26 The Rev. D. Williams on the true Position of the 



Few circumstances in my short geological life have af- 

 forded me greater delight than having been enabled during 

 that excursion to determine with precision, the true grade in 

 the Coddon Hill series, No. 8, of the Ivy Bridge jasper rock, a 

 rock which nearly all geologists who have been in South Devon 

 can hardly have failed to observe, and in whose nether position 

 as regards the killas country on the south of it, probably as 

 many are agreed ; for my own part I felt perfectly satisfied that 

 it must be included in the lower culm measures, for the rea- 

 sons mentioned page 132. No. 129 of your Journal; but I also 

 felt that I might not be able to convey the same assurance to 

 the minds of others. I apprehended that doubts might still be 

 entertained that it was some peculiar altered rock, or tliat 

 from its lower terms resting on the granite, that volcanic ag- 

 gregate in its protrusion, or in its " elevation-crater " move- 

 ment, might have brought up with it something almost as fun- 

 damental as itself. On following the bed of the East Okement 

 river from Oakhampton, geographically up, but geologically 

 down to the granite of Dartmoor, I passed in succession the 

 following series, all dipping north at a mean of 45° to 50°. 



Black slates. 



Olive grit beds, and fine 

 foliated gray grit. 



Ivy Bridge jasper grit 

 Trap. 



Jasper grit 

 Trap. 



Jasper grit, passing 

 into 



Granite. 



The jasper grit up the East Okement is as identical in mi- 

 neral type and composition with the Ivy Bridge rock as it is 

 possible for any two specimens from any continuous bed to 

 be ; it is a strikingly characterised mineral aggregate, in which 

 I have nowhere observed a vestige of organic structure, but 

 at each locality it is precisely the same striped, plated and 

 layered compound, a coarse ribbon jasper, possessing the same 

 variegated colours, the same mineral peculiarities, and the 

 same aberrations from a mean normal type. The section up the 

 East Okement presents us with nothing more than is shown 

 above Ivy Bridge, viz. the same peculiarly marked rock resting 

 upon granite ; but let us ascend the West Okement up to the 

 Posidonia lime-rock quarries at Meldon*, and we at once ap- 

 preciate the force and value of the East Okement and Ivy 

 Bridge sections. We there observe the following descending 

 series dipping north at about 50°. 



* Spelt Elmdon on the Ordnance sheet. 



Floriferous schist and grit. 

 Smoky gray schist. 



