88 Correspondence. 



I do not believe that any geologist will ever be able to trace a 

 boundary in those slates and grits, so as to subdivide them into two 

 clearly marked groups, such as Ilfracombe, Combe Martin, Marwood, 

 Pilton beds, etc., until that is done by a good stratigraphical geo- 

 logist, independently of all fossil evidence. That fossil evidence is 

 not worth a rush in this case, because we are merely reasoning in a 

 circle, drawing a boundary to suit the fossil localities, and then using 

 the fossils to prove the correctness of the boundary. According to 

 the results of my field work, (hasty and imperfect enough, doubtless 

 in Devon, but still based upon the experience acquired by thorough 

 and exhaustive work, carried on patiently for years in Ireland), the 

 Old Ked Sandstone of Porlock, Minehead and Dunster is brought up 

 again by a great fault in the centre of North Devon, and forms 

 a ridge, running from Morte Bay to Wiveliscombe, the Lynton, 

 Combe Martin and Ilfracombe beds being part of the Marwood and 

 Barnstaple beds. 



If I am mistaken in this, then the central red ridge from Morte 

 Bay to Wiveliscombe is different from anything we have in Ireland, 

 and can be used to divide the grey slates of North Devon into an 

 Upper and Lower group, still having the genuine Old Eed Sandstone 

 of Porlock, Minehead, and Dunster below them all, and the Coal- 

 measures conformably above them all. 



These Devonian rocks will then rest, like their contemporaries, the 

 Carboniferous Slate in Ireland, between the top of the Old Eed and 

 the base of the Coal-measures, and will be the muddy and sandy 

 representatives of the Carboniferous Limestone, with a somewhat 

 different fauna, arising partly from difference of habitat and partly 

 from difference of province. The contemporaneity of different 

 assemblages of fossils in closely adjacent areas, which is the explana- 

 tion of Barrande's Colonies, has not yet been sufficiently worked out 

 or attended to. It has been the cause, not only of the Devonian 

 delusion, as I have called it above, but of the confusion among the 

 Cambro- Silurian series of Wales and elsewhere. 



I, for one, cannot feel any confidence in the stratigraphical groups 

 of these rocks, established merely to suit the supposed horizons of 

 certain fossils, and not worked out by honest stratigraphical obser- 

 vations in the field. 



Lastly, let me say that I seem to myself to have been endeavouring 

 to fix the exact place of the so-called Devonian system instead of 

 explaining it away. Yours, 



J. Bebte Jukes. 



Dublin, January 6th, 1867. 



FORM OF THE GROUND AND FAULTS IN THE DRIFT. 

 To the Editor of the Geological Magazine. 



Sir, — Allow me to correct a slight error which has crept into 

 print, although not in the proof of my paper, in your January No. 

 At page 9, line 5, for " became " read hecome, and for " sloping," in 

 line 10, read slope, when it will be seen that (however ill expressed) 



