1851.] SEDGWICK — SLATE ROCKS OF DEVON AND CORNWALL. 19 



limestone ; while the south end of the same Dartmoor granite mine- 

 ralizes and tilts up the base of the Plymouth Group. 



7. Contemporaneous with the elevation of the mineral axis above 

 described, was a mineral axis, ranging nearly east and west, and ele- 

 vating the older groups of North Devon. We do not now see this 

 mineral axis ; but that it existed and produced its effects, at the time 

 indicated, cannot, I think, admit of doubt ; and it explains the high 

 incHnation, the southern dip, and the contortions of the great Ply- 

 mouth Group on the coast-line of North Devon. 



8. Lastly we have indications of an elevatory axis, probably con- 

 temporaneous with the two axes already noticed, ranging along the 

 south coasts of Devonshire and Cornwall. For example, the meta- 

 morphic groups of Bolt Head and Start Point, which occupy the 

 south end of Devonshire, are elevated at a high angle and dip to- 

 wards the north. Whether these metamorphic rocks belong to a 

 very ancient group, or are merely an altered form of the Dartmouth 

 Group, is a question of no moment to my present purpose ; but the 

 position, as well as the structure, of these rocks favours my hypo- 

 thesis of a third mineral axis. The same may be said of the igneous 

 rocks, forming the great plateau of the Lizard district. Considered 

 in the mass, they appear to overlie a series of slate-rocks, probably 

 about the age of the Plymouth Group, But at their southern extre- 

 mity, they bring up a group of slate-rocks of metamorphic structure 

 and of uncertain age. This evidence is no doubt obscure, but the 

 existence of the metamorphic slates, with the other phsenomena of 

 the district, combined with the appearance of metamorphic slates at 

 the south end of Devonshire, are facts which favour the hypothesis 

 of a third mineral axis. 



If these views be even approximately true, they help us to ac- 

 count for the singular contortions of the great culm-trough of North 

 Devon. For it must, on this hypothesis, have been exposed (perhaps 

 for a long period of time) to continued and conflicting movements of 

 elevation ; and its beds may at the same time have been doubled up 

 and compressed by enormous lateral forces, originating from two an- 

 tagonistic lines of elevation. 



Finally, a third mineral axis acting from the south helps us to ex- 

 plain some of the phsenomena above noticed ; and makes it probable 

 that rocks older than those of any Devonian group, may have been 

 brought to the surface along the line of the south-eastern coast of 

 Cornwall ; and, that such rocks do partially exist in the headlands 

 south of St. Austell, we have a proof in the facts stated in the pre- 

 vious parts of the paper. 



A further notice of the pheenomena observed, during the past sum- 

 mer, in the Silurian country, in North Wales, and among the 

 Yorkshire group of slates near Ingleton, and in Upper Ribblesdale, 

 is reserved for a future communication. 



c 2 



