362 PfiOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mai. 7, 



The two uppermost of these groups, Nos. 7 and 8, belong to the 

 Coal-measures, the other six are the rocks subsequently known as 

 Devonian ; and he says, they form '^ one persistent consecutive 

 series," all dipping generally to the south, with a few occasional 

 undulations. The angle of dip, he says, increases from 20° on the 

 north to 80° on the south, being from 20° to 30° in his 1st and 2nd 

 groups, from 45° to 70° in his 3rd group, and generally 80° in his 

 4th, 5th, and 6th ; a ^' lower angle being sometimes observable on 

 approaching an undulation"*. 



All the published accounts, so far as I am aware, have a general 

 agreement with those statements, and I should myself agree generally 

 with them so far as they profess to be descriptions of observed facts. 



Now the distance in a straight line, measured directly across the 

 strike of these beds, from Lynton to South Molton, is about fourteen 

 miles, or 73,920 feet. If we say 70,000 feet, and assign a mean 

 angle of inclination of 45° to the dip of the beds, we get a total 

 thickness of 49,490 (say 50,000) feet for them. But suppose we 

 deduct about a third of the width as an allowance for undulations 

 in the beds, and reduce it to 50,000 feet, and allow the mean angle 

 of inclination to be only 30°, and I think the observable facts will 

 hardly admit of any greater reduction on the hypothesis of a suc- 

 cession of consecutive groups, we must still believe that the beds 

 below the Coal-measures in North Devon have a thickness of 25,000 

 feet, and that without reaching their base, which is concealed some- 

 where under the waters of the Bristol channel. 



It appears to me that this is a result that requires a comparison 

 with some other district where these same rocks can be measured, 

 either for its correction or its verification. 



Comparing them with the south-western part of Ireland, the cor- 

 rection I propose is the inevitable result. 



It detaches the true Old Eed Sandstone from the grey-slates, 

 which contain marine fossils, and thus divides the thickness, in the 

 first instance, between two distinct formations, while, with respect to 

 the grey slates themselves, it shows those of one district to be a re- 

 petition of those of the other, and therefore that the thickness of 

 one is not to be added to that of the other. 



The structure of North Devon, then, will be represented by the 

 figure 17, in which the facts observed in different parts of the 

 country are grouped together and condensed. It does not, then, 

 represent any particular line of country, but offers a key to the 

 structure of the whole. 



The structure is that of a broken "anticlinal, the northern arm of 

 which has sunk in against the southern arm ; the line of fracture 

 running along the crest of the curve. It is by no means an un- 

 common structure. There is an excellent example of it in the hill 

 of Slievnamuck, in the county Tipperary, where in like manUer the 

 northern arm of one of a series of parallel anticlinal ridges has 

 fallen in against the southern arm. In that place a thickness of 

 700 or 800 feet of Coal-measures is brought in direct apposition with 

 * Proceedings Geol. Soc. London, vol. ii. p. 589. 



