130 The Rev. D. Williams's Proofs that the Devonian 



it. The alternations here are commonly between a glossy gray 

 or delicate pale-green killas, and thednll olive sa7idsto?ies with 

 shales and plants, or widi Coddon Hill grits with numganese; 

 it is, in truth, these repeated alternations of the killas and culm 

 slates or schists (the olive griis being to a considerable extent 

 absent), oftentimes reciprocally adulterated by common inter- 

 mixture, or by a series of mutual interlockings, of either rudely 

 or well-defined wedge-shaped masses, along the confines ot 

 the killas and floriferous groups, which furnish such abundant 

 sources of difficulty and perplexity along that parallel, inde- 

 pendent of the unequal contortions of the beds and contra- 

 dictions of dip noticed by Mr. Phillips. It is a circumstance 

 worthy of attention, that while these fantastic inflexions im- 

 plicate the beds of the Coddon grit and floriferous almostevery- 

 where, they very rarely affect the killas; the former, however, 

 abound in cotemporaneous traps and greenstones along that 

 confine line; and if we regard the killas as ejected volcanic 

 ash or mud occurritig at intervals in this region during the 

 deposition of the culm rocks, and eventually superseding them 

 during a long succeeding period, one source of difficulty at 

 least will be removed, and it will perhaps materially assist us 

 in solving manj' embarrassing problems both here and else- 

 where. 



Again, on the south of Callington, little more than a mile, 

 we fall in with a considerable area of the Coddon Hill grit 

 and floriferous, extending about six miles from east to west, 

 and two miles from north to south*; it is entirely insulated in 

 the killas; but to object to its identity on that account would 

 be altogether captious or invidious, for it is critically the same 

 dull olive sandstones with black shales and plants underlaid 

 by the same distinctly defined Coddon Hill grits with man- 

 ganese, that we observe on the north and south of the great 

 culni-field. At a deep cutting in the turnpike road to Ply- 

 mouth, at Penter's Cross, the upper culm-beds are seen, be- 

 yond any reasonable doubt, first to intercept the killas in a 

 trough ol a quarter of a mile wide, then to alternate with it, 

 and finally, to be surmounted by it, both of them being first 

 inflected and then fractured through by the protrusion of the 

 Cotldon Hill grit about a hundred yards west of the cutting. 

 About Pillaton, round the confines of the killas and floriferous, 

 we observe repeated alternations and interlockings of the two, 

 as seen at Stoke Climsland, South Hill, and Linkinghornc. 



It is here convenient that we adjourn to Tavistock : the 

 floriferous rocks are brought into the greater portion of this 

 town, from the west flank of Dartmoor, in uninterrupted con- 

 • See Section, No. 2. 



