Cornish Killas in the Devonian System. 27 



8 



l 



§ 



T3 

 O 

 U 



Black slates, rusly 

 externally. 



Ivy Bridge, jasper grit. 



Trap. 



Coddon Hill grit. 



Posidonia limestone. 



Coddon Hill grit. 



Granite. 



s r 



2 Olive-coloured grits and shale. 

 o ' Smoky gray schist. 



m 



The section after the margin of the river up to the Lime- 

 rock quarries is so clear and unequivocal in its details, that 

 nothing is left to inference or conjecture. From the far greater 

 thickness of No. 8, on the south, and from other facts I have 

 collected, I have now no doubt that this jasper grit is a new 

 term introduced into the Coddon grit series on the south, 

 which is altogether absent in the north of Devon • ; while its 

 position above the Coddon grit and its Posidonia limestones, 

 each in a totally unaltered condition, assures us that it is not 

 a metamorphic rock. 



At Tavistock I observed that, I was in error when I stated 

 in your Journal, No. 129, p. 131, that on "the eastward road, 

 after crossing^ the Tavy behind the Bedford Arms, the culm 

 schists dipped into the killas hill," whereas they dip to the 

 north there, or outwards from Whitchurch Down; they are 

 however at the base of that hill ; and as we ascend it from the 

 first turnpike gate, we observe them to incline over easily to 

 the south, and to be overlaid by a thick accumulation of a 

 dull gray and pale green ash which passes insensibly into a 

 delicate killas, in a flat position, or a gently and long undu- 

 lating outline. I had marked this low southern dip of the 

 carbonaceous schist on my field map, and from its small scale, 

 I inadvertently read it off as the true dip along the south mar- 

 gin of the river, less than a furlong distant. 



From hence I went to South Petherwin near Launceston, 

 and on a more careful examination of the slates, I observed 

 for the first time that at Does House the killas crosses the 

 little brook and valley and ascends and flanks the carbonaceous 

 slate ridge on the north, both dipping together to the south 

 at about 15°, the floriferous or culmy*beds as manifestly un- 

 derlying the killas as any one rock on earth underlies another. 



* In a prospectus of my intended publication on the Geology of West 

 Somerset, Devon and Cornwall, which I circulated at Plymouth in August 

 last, I mentioned this rock as occurring at Landkey Hill near Barnstaple; 

 a subsequent examination of that hill in November last, satisfied me that 

 I was in error in so stating. 



