1866.] JUKES OLD EED SANDSTONE AND DEVONIAN. 363 



the Lower Silurian Rocks. There are places there, where in an 

 open ditch, it is possible to stand with one foot on Lower Silurian 

 slates, and the other on a bed 700 or 800 feet above the base of the 

 Coal-measures. To the north of the fault several beds of Coal-mea- 

 sures may be seen, in many places, cropping out on the slope of the 

 ground; while still farther north a great tract of Carboniferous 

 Limestone rises out from below them, across which it is necessary 

 to travel for some miles before the Old Red Sandstone rises up in the 

 ridge, near Emly. To the south of the fault, on the other hand, 

 the base of this Old Red Sandstone may be seen reposing on the 

 highly inclined Silurian slates ; and the hill above shows several 

 hundred feet of it, the beds dipping south, under another Carboni- 

 ferous Limestone valley, the beautiful Yale of Aherlow. The 

 SKevnamuck fault, then, must have a downthrow to the north of 

 at least 4000, and possibly 5000 feet. (See Explan. of Sh. 155, 

 Geol. Surv. Ireland, or ' Student's Manual,' p. 286.) 



A similar throw in the supposed fault in North Devon would 

 account for all the observed facts. 



11. Objections considered. — The arguments against the hypothesis I 

 propose, may be arranged under three heads, (a) stratigraphical, (h) 

 Uthological, and (c) palaeontological. 



a. The stratigraphical objections have already been sufficiently con- 

 sidered, and it has been shown that while the observable stratigra- 

 phical facts would not of themselves lead to the hypothesis, there are 

 none which are conclusive against it. I may also state my belief that 

 it would inevitably have occurred to any one who had examined North 

 Devon, after having made himself acquainted with the structure of the 

 south-west of Ireland. Had it so happened, for instance, that my 

 old teacher, Professor Sedgwick, and my present chief. Sir R. I. 

 Murchison, had been familiar with the rocks of the county Cork 

 before entering on Devonshire, I believe they would have announced 

 the existence of this great fault as a matter of course. 



h. The lithological arguments against the hypothesis seem at first 

 to be stronger. The aspect of the beds about Baggy Point, Barn- 

 staple, and Dulverton is not exactly the same as that of the Lynton, 

 ILfracombe, andMortehoe beds ; and the limestones of Combe -Martin, 

 &c., appear from published accounts to be stronger than the thin 

 bands of calcareous slates in the Barnstaple district. 



In reply I would urge that the Carboniferous slate of Ireland 

 varies in character in different districts to such an extent that its 

 parts might not be recognized as belonging to the same formation, if 

 their physical continuity could not be traced, and their different 

 graduations followed across the country. 



In Devonshire itself the beds which dip under the Coal-measures 

 on the north, differ in aspect from those which rise from under those 

 same Coal-measures on the south, although they must be on the same 

 general horizon. The limestones which prevail near Combe-Martin, 

 and in South Devon, are local banks of limestone, which did not 

 happen to be formed in the beds about Barnstaple and Dulverton, 

 though the calcareous matter is not totally absent there. 



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