the beds contained 

 in the syncline gradually pinched out in succession, so that the two 

 limbs of the synclinal come together. The contour of the surface of 



