lioclcs of North Devon, 8$c. Ill 



appears to me but right to endeavour briefly to give some 

 further explanation of the grounds upon which 1 formed my 

 own individual opinion, as expressed in the Memoir read be- 

 fore the Geological Society, referred to above, and to which I 

 still adhere ; namely, that the Older Stratified Rocks of North 

 Devon, though distinguished by some peculiarities, belong to 

 the transition epoch. These additional reasons will also 

 make manifest the principles upon which I differ from the 

 new position taken by Professor Sedgwick and Mr. Murchi- 

 son. 



And first, as to gradation. When two formations are in 

 juxta-position, gradation from one to the other may be con- 

 ceived as taking place in two ways ; either by an alternation 

 between the beds of the two formations on the line of their 

 conjunction ; or, where there is no such alternation, by an 

 incorporation and interchange of their individual characters, 

 as dependent on mineral composition and the remains of the 

 organized bodies which they contain respectively. Now, 

 I have not found either of these cases applicable in North 

 Devon to the line of junction of the Trilobite slates with the 

 carboniferous strata. On the contrary, we pass at once from 

 the Trilobite clayslates (partly glossy and finely laminated) 

 and containing other peculiar organic remains, to coarse shales 

 free from any such remains, and more or less indurated and 

 siliceous, in connexion with the carboniferous limestone. 



Secondly, as to conformity. It can scarcely be necessary 

 to remind a geologist of extended practical experience, that in 

 exploring the boundary lines of two conjoining formations of 

 different a3ras, the two may be found both in conformable and 

 in unconformable position with each other hi different portions 

 of their expanse; the extent of conformity or unconformity 

 in any one part depending on the form and direction of their 

 respective stratification in that part. But in determining the 

 relative age of the two formations, I consider unconformity (on 

 however small a scale developed in a given district) as the 

 rule for our guidance, (especially when supported by corro- 

 borative considerations,) the conformity in such case arising 

 merely from coincidence in the stratification. As an in- 

 stance, I will refer to the position of the Old Red Sandstone of 

 Gloucestershire in relation to the subjacent transition beds. 

 In the environs of Tortworth, the strata of the two formations 

 are in the eastern quarter of the field in conformable position, 

 while in the western and north-western quarters they are for 

 the greater part unconformably disposed*. Similar relations 



* Geological Transactions, vol. i, second series. Geological observa- 



