liocks of North Devon, SfC. 117 



If then the Older Stratified Rocks of North Devon be allow- 

 ed to bear analogy to those in the South of Ireland, the rela- 

 tions now stated appear to be conclusive as to their relative 



on the East of Ireland, § (J. 134 and 135. $$. 137 to 139, and §§. 143 to 

 151 : and Geol. Trans., vol. v. second series. Memoir on the South of Ire- 

 land, §§. 10, 13, 20. § §. 47 to 49, and §. 69.) That the same uncon- 

 formed position prevails also in Scotland and in the North of England is 

 admitted by Prof. Sedgwick. (Proceedings of Geol. Society, May 1838, 

 pp. 676, 678.) 



All these instances prove a positive break in the geological succession 

 between the transition and the carboniferous epochs. 



How is this fact, based upon a wide field of observation, to be recon- 

 ciled with that solitary example of gradation which is described by Mr. 

 Murchison as taking place from the Silurian system into the Old Red 

 Sandstone formation, and his consequent division of the latter into three 

 masses numbered in descending order, 1. conglomerate and sandstone, 

 2. cornstone and marl, 3. tilestone ? Perhaps the following view may 

 assist toward removing the seeming discrepancy. Let us consider the 

 subject in reference to relative position, composition, and organic re- 

 mains. 



With respect to position, if we look at the map we perceive at once 

 that this great deposit is flanked on both sides, on the E. and W., by 

 Silurian rocks, while j)rotruding masses of the latter are contained within 

 its area. It is therefore clearly spread in one broad sheet (with a few in- 

 terruptions) over the Silurian rocks as a base. But is this upper deposit 

 strictly conformable throughout to the lower ? From Mr. Murchison's 

 descriptions it evidently is so on the Western flank, and in many instances 

 in the interior also, being in the latter case disposed in conformity to the 

 undulated structure of the subjacent Silurian strata. Yet in those parts 

 where the stratification of the Old Red Sandstone is uninterrupted, it ap- 

 pears arranged upon one inclined plane, unto its junction with the su- 

 perincumbent carboniferous limestone, dipping throughout toward the 

 East, as represented in pi. xxxi. fig. 1. On the Eastern flank of the field, 

 however, where the Old Red Sandstone comes again in contact with the 

 Silurian rocks, there are clear instances of unconformity between the two. 

 The disposition of the Old Red Sandstone, therefore, though apparently 

 mainly governed by the form presented to it in the surfece of the Silurian 

 rocks, has not in all cases been regulated by it ,• and this circumstance, 

 combined with its widely overspreading the Silurian rocks, may be consi- 

 dered a valid proof of the difference of age, agreeing in that respect 

 with observation and experience in other tracts. 



This inference refers more f)articularly to the upper divisions, the corn- 

 stone and marl, and the conglomerate and sandstone ; for it does not ap- 

 pear strictly to apply to the lowest division, to the tilestone; and for the 

 following reasons. The two upper divisions find their analogies in other 

 parts of the world, the uppermost as being non-fossiliferous (with the ex- 

 ception of a few vegetable remains sparingly scattered), the lower as con- 

 taining the remains of fishes ; but the lowest division, the tilestone, has 

 not met with its parallel elsewhere, being distinguished by its fossil shells. 

 It thus appears as an anomaly. It is of much smaller dimensions than the 

 overlying formations, and reposes directly on the Ludlow rocks, and, while 

 containing some peculiar fossils, exhibits also others similar to those of the 

 Ludlow formation. Its affinity to the latter is thus fully shown, while in 

 the cornstone not one of these shells is found. Again, the fossiliferous 



